Galsgaard, 2013; Nóbrega-Siverio, Moreno-Insertis, and Martínez-Sykora, 2016).

####  4.9.2 Coronal Rain

section:rain

Coronal rain is a phenomenon that was initially considered only as a specific
kind of prominence (Levine and Withbroe, 1977) and it was not until the advent
of high resolution instrumentation and multi-temperature coverage of the
transition from chromosphere to corona that its unique characteristics were
recognised. IRIS played a fundamental role in this process, marking the unique
links to multiple fundamental plasma processes.

Coronal rain corresponds to cool and dense, clumpy plasma appearing seemingly
out of nowhere in the solar corona on a timescale of minutes, and falling back
towards the solar surface along coronal loops (Antolin, 2020). It is a
catastrophic cooling phenomenon from coronal to chromospheric temperatures and
can thus, in a way, be considered as the opposite of coronal heating. This
fascinating cooling problem, representing somewhat ‘snowflakes in the oven’,
holds many unsolved mysteries upon which IRIS has shed new light.

The leading explanation for coronal rain is thermal instability in the solar
atmosphere, which can occur when radiative losses locally overcome the heating
gains. Due to the shape of the radiative loss function with respect to
temperature, runaway cooling occurs, which can lead to the growth of unstable
thermal modes (also known as entropy modes), that produce a local loss of
pressure and a subsequent accretion of material to form a condensation (Field,
1965; Müller, Peter, and Hansteen, 2004; Claes, N. and Keppens, R., 2019).

The rapid recombination process on a timescale of minutes was first resolved
through IRIS–SOT observations in Antolin et al. (2015b) by combining the SJI
1400, 1330 and 2796 passbands with the Ca ii H filter of SOT (Fig. fig:rain)
(see also Liu et al., 2014b; Antolin et al., 2014). A fast-slow 2 step cooling
process was observed suggesting either the transition from optically thin to
optically thick cooling, or the presence of a heating source linked to the
compression produced by the rain on the downstream plasma. Strong changes in
the morphology are observed along the magnetic field direction (taken as the
direction of the flow), with longer tails at the wake of the rain at
transition region temperatures, thereby potentially playing a role in the
characteristic filamentary morphology of coronal loops observed in transition
region to low coronal temperatures, as seen in AIA 171 Å. Prior to IRIS and
SST observations, coronal rain was considered as a sporadic phenomenon of
active regions, whereby catastrophic cooling would occur in a loop at most
once every two days (Schrijver, 2001). Despite the large evidence for cool
chromospheric and transition region downflows from coronal heights (Levine and
Withbroe, 1977; Kjeldseth-Moe and Brekke, 1998), and EUV downflow signatures
(both bright from emission or dark from absorption, Ugarte-Urra, Winebarger,
and Warren (e.g. 2006); O’Shea, Banerjee, and Doyle (e.g. 2007); Warren et al.
(e.g. 2007); Tripathi et al. (e.g. 2009); Kamio et al. (e.g. 2011)) there was
no direct link between the two, and thereby no consensus for the physical
mechanism of such multi-thermal downflows.

On the other hand, observations with SST have revealed that these warm flows
contain a large number of small cool chromospheric cores with densities on the
order of $10^{11}~{}$cm-3 or higher (see Fig. fig:rain, Antolin et al., 2015b;
Froment et al., 2020), suggesting a tip-of-the-iceberg clump distribution in
which the bulk at smaller sizes may still be unresolved. Combined, these
chromospheric and transition region return flows to the solar surface show
mass flux values on the order of $1-5\times 10^{9}~{}$g s-1 (similar to
prominences), suggesting that most of the material in the loop undergoes
catastrophic cooling. Therefore, it is now clear that coronal rain plays an
important role in the chromospheric-corona mass and energy cycle.

Figure 17.: (a) Coronal rain along an active region loop at the limb observed
with IRIS in the SJI 1400 (green) and SJI 1330 (blue), and with SOT in the Ca
ii H line (red). (b) A zoomed-in view on the yellow rectangle shown in (a) of
a coronal rain clump observed with the various filters, ordered in ascending
temperature formation order from left to right, Ca ii H (SOT), SJI 2796, SJI
1330, SJI 1400 and AIA 304. Each row corresponds to 3 time instances, with
each column showing the snapshot closest in time to the SOT snapshot. Figure
adapted from Antolin et al. (2015b). fig:rain

In the direction perpendicular to the flow (the latter is expected to trace
the magnetic field), the width of the interface from chromospheric to
transition region temperatures is observed to be very sharp, below
$0^{\prime\prime}.33$, evidencing the highly structured and multi-temperature
corona at unparalleled spatial resolution (Fig. fig:rain and Antolin et al.,
2015b). This fact also illustrates the highly anisotropic nature of thermal
conduction in the solar corona. Another characteristic feature of coronal rain
evidenced by these authors is its multi-stranded structure, particularly in
the chromospheric lines, in the direction perpendicular to the field. Although
still an open question, it has been argued that such morphology should
naturally result from the spatial structure of the unstable thermal modes (van
der Linden and Goossens, 1991).

The dynamics of such cool downflows further challenge our understanding of the
solar atmosphere. Despite the high coronal origins, the falling speeds are on
average on the order of $100$ km s-1, with maxima close to $200$ km s-1 (Schad
et al., 2016; Schad, 2017). While the kinetic energy of these flows can be
comparable to that of a microflare, and indeed lead to brightenings in the
visible and UV spectrum upon impact (Kleint et al., 2014; Deng et al., 2016;
Hou et al., 2016c; Ishikawa et al., 2020; Nelson, Krishna Prasad, and
Mathioudakis, 2020), with termination shocks (Straus, Fleck, and Andretta,
2015; Chitta, Peter, and Young, 2016) and strong chromospheric oscillations
(Kwak et al., 2016), the speeds of the downflows, and in particular the
observed acceleration, are usually a factor of 3 lower than that expected from
gravity, suggesting other forces at play (Antolin, Shibata, and Vissers, 2010;
Antolin and Rouppe van der Voort, 2012). Oliver et al. (2014) have shown that
the condensation process generates a restructuring of the gas pressure
downstream of the rain. The acceleration process therefore occurs mainly in a
timescale given by the sound speed, with heavier rain clumps experiencing
stronger acceleration through a yet unidentified physical effect.

Some of these downflows are observed to be persistent over a puzzlingly long
time, much longer than the draining time of a loop (Straus, Fleck, and
Andretta, 2015; Chitta, Peter, and Young, 2016) and have thus favored a siphon
flow interpretation over that of coronal rain. 2D MHD simulations by Fang,
Xia, and Keppens (2013) show, however, that coronal rain can have a siphon-
like nature, with the siphon flows becoming thermally unstable along the way.
The intermittent and clumpy nature of coronal rain is also observed to become
more continuous and persistent just before impact due to a funnel effect from
the expansion of the magnetic field with height (Antolin et al., 2015b). These
effects combined can therefore provide an explanation to the plume-like and
filamentary structure of coronal loops often observed above sunspots (Foukal
et al., 1974).

The amount of thermally unstable material in the corona at any one time is
still unclear. However, IRIS has made it clear that coronal rain is a far more
common and pervasive phenomenon than previously thought. This is supported by
recent results from Samanta, Tian, and Prasad Choudhary (2018). Using 60 large
IRIS rasters above active regions, it was found that 80 % of the datasets
present high-speed downflows at chromospheric to transition region
temperatures.

Coronal rain is not only an active region phenomenon. Indeed, new IRIS and AIA
observations (in the 304 Å passband) indicate the common occurrence of this
phenomenon in QS coronal loops associated with null point topologies, where
the presence of magnetic dips are common and help accumulate material that
subsequently becomes thermally unstable (Liu et al., 2014a; Mason, Antiochos,
and Viall, 2019; Li et al., 2018a, 2019a, 2020b). In this scenario,
interchange reconnection between open and closed loops is invoked to explain
the presence of coronal rain within the closed loops, although it is yet
unclear if reconnection itself plays a direct role in the generation of
coronal rain. This seems indeed possible, as suggested by IRIS observations
from Kohutova, Verwichte, and Froment (2019).

The link between coronal rain and coronal heating has been highlighted by the
discovery of long period EUV intensity pulsations (Auchère et al., 2014;
Froment et al., 2015). This phenomenon corresponds to highly periodic
pulsations in most EUV passbands lasting up to a week in coronal structures in
both active and quiet Sun regions. Simulations have shown that the driver
mechanism is very likely thermal non-equilibrium (Antiochos, DeVore, and
Klimchuk, 1999; Froment et al., 2017, 2018). This scenario is presented by
footpoint concentrated heating that is frequent enough (with a mean recurrent
time less than the radiative cooling time), leading to cycles of heating and
cooling (also known as evaporation and condensation cycles), with periodic EUV
flows (Pelouze et al., 2020). Although globally the loop is in a non-
equilibrium state, during the cooling stage thermal instability can set in to
produce coronal rain (Antolin, 2020).

Observations of coronal rain further allow a direct high-resolution window on
the coronal topology and dynamics, as evidenced by, e.g., Antolin and
Verwichte (2011) and Nelson et al. (2019) for the detection of Alfvénic waves,
and, e.g., Antolin et al. (2021) fort the detection of braiding-induced
reconnection.

The advent of new instruments on SOLO and DKIST provide exciting new
opportunities to address unresolved issues regarding coronal rain. For
example, SOLO/EUI will provide an effective spatial resolution in the EUV of
200 km or less, which, together with IRIS and DKIST/VBI observations in the
chromospheric lines, will address whether the fine multi-stranded structure of
coronal rain extrapolates to the entire coronal loop structure. If so, this
would establish thermal instability as one of the main causes behind the
filamentary morphology of the corona, and would more strongly constrain the
energy transport scales.

Combined observations with IRIS and DKIST/VISP and DL-NIRSP will allow to
observe the fast recombination process involved in catastrophic cooling
leading to coronal rain. The details of this process - e.g. how cool coronal
rain can get, how does the cooling rate change from the optically thin to
thick regimes - may provide an insight into the plasma composition and
processes beyond MHD such as ambipolar diffusion.

####  4.9.3 Prominence Diagnostics

section:prominence

Another manifestation of ‘snowflakes in the oven’ on the Sun is prominences
(Tandberg-Hanssen, 1995; Vial and Engvold, 2015), some mysteriously cool and
dense material found at elevated heights in the hot and tenuous corona. Unlike
coronal rain that is usually small in size and transient in time, prominences
are relatively large-scale, long-lived structures that generally appear as
bright emission at the limb and dark absorption on the solar disk (thus called
filaments). Prominences play important roles, not only in the mass budget of
the corona with prominence drainage serving as the return flow of the
chromosphere – corona mass cycle (together with coronal rain, e.g., Liu,
Berger, and Low, 2014; Liu et al., 2018b), but also in solar activity with
prominence eruptions forming the cores of CMEs (see §wind and §trigger). For
example, about 10 large quiescent prominences of typically $3\times
10^{16}{\rm g}$ each (e.g., Gopalswamy and Hanaoka, 1998) can hold the mass
equivalent to the entire solar corona. Consisting of numerous downflow streams
at typical speeds of $\sim$$10\,{\rm km}\,{\rm s}^{-1}$, a single large
prominence is capable of draining the entire corona in just about one day, at
a rate twice that of the mass loss to the solar wind.

Figure 18.: High-resolution IRIS observations (left panels) and simulations
(right panels) of prominences have revealed details of fundamental physical
processes such as the generation of turbulence as a result of Kelvin Helmholtz
instability, caused by velocity gradients (right panels) (Hillier and Polito,
2018). fig˙vanessa

The mass supply to prominences is a key aspect of the chromosphere – corona
mass cycle and the transfer of mass into the outer atmosphere in general,
which has remained a topic of debate with coronal condensation, injection from
the lower atmosphere, and flux emergence all considered plausible candidates.
Coronal condensation, driven by thermal instability (see §section:rain), has
found compelling support from SDO/AIA with its multi-temperature coverage
(e.g., Liu, Berger, and Low, 2012; Berger, Liu, and Low, 2012). The high
spatio-temporal resolution of IRIS has recently revealed evidence for
injections of plasma into prominences through quasi-periodic reconnection
(possibly mediated by p-mode oscillations) that lead to the well-known
counterstreaming flows in prominences (Li and Zhang, 2016). Similar injection
of plasma was observed by Zhao et al. (2017) in association with reconnection
at quasi-separatrix layers. The so-called prominence bubbles (Berger et al.,
2008, 2010b, 2011; Dudík et al., 2012; Levens et al., 2016a) are dome-shaped,
void-like structures intruding upward into prominences from below. Some
bubbles exhibit shearing flows at its interface with the prominence resulting
in Kelvin–Helmholtz instability rolls and spawn Rayleigh-Taylor instability
plumes rising into the overlying prominence threads (Berger, Hillier, and Liu,
2017b, see also §instabilities). Prominence bubbles and plumes have been
hypothesized as signatures of flux emergence transporting mass as well as
magnetic flux and helicity into the prominence and its hosting coronal
structure (e.g., a flux rope). The outcome of this is the accumulation of
magnetic flux, helicity, and free energy in the flux rope, with its mass
remaining largely unchanged or even reduced due to drainage through the
prominence. This could ultimately render the flux rope unstable and lead to
its eruption as a CME (e.g., Low, 2001) supported by recent numerical
simulations (Fan, 2020). IRIS has detected about a dozen such bubbles and
revealed preliminary, yet important and unique spectroscopic evidence of such
processes.

IRIS observations have revealed the prevalence of turbulence in prominences
(Schmieder et al., 2014a). Further, IRIS spectra and modeling have shown
evidence for the role of the Kelvin Helmholtz instability (Fig. fig_vanessa)
in generating such turbulence (Hillier and Polito, 2018). Non-LTE modeling
using IRIS diagnostics has also provided much needed constraints on the
temperature, density, and flows in prominences (Levens and Labrosse, 2019).
This type of modeling, coupled with coordinated data, has elucidated the
nature of so-called solar tornadoes (e.g., Su et al., 2012) and identified
that such helical motions are often apparent, caused by LOS superposition, and
thus perhaps not as good measure of helicity or eruptive potential as thought
(Panasenco, Martin, and Velli, 2014; Schmieder et al., 2017). Meanwhile, as
exceptions to this general trend, actual rotational motions were also revealed
by Doppler measurements (Orozco Suárez, Asensio Ramos, and Trujillo Bueno,
2012), including those in two tornadoes detected by IRIS (Yang et al., 2018b).
This suggests that there could be two separate classes of tornado involving
either apparent or real rotational motions.

Figure 19.: Coordinated IRIS, SDO and STEREO observations have provided
unparallelled views of the 3D velocity structure and mass flow during the
onset of CMEs (IRIS slit-jaw, left), exploiting high cadence IRIS spectra
(right) that have revealed the first evidence of Doppler dimming in the Mg ii
lines (Liu et al., 2015a). fig˙wei˙cme

A unique plasma diagnostic for the prominence material that IRIS can offer
relies on the Mg ii k and h lines, as alluded in §diagnostics. Unlike their
on-disk or chromospheric counterparts, the off-limb Mg ii k and h lines from
prominences have no central reversals, suggestive of a low pressure or
thickness and optically-thin regime. In general, the integrated intensity
ratio of the Mg ii k and h lines is expected to be $\sim$2 for optically thin,
collisionally excited (thermal) emission. Reported values include 2 for an
active-region prominence (Vial et al., 1979), 1.7 for a quiescent prominence
(Vial, 1982), and 1.3 for quiescent prominences observed by IRIS (Heinzel,
Vial, and Anzer, 2014; Schmieder et al., 2014b). Liu et al. (2015a) found a
surprisingly low value of 1.2 for the fallback material from an eruptive
prominence, which exhibits a positive linear correlation with the Doppler
velocity and Mg ii k line intensity. They ascribed this behavior to the so-
called Doppler dimming effect, which applies to radiatively excited emission
from moving objects, for which the incident line emission from the solar
surface onto a moving object is Doppler-shifted out of resonance. To fully
exploit the diagnostic potential of the Mg ii k and h lines requires detailed
radiative transfer modeling for realistic prominence geometry (e.g., Heinzel,
Vial, and Anzer, 2014; Heinzel et al., 2015; Vial et al., 2016; Vial, Zhang,
and Buchlin, 2019), in comparison with IRIS observations. With such modeling,
IRIS can also help elucidate the distinction and relation between coronal rain
and prominences.

####  4.9.4 Mass supply to the solar wind

wind

The connection between the low solar atmosphere and the solar wind has been
studied extensively with previous instrumentation, focusing on the role of,
e.g., jets (Cirtain et al., 2007; Raouafi et al., 2016), funnels (Tu et al.,
2005), and active region outflows (Doschek et al., 2007). IRIS is well suited
to study the physical mechanisms at the roots of the fast and slow solar wind,
key to interpreting the downstream properties as measured by PSP and SOLO in-
situ instruments.

Figure 20.: Analysis of observations of AR outflow regions, thought to supply
mass to the slow solar wind, with EIS (a,b) and IRIS (c,d,e) will build on
preliminary results suggesting significant differences in the low TR,
challenging current models. Adapted from Polito et al. (2020).fig˙vanessa2

Jets of all types are common at the roots of the solar wind. IRIS observations
of such jets have been used to elucidate the mass supply to the solar wind.
For example, Narang et al. (2016) studied the evolution and properties of jets
in coronal holes at the footpoints of the fast solar wind. They found that the
transition region counterparts of spicules or so-called TR microjets appear
faster and longer in the open field configuration at the roots of the fast
solar wind. IRIS results have also provided new insights into the formation of
larger-scale jets. For example, coordinated IRIS, SDO and Hi-C observations
have established a clear relationship between magnetic cancellation and the
formation of so-called jetlets (Panesar et al., 2019), larger than spicules
but smaller than coronal jets. Such cancellation and the resulting formation
and eruption of small-scale (”mini”) filaments has been proposed as driving
mechanism for many coronal jets that feed into the solar wind (Sterling et
al., 2015). Recent IRIS/SJI and SDO observations have been used to provide
support for this mechanism (Hong et al., 2019) through imaging of the
evolution of the predicted breakout current sheet that precedes the eruption
of a mini-filament and jet.

However, the formation mechanism and impact of such jets remains poorly
constrained. Coordinated magnetograms from ground-based observatories could be
used to help better understand the variety of magnetic field configurations
that drive coronal jets and determine the relative roles of flux emergence and
cancellation. Detailed studies using very high cadence IRIS spectroheliograms
may also allow diagnostics of the evolution of the mini-filament, if it is
present, before it erupts. Similarly, coordinated IRIS–AIA–PSP observations
during PSP perihelia with front-side connectivity can be used to study whether
these jets are related to the magnetic field “switchbacks” (Sterling and
Moore, 2020) reported by PSP (Bale et al., 2019).

Another potential mass source for the (slow) solar wind are the active region
outflows (Doschek et al., 2007). Recently, Polito et al. (2020) studied the
chromospheric and transition region signatures of these outflows. In the past
these outflows have been detected in coronal lines as strong blueshifts.
Preliminary analysis suggests that, surprisingly, these outflows leave
distinct signatures in the spectral properties of chromospheric and transition
region lines (Fig. fig_vanessa2). It is currently unclear what the physical
cause is for these signatures. The canonical model for these outflows invokes
reconnection at coronal heights and does not seem to provide an explanation
for the IRIS observations (e.g., Baker et al., 2009; Del Zanna et al., 2011).
Follow-up studies are needed to determine whether perhaps the reconnection
also occurs at lower heights and temperatures, or whether the different
properties of open field regions (e.g., spicules) perhaps play a role.

Similar low-atmospheric signatures of the open fields connecting to the solar
wind have been found through analysis of part of the timeseries of more than
80 (almost monthly) IRIS full-disk mosaics and large rasters of equatorial and
polar coronal holes. This analysis suggests that the boundary of open and
closed fields in coronal holes can (sometimes?) be detected in the shape and
properties of the Mg ii h/k line profiles (Bryans et al., 2020a) and their
correlation with the underlying magnetic field properties (Kayshap et al.,
2018). Preliminary results suggest that the different properties of spicules
in coronal holes may play a role in this intriguing detection. Further studies
are needed to obtain a full understanding of the physical cause of this
chromospheric signature, perhaps through investigating the physical parameters
(temperature, density, microturbulence, and velocity) derived from IRIS2(Sainz
Dalda et al., 2019). In addition, high-cadence sit-and-stare IRIS observations
of the boundary between quiet Sun and coronal holes could be used to help
reveal chromospheric and TR signatures of interchange reconnection, thought to
play a key role in energizing the fast solar wind. These studies would benefit
from coordinated magnetic field observations from SDO/HMI, Hinode/SOT,
SOLO/PHI, or GBOs. The SOLO/PHI measurements, once they occur from a vantage
point above or below the ecliptic, would be ideal to help study polar coronal
holes from different vantage points.

One topic that is at the core of the solar wind connectivity to the low solar
atmosphere is that of the enrichment of elements with low first ionization
potential (FIP). While IRIS cannot perform measurements of abundances, the
roots of this FIP effect are thought to lie in the chromosphere where the
ionization of these elements occur. Future coordinated observations with IRIS,
EIS, and SOLO/SPICE can help constrain theoretical models for this important
physical phenomenon.

During space weather events, coronal mass ejections play a significant role in
the mass supply to the solar wind. Initial studies indicate that coordinated
IRIS, SDO and STEREO observations hold promise in estimating the mass budget
of coronal mass ejections. For example, Liu et al. (2015a) exploited the line-
of-sight and plane-of-the-sky velocities of a CME measured with IRIS to
reconstruct the 3D flow-field. They found rapid initial acceleration at very
low heights (constraining CME models) and obtained estimates of the returning
mass from the first observations of Doppler dimming in the Mg ii lines. Future
observations with IRIS, SDO, and SOLO could be used to perform similar studies
for more CMEs, further elucidating the mass supply to the solar wind during
these eruptions.

##  5 Large-scale instabilities

instability

###  5.1 Energy deposition process during flares

flares

Despite its relatively small field-of-view (which covers $\approx$ 5% of the
solar disk), IRIS has so far observed 10 X-class, over 100 M-class and more
than 600 C-class flares since its launch in June 2013 888For a list of flares
observed by IRIS see: https://iris.lmsal.com/data.html#flares. Coupled with
the rich diagnostics that cover conditions from the photosphere to the hottest
parts of the flaring corona and the high spatio-temporal resolution, IRIS
observations have provided many discoveries and a wealth of information that
have expanded our knowledge of how flares are triggered, and how non-thermal
energy is released during flares (see e.g. Fletcher et al., 2011; Benz, 2017,
for a review of some earlier results).

####  5.1.1 Chromospheric evaporation and condensation with IRIS

flares_evaporation Unprecedented observations of the Fe xxi 1354Å line (formed
at $\approx$ 10 MK during flares) have provided new significant insights into
how energy deposition in the chromosphere drives hot plasma into the corona,
i.e., the chromospheric evaporation process. (see also previous observations
with the EIS and CDS spectrometers, e.g. Brosius, 2003; del Zanna et al.,
2006; Milligan and Dennis, 2009; Young et al., 2013, to name a few) Several
studies have shown that the line is observed to be fully blueshifted at the
flare ribbon sites (without the presence of a rest component), with typical
speeds ranging from $\approx$ 100 to 300 km s-1 (e.g., Young, Tian, and
Jaeggli, 2015; Polito et al., 2015; Tian et al., 2015; Graham and Cauzzi,
2015; Li et al., 2015; Brosius and Daw, 2015; Tian and Chen, 2018, to name a
few), in contrast to observations with previous lower resolution
spectrometers, that most often showed an additional stationary component (e.g.
Polito et al., 2016b). The observation of fully blueshifted Fe xxi profiles
indicates that IRIS is finally resolving the emission from the flare
footpoints that contain the upflowing plasma, solving a long-standing problem
and obviating the need for more complex theoretical explanations (e.g. Doschek
et al., 1986; Young et al., 2013).

However, IRIS has also provided several new results that have challenged our
current understanding of chromospheric evaporation. Most notable of this is
the work of Graham and Cauzzi (2015) (Fig. fig:flare_graham15a), who exploited
the high cadence of IRIS sit-and-stare observations of the 2014 September 10
flare to study the Fe xxi velocities in many tens of individual pixels in
flare ribbons. They found that the upflow evolution is strikingly similar
among all of these pixels, possibly suggesting that either the energy input is
remarkably uniform, or the evolution of the evaporation is more governed by
the local conditions than the initial energy input. The upflows are also
observed to smoothly decrease over about 6 minutes (Fig. fig:flare_graham15a).
It should be noted that some other studies (Li, Ning, and Zhang, 2015b;
Brosius and Daw, 2015; Polito et al., 2016b; Dudík et al., 2016) have reported
an initial increase in the blue shifted Fe xxi velocity followed by a
decrease, a phenomenon that still has not been systematically investigated.
Long and gradual decay times for the evaporation flows as those shown by
Graham and Cauzzi (2015) can be hard to explain by single flare loop models,
and seem to be best reproduced by models involving heating of multi-threaded
flare loops (e.g., Reep et al., 2016b, 2018, 2019).

Figure 21.: Panels (a) and (b): Superposed epoch analysis of Fe xxi and Mg ii
flows respectively for selected slit pixels at ribbon of the 2014 September 10
X-class flare (negative/positive velocities showing upflows/downflows). The
grayscale darkens with increasing occurrence within a given velocity interval.
Panel (c): Mg ii intensity spacetime map (inverted B/W color table) with
overlays of: Mg ii downflows at 15 and 30 $\mathrm{km}\ {{\rm{s}}}^{-1}$ (red
contours), Fe xxi upflow velocities above 270, 200, and 100 $\mathrm{km}\
{{\rm{s}}}^{-1}$ (yellow, dark blue, and light blue crosses, respectively).
Adapted from Graham and Cauzzi (2015). fig:flare˙graham15

Finally, multiple episodes of chromospheric evaporation in the IRIS Fe xxi
line, which are suggestive of multiple individual energy injections into the
atmosphere, have sometimes been observed (Tian and Chen, 2018; Brosius and
Inglis, 2018). Such recurring energy injections have been interpreted as being
due to a bursty type of reconnection (see also § flare_reco).

The broadening of the Fe xxi line profiles during the impulsive phase has also
provided new diagnostics of flare heating. Young, Tian, and Jaeggli (2015)
examined the “bar” of enhanced emission that is typically found at the loop-
tops in flare arcades. They found that the Fe xxi emission is completely
resolved at the loop-top, and the line profile in this location is stationary
and does not have excessive non-thermal widths, indicating a lack of
turbulence. The cause of this enhanced emission at the loop-tops is still a
mystery, but this observation rules out the idea that it is caused by
turbulence in tangled magnetic fields at the loop tops (Jakimiec et al.,
1998). Other models for this emission have been proposed (e.g., Reeves,
Warren, and Forbes, 2007; Longcope, 2014) and further IRIS observations of
flare loops will help clarify the mechanism at play.

In addition, Polito et al. (2015) found that the Doppler shift and non-thermal
width of the line gradually decrease as a function of time and are highly
correlated, while no significant non-thermal line broadening was observed at
the loop tops, in agreement with Young, Tian, and Jaeggli (2015).
Superposition of evaporative upflows has long been a popular explanation for
the large non-thermal broadening in flare ribbons (e.g. Doschek et al., 1986;
Milligan, 2011). By using 1D hydrodynamic simulations of multithread flare
loops mimicking a 3D geometry, Polito, Testa, and De Pontieu (2019) found that
this scenario may not be compatible with the IRIS Fe xxi profiles, which
appear to be broad but at the same time symmetric and well-fitted with a
single Gaussian profile (Fig. fig:flare_polito19), and suggested that other
mechanisms such as Alfvénic turbulence or non-equilibrium ionization might
need to be invoked to explain the observations.

Preliminary results from the RADYN arcade model (Kerr, Allred, and Polito,
2020), which is obtained by grafting 1D hydrodynamic simulations of flare
loops heated by electron beams onto an observed AR loop arcade in a 3D domain,
also show discrepancies with the IRIS Fe xxi observations (such as the long
upflows decay and the large non-thermal broadenings), suggesting that
important physical mechanisms might still be missing in these models (see also
Reep et al., 2020, for another recent arcade model). For instance,
hydrodynamic models including non-equilibrium ionization for highly charged
iron atoms using the HYDRAD code (Bradshaw and Mason, 2003) appear to
reproduce some of the observed features, such as the broad and symmetric
profiles in hot lines, assuming short flare loops and a combination of input
parameters (Mandage and Bradshaw, 2020). However, simulations of longer loops
with lengths $\gtrsim$ 50 Mm, such as the ones observed in the 2014 September
10th flare analysed by Polito, Testa, and De Pontieu (2019), still show
profiles with blue wing asymmetries which are not seen in the observations.
Further investigation into the parameter space of these models and inclusion
of additional heating mechanisms, such Alfvén waves, might be needed to solve
some of these discrepancies.

Due to momentum conservation, the evaporation upflows are usually accompanied
by downflows of chromospheric and transition region plasma (“chromospheric
condensation”), which can be observed by IRIS as redshifts of cooler lines
such as Si iv, C ii and Mg ii (e.g., Tian et al., 2015; Brosius, Daw, and
Inglis, 2016; Graham and Cauzzi, 2015; Graham et al., 2020). The Si iv and C
ii spectra sometimes show persistent redshifts or red-asymmetries of a few
tens of km s-1 during the whole impulsive phase (Warren et al., 2016; Yu et
al., 2020), which provide challenging constraints to the 1D loop models (e.g.,
Reep et al., 2016b, 2018). On the other hand, downflows in chromospheric lines
such as Mg ii are usually observed to be more short-lived and last $\approx$
30–60s (e.g., Graham and Cauzzi, 2015; Graham et al., 2020, Fig.
fig:flare_graham15b), in agreement with early models of chromospheric
condensation (Fisher, Canfield, and McClymont, 1985).

In some cases, the IRIS chromospheric lines (including Mg ii, C i, Fe i, Fe
ii, Si ii) also show an enhanced rest component superimposed with a broad and
highly redshifted ($\approx$ 25–50 km s-1) secondary component, which rapidly
decays in 30–60 s (Graham et al., 2020). Such double-component profiles can be
explained by RADYN hydrodynamic simulations assuming that the most energetic
electrons penetrate into the deep chromosphere, while the bulk of the
electrons dissipate their energy higher up into the atmosphere (Kowalski et
al., 2017; Graham et al., 2020). These studies also highlight the diagnostic
potential of the IRIS chromospheric lines such as Fe ii, which are extremely
sensitive to the model input parameters.

Further, several observations have shown significant delays (up to 2 minutes)
between the evaporative upflows and chromospheric condensation flows observed
in Mg ii h and k (e.g., Graham and Cauzzi, 2015; Sadykov et al., 2016, Fig.
fig:flare_graham15c). Such delays are not predicted by the models. Clearly,
more comparisons between targeted or tailored IRIS observations and advanced
models will be required to fully explain the diverse plasma dynamics observed
during the flare impulsive phase and resolve some of these puzzles.

Figure 22.: Broad and symmetric Fe xxi profiles observed by IRIS during the
2014 September 10 flares. Panel (a): SJI 1400 Åimages of the flare ribbons and
the location of the spectrograph slit (vertical line). Panels (b) and (d): Fe
xxi spectral window at the locations indicated by the corresponding colored
symbols in panel (a). Panels (c) and (e): deblended Fe xxi with a Gaussian fit
(dotted curve) superimposed. The insets illustrate the red-blue asymmetry
method showing that the line profiles are mostly symmetric. From Polito,
Testa, and De Pontieu (2019).fig:flare˙polito19

Sometimes upflows in cooler IRIS lines are also observed. For instance, Li et
al. (2019b) recently observed blueshifts in the Si iv, C ii and Mg ii IRIS
lines at the ribbons of a small GOES B-class flare. Blueshifts in the Si iv
have also been observed in smaller micro- or nano-flare size events (Testa et
al., 2014; Testa, Polito, and De Pontieu, 2020, see § nanoflares), suggesting
that similar energy release mechanisms may be at play in energetic events of
different size, from nano- or micro- to larger class flares. In addition, an
earlier study by Li et al. (2015) combined IRIS and EIS spectra of flare
footpoints to observe the temperature dependence of the evaporation and
condensation in the same flare, and found some locations where coronal and TR
lines were blue shifted, and others where the TR lines were redshifted, which
had been interpreted by the authors as being suggestive of “gentle” or
“explosive” evaporation, respectively (e.g. Fisher, Canfield, and McClymont,
1985; Milligan and Dennis, 2009). Recent theoretical studies have shown that
the threshold between different trends of evaporations with temperature mainly
depends on the location of the energy deposition, and is very sensitive to the
details of the heating (e.g., Reep, Bradshaw, and Alexander, 2015; Polito et
al., 2018a).

After the flare peak phase, condensation downflows along the cooling flare
loops are also observed (which are sometimes referred to as coronal rain).
Such cooling loops have been observed in the IRIS Mg ii as TR lines, which
exhibit strong redshifts and increased non-thermal widths (Brannon, 2016;
Mikuła et al., 2017). Another study exploited data from the New Solar
Telescope at BBSO and IRIS to reveal the evolution of coronal rain and its
impact on the chromospheric ribbons (Jing et al., 2016). Importantly, the
widths of the ribbon, the rain, and that of the observed footpoint
brightenings were all in the same range of 80–200 km. The authors speculated
that these observations might be close to resolve the spatial scale of the
energy transport during flares. Future coordinate observations between IRIS
and GBOs during the flare gradual phase may help investigate this further.

####  5.1.2 IRIS lines as diagnostics of flare heating mechanisms

Flare_diagnostics Understanding what physical mechanisms contribute to what
extent to flare heating remains an active topic of debate (e.g. see review by
Fletcher et al., 2011), to which IRIS has provided new and challenging
insights.

Several recent studies have provided support for electron beams being the main
carrier of energy from the flare site to the ribbons (e.g., Tian et al., 2015;
Li, Ning, and Zhang, 2015b; Sadykov et al., 2015; Polito et al., 2016b;
Brosius, Daw, and Inglis, 2016; Brosius and Inglis, 2017; Zhang et al., 2016;
Zhang, Li, and Ning, 2016; Li et al., 2017b; Reep et al., 2019). These studies
were based on correlations between evaporation/condensation flows observed
with IRIS and hard X-ray and radio fluxes (assumed to be associated with non-
thermal electrons), as well as comparison with hydrodynamic models. In
particular, a statistical work comparing IRIS and HXR RHESSI observations with
a grid of hydrodynamic models has shown that non-thermal electrons can
reproduce many aspects of the IRIS observations, such as the correlation
between the non-thermal energy flux and the C ii Doppler shifts, although
significant discrepancies remain, including a puzzling lack of correlation
with the Fe xxi Doppler shifts (Sadykov et al., 2019). Besides, evidence for
both non-thermal electron and thermal heating has also been reported (e.g., Yu
et al., 2020), sometimes within the same flare event (e.g., Battaglia et al.,
2015).

In addition to particle beams and in-situ heating followed by thermal
conduction, it has also been suggested that Alfvén waves may play a role in
transporting energy from flare sites and depositing or thermalizing this
magnetic free energy in the lower atmosphere (Fletcher and Hudson, 2008;
Russell and Fletcher, 2013), but observational evidence has been sparse.
Numerical work has shown that the heating from electron beams alone cannot
account for coronal rain, a common occurrence in flaring loops during their
last cooling stage (Reep, Antolin, and Bradshaw, 2020). Such work suggests the
need for an additional, long lasting (radiative cooling timescale) and
footpoint-concentrated heating mechanism triggered by the flare and future
work needs to consider whether Alfvén waves fit the purpose. Other IRIS
observations find some suggestions that Alfvén waves may be at work in driving
sunquakes, photospheric perturbations in the aftermath of a flare whose origin
has remained difficult to explain since their discovery in the 1990s (Matthews
et al., 2015). Additionally, IRIS observations at an unprecedented temporal
resolution (1.7s) have shown increased and oscillatory Si iv non-thermal
broadening preceding significant brightening in flare ribbons (Jeffrey et al.,
2018). These observations are compatible with a scenario in which turbulent
dissipation of magnetic energy first occurs in the lower atmosphere
footpoints, possibly as the result of interacting waves, rather than
thermalization of electrons that have been accelerated in the corona.

Figure 23.: Example of a k-means clustering of IRIS Mg ii profiles for the
M6.5 flare on 2015 June 6, highlighting the potential in identifying spatio-
temporal patterns in spectroscopic data. The line profiles are assigned to the
representative groups in the bottom panel they most closely resemble,
according to the colors. The black reversed profiles are those observed at the
ribbon fronts. From Panos et al. (2018).fig:flare˙panos18

The IRIS Mg ii h and k lines in flare ribbons provide unique chromospheric
diagnostics, which previously had been seen only once before in 1984 with
OSO-8. The complex and broad line profiles observed in and around the flare
ribbons (e.g., Kerr et al., 2015; Liu et al., 2015b) and loops (e.g. Mikuła et
al., 2017) appear to be difficult to fully reconcile with current numerical
models (e.g., Rubio da Costa et al., 2016). This is also due to the complex
formation mechanisms of these lines, especially under flaring conditions. Kerr
et al. (2016) performed radiative hydrodynamic modeling and non-LTE radiative
synthesis of the Mg ii h & k lines in the aftermath of a flare and found that
the shape of these chromospheric lines appears to be very sensitive to the
detailed physical mechanism that transports and deposits energy in the
chromosphere. Their preliminary results appear to favor Alfvén waves over
electron beams, but much work remains to be done to provide definitive
evidence for a significant role of Alfvén waves in flares.

In addition to previous studies (Rubio da Costa and Kleint, 2017), recent work
by Zhu et al. (2019) has succeeded in reproducing some of these profiles
assuming very large electron energy fluxes (typical of a X-class flare) and
increased Stark widths. Nevertheless, it is still unclear how to reproduce
such single peaked profiles in the case of smaller flares, and how to fully
explain their increased broadening. Kerr, Allred, and Carlsson (2019)
highlighted the importance of improving the realism of the codes that are
currently used to forward model these IRIS lines by taking into account all
the relevant physics (such as non-LTE, PRD and opacity) for these transitions.

Puzzling blue-wing enhancements in the Mg ii lines (Tei et al., 2018; Huang et
al., 2019a) have also been observed and could be indicative of “cool upflows”,
caused by non-thermal electrons penetrating much deeper into the lower
atmosphere than expected and causing an upward motion in the upper
chromosphere (Huang et al., 2019a; Zhu et al., 2019; Hong et al., 2020). A
recent study has also shown that the location where the cool upflows occur
varies in different flare models (Hong et al., 2020).

These preliminary results suggest that the shape of the Mg ii lines may hold a
key to a better understanding of the energy deposition mechanisms in flares.
Exciting advances in identifying these complex profiles through machine
learning has recently enabled a statistical approach that captures the full
spatio-temporal information in IRIS observations. In particular, Panos et al.
(2018, Fig. fig:flare_panos18) analyzed thousands of profiles and discovered a
class of broad profiles with a blueshifted reversal that occurs at the fast-
moving leading edges of ribbons (black profiles in Fig. fig:flare_panos18).
Such peculiar profiles are similar to those observed by Xu et al. (2016) at
the ribbon fronts of a M-class flare, in the same location as increased
absorption (“negative ribbons”) in the He i 10830 chromospheric line was also
observed with the _Goode Solar Telescope_ at BBSO. More observations from
IRIS, combined with ground-based and X-ray satellites may be needed to
understand these puzzling spectral features.

IRIS observations of NUV continuum have also provided exciting diagnostics of
energy deposition in white light flares (WLF). A significant fraction of the
energy deposited in the atmosphere during many flares is in fact observed in
white light emission, which emanates from the photosphere. Since energetic
particles have difficulty penetrating that deep into the atmosphere, some
other mechanism is thought to be responsible for the emission. IRIS has now
been able to provide key new insights into this long-standing puzzle by
exploiting observations that were coordinated with ground-based telescopes,
Hinode, RHESSI and SDO. For example, extending the work of Heinzel and Kleint
(2014), Kleint et al. (2016) combined observations covering wavelengths in the
UV, visible and infrared, as well as RHESSI HXR observations and numerical
models, to show that the white-light emission in the 2014 March 29 flare
originated from temperature increases in the photosphere due to back-warming
from hydrogen recombination (e.g., Balmer continuum) in the chromosphere,
where the flare energy was dissipated. Moreover, the authors found that the
energy contained in the high energy electrons observed by RHESSI is sufficient
to explain the observations. Using data from the same flare, Kleint, Heinzel,
and Krucker (2017) also showed that enhanced spectral line emission can
contribute significantly to the IRIS SJI 2832Å filter, whose emission cannot
therefore be considered as being purely due to Balmer continuum during WLFs.

Further evidence for this process was reported by Kowalski et al. (2017), who
combined IRIS and RHESSI data with electron-beam driven flare simulations and
found that the NUV continuum intensity was consistent with hydrogen Balmer
recombination radiation emitted in two separate atmospheric layers where the
electron beams release their energy (see also § stellar). These findings
appear to support the hyphothesis that WLF emission is associated with energy
deposition from non-thermal electrons streaming from the corona and
propagating down towards the chromosphere (see also Cheng et al., 2015; Lee et
al., 2017). On the other hand, recent IRIS observations of a WLF, showing
spectral features typical of UV bursts, such as enhanced and broadened Si iv
and C ii profiles superimposed by chromospheric absorption lines, combined
with the lack of significant HXR flux in RHESSI, have also provided evidence
for a different scenario, in which the WLF is caused by reconnection occurring
locally in the lower atmosphere (Song et al., 2020). More observations of WLFs
as well as comparison with models will be necessary to investigate these
different scenarios.

As demonstrated by the wealth of results summarized above, IRIS has allowed
significant progress in the understanding of how flare energy is propagated
and deposited in the lower atmosphere, as well as providing challenging new
constraints for current flare heating models. However, many unsolved puzzles
still remain and several new questions have also been raised. For instance,
current hydrodynamic models still cannot fully and consistently explain the
gradual decay of the evaporation flows (Fig. fig:flare_graham15) and the large
symmetric broadenings (Fig. fig:flare_polito19) observed in the Fe xxi line,
as well as the persistent redshifts that are sometimes observed in the
transition region lines, and the puzzling complex and broad Mg ii profiles
(Fig. fig:flare_panos18). In addition, important questions remain regarding
the details of the energy propagation and dissipation in flares of different
classes including WLFs, and the importance of Alfvén waves vs electron-beam
and thermal conduction heating.

In the next few years there will be exciting opportunities to investigate some
of these puzzles even further, thanks to new advancements in the models,
including recent multithreaded and arcade models (e.g. Polito, Testa, and De
Pontieu, 2019; Kerr, Allred, and Polito, 2020; Reep et al., 2020; Mandage and
Bradshaw, 2020), improvements in the modelling and synthesis of the
chromospheric diagnostics (e.g., Zhu et al., 2019; Kerr et al., 2019; Graham
et al., 2020), as well as new approaches to the analysis of the observations
based on statistical studies and machine-learning techniques (e.g., Panos et
al., 2018; Panos and Kleint, 2020; Sadykov et al., 2019). With the Sun going
towards a long period of increased activity, many new coordinated observations
of flares with existing ground-based (including SST, ALMA, BBSO, GREGOR) and
space-based (e.g., SDO, Hinode, PSP, SOLO) satellites, as well as the novel
DKIST instrument, will also be available.

###  5.2 Magnetic reconnection in flares

flare_reco Reconnection is widely believed to be the dominant mechanism to
convert magnetic energy into the kinetic and thermal energy that triggers
flares, although the details of how such processes occur are still not fully
understood (see e.g. Fletcher et al., 2011, for a review). The high spatial
resolution of IRIS has provided a better understanding of the ways in which
reconnection can occur in three dimensions during solar flares, as discussed
in some of the examples below.

In an earlier study, Li et al. (2016) used IRIS 1330 Å slit jaw images to
follow the peculiar motions of an unusual X-shaped flare ribbon, that suggest
the presence of separatrices in the coronal field, with reconnection occurring
all the way down to the Sun’s surface. In the same event, Li et al. (2017c)
observed significantly broadened wings extending to 200 km s-1 in the spectra
of the Si iv line close to the X-point, which they interpret as bidirectional
flows produced by the separator reconnection. Such flows have rarely been
observed by previous spectrometers. Another example of bi-directional flows
was recently reported by Yang et al. (2020), who observed broadened IRIS Si iv
profiles with extended wings superimposed by several blueshifted absorption
lines at the null point of a fan-spine magnetic topology during a small
B-class flare. These absorption features were interpreted as the signature of
cool upward moving material above the reconnection region, and their depth
provided an indication of the amount of cooler material superimposed along the
line of sight. This result implies that the IRIS spectral profiles can help
study the plasma dynamics in the reconnection site.

In another study, Jiang, Zhang, and Yang (2015) use IRIS slit jaw 1400 Å
images to trace the evolution of a small active region with a fan-spine
topology as an emerging flux region appears. IRIS observations of reconnection
and many recurring jets as the new flux emerges near the fan-spine structure
indicate the latter is eventually destroyed. These observations are consistent
with models that show that fan-spine topologies can be destroyed if a current
sheet forms at the null point of the magnetic field.

Figure 24.: Signatures of large flows above the flare loop-top. Left: AIA 131Å
image of the hot flare loops with the position of the IRIS slit overlaid.
Right: Fe xxi spectra observed in the locations indicated in the left panel.
In particular, the green spectra shows the redshifted line above the loop-top.
Adapted from Tian et al. (2014c).fig:flare˙tian

New exciting results on reconnection theory have also been obtained by
exploiting high-resolution observations of the Fe xxi hot line above the flare
loop-tops. An observation by Tian et al. (2014c) showed red-shifted Fe xxi
features that coincided with an X-ray source as observed with RHESSI (Fig.
fig:flare_tian). The line of sight speed of this feature as calculated from
the Doppler shift was $\sim$125 km s-1, with $\approx$ 100 km s-1 non-thermal
width, and it was interpreted as a possible signature of the downward-moving
reconnection outflows or the hot retracting loops, as predicted by the flare
models.

Large redshifts and simultaneous blueshifts ($\approx$ 200–250 km s-1) of the
Fe xxi in the loop-top region were also observed by Polito et al. (2018b)
during the well-studied 2014 March 29 X-class flare. The locations of the
flows were also coincident with HXR loop-top sources observed by RHESSI, and
were interpreted by the authors as possible signatures of the deflection flows
in the downstream of a flare termination shock, as predicted by Guo et al.
(2017). While fast-mode shocks and associated flows are expected by numerical
simulations, unequivocal evidence has been scarce because of the limited
spatio-temporal resolution of previous spectrographs. Hints of the presence of
a termination shock in supra-arcade fans were also reported by Cai et al.
(2019) during the X-class flare on 2017 September 10, the second largest flare
of this solar cycle. Using IRIS SJI images, Cai et al. (2019) found quasi-
periodic oscillating features above the flare arcade which were suggestive of
a bursty and turbulent reconnection process and possibly the presence of a
termination shock. Another recent study reported puzzling oscillations in the
hot plasma flows observed in the Fe xxi line above the loop-top region in the
same flare event (Reeves et al., 2020, Fig. fig:flare_kathy). The IRIS
resolution made it possible to closely follow the evolution of these flows,
which ranged between 20–60 km s-1 in a damped oscillation pattern with periods
of 400 s, independent of the loop length. These observations seem to be
consistent with a scenario in which the dynamics of the loop-top plasma are
disturbed by the reconnection outflows impinging on the closed loops below, in
agreement with the “magnetic tuning fork” model of Takasao and Shibata (2016).

Figure 25.: Oscillations in the hot plasma above the flare arcade. Left panel:
IRIS SJI 1330Å image during the flare showing the hot plasma emission above
the loops, with superimposed the field of view of the IRIS raster. Middle
panel: example of damped oscillations observed by IRIS in the Fe xxi line.
Right panel: schematic representation of the magnetic tuning fork model
suggested by the authors as possible interpretation for the observed
oscillations. Adapted from Reeves et al. (2020).fig:flare˙kathy

Evidence of transverse MHD waves in flaring loops has also been provided by
IRIS in the Fe xxi 1354.08 Å line. The first fully resolved standing
fundamental fast sausage mode was detected by Tian et al. (2016b) during an
M1.6 flare, characterised by the $\pi/2$ phase difference between the Doppler
velocity and the intensity, with a short period around 25 s. Such waves are
leading candidates to explain the quasi-periodic pulsation (QPP) phenomena and
represent unique seismological tools of flaring loops (see also Li et al.,
2020a). Interestingly, an increase of the wave period with time was detected,
suggesting a continuous generation of fast sausage modes in the gradually
longer flaring loops issued from the reconnection process. Another example was
provided by Li et al. (2017a) with IRIS observations of a fundamental standing
kink mode in a M7.1 flare. These waves are characterised by a periodic Doppler
signal (with 3.1 min period) and negligible intensity variation.

Inspired by the work of Tian et al. (2016b), Shi et al. (2019) forward
modelled the response of the IRIS Fe xxi line to fast sausage modes in flare
loops under NEI conditions. They found that the synthetic spectral signatures,
in particular the phase difference between the intensity and Doppler shift
oscillations, are mostly consistent with the IRIS observations. However,
differences between the model predictions and observations still remain, such
as the presence of asymmetries in the wave period of the Doppler widths (which
results from the competition between the broadening due to the superposition
of bulk flows and that due to temperature), which are not found in the
observations. Additional high cadence spectral observations of the hot flare
loops with IRIS might help addressing some of these discrepancies.

Figure 26.: Quasi-periodic slipping motions at the ribbons of the 2014
September 10 flare. Temporal evolution of peak intensity (a), Doppler shift
(b), and line width (c) of the IRIS Si iv line. From Li and Zhang (2015b).
fig:flare˙slipping

Other types of oscillations detected with IRIS have been associated with
heating signatures in the lower atmosphere. For example, Li, Ning, and Zhang
(2015a) observed 4-minute QPPs in HXR, radio, EUV and UV light curves, which
corresponded to broadened and redshifted profiles in the C i, Si iv, O iv and
Fe xxi lines observed by IRIS at the flare ribbons. These observations support
the scenario in which the QPPs are produced by non-thermal electron beams
accelerated from the corona, where quasi-periodic magnetic reconnection
occurs. A possible explanation put forward by the authors is that the
reconnection is quasi-periodically modulated by slow p-modes, although other
scenarios (for example involving fast waves) cannot be ruled out.

Signatures of quasi-periodic slipping reconnection has been found by analyzing
the motions observed in the IRIS images and spectra throughout the impulsive
phase of the 2014 September 10 X-class flare (e.g., Li and Zhang, 2015b; Dudík
et al., 2016). In this flare event, the footpoints of the slipping flare loops
appeared as small-scale bright knots moving along the ribbon following a
quasi-periodic pattern, as best observed in the IRIS SJI 1400Å images. While
the bright slipping footpoints progressively travelled under the IRIS slit,
the Si iv spectra also revealed that the line is redshifted and broadened at
those locations (Fig. fig:flare_slipping). These observations were interpreted
by Dudík et al. (2016) in terms of the standard solar flare model in 3D (e.g.,
Janvier, Aulanier, and Démoulin, 2015). On the other hand, puzzling
observations of quasi-periodic slipping motions of ribbon substructures for a
non-eruptive flare were reported by Li et al. (2018b). Despite these
observations being reminiscent of the signatures of slipping reconnection,
they cannot be simply explained by the 3D standard flare picture, which
requires the presence of an eruptive flux rope. Alternative explanations for
the observed quasi-periodic property of the flare kernels include MHD waves or
the tearing mode instability. For instance, Parker and Longcope (2017) invoked
this instability to explain IRIS observations of a flare exhibiting sawtooth-
like ribbon motion and associated periodicity (Brannon, Longcope, and Qiu,
2015; Brosius and Daw, 2015). Additional high-resolution observations of
quasi-periodic motions in both confined and eruptive flares and advanced
modelling are required to distinguish between these competing hypotheses and
settle this issue.

###  5.3 Initiation of coronal mass ejections and flares

trigger

Coronal mass ejections occur when energy is built up in the corona in excess
of the energy in the potential field, and then suddenly released.
Determinations of this “free energy” are typically based on non-linear force-
free field (NLFFF) extrapolations that use photospheric magnetograms as a
lower boundary condition (e.g. Sun et al., 2012; Gibb et al., 2014). However,
since the photosphere and lower chromosphere are not force-free, the
reliability of such extrapolations is questionable. There are two distinct
approaches to improving NLFFF extrapolations, one using coronal constraints,
and the other constrained by chromospheric observables (e.g., from IRIS,
Aschwanden, 2015; Aschwanden, Reardon, and Jess, 2016). However, these
approaches lead to very different estimates of the free energy. The resolution
of this dilemma may have to await the development of reliable magnetic field
mapping in the upper chromosphere or corona.

Determining the mechanism that suddenly releases the free energy in the corona
is fundamental to understanding coronal mass ejection initiation.
Observational precursors of CMEs cataloged prior to the IRIS mission can be
found in the review by Chen (2011). IRIS has observed evidence for several CME
initiation mechanisms, suggesting a variety of processes may be important in
triggering eruptions. For example, pre-eruption reconnection (often referred
to as “tether-cutting”) has been identified as a possible trigger in several
instances (e.g., Li and Zhang, 2015a; Dudík et al., 2016; Cheng and Ding,
2016). Many of these examples exploit the sensitivity of IRIS to reconnection-
induced flows and non-thermal motions to provide support to the tether cutting
scenario in which flux ropes are formed prior to the eruption (e.g., Kumar et
al., 2015, using coordinated observations from IRIS, NST and SDO). Zhou,
Zhang, and Wang (2016) interpret oscillations in the Fe xxi line in a sigmoid
minutes before an eruption as evidence for reconnection between the flux rope
and the overlying field. Blue-shifted Fe xxi line profiles and non-thermal
line broadening are observed in another flux rope (Cheng and Ding, 2016) just
prior to the eruption, suggesting that reconnection heats threads of the flux
rope to 10 MK. In another event, Cheng, Ding, and Fang (2015) interpret red
shifts and enhanced line widths in IRIS lines at the footpoint of a magnetic
flux rope prior to the eruption as evidence for pre-event reconnection along
the flux rope.

Figure 27.:  IRIS observations of a CME (top left) reveals fast outflows (top
middle) as the erupting prominence collided with the overlying loops
containing coronal rain. Reeves, McCauley, and Tian (2015) find a brightening
under the prominence before the eruption, suggesting that a tether cutting
mechanism is responsible for initiating the eruption (top right). An alternate
interpretation is given by Kumar et al. (2019). The pre-eruption configuration
of the filament and coronal rain in the 2796 Å channel (bottom left) shows the
ends of the filament (F and F’) and a null point (N). The evolution of the
coronal rain in the 1330 Å channel (bottom middle and bottom right) shows
brightenings near J1, which are interpreted as plasmoids resulting from
reconnection above the prominence, an indication of breakout
reconnection.fig˙kathy2015

There is also evidence for pre-eruption slipping magnetic reconnection, found
by analyzing the motions of flare ribbon kernels in the IRIS images and
spectra (e.g., Li and Zhang, 2015b; Dudík et al., 2016). IRIS observes
substructures moving bi-directionally along the flare ribbons in a couple of
events, indicating that reconnection in the quasi-separatrix layer between two
flux systems has caused slip-running reconnection to occur (Li et al., 2018b,
2019b). A fan-spine topology was identified in another event using IRIS 2796 Å
SJI observations, and because of this topology, slip-running reconnection is
identified as a possible trigger mechanism (Zuccarello et al., 2020).

Emerging flux can destabilize the magnetic field and cause an eruption. In an
event documented by Hou et al. (2019), an embedded fan-spine structure forms
via flux emergence and subsequent reconnection with the overlying flux,
triggering a confined flare. In this case, high spatial resolution images from
IRIS confirm predictions from NLFFF modeling that imply the existence of a
small fan-spine system embedded in a larger one. In another event reported by
Kleint et al. (2015), emerging flux destabilizes a filament, leading to a
flare. Increasing Doppler velocities in the IRIS Si IV line indicate a very
high acceleration of the filament (3-5 km s-2) before the impulsive phase of
the flare, indicating that the flare was a consequence of the filament
eruption and not a cause.

Another possible eruption trigger is breakout reconnection, where reconnection
high in the corona destabilizes a sheared magnetic field, causing an eruption.
Breakout reconnection has been identified as the trigger for a small eruption
at the limb based on bi-directional flows and small blobs observed in IRIS
slit-jaw images (Kumar et al., 2019). However, other authors attribute the
triggering of this eruption to tether cutting, based on the identification of
brightenings seen in the slit jaw images that occur just as the fast rise
phase of the eruption begins (Reeves, McCauley, and Tian, 2015, Fig.
fig_kathy2015). In a different event, Woods et al. (2020) observe a
brightening in IRIS located between two flux systems for a series of three
flares, the last of which was associated with a CME. Magnetic modeling
indicates that a breakout like mechanism is most likely occurring in this
case.

In a few cases, ideal MHD instabilities have been identified as eruption
triggers. An observation of an erupting prominence at the limb finds
alternating patterns of red and blue shifts in the IRIS Mg ii line, which is
interpreted as initiation via the kink instability (Zhang, Buchlin, and Vial,
2019). Woods et al. (2018) find that a combination of tether cutting
reconnection and the recently proposed double arc instability (Ishiguro and
Kusano, 2017) is the likely trigger for an X-class flare, based on morphology
and brightenings observed by IRIS.

A key component in understanding the (in)stability of flux ropes is the long-
term evolution of prominences, which often erupt and drive CMEs. Recent
combined IRIS and Hinode/SOT observations exploit the high sensitivity and
spatial resolution of IRIS Doppler shift measurements in quiescent prominences
to show how these common features can shed helicity through reconnection with
the ambient coronal field, thus preventing eruptions (Okamoto, Liu, and
Tsuneta, 2016). Coordinated IRIS observations with ground-based magnetic field
measurements have also provided new insights into the injection of helicity at
the base of prominences (Levens et al., 2016b). Liu et al. (2015a) examines a
prominence eruption at the limb associated with a X1.6 class flare. The
combination of Doppler shifts and cork-screw shaped threads imaged in the IRIS
SJIs lead to the conclusion that the prominence is a left-handed helical
structure that unwinds in the counter-clockwise direction during the eruption.
This result is interesting because the source active region is in the southern
hemisphere, and left-handedness would be opposite to the prevailing magnetic
helicity for that hemisphere (e.g., Pevtsov, Canfield, and Metcalf, 1995). The
authors speculate that this anomalous chirality may contribute to the
prominence’s eruptive potential.

Figure 28.: Drifting motions and fine structure (e.g., hook-shapes) of flare
ribbons observed at the high IRIS spatial resolution (right), through
comparisons with 3D MHD models (left) (Aulanier and Dudík, 2019), provide key
information about reconnection occurring in the corona during an eruption and
the structure of flux ropes as they evolve to form ICMEs, critical in the era
of PSP and SOLO. fig˙guillaume

Most attempts at flare forecasting prior to IRIS primarily used the line of
sight magnetic field data (see e.g. Barnes et al., 2016), but recent IRIS
results show exciting promise for accurate predictions of the timing of the
onset of solar flares using UV spectroscopic data. Panos and Kleint (2020)
applied machine learning techniques to show how chromospheric spectra (Mg ii
h/k and, in particular, triplet emission) in pre-flare regions (40 min before
flare) are clearly distinct, and could, in principle, be used to predict flare
occurrence with high accuracy.

Woods et al. (2017) studied the pre-flare phase of an X1 flare observed by
IRIS and found strongly blue shifted plasma flows in the IRIS Si iv line with
velocities up to 200 km s-1 40 minutes before the eruption. These flows are
along the active region filament, and modeling indicates that the flows are
due to internal reconnection in the flux rope or tether cutting along the
filament. Bamba et al. (2017) examine an X1.6 flare and find blue shifts in a
variey of IRIS and Hinode/EIS lines at the location of an isolated positive
polarity region about 30 minutes prior to the flare. They find blue shifts of
$\sim$100 kms-1 in all the IRIS and EIS lines, indicating a jet-like
reconnection outflow which may change the topology of the magnetic field
enough to trigger the subsequent eruption.

The initiation and propagation of CMEs will continue to be a robust topic of
study for IRIS as solar cycle 25 ramps up, especially considering the recent
launches of PSP and SOLO. Future work will combine the powerful spectroscopic
diagnostics of IRIS with highly complementary high-resolution transition
region and coronal measurements with instruments on SOLO. IRIS observations of
eruptions focused on possible source regions during PSP perihelia will provide
the groundwork for holistic views of eruptions from the chromosphere to the
heliosphere. Combinations of IRIS observations and MHD modeling have implied
that the flare ribbon geometry is an indicator of how the CME evolves as it
erupts (Aulanier and Dudík, 2019), and coordinated observations between IRIS,
PSP and SOLO will be the perfect test bed for this idea.

IRIS observations to date have provided evidence for a variety of triggering
mechanisms for solar eruptions. It would be very interesting to conduct
statistical studies using existing (and future, as the cycle ramps up) IRIS
observations of eruptions, in coordination with SDO/AIA, Hinode/XRT, and SOLO
imaging, in order to understand which, if any, of the triggering mechanisms
that have been investigated so far are the most common. Future efforts could
also focus on the formation and evolution of flux ropes, the seed structures
of solar eruptions. These studies would benefit from a combination of IRIS,
HSO, and GBO observations, and 3D numerical models to study the build-up,
nature and trigger mechanisms of pre-eruptive magnetic configurations. GBO
measurements of the field in the photosphere and chromosphere, as well as
signatures of cancellation, reconnection, and heating (e.g., IRIS observations
of UV bursts, rapid outflows) around the polarity inversion line are key to
study flux rope formation, and this research will benefit from new coordinated
observations, especially from DKIST.

##  6 Solar cycle variations of far- and near ultraviolet radiation

irradiance

Like many solar radiative outputs, the IRIS far and near ultraviolet passbands
are subject to variation over the course of the sunspot cycle. Variations of
the solar ultraviolet emission impact the Earth’s atmosphere and space
environment in a variety of ways, including thermospheric density changes that
impact satellite drag and the chemistry of stratospheric ozone. The
accessibility of strong ultraviolet Mg ii emission and the relative ease of
measurement from low earth orbits led to the creation of the “Mg ii index”,
the ratio between the Mg ii h/k lines and their nearby continua (see, e.g., de
Toma et al., 1997). This index has been measured by NOAA on a daily basis
since 1978 because it is a strong proxy of solar variability over the entire
UV spectral domain, but especially at solar minimum when coronal activity
proxies, like the 10.7cm radio flux (“f10.7”), are insensitive.

However, since little is understood about the physical underpinnings of
ultraviolet variability or the Mg ii index - especially its spatial, temporal
and spectral relationship to the magnetized and unmagnetized solar atmosphere
- the IRIS synoptic program was initiated shortly after first light. IRIS
regularly obtains a variety of observations to characterize the cyclical
variations of spatially resolved NUV and FUV spectra, including quiet Sun
measurements, full-disk spectroheliograms, active region disk passages, etc
where each will help us to characterize the origins of solar variability.

The time since IRIS launch has covered the descent of Sunspot Cycle 24, the
depths of solar minimum and the very earliest sprouts of Sunspot Cycle 25.
Over this time IRIS has completed more than ninety full disk spectroheliograms
(see, e.g., Schmit et al., 2015; Bryans et al., 2020b). These observations
stitch together a large number of individual IRIS pointings to build a mosaic
of the solar atmosphere at high spectral and spatial resolution over the
course of a day, depending on the characteristics of the IRIS observations
employed. Preliminary studies of the full-disk spectroheliogram archive have
been focused on the chromospheric footprint of coronal holes (Bryans et al.,
2020b) and the impact of magnetism (Schmit et al., 2015) on the nature of
chromospheric spectra. In addition, Kayshap et al. (2018) have found a
significant difference, for a given magnetic field strength, between the Mg ii
h/k spectral properties (and thus Mg ii index) of QS and CH, when observed at
the 1Å resolution of space weather measurements with the commonly used “sun-
as-a-star” SOLSTICE instrument (Kayshap et al., 2018). Since the Mg ii index
is often used as a proxy for plage areas (which vary greatly throughout the
cycle), this intensity contrast between CH and QS must be taken into account
in future irradiance studies that use this index, in order to properly account
for solar activity.

The comprehensive archive of full disk IRIS spectroheliograms sees research
launching on the impact of center-to-limb effects, active region plage,
coronal holes, and sunspots on the Mg ii index, thereby elucidating what
drives the Mg ii index variability throughout the cycle, and especially in the
rising phase of Sunspot Cycle 25.

Figure 29.: IRIS spectroheliograms provide the opportunity to explore the
relationships between the Ultraviolet irradiance over the course of the solar
cycle. (Left) An image constructed from the ratio of the integrated Mg ii h
and k line cores. (Right) the variance of the disk-averaged (integrated) Mg ii
h line core (red) since launch from the 88 IRIS spectroheliograms to date in
comparison with scaled versions of the Sunspot number (blue) and the
University of Bremen Mg ii index over the same epoch, we see that the IRIS
measurements follow the observed trend in the Mg ii index, permitting future
investigations into the physical origins of Mg ii variance. fig˙scott

IRIS has obtained a timeseries of full-disk spectroheliograms on a monthly
basis since 2013, and will continue to obtain such data. This unique
timeseries provides spatially resolved spectra in the NUV and FUV, including
Mg ii h & k spectra, which can be used to determine the Mg ii index, a
commonly used proxy to gauge impact of variations of solar irradiance on the
Earth’s upper atmosphere. Comparisons between high-resolution IRIS spectra and
HMI magnetic fields can be used to determine how variability in magnetic
structures (plage, ARs) impact the Mg ii index, and through SOLSTICE sun-as-a-
star irradiance spectra, develop an improved Mg ii proxy for irradiance
modeling.

IRIS will continue weekly synoptic observations of the chromospheric
counterparts to coronal bright points to help study the onset and strength of
the next solar cycle, properties that have been difficult to predict (e.g.,
McIntosh et al., 2020). This will build on recent research suggesting that
such bright points appear at mid-latitudes long before onset of the new cycle
as part of an “extended solar cycle” (Srivastava et al., 2018b). Weekly
observations (part of two IHOPs) are focused on detecting small magnetic
features that, according to recent results (McIntosh and Leamon, 2014), arise
due to coherent patterns of flux emergence that are related to the dynamo
process. These features are hypothesized to relate to underlying toroidal
magnetic flux systems. The IRIS synoptic effort will help to contrast changes
in structure, emission and timing of these magnetic systems between the active
latitudes and the equator (McIntosh et al., 2019). The precise location of
these magnetic systems may be important for predicting the timing of solar
minimum, the strength of the upcoming cycle and, more generally, the long-term
variability of the Sun. Since 2017 these IHOPs have been run on a weekly
basis, and will continue to be run as the new cycle ramps up activity.
Detailed analysis of the timing and location of bright point activity may
provide constraints on dynamo models.

##  7 Solar-Stellar Connections

stellar

The Sun is the one star where we can dig down into the fine-scale nature of
the top-level processes evident in full-disk stellar spectra and broadband
photometry (for general reviews see Engvold, Vial, and Skumanich, 2019; Hall,
2008). Observations of the solar outer atmosphere offer a close-up view of the
physical processes at work in stellar atmospheres, and it allows us to explore
the extent and limitation of the solar-stellar analogy (”solar-stellar
connection”) (e.g. Testa, Saar, and Drake, 2015, and references therein).
Previous work on the solar-stellar connection has focused on spectral
irradiance variations over magnetic cycles (Baliunas et al., 1995; Böhm-
Vitense, 2007; Egeland et al., 2015; Ayres, 2020), magnetic field dynamo
modeling (Charbonneau, 2013; Brun and Browning, 2017), surface magnetic flux
transport modeling (Schrijver, 2001), magnetic activity diagnostics in young
solar analogs (Güdel, 2007), asteroseismology (van Saders et al., 2016; García
and Ballot, 2019), stellar winds and mass loss (Wood, 2004; Matt et al., 2012;
Cranmer, 2017), abundance effects (Laming, 2015), the link between Sun-as-a-
star flare light curves and coronal mass ejection occurrence (Harra et al.,
2016), solar flare processes such as the Neupert effect (Hawley et al., 1995;
Guedel et al., 1996; Güdel et al., 2004), and flare scaling relationships
(Shibata and Yokoyama, 2002; Namekata et al., 2017; Notsu et al., 2019;
Okamoto et al., 2021). These comparisons have largely resulted from
observations of hotter plasma emitting in X-rays (GOES, Chandra, XMM-Newton),
the extreme ultraviolet (e.g., EVE, EUVE), and a narrow spectral region around
Ca ii H and K. Yet, many types of lower-atmospheric solar phenomena may have
stellar analogs that are only revealed and constrained in the FUV and NUV with
broad spectral coverage, such as from IUE and the HST. Complementary,
spatially resolved solar spectra have long been lacking at these wavelengths,
precluding direct comparisons to stellar observations. And now, IRIS provides
for the first time the combination of high spectral resolution in crucial
ultraviolet bands, comparable to that delivered by the best modern space
instruments for the stars, as well as high spatial resolution and fast
temporal response. With IRIS, the vagaries that result from mixing radiation
from different locations along a flare ribbon, or from basal and active region
components, are not an issue. The heating properties as a function of
atmospheric depth can be uniquely determined on the Sun.

The basic properties of stars, such as radius, temperature, gravity, mass,
magnetic field, and chemical composition, are critically important to
establishing their age, formation history, and angular momentum evolution, and
consequently the history of the Galaxy as a whole. Most stellar parameter
determinations rely on interpreting the spectra of stars, exploiting the
record it carries of the photospheric temperature and pressure, and the
imprint of its elemental composition, all recorded in the strengths of
absorption features due to individual lines. The basic properties of stars are
then determined from comparisons of observed stellar spectra with calculations
from first principles, i.e., stellar spectral synthesis codes: temperature
from the relative strength of lines with low and high excitation; gravity from
lines of low and high ionization; elemental abundances follow from predictions
of ionization and excitation (see e.g., Walkowicz, 2008; Souto et al., 2020,
for recent work in the UV and NIR).

However, synthesis codes are complex, adopted line parameters vary widely
among models, and it is unclear whether the inherent limitations of non-
magnetic 1D model calculations can adequately reproduce line formation over
the entire photosphere and chromosphere (Uitenbroek and Criscuoli, 2011;
Lebzelter et al., 2012). Furthermore, most spectral synthesis analyses must
simultaneously determine temperature, gravity, magnetic field, and filling
factors all at once. For the Sun, however, the temperature and gravity are
known accurately through other means so that deficiencies in the spectral line
modeling can be more easily identified. Spatially resolved IRIS observations
of photospheric lines thus provide a unique opportunity to benchmark spectral
observations and help determine non-LTE corrections to existing model
opacities in the ultraviolet (e.g., Bruls, Rutten, and Shchukina, 1992;
Criscuoli et al., 2020), one of the main challenges in stellar atmospheric
physics and solar and stellar irradiance modeling. Furthermore, the lack of
direct spatial information for other stars requires unconstrained filling
factors of different magnetic field strengths (e.g., Johns-Krull and Valenti,
2000) and mixed emission components from spots, plage, and basal regions
(Walkowicz, 2008; Ayres, 2020), whereas the spatial extent of many solar
features can be directly measured (e.g., Fontenla, Stancil, and Landi, 2015).

IRIS results have also been useful to understand stellar flux-flux relations.
Correlations between magnetic field and IRIS diagnostics show many
similarities with those found for stars, but also intriguing discrepancies for
TR lines (Barczynski et al., 2018). The spatially resolved nature of the IRIS
observations suggests that these differences are caused by global averaging
inherent in stellar observations, e.g., center-to-limb variations. Other
studies exploit IRIS and SORCE solar observations, and HST and Chandra
observations of $\alpha$ Cen A and B to reveal novel flux-flux power laws over
activity cycles, allowing improvements of proxy-based irradiance models,
estimates of ionizing radiation of exoplanet hosts, and quantitative
assessment of multi-component magnetic dynamo models of the Sun and solar
analogs over stellar evolution (Ayres, 2020).

Figure 30.: Composite NUV flare spectrum of an event from the dM4e star GJ
1243 (from Kowalski et al. (2019b)), observed by the Hubble Space
Telescope/COS ($\lambda<2840$Å) and the ARC 3.5m at the Apache Point
Observatory ($\lambda>3400$Å). This observation shows that the $U$-band
consists of hydrogen Balmer recombination radiation that decreases toward the
IRIS NUV range where additional Mg ii and Fe ii radiation is prominent.
However, the Balmer jump is much smaller than predicted by a model (red,
RHDF11) of chromospheric heating by a reasonable flux of non-thermal
electrons. This flare has an energy of $E_{U}\sim 2\times 10^{31}$ erg, which
is likely comparable to a large solar flare. fig:stellarflare

IRIS NUV spectra during solar flares provide constraints on a major missing
observational aspect of a dominant mode of radiative energy release in both
solar and stellar flares (Osten and Wolk, 2015). Whereas spectra of flares on
other stars (primarily dwarf M emission (dMe) stars) cover a broad wavelength
range that includes the optical and NUV wavelengths that are observable from
the ground (i.e., wavelengths longer than $\sim 350$ nm), IRIS spectra probe
shorter ultraviolet wavelengths where currently the Hubble Space Telescope can
safely observe the dMe flares that are relatively small in amplitude with a
bright Balmer continuum component, as in Figure fig:stellarflare (instrumental
bright limits prevent frequent monitoring of nearby stars in the NUV due to
the possibility of a large flare; Osten 2017, Instrument Science Reports
2017-02)999https://www.stsci.edu/hst/instrumentation/stis/documentation/instrument-
science-reports. From extrapolations of optical spectra of much more impulsive
(and often, larger and more energetic) dMe flares, the peak of the white-light
continuum radiation may occur much further into the ultraviolet, near 300 nm
or shorter (Hawley and Fisher, 1992; Hawley et al., 2003; Fuhrmeister et al.,
2008; Kowalski et al., 2013, 2016), which is a critical challenge for models
of solar flare heating (see also § Flare_diagnostics Hawley and Fisher, 1992;
Allred et al., 2005, 2006).

Detailed modeling of the IRIS observations of NUV properties of solar flares
serve as “ground truth” for modeling approaches to these white-light
properties in stellar flares. In particular, Kowalski et al. (2017) and Graham
et al. (2020) studied IRIS, RHESSI, and Fermi/GBM data of two X-class flares
to constrain RADYN simulations of the chromospheric response to the impact of
moderate-to-high fluxes of electron beams and found that the NUV continuum
intensity and red-shifted emission line satellite components were rather well-
reproduced by two transient flaring layers at chromospheric heights: a
stationary heated layer below a chromospheric condensation (see also §
flares_evaporation).

A similar scenario was recently proposed to explain the 10,000 K blackbody-
like white light radiation (Hawley and Pettersen, 1991) and a peak near 300 nm
in flares in dMe stars: RADYN simulations of M dwarf flares with even higher
electron beam fluxes show a similar scenario with two flaring layers, which
are deeper and denser in the M dwarf atmosphere (Kowalski et al., 2015).

Because dMe flares are rarely observed with both high cadence and high
resolving power (and never with any direct spatial resolution), the red-
shifted satellite components that are expected from extreme (and possibly
unrealistic; Krucker et al., 2011) electron beam fluxes are difficult to
validate using only stellar data. A line-to-continuum ratio in IRIS NUV
spectra was recently investigated to better diagnose the amount of heating in
the low chromosphere or upper photosphere in solar flares (Kowalski et al.,
2019a), demonstrating the relative heating rate of these lower layers to be
significant relative to that of the upper chromosphere. Other sources of
evidence have previously hinted at significant heating below the upper
chromosphere (Martínez Oliveros et al., 2012; Warren, 2014); resolving this
issue on the Sun would provide important clues for modeling the origin of
10,000 K blackbody-like component101010Kretzschmar (2011) reported that Sun-
as-a-star data of many solar flares exhibits a color temperature of 9000 K,
but this likely results from an error in the analysis (Hawley et al., 1995;
Kleint et al., 2016; Castellanos Durán and Kleint, 2020). that clearly
dominates the energetics in some dMe flares.

IRIS provides robust continuum measurements of flares in the FUV as well (Tian
et al., 2015). Multifarious types of FUV spectral shapes (in units of Å-1)
have been reported in stellar flares: slightly rising toward shorter
wavelengths (Froning et al., 2019), nearly flat continua (Hawley and
Pettersen, 1991) and continua falling toward shorter wavelengths (Loyd et al.,
2018b). However, in all cases the spectra are bluer than an extrapolation of
the NUV optically thin Balmer continuum recombination models, as inferred in
IRIS data of solar flares (Heinzel and Kleint, 2014; Kowalski et al., 2017,
see also § Flare_diagnostics). Also, the FUV continuum radiation is always
observed to evolve on shorter timescales than the NUV and optical continuum
and emission lines radiation (Hawley et al., 2003), which can be investigated
further with IRIS (e.g., Tian et al., 2015). Another challenging observation
of the FUV in large stellar flares comes from the rapidly rotating young solar
analog EK Dra, observed serendipitously with HST/COS (Ayres, 2015b).
Superimposed on the decay phase were bright bursts that showed up only in the
FUV continuum without a response in higher temperature lines, like Fe xxi and
C ii. Curiously, the redshifted transition region line emission at tens of km
s-1 speeds that are often111111Redshifts of several thousand km s-1 have been
occasionally reported in HST spectra of TR lines in dMe flares (Woodgate et
al., 1992; Bookbinder, Walter, and Brown, 1992). reported in stellar flares is
interpreted as chromospheric condensation from electron beam impact (Hawley et
al., 2003) or as condensation in cooling “post-flare” loops (Ayres, 2015b), in
line with the solar flare paradigm.

IRIS is uniquely suited for comparisons of solar flare FUV and NUV spectra to
stellar flare spectra. For example, intriguing observations of temporal
differences in NUV and FUV continuum emission in M dwarf flares are also seen
in IRIS spectra of solar flares. Future work will help determine the cause of
these effects in M dwarf flares, building on preliminary IRIS results
suggesting that these are related to chromospheric condensations that occur as
a result of impulsive flare dynamics. Investigations into the physics of very
broad Mg ii lines in solar flares (Rubio da Costa and Kleint, 2017; Zhu et
al., 2019; Kerr, Allred, and Carlsson, 2019, see also § Flare_diagnostics) may
provide insight into the even more extreme broadening that is observed in
archival HST echelle spectra of M dwarf flares in the NUV around Mg ii (Hawley
et al., 2007).

Since many M dwarfs are hosts for planets in the habitable zone, the spectral
variation from FUV to NUV is important for accurate assessments of
biosignatures in future exoplanet transit spectroscopy (Tian et al., 2014a;
Harman et al., 2015). There is now a large database of time-tagged FUV spectra
of old and young exoplanet host stars observed with the HST (Osten et al.,
2005; France et al., 2016, 2018, 2020; Loyd et al., 2018b, a) as part of the
MUSCLES Treasury Survey and HAZMAT program. Spatially resolved IRIS spectra of
a wide range of targets (from QS to AR plage) can also be used to better
understand the Wilson-Bappu effect in the Mg ii k line, which ties the line
width to the absolute visual magnitude of a star and can be used for
determining the distance of stars. Further, to our knowledge, the ubiquitous
property of (non-flaring) stellar TR lines to be composed of narrow and broad
line components (Wood, Linsky, and Ayres, 1997; Pagano et al., 2004; Peter,
2006) during all activity phases has the great potential to be finally
resolved with IRIS data (see discussions in Ayres, 2015a).

##  8 Conclusions

conclusions

IRIS is the highest resolution observatory to provide seamless coverage of
spectra and images from the photosphere into the corona. The unique
combination of near and far-UV spectra and images at 0.33-0.4′′ resolution at
high cadence allows the tracing of mass and energy through the critical
interface between the solar surface and the corona or solar wind. IRIS has
enabled crucial research into the fundamental physics processes thought to
play a role in the low solar atmosphere such as ion-neutral interactions,
magnetic reconnection (e.g., resulting from braiding or driving flares and
smaller scale events), the generation, propagation, and dissipation of waves,
the acceleration of non-thermal particles, and various small-scale
instabilities. These new findings have helped provide novel insights into a
very wide range of phenomena including the discovery of non-thermal particles
in coronal nanoflares, the formation and impact of spicules and other jets,
resonant absorption and dissipation of Alfvénic waves, energy release and jet-
like dynamics associated with braiding of magnetic field lines, the importance
of thermal instability in the chromosphere-corona mass and energy cycle, the
role of turbulence and the tearing mode instability in magnetic reconnection,
the contribution of waves, turbulence, and non-thermal particles in the energy
deposition during flares and smaller-scale events such as Ellerman bombs or UV
bursts, and the role of flux ropes and various other mechanisms in triggering
and driving CMEs.

Machine learning techniques are increasingly being utilized to exploit the
very rich IRIS observations, including a revolution in chromospheric
thermodynamical diagnostics through inversions and machine learning, and novel
flare predictions that provide insight into the trigger mechanisms behind
these space weather events. Advanced numerical simulations and radiative
transfer calculations have been key for interpreting the IRIS observations,
including exciting insights into the coldest regions and the key role of
ambipolar diffusion and ion-neutral interactions in heating the chromosphere
and driving jets, the role of braiding-driven ion-cyclotron waves and
dissipation in heating transition region loops, and the nature of the physical
mechanisms driving white light flares (including on M-dwarf stars, affecting
exoplanet habitability).

Many of these results are surprising, challenge current models and provide
novel avenues to explore, often requiring new observing modes that utilize the
flexibility of IRIS on different targets or events.

IRIS observes the region that is at the interface between the photospheric
driver of solar atmospheric heating and space weather events and the corona,
and IRIS observations are thus best analyzed in coordination with observations
from other space-based observatories and ground-based observatories. The next
few years promise an exciting new era of solar research: IRIS will continue to
provide seeing-free imaging and spectroscopy at high spectral resolution of
the chromosphere, transition region, and flaring corona, revealing velocity,
temperature and density diagnostics at high spatio-temporal resolution in
thermal domains that are complementary to existing instruments (SDO, Hinode,
NuSTAR, ALMA, SST, BBSO) and new instrumentation onboard SOLO and PSP, and
from the ground, such as NSF’s 4m DKIST telescope. As outlined in the various
sections of this paper, unique coordinated datasets of IRIS with novel
photospheric, chromospheric, transition region, coronal and solar wind
diagnostics from new instrumentation, coupled with increasingly sophisticated
numerical models and machine learning and inversion techniques, will be able
to build on the advances made with IRIS in the past few years and address many
of the unresolved issues. This will lead to key advances in understanding
physical mechanisms such as multi-fluid interactions, reconnection and
turbulence, energy transfer between the surface and corona or wind,
instability of the solar atmosphere, solar-stellar studies, and the drivers of
the irradiance that impacts Earth’s upper atmosphere.

Disclosure of Potential Conflicts of Interest The authors declare they have no
conflicts of interest.

##### Acknowledgements

The authors would like to thank the IRIS science and operations team,
including all science planners, for enabling the IRIS science mission. We are
grateful to Ed DeLuca for careful reading of the manuscript. We also would
like to thank Tom Ayres, Paul Bryans, Georgios Chintzoglou, Milan Gosic, Chad
Madsen, Navdeep Panesar, Ruth Peterson, Alberto Sainz-Dalda, Alan Title,
Sanjiv Tiwari, and Magnus Woods for discussing some of the IRIS results
reviewed here. We gratefully acknowledge Alberto Sainz Dalda for providing
Fig. fig_alba. IRIS is a NASA small explorer mission developed and operated by
LMSAL with mission operations executed at NASA Ames Research center and major
contributions to downlink communications funded by ESA and the Norwegian Space
Centre. This work is supported by NASA contract NNG09FA40C (IRIS). The
simulations have been run on clusters from the Notur project, and the Pleiades
cluster through the computing project s1061, s2053 and s8305 from the High End
Computing (HEC) division of NASA. This research was in part supported by the
Research Council of Norway through its Centres of Excellence scheme, project
number 262622, and through grants of computing time from the Programme for
Supercomputing. PA acknowledges STFC support through Ernest Rutherford
Fellowship grant ST/R004285/2. Numerical computations for Fig. fig:resonantkhi
were carried out on Cray XC50 at the Center for Computational Astrophysics,
NAOJ.

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