Solar Physics, Plasma Physics, Space Science Yuandeng Shen

# Observation and Modeling of Solar Jets

Yuandeng Shen1 1Yunnan Observatories, Chinese Academy of Sciences, Kunming,
650216, China<EMAIL_ADDRESS>
###### Abstract

The solar atmosphere is full of complicated transients manifesting the
reconfiguration of solar magnetic field and plasma. Solar jets represent
collimated, beam-like plasma ejections; they are ubiquitous in the solar
atmosphere and important for the understanding of solar activities at
different scales, magnetic reconnection process, particle acceleration,
coronal heating, solar wind acceleration, as well as other related phenomena.
Recent high spatiotemporal resolution, wide-temperature coverage,
spectroscopic, and stereoscopic observations taken by ground-based and space-
borne solar telescopes have revealed many valuable new clues to restrict the
development of theoretical models. This review aims at providing the reader
with the main observational characteristics of solar jets, physical
interpretations and models, as well as unsolved outstanding questions in
future studies.

###### keywords:

Flares, Coronal Mass Ejections, Magnetic Fields, Filaments/Prominences, Solar
Energetic Particles, Magnetic Reconnection, Magnetohydrodynamic Simulation

##  1 Introduction

The dynamic solar atmosphere hosts many jetting phenomena that manifest as
collimated plasma beams with a width ranging from several hundred to few times
$10^{5}$ km [1, 2, 3, 4, 5]; they are frequently accompanied by micro-flares,
photospheric magnetic flux cancellations, and type III radio bursts, and can
occur in all types of solar regions including active regions, coronal holes,
and quiet-Sun regions. Since these jetting activities continuously supply mass
and energy into the upper atmosphere, they are thought to be one of the
important source for heating coronal plasma and accelerating solar wind [6, 7,
1, 8, 9].

This review mainly focuses on bigger solar jets including surges, coronal
jets, and macro-spicules. Although these jet activities are observed at
different scale and temperature range, they can be viewed as the same type of
solar jets due to their similar observational characteristic and generation
mechanism, i.e., magnetic reconnection dominated jet-like activities with an
inverted-Y structure. For smaller, lower-energy jet-like activities like
spicules and dynamic fibrils, their generation mechanisms are still open
questions. Previous studies suggested that spicules and dynamic fibrils are
possibly launched by upward propagating shocked pressure-driven waves leaking
from the photosphere [10, 11]. However, some recent studies indicated that a
portion of spicules are possibly produced by the same mechanism resembling
bigger jets, since they also showed an inverted-Y shape and associated with
flux cancellations [12, 9, 13]. Hence, the generation of these small jet-like
activities needs further investigation, and the present review will not
introduce them in detail. Readers who are interested in this topic can refer
to several previous reviews [14, 15, 16].

The observation of solar jets can date back to 1940s; they were dubbed surges
in history [17]. At the very beginning, surges were found to be associated
with micro-flares or sudden brightenings near their footpoints [18].
Afterwards, observations suggested that surges were dominated by local
magnetic fields [19]; they move along magnetic field lines and tend to occur
above satellite sunspots or in regions of evolving magnetic features [20, 21].
Before 1990s, observations were mainly taken by small-aperture ground-based
H$\alpha$ telescopes and a few low-resolution space instruments such as the
Skylab (1991-2001) and SMM (1980-1989). Solar jets were studied intensively
since 1990s due to the launch of a series of space telescopes, including the
Yohkoh satellite [22], the Solar and Heliospheric Observatory [23] (SOHO; 1995
to now), the Transition Region and Coronal Explorer [24] (TRACE; 1998-2010),
the Reuven Ramaty High Energy Solar Spectroscopic Imager [25] (RHESSI;
2002-2018), the Hinode [26] (2006 to now), the Solar Terrestrial Relations
Observatory [27] (STEREO; 2006 to now), the Solar Dynamics Observatory [28]
(SDO; 2010 to now), and the Interface Region Imaging Spectrograph [29] (IRIS;
2013 to now). In the meantime, more and more ground-based large-aperture solar
telescopes were put into routine observation, for example, the Swedish Solar
Telescope [30] (SST; 1-meter), the Goode Solar Telescope [31] (GST;
1.6-meter), and the New Vacuum Solar Telescope [32] (NVST; 1-meter). So far,
the temporal (spatial size) resolution has been largely improved from tens of
minutes (several arcseconds) to a few seconds (sub-arcsecond), and we now
observe the Sun from multiple view angles with imaging and spectroscopic
instruments covering a broad waveband from radio to hard X-ray (HXR). The
current high spatiotemporal resolution imaging, spectroscopic, and
stereoscopic observations continuously increase our knowledge about solar
jets. In addition, due to the tremendous improvement of computing power and
calculation techniques, numerical modeling of solar jets has also achieved
great progress in recent years. Nowadays, the study of solar jets has become a
main research field in solar physics.

Over the past three decades, significant advances achieved in observational,
theoretical, and numerical analyses have contributed to shaping our evolving
understanding of the different aspects of solar jets, such as their triggering
and driving mechanisms, fine structures, and their relationship with other
solar activities [33]. Now, we recognize that the basic energy release
mechanism in solar jets is magnetic reconnection, which converts magnetic free
energy into kinetic energy, thermal energy, and radiant energy of particles
[34, 35]. Recent high spatiotemporal resolution observations showed that solar
jets are probably miniature version of large-scale eruptions such as filament
eruptions and coronal mass ejections (CME) [36, 37]. In addition, recent
ultrahigh resolution observations further indicated that the eruption of small
spicules is possibly the same as solar jets, since they were evidenced to be
associated magnetic flux cancellations and possibly mini-filament eruptions in
several observational studies [9, 38, 13, 39]. Therefore, these results may
suggest a scale invariance of solar eruptions, and our understanding of solar
jets can probably be applied to interpret the complicated and energetic large-
scale solar eruptions and currently unresolved small spicules. Solar jets are
also important for space weather forecasting, because they often eject large-
scale mass and energetic particles into the interplanetary space.

Despite the great progress achieved in the past, the detailed physics behind
solar jets is still not completely understood. For example, questions about
their triggering and driving mechanisms, evolution behaviors, fine structures,
and particle acceleration; how do small-scale solar jets evolve into large-
scale CMEs? how do they contribute to coronal heating and solar wind
acceleration? In addition, whether different jetting phenomena from the
photosphere to the low corona are interconnected, or are they driven by the
same mechanism but different emission components of the same basic physical
process? Could the current models of solar jets be applied to explain small
spicules and large filament/CME eruptions? The main aim of this review is to
summarize our current knowledge of solar jets, and attempt to discuss how
close we are to the answers to the above questions. Readers who are interested
in the relevant research fields can refer to several previous reviews [40, 16,
41, 33, 42].

##  2 Observational Feature

###  2.1 Morphology and Classification

Solar jets are generally described as collimated, beam-like ejecting plasma
flows along straight or slightly oblique magnetic field lines. Due to the huge
improvement of the observing capabilities, solar jets can be imaged in a wide
temperature range from the photosphere to the outer corona. According to
different classified methods, solar jets were classified into different types
in history. Firstly, solar jets were divided into photospheric jets,
chromospheric jets (or surges), transition region jets, coronal jets, and
white-light jets, based on the temperature of the atmosphere in which they
occur. Secondly, they were classified as coronal hole jets, active region
jets, and quiet-Sun region jets, based on regions where they occur. Thirdly,
they were classified as H$\alpha$ jets, extreme ultraviolet (EUV) jets, and
X-ray jets, depending on different observing wavebands. Nevertheless, since
solar jets are often observed simultaneously at different wavebands covering a
wide temperature range, and they can occur in all types of solar regions, it
seems that the above classified methods are not very reasonable if one
considers the physical properties and morphologies.

Figure 1: SDO/AIA 304 Å (left) and 335 Å (right) images show an anemone jet on
2015 January 30 and a two-sided-loop jet on 2013 June 02 [43], respectively.
The arrows indicate the bright points in the eruption source regions.

Based on morphology, Shibata et al. [44] classified coronal jets into straight
anemone jets and two-sided-loop jets (see Figure 1). An anemone jet exhibits
as an inverted-Y shape consisting of a straight plasma beam and a bright dome-
like base. In contrast, a two-sided-loop jet appears as a pair of plasma beams
ejecting in opposite directions from the eruption source region [45, 46, 47,
43, 48, 49]. Recently, high resolution observations combined with extrapolated
three-dimensional (3D) coronal magnetic fields together revealed the fan-spine
topology magnetic system of straight anemone jets [50], which consists of a
coronal nullpoint, a dome-like fan that represents the closed separatrix
surface, and inner and outer spines belonging to different connectivity
domains [51, 52, 53, 54, 55, 56, 57, 58, 59, 60]. A fan-spine topology often
arises when a parasitic magnetic polarity emerges (or carries) into a
preexisting magnetic field region with opposite polarity, and a jet occurring
within it can lead to three flare ribbons due to the low-altitude impact of
particle beams accelerated through the nullpoint magnetic reconnection,
namely, an inner bright patch surrounded by a circular ribbon relevant to the
inner spine and the dome-like fan structure, and a remote elongated bright
ribbon associated with the outer spine. In principle, the fan-spine topology
represents the 3D magnetic structure of all straight anemone jets. For
straight jets in coronal hole, their outer spines are very long and can be
regarded as open fields in the outer corona; therefore, their remote
footpoints (brightenings) of the outer spines can not be identified, and these
jets can be considered as eruptive jets. In comparison, for straight jets in
or around active regions, one can often identify their entire fan-spine
structures due to their shorter outer spines. For these jets, they can be
considered as confined jets, because their ejecting material is typically
observed to be confined within the fan-spine system. Straight anemone jets
could be further divided into inverted-Y and $\lambda$ types, in which an
inverted-Y ($\lambda$) type jet was commonly interpreted as a small-scale
magnetic bipole reconnecting with the ambient open coronal magnetic fields
around the bipole top (footpoint). Hence, the different shapes could possibly
be used to distinguish the reconnection sites in solar jets [61].

Figure 2: An example of blowout jets [62]. (a) is a SDO/HMI magnetogram, in
which the inset shows a small bipole near the left end of the mini-filament
(white contour). (b1)–(b5) are H$\alpha$ images from Big Bear Solar
Observatory. (c1)–(c6) and (d1)–(d6) are SDO/AIA 304 Å and 193 Å images,
respectively. The white arrows in (b1), (c1), and (d1) point to a bright patch
at the location of the small bipole prior to the jet. The black arrow in (b1)
and the one in (c3) indicate the pre-eruption and rising phases of the mini-
filament, respectively. The arrow in (d4) indicates the first appearance of
the hot jet, and the two arrows in (d5) point to the jet’s hot and cool
components, respectively.

Moore et al. [63] classified straight anemone jets into standard jets and
blowout jets based on their different physical properties. According to their
definition, blowout jets exhibited several different distinguishing
characteristics relative to standard jets that are the same as typical anemone
jets, including 1) an additional brighten point inside the base arch besides
the outside one, 2) the blowout eruption of the base arch that often host a
twisting mini-filament, and 3) and extra jet-spire strand rooted close to the
outside bright point (see Figure 2). At the beginning, Moore et al [63] found
that about two (one) third anemone jets belong to standard (blowout) type
jets; however, this result was then updated to be about 50% each when the
statistical samples were expanded [64]. Sterling et al. [36] studied 20 polar
jets and proposed that all jets are generated in the same way as blowout jets;
they argued that the successful (failed) eruptions of the mini-filaments
inside the base arches can account for the observational characteristics of
blowout (standard) jets. In a subsequent paper, Moore et al. [65] further
examined 15 of the 20 jets studied by Sterling et al. [36] to study the onset
of the magnetic explosion in polar coronal jets; they found that a large
majority of polar jets work the same way as large-scale magnetic breakout
eruptions in association with energetic flares and CMEs, in which external
breakout reconnection proceeds and is involved in the triggering of the
eruption. Taken together with the results of Panesar et al. [66, 67, 68] they
also claim that flux cancellation is the main process whereby the energy is
stored prior to eruption in all jets and CMEs, and may also be involved in the
triggering process. So far, the finding of blowout jets has been confirmed by
many observational studies, and now we recognized that the vast majority of
solar jets are caused by magnetic flux cancellation rather than flux emergence
[62, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84].
Recently, high resolution observational studies showed that two-sided-loop
jets are also associated with flux cancellations and include the eruption of
mini-filaments inside the base arches [46, 49, 43, 85, 86], and two-sided-loop
jets occurring in filament channels may important for causing the eruption of
large filaments [87].

###  2.2 Precursor

Solar jets are evidenced to be launched from various pre-eruption structures
including satellite sunspots (or small opposite-polarity magnetic elements),
mini-filaments, coronal bright points, and mini-sigmoids. Observationally,
these structures can be regarded as the progenitor of solar jets, and studying
of them can contribute to the prediction of the occurrence and evolution
characteristics of solar jets.

In the photosphere, satellite sunspots can be considered as a conspicuous
progenitor for surges or many active region coronal jets. Rust [20] reported
that surges are liable to occur at nullpoints above satellite sunspots. Roy
[21] confirmed this finding and further proposed that significant magnetic
flux change over a short time interval is also important for producing surges.
Subsequent studies based on high resolution observations revealed that the
appearance of evolving satellite sunspots are liable to launch recurrent jets
through continuously collision with the main sunspots [88, 89, 90, 91, 92, 93,
94, 55]. In quiet-Sun and coronal hole regions, small opposite-polarity
magnetic elements can be recognized as the most conspicuous photospheric
progenitor for many lower-energy, small-scale coronal jets. Generally,
observations suggest that the onset of these jets are often tightly associated
with magnetic flux cancellations caused by the converging and/or shearing
motions of the magnetic elements’ opposite polarities [62, 66, 65, 68].

Mini-filaments in the chromosphere and the corona can be recognized as an
important progenitor for producing coronal jets. Mini-filaments were found to
be eruptive in nature, and their eruption characteristics are similar to those
evidenced in large-scale filament eruptions [95]. Several earlier observations
showed that mini-filament eruptions are tightly associated with coronal jets;
however, the authors did not clarify the physical relationship between them
[89, 96]. Shen et al. [62, 69] studied two blowout jets and found that the
erupting mini-filaments directly form the cool component of coronal jets (see
Figure 2). Recently, more and more observational studies confirmed that
blowout jets are driven by mini-filament eruptions or filament channels [73,
74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 97, 98]; some authors even
proposed that all coronal jets are originated from mini-filament eruptions,
and their generation resembles the eruption of large-scale, energetic
filament/CME eruptions [36, 65, 99].

Figure 3: Examples of solar jets originated from EUV bright points [74] (top
and middle rows) and mini-sigmoid structures [100] (bottom row). (a1)–(a5),
(b1)–(b4), and (d) are SDO/AIA 193 Å, 304 Å, and magnetogram images,
respectively. A mini-filament is indicated by the arrows in (a2) and (b2), and
the jet spire is indicated by the arrow in (a4). The circles mark the bright
point region. The blue curve in (d) indicate the position of the mini-filament
on the magnetogram. (i)–(l) are Hinode soft X-ray images from the Hinode.

Solar jets are frequently observed to be ejected from coronal bright points
and micro-sigmoids. Coronal bright points represent a set of small-scale low-
corona loops with enhanced emission in the EUV and X-ray spectra [101, 102],
and plasma ejections were found to be originated from them [103, 6, 104].
Recent high resolution observations showed that coronal bright points are
ubiquitous in the corona [105], and they are found to be liable to produce
solar jets [106, 107, 108, 109]. Statistical studies indicated that a majority
of coronal bright points produce at least one eruption during their lifetime
($\approx$21 hours) [110]. Hong et al. [74] found that about one quarter to
one third of coronal bright points produce one or more filament-driven blowout
jets during their lifetimes (see the top and middle panels of Figure 3).

A coronal sigmoid consists of many differently oriented loops that all
together form two opposite J-shaped bundles or an overall S-shaped structure
[111], which is more likely to be eruptive and is a main progenitor of solar
eruptions [112]. Micro-sigmoids have a typical size of about one fifth of the
large-scale ones; they can be formed through injecting twist into simple
coronal bright points [113], or via tether-cutting reconnection mechanism
[109]. Using the Hinode X-ray observations, Raouafi et al. [100] identified
that some coronal jets are evolved from mini-sigmoids (see the bottom panels
of Figure 3). Liu et al. [114] reported a special case in which a pair of twin
blowout jets were successively generated from a sigmoid structure; the authors
proposed that the two jets were produced by the reconnection between the
ambient open fields and the two opposite J-shaped twisted sigmoidal magnetic
fluxes, respectively. These observations indicate that coronal mini-sigmoids
can be recognized as a progenitor of coronal jets [100].

###  2.3 Fine Structure

####  2.3.1 Cool and Hot Components

Sometimes, cool and hot plasma flows can be identified simultaneously in a
single jet. The co-existed cool and hot components can be observed in EUV
images, or separately appear in H$\alpha$ images as a surge and in EUV or soft
X-ray images as a coronal jet [88, 115, 116, 117, 62, 69]. Some earlier
studies indicated that the cool and hot components of solar jets are
correlated in time and space, and with similar kinematic behaviors [118, 119,
120]. The cospatial relationship was confirmed by some recent high
spatiotemporal resolution observations [121], although the two components were
not exactly cospatial over the entire length [115], or the hot component had a
higher speed than the cool one [122]. Nevertheless, the two components of soar
jets were also found to be adjacent to each other in some events [88, 123, 96,
124, 117, 62, 69]. Particularly, Chae et al. [123] reported several jets whose
hot EUV components were identified with the cool H$\alpha$ bright jet-like
features. Jiang et al. [117] studied three jets whose cool and hot components
showed different evolutions not only in space but also in time, in which the
cool H$\alpha$ component had a smaller size than the larger, hot one, and the
former moved along the edges of the latter.

The appearance of the cool component in a jet is often after the corresponding
hot one a few minutes [125, 115, 117, 126, 127, 62, 69]. Specially, in the
outer corona at a height of ${1.71}\,R_{\odot}$, Dobrzycka et al. [128]
observed 5 polar jets in which the arriving of cool components were after the
hot ones about 25 minutes. There is one case in which the cool component
(surge) appeared about 2 hours before the corresponding hot X-ray jet [90].
This abnormal result was probably caused by the unsteady cadence and low
spatial resolution of the Yohkoh X-ray data. Since solar jets can occur
repeatedly from the same source region, and their lifetimes are usually of 40
minutes or less according to high resolution observations, the 2 hours time
interval seems too long for a single jet. Hence, the cool surge and the hot
X-ray jet in Zhang et al. [90] were very possibly two different jets
originated from the same source region, rather than simultaneous cool and hot
components in a single jet.

The different spatial relationships can possibly be reconciled by considering
the projection effect. In principle, observational results suggested that the
two components are along different magnetic field lines and dynamically
connected. Therefore, their spatial relationship should be not cospatial but
adjacent to each other. The cospatial case can be expected when the two
components are overlapped to each other along the line-of-sight. For the
delayed appearance of the cool component, one can understand it based on the
formation mechanism of solar jets. Previously, the delayed appearance of the
cool component was explained as the cooling of the earlier, hotter one [125,
115, 117], the emerging chromospheric or transition region cool plasma
accelerated magnetic tension force of the newly formed magnetic reconnection
field lines [129, 123, 127], or the different Alfvén speeds in the cool (high-
density, low Alfvén speed) and hot (low-density, high Alfvén speed) plasma
flows [130]. Based on high spatiotemporal resolution observations (see Figure
2(d5)), several works provide evidence that the cool component of jets is
directly formed by the erupting mini-filaments confined within the jet base
[63, 62, 64, 74, 36, 110, 43, 69, 38, 39, 131]. According to the
interpretation of Shen et al. [62, 69, 43], a mini-filament is confined by a
small arch surrounded by open field lines. Due to some reasons, the arch
starts to reconnect with the ambient open field lines (external reconnection),
which produces the hot jet component like the generation of a standard jet.
The external reconnection not only produces the hot jet but also removes the
confining field lines of the mini-filament, which further results in the
instability and eruption of the filament due to the internal reconnection
between the two legs of the confining field lines. Therefore, the appearance
of the cool component is naturally after the hot one, and their spatial
relationship is adjacent to each other. So far, more and more observations
indicated the appearance of cool component in solar jets, especially in
blowout jets that often involve the eruption of mini-filaments [63, 64, 36,
74, 110, 38, 39, 131].

####  2.3.2 Plasmoid

Supported by various observational evidences [40], it has been widely accepted
that solar jets are produced by magnetic reconnection [33]. A typical evidence
supporting the reconnection scenario is the formation and ejection of outward
plasmoids (also called magnetic islands, or plasma blobs) due to tearing-mode
instability of the current sheets. Singh et al. [132] observed multiple bright
plasmoids in several jets imaged by the Ca II H filtergram of the Hinode/SOT,
whose typical lifetime, size, and intensity enhancement relative to the
background are about 20–60 seconds and 0.3–1.5 Mm, and 30%, respectively.
Plasmoids has a multi-thermal (1.4–3.4 MK) structure with a number density in
the range of 1.7 – $2.89\times 10^{10}$ $\rm cm^{-3}$ [133, 134, 69].
Subarcsecond plasmoids were detected by the IRIS, whose average size is about
0.57 Mm, and their ejecting speed ranges from 10 to 220 km s-1 [135]. Besides
the observation of plasmoids in straight anemone jets [136, 81, 137, 138,
139], they were also observed in two-sided-loop jets [43] (see Figure 4).
Analysis results indicated that plasmoids observed in different types of jets
were similar, and they were thought to be created by magnetic reconnection as
a result of tearing instability of the current sheets.

Numerical experiments also revealed the appearance of plasmoids in solar jets.
Yokoyama & Shibata [129, 140] performed the two-dimensional (2D) simulations
of solar jets, in which they evidenced the creation, coalesce, and ejection of
plasmoids in the current sheets. In another simulation, high-temperature and
high-density plasmoids were generated repeatedly at the same location and were
ejected upward and downward simultaneously [141]. The authors claimed that the
merged upward moving plasmoids correspond to the anemone jets as in
observations. Ni et al. [142, 143] tested the low and high plasma $\beta$
cases to study the formation of plasmoids in solar jets; they found that
plasmoids with similar characteristic parameters as in observations are easily
created in the low $\beta$ case, while the high $\beta$ case created vortex-
like structures due to Kelvin-Helmholtz (KH) instability. According to this
study, the observed discrete high density features in solar jets could either
be plasmoids or vortex structures at different wavebands. In 3D simulations,
plasmoids were evidenced as twisted flux ropes resembling the shape of
solenoids, and they are formed most likely the result of resistive tearing
mode instabilities in the current sheets located between closed and open
fields [144, 145]. Wyper et al. [146] pointed out the tearing process should
occur at the separatrix surface between the closed and open flux systems, and
the repeated formation and ejection of flux ropes can naturally explain the
intermittent outflows, bright blobs, and filamentary structure observed in
some jets.

Figure 4: Examples of plasmoids in anemone [69] (left three columns) and two-
sided-loop jets [43] (right two columns). The anemone jet is displayed with
the SDO 171 Å (top row) and 335 Å (bottom row) images, in which the green
arrows indicate the plasmoids along the jet spire. The two-sided-loop jet is
shown with the SDO composite high-temperature images made from the AIA 94 Å
(red), 335 Å (green), and 193 Å (blue) channels (left column) and the 304 Å
(right column) images. The arrows in the top row point to the plasmoids in the
current sheet, while those in the middle and bottom rows indicate the current
sheet.

####  2.3.3 Kelvin-Helmholtz Instability

KH instability is a basic physical process that occurs when there is velocity
shear in a single continuous fluid, or where there is a velocity difference
across the interface between two fluids [147]. Recent observations revealed
the occurrence of KH instability in the solar atmosphere, such as at the
interface between erupting region and the surrounding corona [148], at the
outer edge of CMEs [149], in prominences [150], in coronal streamer [151], and
in solar jets [152, 153].

Vortex structures caused by KH instability can be regarded as a basic fine
structure of solar jets, which were frequently observed within or at the outer
edge of solar jets [154, 155]. Using IRIS observations, Li et al. [152]
reported the developing process of KH instability in a blowout jet due to the
strong velocity shear of two plasma flows along the jet spire, in which the
developing process was about 80 seconds, and the distortion scale was less
than 1.6 Mm. Using H$\alpha$ observations taken by the NVST, Yuan et al. [153]
studied the KH instability at the outer edge of a small solar jet, in which
the KH instability was thought to be caused by the shearing motion between
cool chromospheric and hot coronal plasma flows. During the mature stage,
plasma heating was evidenced around the region of the vortex structures,
supporting the scenario that KH instability can effectively transfer plasma
kinetic energy into thermal energy and heat the coronal plasma. Since velocity
shear can occur at a variety of length scales and different regions in the
solar atmosphere, this finding led the authors to conjecture that KH
instability could be an effective way to supply energy to heat the corona
plasma.

Theoretical and numerical works were performed to study the KH instability in
rotating solar jets. Zaqarashvili et al. [156] found that rotating jets are
unstable to KH instability when the kinetic energy of rotation is more than
the magnetic energy of the twist; the growth time of KH instability is several
seconds for miniature jet-like events and a few minutes or less for large
jets. The authors argued that rotating jets may provide energy for
chromospheric and coronal heating, since KH vortices can lead to enhanced
turbulence development and heating of the surrounding plasma.

###  2.4 Dynamical Characteristic

####  2.4.1 General Property

Based on Yohkoh soft X-ray observations, Shimojo et al. [157] concluded
several typical properties of X-ray jets, including that 1) most of jets are
associated with micro-flares whose brightest parts show a gap between the
exact footpoints of the jets; 2) the lengths (widths) are in the range of a
few $\times 10^{4}$–$4\times 10^{5}$ ($5\times 10^{3}$–$10^{5}$) km; 3) the
apparent velocities are of 10 to 1000 km s-1 with a mean value of about 200 km
s-1; 4) the distribution of the lifetimes is a power law with an index of
about 1.2; 5) most active region jets are observed to the west of the active
regions; 6) 76% jets show constant or converging spires whose widths get
narrower from the photosphere to the corona, and their intensity distribution
often show an exponential decrease with distance from the footpoints. In a
subsequent paper [4], they further concluded that 1) the temperatures of the
jets are 3–8 MK with an average value of 5.6 MK, similar to those of the
associate flares, and it shows a correlation with the sizes of the associated
flares; 2) the density is in the range of 0.7–4.0$\times 10^{9}\rm~{}cm^{-3}$
with an average value of $1.7\times 10^{9}\rm~{}cm^{-3}$; 3) the thermal
energies of the jets are $10^{27}-10^{29}$ ergs, far less than those of the
associated flares; and 4) the apparent velocity of the jets is usually slower
than the sound speed. The physical parameters were further studied based on a
large sample of 7197 coronal hole X-ray jets observed by the Hinode [158], the
authors found that the peaked distributions with maxima of the outward
velocities, the lengths, widths, and lifetimes of the jets are 160 km s-1,
$5\times 10^{4}$ km, $8\times 10^{3}$ km, and 10 minutes, respectively. In
addition, the velocities of transverse motions perpendicular to the jet axis
are ranged from 0–35 km s-1.

Using Ca II H broadband filter observations taken by the Hinode/SOT, Nishizuka
et al. [159] made a statistically study of chromospheric anemone jets [1].
Different from spicules [15, 16] and dynamic fibrils [160], this type of jets
are usually observed in active regions and show bright cusp-like or inverted
Y-shaped structures, and are smaller and occur much more frequently than
surges. The authors found that the shape of chromospheric anemone jets are
similar to X-ray jets, suggesting their common formation mechanism. The
typical parameters of lengths, widths, lifetimes, and velocities of
chromospheric anemone jets are in the ranges of about 1–4 Mm, 100–400 km,
100–500 seconds, and 5–20 km s-1, respectively. In addition, the velocities
are found to be comparable to the local Alfvén speed in the lower solar
chromosphere.

Nisticó et al. [61] performed a statistically study of energetic polar EUV
jets using STEREO observations; they found that the appearance of EUV jets are
always correlated with small-scale chromospheric bright points. The typical
lifetimes of the studied EUV jets are 20 (30) minutes at 171 (304) Å while
that of the white-light jets observed in coronagraph are peaked at around
70–80 minutes. It was found that the speeds are 400 and 270 km s-1 for the hot
171 Å and cool 304 Å components, respectively. The speeds measured from 171 Å
observations are comparable to those derived from coronagraph observations
(390 km s-1). Mulay et al. [161] studied active region EUV jets observed by
the SDO, their results indicated that the lifetimes and velocities are in the
ranges of 5–39 minutes and 87–532 km s-1, and the corresponding average values
are 18 minutes and 271 km s-1, respectively. Typically, all the studied jets
were co-temporally associated with H$\alpha$ jets and nonthermal type III
radio bursts, and 50% (30%) events in their samples were originated in regions
of flux cancellation (emergence). Other similar statistical studies based on
STEREO and SDO observations can also be found in the literature [162, 163]. In
statistical studies of white-light jets based on coronagraphs onboard the
SOHO, the speeds of jets at solar minimum of activity are in the range of 400
to 1100 km s-1 for the leading edge and 250 km s-1 for the bulk of their
material, while the typical speeds at maximum of activity are around 600 km
s-1 [164, 165]. In addition, Kiss et al. [166] statistically studied 301
macrospicular jets using the AIA 304 Å observations from 2010 June to 2015
December. The authors found a strong asymmetry in the spatial distribution in
terms of solar north/south hemispheres, and the average lifetime, width,
length, and velocity of the studied macrospicular jets are $16.75\pm 4.5$
minute, $6.1\pm 4$ Mm, $28.05\pm 7.67$ Mm, and $73.14\pm 25.92$ km s-1,
respectively.

####  2.4.2 Rotating Motion

Rotating motion is a typical dynamical characteristic of solar jets. Earlier
observations evidenced the appearance of rotating motion in prominences like a
tornado [167, 168, 169], and this kind of motion is frequently evidenced in
erupting filaments [170, 171, 172, 173, 174, 175, 176]. Xu et al. [177]
detected the rotating motion in a surge using the H$\alpha$ spectral
observations. With the improvement of the quality of imaging and spectral
observations, rotating motion was widely observed in H$\alpha$ surges [178,
90, 179], EUV jets [115, 88, 117, 180, 2, 181], and macrospicules [182, 183].
Some observations suggested that the eruption of twisted filaments sometime
show as rotating jets [184, 84, 174], and it can be explained as a result of
the reconnection between twisted filaments and their surrounding open fields
[185]. In such a process, the magnetic twist stored in the closed filaments or
loops are released into open magnetic fields, and plasma is driven out in the
relaxation process [88, 179, 2].

Stereoscopic observations taken by STEREO imaged the fine helical structure of
rotating jets, which exhibited different morphologies when they are observed
from different view angles [180]. A statistical study based on STEREO
observations indicated that at least about half of EUV jets exhibited a
helical magnetic field structure [61]. Using the SDO data, Shen et al. [2]
studied a rotating polar coronal hole jet which exhibited distinct bright
helical structures around the jet axis; the authors proposed that the rotating
jet was driven by the releasing of the magnetic twist stored in the pre-
existing arch into the ambient open field through magnetic reconnection.
Measurement results indicated that the period of rotation and the twists
transferred into the open fields are about 564 seconds and 1.17–2.55 turns,
respectively. Specifically, the statistical studies made by Moore et al. [64,
186] found the existence of obvious axial rotation in both standard and
blowout jets; their results showed that the number of turns of axial rotation
ranges from 0.25 to 2.5. Other case studies showed that the number of turns of
axial rotation ranges from about 0.34 to 4.7 [187, 188, 189, 190], and the
periods were measured to be 1–20 minutes [191, 192, 174].

The rotating motion of solar jets manifests as simultaneous blue- and redshift
on the either side of the jet body in doppler velocity maps [88, 182, 183,
108, 181, 193]. Pike & Mason [182] reported the appearance of simultaneous
blue- and red-shifted emission on either side of macrospicules, which was
interpreted as the presence of rotation plasma flows. Kamio et al. [183]
reported similar doppler velocity patterns and measured that the blue- and
redshift are about -120 and 50 km s-1, respectively. Cheung et al. [194]
studied four homologous helical jets at transition region temperatures, which
showed evidence of oppositely directed flows with components reaching doppler
velocities of $\pm$100 km s-1, and the magnetic twist needed for the helical
jets were found to be supplied by emerging current-carrying magnetic fields
(see Figure 5). Lu et al. [195] studied a recurrent jet event using
spectroscopic and stereoscopic observations, in which the doppler velocities
were about $\pm$90 km s-1, consistent with the value derived from stereoscopic
imaging observations. Recent statistical study performed by Kayshap et al.
[196] indicated that rotational motion is omnipresent in network jets, which
can be detected as blueshift on one edge and redshift on the other at a mean
rotational velocity of about 49.56 km s-1.

Figure 5: Spectroscopic observation of two rotating jets [194]. For each jet,
total intensity and mean Doppler velocity are computed from IRIS Si IV 1394
line. In the doppler velocity maps, blue- and redshift signals are prominent
around the northern and south edges of the jets.

####  2.4.3 Transverse Motion

Lateral expansion is a typical characteristic of solar jets, which manifests
the whiplike upward motion of the newly formed field lines [129, 88]. The
typical expansion speeds was found to be tens of km s-1 [177, 106, 158].
Savcheva et al. [158] found that most X-ray jets exhibited lateral motions,
which show some acceleration and deceleration before and after a period of
constant lateral motion. Shimojo et al. [197] reported the successive slow (10
km s-1) and fast (20 km s-1) lateral expansions in an X-ray jet, in which the
slow expanding stage was explained as the loop escaping from the anti-parallel
magnetic field, while the fast stage was corresponding to the whiplike motion
of the reconnected field lines. In contrast, Chandrashekhar et al. [198]
proposed that the progressively reconnection occurring in magnetic structures
along the neutral line could account for the slow lateral expansion motion of
solar jets. Similar slow (16 km s-1) and fast (135 km s-1) expanding motions
of loop systems were also observed in small chromospheric anemone jets [191],
in which the transition from slow to fast expansion stage occurred at the
start of the accompanying flares.

Figure 6: Examples of lateral expansion [2] (left) and transverse oscillation
[199] (right) of solar jets, showing with time-distance plots perpendicular to
the main axis of the jets at different heights. (a)–(e) and (f) are made from
the SDO 304 Å and 193 Å observations, respectively. The expanding speeds and
final width of the jet are indicated. The right panels are made from the
Hinode Ca II H images. The black and blue arrows point to the crests of the
first two inverted-V-shaped tracks. The dotted lines are linear fits to the
propagation of the two crests, and the derived phase speeds are marked.

Shen et al. [2] reported an EUV jet in which the lateral expansion showed
three distinct stages: slow (10 km s-1), fast (25 km s-1), and constant
stages. Both the slow and fast expansion stages lasted for about 12 minutes,
and the jet kept a constant width of about $4\times 10^{4}$ km during the
constant stage (left panels in Figure 6). The fast transition from the slow to
the fast expansion stage was explained as the sudden acceleration of the
magnetic reconnection between the emerging arch and the ambient open field. In
other words, the slow expansion stage corresponded to the emerging period of
the arch, during which its reconnection with the ambient open field was
slowly, while the fast expansion stage manifested the impulsive reconnection
between the two magnetic systems. The constant stage indicated the fully
opening of the closed arch and the end of the twist transfer into the open
fields, and its width corresponds to the distance between the footpoints of
the open field line and the remote footpoint of the closed arch. In a
statistical study [64], the authors found that all blowout jets showed obvious
lateral expansions but none in standard jets; the lack of lateral expansion in
standard jets was possibly due to the smaller lateral plasma pressure in the
jets than the magnetic pressure of the surrounding open field lines.

Transverse oscillation is another distinct characteristic of solar jets (right
panels of Figure 6). Cirtain et al. [7] proposed that the transverse
oscillation manifests the formation of Alfvén wave during the relaxation of
the reconnected magnetic fields. Using the magneto seismology technique, the
transverse oscillation of solar jets were used to derive some important
physical parameters. For example, considering the transverse oscillation as
kink mode oscillation [200], Morton et al. [201] estimated the temperature of
a dark thread in a jet to be $2-3\times 10^{4}$ K, therefore, the authors
proposed that the dark thread was likely to have originated in the
chromosphere. Using the measured wave parameters of a on-disk coronal hole jet
and the magneto-seismological inversion technique, Chandrashekhar et al. [202]
estimated the magnetic field strength along the jet spire to be about 1.2
Gauss. The speed and period of the transverse oscillations were measured to
be, respectively, about 100-800 km s-1 and 200–536 seconds [7, 199, 191, 201,
203, 202], consistent with theoretical prediction results of Alfvén waves in
solar jets [88, 7].

##  3 Relation to Other Phenomena

###  3.1 Plume

Coronal plumes are thin ray-like structures pervasively within polar and
equatorial coronal holes, as well as quiet-Sun regions [204, 205, 206]; they
root in chromospheric networks and can be identified over distances of several
solar radii, and even in the interplanetary space. Solar jets show some common
properties with plumes, for example, both are collimated magnetic structures
resulting from magnetic reconnection between closed and open field lines [207,
208].

Lites et al. [209] observed an EUV jet that embedded in a polar plume and
caused notable density fluctuations within the plume structure. Ubiquitous
episodic jets rooted in magnetized regions of the quiet corona were detected
in plumes and interplume regions [205]. Raouafi et al. [210] studied 28 jets
during the deep solar minimum and found that over 90% jets in their sample
were associated with plumes, in which about 70% were followed by the formation
of plumes with a time delay of minutes to hours, while the remaining jets
occurred in pre-existing plumes and caused the brightness enhancement of the
latter. Therefore, the authors proposed that solar jets are precursors of
plumes. In addition, short-lived, jet-like events and transient bright points
were identified at different locations within the base of preexisting long-
lived plumes, which was thought to be important for the maintenance and change
of the plume brightness. Raouafi & Stenborg [211] further found a large number
of short-lived small jets and transient bright points caused by quasi-random
cancellations between minority magnetic polarity with the ambient dominant
open magnetic fields, confirming their previous finding that plumes are
dependent on the occurrence of transients resulting from low-rate magnetic
reconnection. However, solar jets may not be a necessary step for the
formation and maintenance of plumes, because not all jets are accompanied by
the formation of plumes, and the birth of plumes is sort of a follow up of the
jet occurrence. Therefore, the relationship between jets and plumes needs
further in-depth investigations.

###  3.2 Filament

Solar jets are tightly associated with filaments. On the one hand, as what has
been discussed in Section 2(b) and (c) (i), many solar jets are result from
mini-filament eruptions, and the erupting filament material forms their cool
component. On the other hand, solar jets can cause the oscillation, formation,
and eruption of large-scale filaments [212, 213, 214, 215].

Luna et al. [214] reported a case of large-amplitude longitudinal oscillation
in a filament that was triggered by episodic jets along the filament axis;
they proposed that the restoring force of the large-amplitude longitudinal
filament oscillations was solar gravity, while the damping mechanism was the
ongoing accumulation of mass onto the oscillating filament threads [216]. A
similar event was reported by Awasthi et al. [217], in which the damping of
the longitudinal filament oscillation was explained with the continued mass
accretion supplied by the associated jets. Zhang et al. [215] reported the
simultaneous transverse and longitudinal oscillations in a quiescent filament
triggered by a coronal jet. Simultaneous transverse and longitudinal
oscillations in filaments can also be excited by EUV waves [218]; it was found
that the angle between the incoming waves and the filament axis is important
to trigger which kind of oscillations [218, 219]. This thought can be used to
the generation of simultaneous transverse and longitudinal filament
oscillations caused by jets. If a jet interacts with a filament along
(perpendicular) to the filament axis, large-amplitude longitudinal
(transverse) oscillation can be expected; if the jet interacts the filament
with an acute angle with respect to the filament axis, simultaneous
longitudinal and transverse oscillations can be launched.

Solar jets not only supply sufficient mass for filament formation but also
cause the instability and eruption of large-scale filaments. Zirin [212]
reported the formation of a short-lived filament caused by a surge through
filling a semi-stable magnetic trap. Liu et al. [213] reported several similar
events and found that the newly formed filaments exhibited distinct helical
structures, whose lifetimes and average lengths were more than 20 hours and
145 Mm, respectively (see Figure 7). The authors proposed two necessary
conditions for new filament formation by jets, namely, an empty filament
channel (or magnetic trap) and enough mass supplied by surges. Guo et al.
[220] studied the formation and eruption of a large filament associated with a
recurrent surge event; they confirmed that surge activities can efficiently
supply enough mass for the filament formation, and continuous mass with
momentum supplied by surges can result in the instability and even the
eruption of the newly formed filament. Other similar studies suggest that the
material for filament formation could be supplied by both of cool surges and
hot coronal jets [221, 222]. All the above studies showed that jet material
was injected into filaments from one end of the filament channels. Recently,
two-sided-loop jets in filament channels were found to be important the mass
maintaining and eruption of large-scale filament. Shen et al. [43] reported
that a two-sided-loop jet ejects material into an overlying large filament
from below through magnetic reconnection, which provided an alternative way to
understand how jet material injects into filament structures. In a recurrent
two-sided-loop jet event occurred in a filament channel, Tian et al. [87]
found that the first jet firstly caused the splitting of an overlying large
filament into a double-decker filament, and then the following jets finally
led to the fully eruption of the filament. These studies showed the close
relationship between solar jets and filaments, but detailed physical
connection between them still needs further observational and theoretical
investigations.

Figure 7: The formation of a filament due to the injection of jet material
[213]. (a)–(c) are H$\alpha$-center images from the Flare Monitor Telescope;
(d) and (e) are the blue- and red-wing images at $\pm 0.8$ Å from the
H$\alpha$ center. The arrows in (b) and (c) indicate the surge and the newly
formed filament, respectively. (f) is a SOHO magnetogram overlaid with the
outline of the filament.

###  3.3 Magnetohydrodynamic Wave

Solar jets are closely related to MHD waves. Observational studies indicated
that solar jets can act as a driver to excite torsion Alfvén waves in
themselves (see Section 2 (d) (iii)), kink waves in remote coronal loops and
filaments, and large-scale EUV waves. Statistical analysis indicated that the
most probable mechanism for exciting kink oscillations of coronal loops is the
deviation of loops from their equilibrium by nearby eruptions of plasma
ejections [223, 224]. Using the magneto seismology technique, the measured
oscillation parameters could be used to derive the magnetic field strength of
the loops/filaments. For example, Sarkar et al. [225] reported a case of jet-
driven transverse oscillation of a coronal loop whose magnetic field strength
was estimated to be about 2.68–4.5 Gauss. Luna et al. [214] estimated the
minimum magnetic field strength of an oscillating filament to be about 14
Gauss. Zhang et al. [215] derived the curvature radius of the long arcade
supporting the filament to be about 244 Mm.

Figure 8: EUV waves driven by recurrent jets [94]. The top row shows three
recurrent jets with the SDO 304 Å images, while the bottom row displays the
corresponding EUV waves with the SDO 171 Å running difference images. The
white boxes in the 171 Å images indicate the field-of-view of the 304 Å
images. The blue contours in panels (a) and (c) indicate the RHESSI hard X-ray
(HXR) sources, while the dashed red curves in the 171 Å images indicate the
EUV waves.

Although many previous studies showed that large-scale EUV waves are driven by
CMEs [226, 227, 228, 229, 230], recent observations suggest that they can also
be launched by solar jets directly or indirectly. Shen et al. [231] reported
that large-scale non-CME-association EUV waves were excited by the sudden
lateral expansion of transequatorial loops due to the impingement of solar
jets, in which the generation of the waves were caused by the sudden increase
of gas and magnetic pressures around the expansion section of the loop. In a
subsequent study, they further reported the generation of recurrent fast-mode
EUV waves ahead of homologous jets along a large-scale transequatorial loop
system (see Figure 8); they explained that the excitation mechanism of these
waves resemble the generation mechanism of a piston shock in a tube [94]. Li
et al. [232] also reported a nonlinear shock wave in a closed loop system
driven by a coronal jet at one of the footpoint of the loop, the authors
proposed that such kink of wave can quickly heat the corona plasma through the
rarefaction wave. Simultaneous EUV wave, quasi-periodic fast-propagating wave,
and kink waves were found to be launched during the interaction of a jet upon
a coronal loop [224]. In addition, the expansion of the strongly curved
reconnected loops in solar jets can also launch large-scale EUV waves [233]. A
typical characteristic of these non-CME-association EUV waves is that their
lifetimes (a few minutes) are much shorter than those driven by CMEs [231].
This is possibly because of that transient solar jets can not provide
continuous driving to EUV waves like those driven by CMEs. EUV waves were
evidenced to be important to trigger sympathetic solar activities, it was
observed that coronal hole jets could be launched due to the passing of EUV
waves [234]. This suggests that solar jets can also be produced by external
disturbances except for internal magnetic activities such as flux
cancellations and mini-filament eruptions.

###  3.4 Coronal Mass Ejection

CMEs represent large-scale plasma and magnetic field releasing from the Sun
into the interplanetary space [235, 236, 237]. CMEs with apparent angular
width of $15^{\circ}$ or less are typically associated with solar jets [238].
Observations suggested that solar jets can not only cause narrow jet-like CMEs
[164] (see the left panels in Figure 9), but also standard broad CMEs with a
typical three-part structure [239]. Sometimes, a paired narrow and board CMEs
can be simultaneously launched by a single blowout jet [62, 240] (see the
right panels in Figure 9).

Narrow jet-like CMEs are simply the outward extensions of solar jets in the
outer corona [164, 94]. Statistical studies showed that during solar activity
minimum the leading edges of jet-like CMEs propagate at speeds of 400-1100 km
s-1, while the bulk of their material travels at an average speeds of about
250 km s-1 at heliocentric distances of ${2.9-3.7}\,R_{\odot}$ [164]. In
contrast, narrow jet-like CMEs during solar activity maximum have a typical
speed of about 600 km s-1, and they tend to be brighter and wider than those
in solar activity minimum [165]. The propagation of jet-like CMEs did not
regulated by the gravity along, since some of them exhibit accelerations
rather than decelerations above ${3}\,R_{\odot}$ [241]. In addition, the
direction of CMEs originated from solar jets can be significantly changed
through interacting with other magnetic structures [242, 243]. The on-disk
progenitors of jet-like CMEs include flux emergence [244, 245, 246] and the
eruption of mini-filaments [96, 247, 75, 248, 84]. Recently, Panesar et al.
[249] studied many jets at the edge of an active region; they found that six
of the homologous jets resulted in the so-called streamer-puff CMEs, and the
CME-producing jets tended to be faster and longer-lasting than the non-CME-
producing jets. Their observations also indicated that streamer-puff CMEs are
due to the blowout of twisted streamer-base loops through magnetic
reconnection.

Figure 9: Left two columns: SOHO LASCO/C2 and 195 Å difference images show two
narrow white-light jets in the coronagraph and on the disk limb [165]. Right
two columns: STEREO 304 Å and the inner coronagraph difference images show a
pair of simultaneous narrow and broad CMEs from two different view angles
[240]. The bright feature in the source region and the jet are indicated by
the arrows in (a1), while the narrow jet-like CME and the broad bubble-like
CME are indicated by the arrows in (a2) and (b2), respectively.

Broad bubble-like CMEs with a typical three-part structure but on much smaller
scales were found to be caused by solar jets [61], which are not simply the
extension of jets into the outer corona, and their generation mechanism is
possibly similar to large-scale CMEs. Jiang et al. [250] reported that
sympathetic bubble-like CMEs can be launched through the impingement of a jet
on remote interconnecting loops. Hong et al. [247] reported a micro-CME caused
by a blowout jet that exhibited many observational characteristics as those
identified in large-scale CMEs, suggesting the similarity between jet-driven
micro-CMEs and large-scale CMEs [37]. Liu et al. [239] observed a jet-
associated bubble-like CME whose bright core was evolved from the jet. Solar
jets in or around active regions in association with fan-spine magnetic
systems are often confined [55]. However, a few studies found that some broad
CMEs are evolved from fan-spine eruptions [251]. For example, Li et al. [252]
observed a broad CME that was caused by the eruption of a complicated fan-
spine system in which a large fan-spine system hosted a small one below its
fan. The event started from the eruption of a mini-filament underneath the fan
of the small fan-spine system, which firstly triggered the nullpoint
reconnection within the small fan-spine system; then the eruption of the small
fan-spine system further triggered the nullpoint reconnection within the large
fan-spine system. Here, the successful formation of CMEs from fan-spine
eruptions might be due to the weak magnetic confinement of the overlying
magnetic fields or sufficient energy released during the associated flares
[236].

Shen et al. [62] reported an interesting event in which a pair of narrow and
broad CMEs were dynamically connected to a single blowout jet which showed
cool and hot components. Similar event was possibly observed by Ko et al.
[116], where they detected both cool and hot components in a jet and the
appearance of both jet-like and bubble-like CME pair in the coronagraph.
However, due to the low resolution observations they used, the authors did not
establish the physical relation between the CMEs and the jet. Shen et al. [62]
proposed a cartoon model to interpret the generation of the cool and hot
components and the formation of the paired CMEs. According to their
interpretation, the hot component is the outward moving heated plasma flow
generated and accelerated by the external reconnection between the base arch
and the ambient open field lines, and it further evolves into the narrow jet-
like CME in the outer corona. In the meantime, the external reconnection
removes the confining fields of the mini-filament, which therefore leads to
the rising of the mini-filament and the formation of an internal current sheet
between the two legs of the confining field lines. Finally, the reconnection
in the internal current sheet further results in the full eruption of the
mini-filament and the formation of the broad bubble-like CME, during which the
erupting filament material forms the jet’s cool component. Recently, more and
more observations evidenced the appearance of paired narrow and broad CMEs in
association with on-disk blowout jets [253, 240, 254, 155, 243], and the
phenomenological model of Shen et al. [62] provides a possible explanation for
these observations but further observational and numerical investigation is
required to confirm this scenario.

###  3.5 Particle Acceleration

Solar energetic particles (SEP) carry important information about the particle
energization inside the solar corona, as well as the properties of the
acceleration volume. SEP events are divided into ”gradual” and ”impulsive”
types. Gradual SEP events are long-lasting, intense, more closely correlated
with CMEs, and characterized by the abundances and charge states of the solar
wind. Therefore, they are thought to be accelerated by CME-driven
coronal/interplanetary shock waves. In contrast, impulsive SEP events are
short lived, less intense, closely related to flaring active regions,
characterized by high ${}^{3}\rm He/^{4}\rm He$ ratios and high ionization
states, and tightly correlated with type III radio bursts [255].

Flaring regions accompanied by solar jets are found to be the most possible
candidate solar source for producing impulsive SEP events [256], since the
magnetic field along which a jet emerges is open to interplanetary space,
offering a clear ”escape route” for flare accelerated particles. In radio
observations, type III bursts are produced by electrons streaming along open
field lines extending to interplanetary space. Many studies indicated that
type III radio bursts and SEP events are spatially and temporally associated
with solar jets [257, 258, 259, 260, 261, 69, 262]. Wang et al. [263]
investigated 25 ${}^{3}\rm He$-rich events and found that their sources lie
close to coronal holes and characterized by jet-like ejections along Earth-
directed open field lines. Some studies suggested that impulsive SEP events
are associated with narrow jet-like CMEs [264, 265, 266, 263]. Nitta et al.
[267] found that the solar source regions of SEP events are often accompanied
by solar jets preceded by type III radio bursts, and about 80% events showed
open field lines in or around their source regions. In addition, ${}^{3}\rm
He$-rich SEPs were also observed to be associated with helical jets [268,
268], and the solar source regions could be small active regions near coronal
holes [266, 263, 268], plage regions [269], and sunspots [270, 271].

Type III radio burst is an important diagnostic tool for SEPs. It is a
signature of propagating nonthermal electron beams in a wide range of heights
of the solar atmosphere (from the low corona to the interplanetary space), and
is excited at the fundamental and second harmonic of the local electron plasma
frequency ($f_{pe}\approx 9\sqrt{n_{e}}$ kHz, here $n_{e}$ is the electro
number density) by the Langmuir waves generated by the electron beam
instabilities [272]. Since the electron number density of the corona decreases
rapidly with the increasing height, a type III radio burst drifts from high
(low) to low (high) frequencies reflecting the upward (downward) moving
electron beams. Chen et al. [260, 262] derived the trajectories of electron
beams in the low corona and found that each group of electron beams diverges
from an extremely compact region that located behind the erupting jet spire
but above the closed arcades, coinciding with the location of magnetic
reconnection predicted in jet models.

Figure 10: Schematic describing prompt SEP events [273]. Time series data
track energetic electrons from the Sun into interplanetary space are plotted
on the left and right, including soft and HXRs, radio waves, and non-thermal
electrons seen near 1 AU. The center shows the results of HXR and EUV
observations and a schematic of jet model. The reconnection region, the flare
loop, the HXR footpoints, and the jet are labeled.

Type III radio burst is an excellent qualitative maker of accelerated
electrons and their paths, but it can not be used to quantitatively measure
the emitting electron distributions due to the nonlinear processes in its
generation [272]. A complementary diagnostic tool for studying accelerated
electron distributions is HXR observation, which is dominated by footpoint
emission in the dense chromosphere due to the downward propagating electron
beams, but emission from escaping electron beams in the low density corona is
typically too faint to be observed [274]. Glesener et al. [275] analyzed the
accelerated electron distributions in a jet-associated event using
simultaneous HXR and microwave data and found that the HXR time profile above
20 KeV matches that of the accompanying type III and broadband gyrosynchrotron
radio emission, indicating both accelerated electrons escaping outward along
the jet path and those trapped in the flare loop. Using combined radio and HXR
observations, Krucker et al. [273] observationally confirmed the three
expectant HXR sources in solar jets as those predicted in jet models, in which
two sources are at the footpoints of the post-flare loop, and the other one is
at the footpoint of the newly formed opened field lines (see Figure 10).

Above studies highly suggested that impulsive SEP events are tightly
associated with solar jets, and they are mostly accelerated by the mechanism
of interchange reconnection. However, the detailed acceleration mechanism of
SEPs is still an unresolved question, although several possible theoretical
mechanisms have been proposed to account for the acceleration of SEPs [276,
277, 278].

###  3.6 Coronal Heating and Solar Wind

The problems of coronal heating and the acceleration of solar wind are two
highly controversial topics in solar physics. Since energy must come from the
solar interior, it is hard to understand why coronal temperature is extremely
hotter than the solar surface. The problem is primarily concerned with how
energy is continuously transported up into the corona through nonthermal
processes from the solar interior and then convert it into heat within a few
solar radii. In the last half-century, many coronal heating theories have been
proposed, but two theories have remained as the most likely candidates: wave
heating and magnetic reconnection [279]. Solar wind is composed of charged
particles including neutral atoms, positive charged ions, and free electrons,
which is released from the upper solar atmosphere and fill the majority of the
volume of the solar system. Solar wind has two fundamental states: slow and
fast solar winds. While their compositions and temperatures are similar to the
corona, their average velocities in near-Earth space are respectively about
300–500 km s-1 and 750 km s-1 for slow and fast solar winds [280]. Previous
studies suggested that the slow solar wind appears to originate from a region
around the Sun’s equatorial belt that is known as the streamer belt, while
coronal holes that consist of funnel-like regions of open field lines are
regarded as the solar source of the fast solar wind. However, the detailed
origin and acceleration of solar wind is still not understood and cannot be
fully explained by current theory [281]. Recent observations suggested that
high frequency but small-scale solar jets (also called spicules, fibrils, and
microjets) seems significant important for supplying mass and energy to power
the corona and the solar wind [282].

Ultraviolet spectrum observations revealed prevalent high-energy jets in the
corona at an average speed of 400 km s-1, whose energy and mass can satisfy
the power ($6\times 10^{27}\rm erg~{}s^{-1}$) and mass flux ($2\times
10^{12}\rm g~{}s^{-1}$) requirements of the corona and solar wind if one
assume a birthrate of 24 events per second over the whole Sun [6]. Shibata et
al. [1] proposed that chromospheric jets, which have a width (length) of
0.15–0.3 (2–5) Mm and eject at a speed of 10–20 km s-1, may play an important
role in heating coronal plasma as the nanoflare scenario [283]. The one-to-one
relation between chromospheric jets and their coronal counterparts were
examined in detail [284], which showed that chromospheric plasma was propelled
upward with speeds of about 50–100 km s-1, and with the bulk of the mass
rapidly heated to transition region temperature. A little bit later, plasma
directly associated with these jets was heated to coronal temperature of at
least 1–2 MK, at the bottom during the initial states, and both along and
toward the top of the chromospheric feature. Very recently, Samanta et al. [9]
found that enhanced coronal emission generally appeared at the top of
chromospheric spicular jets, which implied that the chromospheric spicular
jets channelled hot plasma into the corona, and it provided a link between
magnetic activities in the lower atmosphere and coronal heating (see Figure
11). In addition, solar jets also excite shocks ahead of them and drive KH
instability at their boundaries; the dissipation of shocks and reconnection
within the vortex structures also release energy to heat the corona [6, 94,
153].

Figure 11: Heating effect caused by spicular activities [9]. The left panel
shows a SDO 171 Å image (yellow) overlaid with the H$\alpha$ blue wing image
(grayscale) from GST. The H$\alpha$ blue wing and the same image overlain with
the SDO 171 Å image are shown in each pair of panels on the right, in which
the white dotted boxes correspond to the box regions (R1–R4) in the left
panel.

Alfvén waves, which propagate along magnetic field lines over large distances
and transport magnetoconvective energy from near the photosphere into the
corona, have been invoked as a possible candidate to heat coronal plasma to
millions of degrees and to accelerate the solar wind to hundreds of km s-1.
Transverse oscillations of spicular jets were regarded as the presence or
passage of Alfvén waves; the energy carried by these Alfvén waves was found to
be enough to accelerate solar wind and to heat the quiet corona [285, 286,
287, 288]. Cirtain et al. [7] detected two distinct speeds of solar X-ray
jets, in which one is near the Alfvén speed ($\sim$800 km s-1) and the other
near the sound speed ($\sim$200 km s-1). The authors claimed that a large
number of X-ray jets with high velocities may contribute to high-speed solar
wind. McIntosh et al. [289] found that a significant portion of the energy
responsible for the transport of heated mass into the fast solar wind was
provided by episodically occurring small-scale jets in the upper chromosphere
and transition region. Tian et al. [290] found two types of doppler shift
oscillations in the corona, in which one was at the loop footpoint regions
with a dominant period around 10 minutes, while the other was associated with
the upper part of loops with a period of 3–6 minutes. The authors argued that
the first type is quasi-periodic upflows associated with small-scale jets and
it plays an important role in the supply of mass and energy to the hot corona,
while the second type is kink/Alfvén waves (see also De Moortel et al. [291]
and Threlfall et al. [292]). Recent IRIS observations also reveled the
prevalence of small-scale jets from the networks of solar transition region
and chromosphere [8]; they originate from small-scale bright regions and
preceded by footpoint brightenings, ejecting at a speed of 80–250 km s-1 and
are accompanied by transverse waves with amplitudes of about 20 km s-1. They
were thought to be an intermittent but persistent source of mass and energy
for solar wind.

For big jets that often reach up to a few solar radii and can be observed as
white-light jets or jet-like CMEs; their contribution to solar wind often
exhibit as microstreams or speed enhancements [293, 294, 211, 295]. It was
found that these jets are not sufficient to explain the fast solar wind [296].
Observations indicated that the motions of white-light jets are not consistent
with the ballistic behavior, and some of them even exhibit slight
accelerations instead of decelerations above ${3}\,R_{\odot}$. This suggested
that the motions of white-light jets are regulated by other forces besides the
gravity. In addition, the bulk of almost all white-light jets travel at lower
velocities averaging around 250 km s-1 at heliocentric distances of a few
solar radii. These observational facts may imply that the moving jets have
been incorporated into the ambient solar wind [164, 241]. Yu et al. [297]
found that fast solar polar jets show a positive correlation with high-speed
responses traced into the interplanetary medium, and they contributed about
3.2% (1.6%) of the mass (energy) of solar wind. The authors further analyzed
the responses in the solar wind resulting from a high-speed jet at a speed of
about 1200 km s-1; they found an ubiquitous presence in polar coronal regions
at about 100-fold mass and energy greater than the coronal response itself.
This suggests that the primary acceleration of solar wind should induce the
dissipation of high-speed solar jets [298].

##  4 Physical Interpretation and Modeling

With the unceasing improvement of solar telescopes and numerical modeling, the
physical interpretation of solar jets have achieved significant progresses in
recent years. In theoretical studies, the mechanism of flux emergence and
onset of instability or loss of equilibrium were investigated in detail, in
which the slingshot effect, untwisting, and chromospheric evaporation were
considered as the possible acceleration mechanisms [33]. As more and more
observational studies revealed that solar jets are caused by the eruption of
mini-filaments in association with flux cancellations, new models were also
developed to account for these new features. Although there are various
aspects that have not yet been fully addressed, the results of the current
simulations have been in reasonably good agreement with the observations
including morphology, velocities, and basic plasma properties.

###  4.1 Emerging-Reconnection Model

Heyvaerts et al. [299] proposed an emerging-reconnection model for explaining
solar flares and surges. This mechanism was also proposed for the onset of
CMEs [300]. Shibata et al. [106] found that many X-ray jets are associated
with emerging flux regions, and started with formation and ejection of
magnetic plasmoids; they therefore proposed that the emerging-reconnection
scenario could be a possible explanation for solar jets. This scenario was
tested with 2D MHD simulation without considering the effect of heat
conduction and radiative cooling [129, 140], which showed that simultaneous
hot X-ray jet and cool H$\alpha$ surge are generated by magnetic reconnection
between emerging fluxes and ambient pre-existing magnetic fields (see Figure
12). The hot jet is the secondary jet accelerated by the enhanced thermal
pressure gradient behind the fast shock caused by the collision of the
reconnection outflow with the ambient magnetic field, while the cool surge is
formed by the cold chromospheric plasma that is carried up by the emerging
flux and accelerated by the tension force of the reconnected field lines
(slingshot effect). The reconnection outflow from the current sheet is
composed of many plasmoids produced by tearing and coalescence instabilities,
which represent miniature flux ropes in 3D [106, 144, 145, 146]. The emerging-
reconnection scenario was intensively studied in previous articles with 2D and
3D simulations by considering more realistic physical condition, and the
results could be applied to explain many characteristics of solar jets (see
[301, 302, 145, 303, 33] for details).

Figure 12: Schematic diagrams for the numerical models of two-sided-loop jets
(left) and anemone jets (right), in which the arrows show the moving direction
of the magnetic field lines, and features including the X-point, slow and fast
shocks, cool and hot jets are indicated [140].

Although there are many theoretical studies of the emerging-reconnection
scenario in the literature, the explicit observational evidence for flux
emergence directly driving jets is so far limited. In an emerging active
region, Li et al. [163] observed 575 jets, in which most of them occurred at
the periphery region of the emerging fluxes. However, the authors did not
figure out the relationship between the jets and the flux emergence. In many
observations, opposite polarities firstly show emergence and then followed by
flux cancellation, and the associated jets often occurred right after the
start of flux cancellation [62, 69, 46, 87, 37]. Such a magnetic flux
variation pattern often suggest the triggering of the jets should be flux
cancellations. One possible example of this case was presented by Cheung et
al. [194]; however, since the jet eruption source region was very dynamic with
mixed polarity, it hard to say if the jet was directly caused by flux
emergence or not. Panesar et al. [98] augured that the reason for flux
emergence not directly causing jets is because the speed of emergence is
seldom, if ever, fast enough. The emerging-reconnection scenario was
challenged by the discovery of blowout jets that often involve the eruption of
mini-filaments or filament-channels [63, 62, 36, 69, 97] in association with
flux cancellations [247, 304, 305, 306, 66, 68, 307, 308, 43], and the cool
component of blowout jets is actually the erupting filament material itself
[62, 69, 43] rather than chromospheric material carrying up by emerging flux
and accelerated by magnetic tension force as proposed in the emerging-
reconnection model [129]. In a statistical study of 27 equatorial coronal hole
jets, Kumar et al. [97] found six jets (22%) were apparently associated with
flux cancellations, while the remaining events did not show measurable flux
emergence or cancellation associated with the eruption. Therefore, the
ultimate trigger source of solar jets still requires further investigation.
One should keep in mind that the emergence aspects of the models introduced in
following sub-sections is still subject to verification.

###  4.2 Embedded-Bipole Model

Pariat et al. [309] proposed the embedded-dipole model to interpret rotating
solar jets. The model adopted an axisymmetrical fan-spine topology that hosts
a nullpoint within the system, in which magnetic free energy is injected
slowly by footpoint motions that introduce twist within the closed dome, and
is released rapidly by the onset of an ideal kink instability. Since
reconnection is forbidden for the axisymmetrical nullpoint topology, explosive
energy release via reconnection can only occur when the symmetry of the system
is broken by the occurring of kink instability until the magnetic stress
builds up to a high level. The reconnection between the twisted, close and the
ambient untwisted, open field lines launches a torsional Alfvén wave which
compresses and accelerates the plasma along the reconnected open field lines
upwardly. Eventually, an upward ejecting helical rotating jet is generated,
and it has similar geometrical features as observations, such as the
inverted-Y shape, the drift of the jet axis [106], helical structure [180, 2,
62], and Alfvén waves within jets [7, 199]. It was found that this mechanism
can efficiently release about 90% free energy stored in the embedded bipole
topology. If a stress is constantly applied at the photospheric boundary,
recurrent rotating jets can be launched [310]. In such a symmetric
configuration, Rachmeler et al. [311] found that reconnection is fundamental
for jet formation. Recently, the embedded-bipole model was subsequently
extended for studying the influence of magnetic field geometry [312], plasma
beta [313], gravity [314], and the characteristic lengths of the spine and fan
structures [315]. In addition, the possible applications of the embedded-
dipole model to interpret standard and blowout jets [312], the formation of
plasmoids [146], and microstreams and torsional Alfvén waves in the solar wind
[314] were also explored in great detail.

###  4.3 Breakout Jet Model

The magnetic breakout model was originally proposed to interpret the
initiation of large-scale CMEs, in which magnetic reconnection between the
unsheared field and neighboring flux systems decreases the amount of the
overlying field and, thereby, allows the low-lying sheared flux to breakout
[316]. So far, the magnetic breakout model has been confirmed by many
observational studies [237, 317].

Figure 13: The evolution configuration of the breakout jet model [318]. Field
lines with different colors represent different connectivity domains, and the
positive and negative polarities are respectively indicated by the plus and
minus symbols. The yellow curves depict the filament or flux rope formed
beneath the central arcade. The current density is displayed as semi-
transparent shading (color scale), and high current density regions can be
identified as thin strips under beneath the filament (d) and the center of the
simulation domain (b and c).

Recently, high-resolution observational and statistical studies suggested that
all coronal jets are probably driven by mini-filament eruptions, and they
share many common characteristics with large-scale eruptions. Therefore,
coronal jets are proposed to be the miniature version of large-scale eruptions
[63, 247, 62, 64, 74, 186, 36, 33]. In this line of thought, Wyper et al.
[318] performed an ultrahigh-resolution 3D MHD simulation to test this
hypothesis, using the above mentioned embedded-dipole scenario (see Figure
13). The initial magnetic configuration is a fan-spine structure, which is
current-free and therefore has no filament and no free energy within it to
power an eruption. Through shearing the footpoints of field lines connecting
to the parasitic polarity over a finite time interval, the system is energized
and a twisted filament structure is generated underneath the fan structure.
The confining field lines of the filament expands upward towards the nullpoint
and creates a current sheet between the confining field and the ambient open
field. The (external) reconnection in this current sheet removes the confining
field of the filament, allowing the filament to rise. Subsequently, (internal)
reconnection starts underneath the filament possibly enhanced by the kink or
torus instability, which eventually leads to the violent eruption of the
system and the formation of a rotating jet along the reconnected open field
lines. The simulated jet is accelerated by torsional Alfvén wave launched when
the twist in the filament begins to transfer into the ambient open field
through magnetic reconnection, and the jet body is composed of hot and cool
plasma flows originated form the reconnection region and the filament,
respectively. In this model, the eruption is due to reconnection rather than
ideal instability as proposed in the embedded bipole model [309], and the
physical process is similar to the magnetic breakout model. Therefore, the
authors named their model as breakout jet model, and claimed that the magnetic
breakout model is a universal model for solar eruptions regardless of their
scales. In subsequent studies, the authors further used their model to explain
observational features of solar jets [319, 320, 321].

###  4.4 Data-constrained and Data-driven models

To obtain realistic numerical results that are more comparable with real
observations, some works managed to use multi-wavelength observations in
tandem with MHD simulations to investigate the formation and evolution of
solar jets. Such kind of simulations are known as data-driven models, which
use continuously time-varying solar observations as input to reproduce solar
jets. In contrast, if one use one instantaneous cadence of observation as
input, it should be called data-constrained modeling.

Jiang et al. [322] simulated a jet-like eruption in a realistic and self-
consistent way from its origin to onset with a data-driven MHD model; their
result is well consistent with EUV observations. The authors found that the
transition from the pre-eruptive to eruptive state is due to the magnetic
reconnection between a stressed emerging and expanding arcade and the ambient
pre-existing open field, in agreement with the physical picture described in
anemone jet models [129, 301]. In addition, their simulation also revealed
that the non-potential magnetic flux emergence not only continuously injects
magnetic free energy/helicity into the system due to photospheric shearing
motions, but also stresses the field to form an intense current sheet.

Using extrapolated non-force-free magnetic field as the initial condition,
Nayak et al. [323] performed a data-constrained MHD simulation to study
blowout jets. In their simulation, the plasma is idealized to be
incompressible, thermally homogeneous and having perfect electrical
conductivity. They found that the initiation of the jet is due to the magnetic
reconnection near a set of two 3D magnetic nullpoints, and the jet itself is
evolved from a flux rope near the nullpoints through changing the flux rope’s
magnetic field lines from an anchored to an open topology. In addition, the
generation of flare ribbons is found to be attributed to reconnections at a 3D
nullpoint and a quasi-separatrix layer, consistent with previous data-
constrained simulation of circular flares [50] and the observations of
confined fan-spine jets [55].

Cheung et al. [194] presented the data-constrained simulations of four
homologous helical jets originated from a fan-spine magnetic system. Based on
the extrapolated potential magnetic field, the authors used the time-dependent
magnetofrictional method [324] to carry out the numerical simulation of the
coronal field evolution. Their result showed that the emergence of current-
carrying magnetic field supplies the magnetic twist needed for the formation
of recurrent helical jets. Since the magnetofrictional method calculates the
evolution of the magnetic field through a series of quasi-static equilibria in
response to photospheric footpoint motions, it can capture the response of the
relax of a magnetic configuration to the Lorentz force, but it can not reveal
the heating process of the cool plasma by the stored magnetic energy, as well
as the acceleration mechanism of the ejecting plasma. Meyer et al. [325]
presented eight different simulations to demonstrate the structure of coronal
jets in unipolar regions, in which the coronal magnetic field is evolved in
time using the magnetofrictional technique. The investigated photospheric
magnetic field configurations include a single parasitic polarity rotating or
moving in a circular path, opposite polarity pairs involved in flyby
(shearing), cancellation, and emergence. Although the simulations can not
model the dynamic eruptive stage of the jets, it can be used to diagnose the
building of magnetic energy and the formation of the jet structures. The
authors found that certain configurations and motions, such as twisting and
shearing, can produce a twisted flux rope and allow the significant buildup of
free energy, and they can be viewed as the progenitors of blowout jets; other
simpler configurations are more comparable to the standard jets.

###  4.5 Large-scale Interplanetary Jet

Most previous simulations were performed within a small numerical domains in
Cartesian geometry to study the generation mechanism and evolution process. So
far, only a few publications considered a large simulation domain extension to
the interplanetary space using spherical geometry to investigate the
interplanetary effects caused by solar jets.

Török et al. [326] and Lionello et al. [327] performed a 3D, viscous,
resistive MHD simulation in spherical coordinates. The simulation domain
covers the corona from ${1-20}\,R_{\odot}$, and the effects of radiative
losses, thermal conduction parallel to the magnetic field, and an empirical
coronal heating function are all considered. The simulation adopted the flux
emergence scenario to generate the jet in the low corona, in which the authors
evidenced the transition of a standard jet to blowout jet if the emergence is
imposed for a long time, resembling other 3D emerging-reconnection models
[145]. A white-light CME is identified two hours after the launch of the
standard jet. Several plasmoids are identified along the CME, which manifested
the episodic reconnection outflows at larger heights. It was estimated that
the total energy and mass provided by the jet to the background solar wind are
about (0.3–1.0)% and (0.3–3.0)%, respectively. In addition, the authors found
that blowout jets can produce a stronger perturbation in the solar wind than
standard ones.

To investigate the influence of solar jets to the solar wind, Karpen et al.
[314] extended the embedded bipole model [309] by including spherical
geometry, gravity, and solar wind in a nonuniform, coronal hole-like ambient
atmosphere. Similar to previous works, they launched a helical jet due to the
resistive kink-like instability that drives fast reconnection across the
closed-open separatrix; they found that the jet propagation is sustained
through the outer corona, in the form of a traveling nonlinear Alfvén wave
front trailed by slower-moving plasma density enhancements that are compressed
and accelerated by the wave. The authors claimed that their results agree well
with observations of white-light jets, and can explain microstreams and
torsional Alfvén waves detected in situ in the solar wind. Using another code
that employs Alfvén wave dissipation to produce a realistic solar wind
background, Szente et al. [328] studied the effects of coronal jets on the
global corona and their contribution to the solar wind. A reconnection-driven
blowout jet similar to that described by Pariat et al. [309] is generated, and
its physical structure, dynamics, and emission are close matching with the
observed EUV and X-ray jets. The authors found that the large-scale corona is
affected significantly by the outwardly propagating torsional Alfvén waves
generated by the jet (across 40∘ in latitude and out to ${24}\,R_{\odot}$).
The simulation also showed that the magnetic untwisting loses most of its
energy in the low corona below ${2.2}\,R_{\odot}$, but the introduced magnetic
perturbation can propagate out to ${24}\,R_{\odot}$ within 3 hours. Consistent
with observational results [297, 298], the above simulations confirmed the
conjecture that coronal jets provide only a small amount of mass and energy to
the solar wind.

##  5 Conclusion and Prospects

High spatiotemporal resolution imaging, spectroscopic, and stereoscopic
observations covering a wide temperature range over the last several decades
significantly improved our understanding of solar jets, including various
aspects such as their triggering, formation, evolution, fine structure,
relationships with other solar eruptive activities, and the possible
contribution to the coronal heating and the acceleration of solar wind.
Nowadays, we recognize that the basic energy release mechanism in solar jets
is magnetic reconnection; they are triggered by photospheric magnetic
activities exhibiting as flux flux cancellation and shearing motions of
opposite polarities, and accelerated alone or in combination by possible
mechanisms of untwisting, chromospheric evaporation, and slingshot effect.
Observationally, solar jets can be divided into eruptive jets and confined
jets, or straight anemone jets and two-sided-loop jets; they can evolve from
different progenitors including satellite sunspots (or small opposite-polarity
magnetic elements), mini-filaments, coronal bright points, and mini-sigmoids,
exhibit various fine structures including cool and hot components, plasmoid,
and KH vortex structures, and show interesting rotating and transverse
oscillation motions. Solar jets not only provide necessary mass and energy to
the corona and solar wind, trigger other eruptive phenomena such as EUV waves,
filament and loop oscillations, and CMEs, but also significantly affect the
interplanetary space through launching CMEs and energetic particles. One of
the new knowledge we have gained in recent years is that solar jets are often
driven by mini-filament eruptions in association with photospheric magnetic
flux cancellations; besides narrow white-light jets, broad CMEs with typical
three-part structure and simultaneous paired narrow and broad CMEs are found
to be dynamically associated with solar jets. These findings lead to an
important conclusion that solar jets may represent the miniature version of
large-scale solar eruptions, and it probably hints at a scale invariance of
solar eruptions. In this sense, investigating solar jets can provide important
clues to understand complicated large-scale solar eruptions (e.g., CMEs) and
currently indistinguishable small-scale transients (e.g., spicules).

Numerical modeling of solar jets is also achieved many significant advances in
recent years. MHD models of solar jets have been developed from 1D to 3D with
different scenarios such as the emerging-reconnection and onset of instability
mechanisms, which can be applied to interpret the formation, evolution,
morphology, and plasma properties of standard and blowout jets in coronal
holes and active regions. Recently, some numerical works further consider the
effects of heat conduction, radiative losses, and background heating, and more
realistic data-constrained and data-driven MHD simulations are being developed
to understand solar jets. These great efforts make the obtained numerical
results are more morphologically and quantificational comparable with real
observations. In addition, numerical works by considering a large domain
extension to the interplanetary space using spherical geometry are also
developed for understanding the interplanetary disturbances resulted from
solar jets.

Despite the great advances obtained in previous observational and numerical
studies, there are still many aspects of solar jets deserve further
investigations. The following is a list of some outstanding questions:

  1. 1.

Observations showed that solar jets are tightly associated with magnetic flux
cancellation, especially in mini-filament-driven jets. Nevertheless, what kind
of physical process undergoes during the triggering stage is still unclear.
Physically, flux cancellation represents three possible processes: emergence
of U-shaped loops, submergence of $\Omega$-shaped loops, and reconnection in
the magnetogram layer [329]. Therefore, which process and how flux
cancellation trigger a solar jet need to be clarified in future observational
and numerical works. In addition, although there are many models have been
developed based on the emergence-reconnection scenario, explicit evidence for
flux emergence directly driving jets is still very limited. Therefore, these
models should be verified with more observational evidences.

  2. 2.

Observational studies indicated that solar jets can not only cause narrow
white-light jets in the outer corona, they can also result in broad CMEs with
typical three-part structure. Sometimes, a single mini-filament-driven jet can
cause a pair of simultaneous narrow and broad CMEs [62]. Narrow white-light
jets are simply the extension of solar jets into the outer corona; however,
the physical relationship between solar jets and broad and simultaneous paired
narrow and broad CMEs are still unclear. Although the formation of solar jets
in the low corona has been intensively studied with 3D MHD simulations, there
still no theoretical or simulation works for understanding how can a straight,
linear solar jet cause broad and paired narrow and broad CMEs in the outer
corona. Therefore, this aspect deserves further observational and theoretical
investigations, and this kind of study can also help us to understand the
similarity between small- and large-scale solar eruptions.

  3. 3.

Although more and more observational showed the similarity between small-scale
solar jets and large-scale filament/CME eruptions, the possible scale
invariance of solar eruptions should be further tested theoretically and
observationally. It should be significative to check whether the current jet
models can be applied to small-scale explosions such as spicules and nano-
flares which are believed to be important for coronal heating. On the other
hand, it is also important to check if the current jet models are suitable for
explaining complicated large-scale solar eruptions and astrophysical jets.

  4. 4.

The contribution of solar jets to coronal heating and the formation and
acceleration of solar wind, and the jet-associated acceleration mechanism of
solar energetic particles should be investigated in-depth. There are too much
guessing and uncertainties in the existing studies on these topics.

  5. 5.

Most of the current MHD simulations only deal with idealized boundary and
initial conditions using a relatively small numerical domain. Future
investigations should consider more realistic data-constrained and data-driven
MHD simulations, and using a large simulation domain so that one can study the
interplanetary disturbances caused by solar jets.

The investigation of solar jets will benefit from future ground-based large-
aperture solar telescopes and advanced space missions. For example, the Parker
Solar Probe (PSP [330]) launched in 2018 will observe the Sun within
${9.86}\,R_{\odot}$ by 2025, which means that it will fly through coronal
structures such as solar jets and CMEs, which can provide in suit detection of
physical parameters. The Solar Orbiter [331] launched in 2020 will operate
both in and out of the ecliptic plane, which will image the polar regions of
the Sun where solar jets are prominent, and can provide an opportunity for
stereoscopic diagnosing of solar jets in combination with other telescopes on
geosynchronous orbit. The 4-meter ground-based Daniel K. Inouye Solar
Telescope [332] under construction has obtained its first images which can
distinguish solar features as small as 30 km in size. The ultrahigh spatial
resolution observations will help us to resolve the triggering and formation
problems of solar jets, as well as small spicules. The Advanced Space-based
Solar Observatory (ASO-S [333]) that will be launched in 2021 will provided
coronagraph, photospheric magnetic field, and hard X-ray observations for the
investigation of solar jets. A combination measurements of magnetic field,
spectroscopy, imaging, and in situ observations provided by above telescopes
will undoubtedly make significant breakthrough in our understanding of the
physics of solar jets and the related phenomena.

This article has no additional data.

We declare we have no competing interests.

This work was supported by the Natural Science Foundation of China (11922307,
11773068, and 11633008), the Yunnan Science Foundation (2017FB006), and the
West Light Foundation of Chinese Academy of Sciences.

The author thanks the referees who provide many valuable suggestions and
comments for improving the quality of the present paper, and appreciate the
helpful discussions with Dr. Y. Liu, L. Yang, J. Hong and their careful
reading of the manuscript.

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