FR structure of CMEs (Thernisien2006; Thernisien2011). It involves three
geometric parameters: ‘h’, the height of the leading edge, ‘$\kappa$’, the
aspect ratio, and ‘$\delta$’, the half edge-on angular width, and three
positioning parameters: ‘$\theta$’, ‘$\phi$’, and ‘$\gamma$’, the Stonyhurst
latitude and longitude of the source region, and the tilt angle of the source
region neutral line respectively. The GCS model is usually used to study
morphology, position, and kinematics of a CME based on the best fitting result
of a CME transient recorded in white-light images. The ice-cream cone model is
another model of CMEs, which composed of a ball that we call the ice-cream
ball and circular cone tangent to the ball with a conic node on the solar
surface (Fisher1984). The GCS model becomes equivalent to the ice-cream cone
model when its parameter $\delta$, equals 0 (Thernisien2011). For our case
study, we use the ice-cream cone model which is a simplified form of the GCS
model and estimated the three-dimensional height and direction of the CME with
LASCO C2, C3 and _STEREO_ –B COR2 images. The best-fitted GCS model is
displayed in Figure 4.10. The corrected CME speed from the GCS model comes out
to be 450 km s-1.

Figure 4.11: Panel (a): The complete kinematics of the jet and the CME. The
projection corrected speeds are plotted with blue color, while the red points
are used for uncorrected data. Panel (b): Light curves for different
wavelengths.

Figure 4.11 (a) depicts the complete kinematics of the jet and the CME with
the different data points of various instruments. We have corrected the
projection effect for the jet and CME with SCC-MEASURE and GCS model fitting,
respectively. The corrected jet speed comes out to be $\approx$ 200 km s-1
from SCC-MEASURE associated with a CME of speed $\approx$ 450 km s-1. The blue
and red colors are used for corrected and uncorrected data points. This plot
of temporal evolution shows the clear link between the jet and the narrow CME.
Figure 4.11 (b) shows the intensity variation at the jet base. The impulsive
peaks at the jet base show the jet peak time in various AIA wavebands. The
enhancement in the light curve of EUV emission suggests that the energy
injection was at the very beginning only, and not responsible for the
continuous acceleration of the jet to escape from the Sun.

The speed of the CME (450 km s-1) is much larger than that of a jet (200 km
s-1). This is because the speed of different parts of erupting structures are
measured. The speed of CME obtained from the _STEREO_ and LASCO observations
is at its leading edge (vfront). It consists of the propagation speed of the
CME center (vcenter) and the expansion speed (vexp) of the CME, so vfront =
vcenter \+ vexp. A cartoon illustrating the CME speed at the leading edge,
which includes the CME propagation speed and expansion speed is given in
Yuming2015. Gopalswamy2009 derived a relation between CME propagation speed
and expansion which is confirmed in many studies till now (Michalek2009;
Makela2016). With an approximation of the CME shape by a shallow ice cream
cone, the relationship is defined as vexp = 2 vfront sin$(w/2)$, where ‘$w$’
is the CME width (25∘ in present case). Therefore vexp comes out to be 230 km
s-1 and vcenter should be 220 km s-1. The jet triggered and developed into the
CME and its trajectory should be followed by the CME center and not by the
leading edge of the CME. Thus, the jet velocity (200 km s-1) is comparable
with vcenter (vcenter $<$ vfront). That explains the difference between the
jet and CME speeds.

####  4.3.4 Magnetic configuration of the jet source region

For a better understanding of the trigger mechanism of the solar jet, we did
the magnetic field analysis of the source region using the HMI SHARP data of
the AR 11731 on April 28, 2013. The continuous cancellation of the negative
magnetic polarity by the emerging positive magnetic spot is observed (Figure
4.12). The positive magnetic polarity ate the negative magnetic polarity which
was already distributed in the jet source region (plotted inside the green
circle in the panel (a) and (g)). Afterwards, small negative polarities
emerges from the large negative ball and get cancelled with the big positive
polarity area. The emergence of small negative polarities is shown with yellow
arrows and the cancellation is indicated with cyan arrows. To look at the
variation of the magnetic flux with time, we calculated the positive, and
negative unsigned magnetic flux at the jet source region, which is indicated
as the red rectangular box in panel (e). This is the same dimensional area we
used to calculate the light curve in Figure 4.11(b). The flux variation with
time in panel (j) shows that, there is a continuous cancellation and emergence
of the negative magnetic flux (blue line) while the positive magnetic flux
emerges throughout (red curve). The positive and negative magnetic flux show
the simultaneous cancellation and emergence of magnetic polarities at the jet
source region. The emergence of the positive magnetic flux dominated over the
cancellation throughout. The initiation of jet time is shown with a green
vertical line.

Figure 4.12: The left panels (a)–(i) show the magnetic field configuration at
the jet site. Cyan and yellow arrows show the cancellation and emergence of
negative magnetic polarity. Right panel (j) is the magnetic flux variation
with time calculated at the jet source region. Figure 4.13: The PFSS
extrapolation of the large FOV is shown in panel (a). The white and pink lines
are the closed and open magnetic field lines and the open lines are resembling
of the jet propagation. Panel (b) and (c) are showing the source region in AIA
304 Å and HMI magnetogram.

We analysed the magnetic topology at the jet location and applied two
different methods of potential extrapolation, one for the global overview of
the jet eruption, and the other for the local view at the jet base region. We
apply the Potential Field Source Surface (PFSS) model (Schrijver2003), to
investigate the global magnetic topology near the jet source region. This PFSS
technique uses the HMI synoptic magnetic maps processed with a software
package available in SSWIDL. PFSS technique is used to see the reconnection
between close loops and open field lines because at large scale the corona is
in potential state (Schmieder1996). The PFSS model for this case study is
presented in Figure 4.13 (a), with open (pink) and close (white) magnetic
field lines. These open field lines resemble exactly the path exactly the same
as the jet, which was along the north direction in the beginning and deflected
towards the north–east afterwards.

To describe the magnetic topology of the jet base region, we extrapolate the
coronal potential field using the photospheric LOS magnetogram as a boundary
condition. The method based on the Fourier transformation (FT) method proposed
by (Alissandrakis1981). The FT method requires the vertical component of the
photospheric vector field as the input parameter. However, due to HMI vector
magnetic field limited field of view, the extrapolation hard to meet
divergence–free condition. Hence, we cut a larger patch of the LOS magnetogram
instead. As the AR is close to the central meridian, the LOS magnetic field
could represent the vertical field to a large extent. In the extrapolated
magnetic field, we find open field lines coincide well with the extension
direction of the jet shown in Figures 4.13 (b) and (c). We have tried for the
NLFFF extrapolation, but it failed to reproduce the magnetic field topology of
the AR. The field lines of the NLFFF did not resemble with the loops observed
in the EUV passbands. It might because that the FOV of the photospheric vector
magnetic field provided by HMI SHARP data is too small as it is available for
significant AR patch of solar magnetic field. Hence the divergence-free
condition is not completely satisfied in the extrapolation, which makes the
NLFFF results unreliable. On the other hand, we mainly focus on the
propagation of the jet, which is more likely to be relevant to the nearly
potential, large scale magnetic field connectivity. Therefore we believe that
the potential field extrapolation might be sufficient and the direction of the
jet ejection is the same as of the open magnetic field lines we have obtained
from the potential field extrapolation.

###  4.4 Results and conclusion

In this chapter, the role of solar jets for triggering the large scale solar
eruption is presented. Two different case studies are analysed, where in the
first study the solar jet triggered a filament eruption with the largest
geomagnetic solar storm of solar cycle 24 (Chandra2017) and in the second
study the jet was directly associated with a CME (Joshi2020ApJ). The main
results of the study are as follows:

The initiation of filament eruption on March 14, 2015 and it’s full eruption
on March 15, 2015 was associated with jet activity in the AR 12297. The decay
index distribution suggests that on March 14, 2015 the filament first enters
into the instability zone with a push from a jet and after reaching some
height it finds itself in the stability zone. Again on March 15, 2015 the
filament enters into the instability zone and finally it erupts, when an
another jet activity again hits it. The major part of filament which had not
been destroyed on March 14, 2015 was activated on March 15 but could not
erupt. Therefore it was a failed eruption. The coronal magnetic field
calculation shows evidence that the decay index at the filament location is
below the threshold of the torus instability and hence the filament fails to
erupt. The observation of the same sign of the twist/helicity in the
chromosphere, higher solar atmosphere and in the magnetic cloud evidence the
conservation property of the helicity.

From the decay index distribution, we establish that the western section of
the filament before the separation and the western filament after the
separation are relatively low (30 Mm) and were located in the zone of
stability within the coronal magnetic field. The eastern section of the
filament was less stable because on the one hand it is higher and on the other
hand the decay index in this area was also higher. That is why, that the
disturbance (solar jet) coming from inner parts of the AR led to the partial
and failed eruption of the eastern section of the filament, which resulted in
the separation of the filament into two parts. The erupted eastern section of
the filament found a new equilibrium position at a greater height. In the new
position of the eastern section, this height (about 80 Mm), considered as the
southern filament F1, was within the zone of stability for the decay index
threshold nc = 1.5 and on the edge of stability for the decay index threshold
nc = 1. The next the disturbance from the AR easily causes the start of the
eruption of the southern filament and this eruption is full because there is
no a zone of stability at heights above 100 Mm. So, the eastern section of the
filament showed the two step eruption with the metastable state at the height
of 80 Mm for 12 hrs. Possibly it could stay there longer if the strong
disturbance (second jet) did not come from inner parts of the AR. The western
filament was deep within the zone of stability therefore it did not erupt
despite the strong activation by the energetic disturbance.

The observed jet speed on April 28, 2013 computed using the multi-view point
observations is about 200 km s-1 at the height of 2 R⊙. The escape velocity
computed at the height of 2 R⊙ comes $\approx$ 430 km s-1. Therefore, we
conclude that the complete jet cannot be escaped from the solar surface. This
could be the reason we have observed the backward motion of the jet material
from the propagation direction towards the source region. Even the jet speed
is lower than the escape speed, we observed the clear CME associated with the
jet by all the space-borne coronagraphs. The possible mechanism for the jet
continuously accelerating to reach the escape speed and form the narrow CME is
that the falling back material makes the upward material of the jet moving
faster to keep the momentum of the whole jet conserved. We concluded that the
observed speed of the CME is containing the speed of the CME center and the
expansion speed, and is much larger than the jet speed, because the different
parts of the erupting structures are being measured. The speed of CME center
(the trajectory followed by the jet) is 220 km s-1 and is equivalent to the
speed of the jet (200 km s-1). This provides a clear evidence of the jet-CME
association.

For the magnetic configuration at the jet origin site, two views are popular.
One is the magnetic flux emergence observed in many observations and also
proposed in the MHD simulations (Shibata1992; Moreno2013; Ruan2019;
Joshi2020MHD). Another is the magnetic flux cancellation, which is also
reported in the observations MHD simulations (Pariat2009; Chandra2017;
McGlasson2019). We have observed that there is a continuous emergence and
cancellation of the negative magnetic flux and the positive flux is emerging
throughout. Therefore, we believe that both the flux emergence and the
cancellation are responsible in this case. We also observed the rotation in
the jet material on April 28, 2013 when it is propagating towards the north
direction from the source region. The untwisting of the jet suggests the
injection of helicity to the upper atmosphere.

For the future study, to look forward for finding the clear in situ
measurements for such association of filament eruptions with solar jets and
jet–like CMEs from the newly launched Parker Solar Probe will be a major field
of interest which will help to contribute for unwinding the mystery of coronal
heating problem.

##  Chapter 5 Transfer of twist to a solar jet from a remote stable magnetic
flux rope

###  5.1 Introduction

An overall common property for solar jet is to exhibit a twist or rotation
(Raouafi2016). The twist of the jet may be due to helical motions
(Patsourakos2008; Nistico2009). Twisting motions have been found in a large
velocity range of jets or surges (Chen2012; Hong2013; Zhang2014). In the study
done by Schmieder2013, a jet analysis revealed a striped pattern of dark and
bright strands propagating along the jet, as well as apparent damped
oscillations across the jet. They concluded that this is suggestive of a
(un)twisting motion in the jet, possibly an Alfvén wave. Spectroscopic data
also provide signatures for detecting the twist in jets. For example, blue and
red shifts observed along the axis of a jet in Hα as well as in Mg II lines
were interpreted as confirmation of the existence of twist along the jet
(Ruan2019).

Spectroscopic and imaging observations of small-scale events reveal
bidirectional flows in transition region lines at the jet base which could
correspond to an explosive reconnection (Li2018; Ruan2019). There are
different conditions for the magnetic configuration of an AR to trigger
magnetic reconnection. We may quote three types of conditions: magnetic flux
emergence (Archontis2004; Archontis2005; Moreno2008; Torok2009; Moreno2013),
magnetic flux cancellation (Priest2018; Syntelis2019), and magnetic
instability (Pariat2010; Pariat2015; Pariat2016). The first two mechanisms
predict hot and cool jets simultaneously. However, the presence of surges and
jets is not frequently reported. Some papers report on the X-ray jets observed
by Yohkoh and associated with a surge (Schmieder1995; Canfield1996; Ruan2019).
Radiative MHD simulations based on flux emergence (Nobrega2017; Nobrega2018)
as well as the flux cancellation model (Syntelis2019) show that surges can
exist at the same time with hot jets. The cool plasma is advected over the
emergence domain without passing near the reconnection site and then flows
along the reconnected magnetic field lines. These models fit with the
observations of X-ray jets observed with Hinode and with Hα jets from the
Swedish 1-m Solar Telescope (SST) (Nobrega2017).

Recently, Joshi2020MHD presented a case-study of collimated hot jets and
associated cool surges which fit in perfectly with the simulation of jets
formed by flux emergence. The double-chambered structure found in the
observations corresponds to the cool and hot loop regions found under the
reconnection site in the models of Moreno2008. In the model of Wyper2019, the
overlying magnetic field is, in fact, expelled by a gentle reconnection above
the closed AR via a breakout mechanism before the instability occurs.
Pariat2015, and Wyper2019 show the importance of the inclination of jets
favouring the jet onset for $\theta$ = 0 - 20 degrees. These models are based
on the instability of the system; a FR formed by shear under the reconnection
point is the trigger of the helical jet. However, based on several
observations, it becomes clear that the twist is not present before the
reconnection but the twist of the jet is transferred during the reconnection.
For example, in Ruan2019, the twist was transferred from twisted overlying
magnetic field lines remnant of the eruption of a filament two hours before
the onset of the jet. The null-point is the favourable location for the
occurrence of magnetic reconnection.

Wyper2019 recently showed that reconnection can be in a region where the
magnetic field lines are tangent to the photosphere. This kind of region is
called BP region. It favours reconnection as a mechanism for initiating jets
and surges (Mandrini2002; Chandra2017; Zhao2017). In these studies, the
magnetic topology was derived by LFFF or NLFFF magnetic field extrapolations
in the corona (Mandrini2002; Chandra2017) or by directly analysing the
observed magnetic field vector maps (Zhao2017). The occurrence of the
reconnection was clearly taking place in the BP regions. It was also recently
proposed that the trigger of jets can be due to the eruption of mini-filament
at the jet base (Sterling2016). That model fits well with the blowout jets
where the entire region below the dome of reconnection is expelled during the
eruption (Moore2010).

In this chapter, the observations of a twisted jet, surge, and a mini-flare
observed in multi-wavelengths, and with the New Vacuum Solar Telescope (NVST,
NVST2014) ground based telescope are analysed.

###  5.2 Observations of the twisted jet

####  5.2.1 EUV and chromospheric observations

In the AR NOAA 12736 a jet along with a surge is well observed in the multi-
wavelength filters of AIA aboard SDO. AIA data consists of a sample of filters
with passbands centered at different EUV lines. The IRIS FOV was focused on AR
NOAA 12736 and the pointing of the telescope was at 709′′, 228′′ with a FOV of
60′′ $\times$68′′ for the slit-jaw images (SJIs). The observational
characteristics are presented in Table 5.1. We used the 1330 Å and 2796 Å SJIs
for this study. There was no data for the IRIS SJI Si IV 1400 filter. The SJI
1330 Å includes the C II line formed at T= 30000 K, and the SJI 2796 Å
emission mainly comes from the Mg II k line. The Mg II h and k lines are
formed at chromospheric temperatures, that is, between 8000 K and 15000 K
(Pontieu2014; Alissandrakis2018). The co-alignment between the different
optical channels of IRIS was achieved by using the drot_map in solar software
to correct the differential rotation. The SJIs in the broadband filters (1330
Å, and 2796 Å) were taken at a cadence of 14 s. IRIS performed medium coarse
rasters of 4 steps from 01:43:27 UT to 02:42:30 UT on March 22, 2019. The
raster step size is 2′′ so each spectral raster spans a field of view of 6
′${}^{\prime}\times$62 ′′. The nominal spatial resolution is 0.′′33\. IRIS
provides line profiles in Mg II k 2796.4 Å and Mg II h 2803.5 Å, Si IV
(1393.76 Å, 1402.77 Å) and C II (1334.54 Å, 1335.72 Å) lines along four slit
positions. Calibrated level 2 data are used in this study, with corrected dark
current subtraction (Pontieu2014).

####  5.2.2 Magnetic field observations

The longitudinal magnetic field is provided by the HMI team with a cadence of
45 s and a pixel size of 0.5$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$. To
obtain the magnetic field vectors in full, we inverted the HMI level-1p IQUV
data, averaged on a 12 minute cadence, by applying the Milne-Eddington
inversion code UNNOFIT (Bommier2007). We selected a large area covering the AR
12736 and applied a solar rotation compensation to select the same region over
more than six hours of observation. We thus treated 22 maps from March 21,
2019 at 23:00 UT to March 22, 2019 at 03:12 UT and three later maps of the
same region from 05:00 UT to 05:24 UT. The specificity of UNNOFIT is that a
magnetic filling factor is introduced to take into account the unresolved
magnetic structures as a free parameter of the Levenberg-Marquardt algorithm
that fits the observed set of profiles with a theoretical one. However, for
further application, we used only the averaged field, that is, the product of
the field with the magnetic filling factor, as recommended by Bommier2007. The
interest of the method lies in a better determination of the field
inclination. After the inversion, the 180∘ remaining azimuth ambiguity was
resolved by applying the ME0 code developed by Metcalf, Leka, Barnes, and
Crouch (Leka2009) and available at http://www.cora.nwra.com/AMBIG/. After
resolving the ambiguity, the magnetic field vectors were rotated into the
local reference frame, where the local vertical axis is the $o_{z}$ axis.

####  5.2.3 Hα observations

The Hα observations were taken with the NVST telescope in China, pointed at
the AR 12736 at N09 W60 on March 22, 2019 from 00:57:00 UT to 04:37:00 UT. We
used the line–center Hα observations at 6562.8 Å that were obtained in a field
of view (FOV) of 126$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ $\times$
126$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ with a cadence of 29
seconds. It displays the Hα fine structures and shows their evolution very
well. For the current analysis, we used the level 1+ data.To focus on the jet
region, we cut the data cube after rotating it with north upwards, as in the
space data (AIA and HMI observations) and used the data of FOV of
65$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ $\times$
65$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$.

Table 5.1:  IRIS observation of AR NOAA 12736 on March 22, 2019.
Location |  Time |  Raster |  SJI
---|---|---|---
|  (UT) |  |
x=709$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ |  01:43 - |  FOV: 6$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ $\times$ 62$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ |  FOV: 60$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ $\times$ 68$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$
y=228$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ |  02:42 |  Steps: 4 $\times$ 2$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ |  C II 1330 Å
|  |  Spatial |  Mg II 2796 Å
|  |  Resolution: 0.$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$33 |  Time
|  |  Cadence: 3.6 s |  Resolution: 14 s
Figure 5.1: Panels (a–e): HMI longitudinal magnetograms of AR NOAA 12736.
Reconnection is occurring between the two large emerging flux areas EMF1 (P1,
N1) and EMF2 (P2, N2) encompassed in the two ovals drawn in panels (b) and
(d). Figure 5.2: Magnetic flux cancellation in two areas including the major
bipole (P1, N2) (red box (b)) and the small jet bipole (JP1, JP2) (blue box
(c)) respectively. Panels (b and c): variation of the magnetic flux in the red
and blue boxes.

###  5.3 Birth of the AR

A mini-flare (B6.7 X-ray class) and its associated jet was initiated in AR
NOAA 12736 located at N09 W60 on March 22, 2019 around 02:02 UT. The AR 12736
was emerging progressively since March 19, 2019. On March 21, we note two
emerging flux regions elongated along the north-east to south-west direction
(the ovals in Figure 5.1 (b and d)). The first emerging flux region (EMF1) is
the main component of the AR, with negative leading polarity and positive
following polarity. The second emerging flux region (EMF2) consists of many
fragmented negative polarities which are travelling very fast as the emerging
flux is expanding towards south and squeeze the positive polarity of EMF1.
Consequently, a very high magnetic field gradient is observed perpendicularly
to the polarity inversion line (PIL) between the squeezed polarities: ‘P1’
positive polarity belonging to EMF1 and ‘N2’ negative polarity belonging to
EMF2 (the red box in Figure 5.1 (e)). The negative polarity N2 is surrounded
by positive polarities P1 on the right side and P2 on the left side and top.
This topology is classical with the aim of getting a null point, as we see
discuss further on in this chapter. In the HMI observations, we note the fast
sliding motion of negative polarity N2 towards the south and the motion of
positive polarity P1 in the opposite direction, which creates a strong shear
between them. Along this PIL, we distinguish that at the time of the flare
observations, we principally observe two bipoles emanating from the two EMFs
in the diagonal of the box (NE-SW): a large north bipole (P1, N2) and a very
tiny bipole (JP1, JN2) in the south, which was detached progressively from the
northern N2 polarity a few hours before (explained in Section 5.4). We
computed the flux budget for these two bipoles and we found a significant
decrease of the positive flux in the two boxes, each of them including a
bipole (Figure 5.2). We interpret these decreases by magnetic cancelling flux.
The tiny bipole is labeled with ‘J’ like “jet" because this is the the
location where the jet took place.

The AIA observations cover the AR and the full development of the jet in
multi-temperatures provided by all the sample of AIA filters from 304 Å to 94
Å, all along the range of temperatures from 105 K to 107 K (Figure 5.3 and
5.4). Contours of longitudinal magnetic fields ($\pm$ 300 Gauss) are overlaid
on the AIA images to specify the location of the small bipole JP1-JN2 at the
jet base. Arch filament system (AFS) are well visible over the two emerging
flux EMF1 and EMF2 with cool and hot low lying loops joining the positive and
negative polarities for each of them, P1 and N1 on the west side and P2 and N2
on the east side. Filaments belonging to these AFS are particularly visible as
dark structures due to absorption mechanism in 171 Å, 131 Å, and 193 Å at
02:06 UT (Figure 5.3 panel e and Figure 5.4 panels b, e). These filaments are
parallel to each other, oriented more or less NE to SW from P1 to N1 and from
P2 to N2 in the direction of the extension of each EMF. They do not lie along
any PIL and, therefore, they do not correspond to the usual definition of
filaments; rather, they are more or less perpendicular to the PIL in each EMF.
Filters AIA 171 Å and 193 Å are good proxies for detecting cool structures
visible in Hα. At these wavelengths, the EUV emission is absorbed by the
hydrogen and helium continua (Anzer2005). The opacity of the hydrogen and
helium resonance continua at 171 Å is almost two orders of magnitude lower
than the Lyman continuum opacity at 912 Å and thus similar to the Hα line
opacity (Schmieder2004). We confirm the presence of Hα filaments/AFS by
looking at the Hα images (Figure 5.5). The two AFS over the two EMFs are well
identified. On the west side the AFS are dense and long, with some narrow arch
filaments overlying the bright corridor of the PIL (N2-P1) and the dome of
EMF2 before the burst (panel b). The AFS over EMF2 on the east side have a fan
structure with an anchorage all around the negative polarities N2 and the
other in the positive polarities P1 and P2. It gives the impression of an
‘anemone’ structure which is frequently observed for jets triggered by
emerging flux (Shibata1982; Schmieder2013; Joshi2020MHD). In order to follow
the jet development with AIA, we focus on the FOV covering the two bipoles
identified in the previous section.

Figure 5.3: Solar jet and surge observed in different AIA/EUV channels (304 Å,
171 Å, and 211 Å) on March 22, 2019. The black arrows in (b) and (h) indicate
a dark area corresponding to a surge, the white arrow points the jet in (e).
Figure 5.4: Solar jet and surge observed in the other AIA/EUV channels (131 Å,
193 Åand 94 Å). Panel (e) shows a dome structure at the jet base highlighted
by a white dashed contour.

####  5.3.1 Morphology of the twisted solar jet

It is interesting to see that activity had started before the onset of the
jet, with very bright north-south tiny threads observed above the PIL between
the two EMFs and, more precisely, between the part of the PIL in the northern
bipole (P1-N2) and continuing into the south tiny bipole (JP1-JN2) around
02:01 UT until 02:04 UT. The bright signature around a dome structure
overlying the EMF2 at the jet base is highlighted by a white dashed contour
around it, as seen in Figure 5.4 (e). This dome is highlighted by the small
fibrils with an asymmetrical anemone shape visible in the NVST images (Figure
5.5 b). Along the west side of the dome, the brightening with an north-south
arch-shape is visible in all AIA channels before the burst indicates already
the presence of hot plasma (T between 104 to 106 K). This region between the
two EMFs corresponds to QSLs (Demoulin1996), where a strong high electric
current develops and heats the plasma, as shown here by the arch-shape
brightening. This is subsequently confirmed with the analysis of the
photospheric vector magnetic field maps in Section 5.4. These QSLs have been
calculated in Yang2020 and are well-identified in this region. QSLs are robust
structures but their localisation is not commonly defined with any substantial
accuracy (Dalmasse2015; Joshi2019). In our case, the moving polarities is a
problem for the exact localisation of QSLs.

Figure 5.5: Hα line center observations of the AR NOAA 12736 with the NVST
telescope for four times before (a-d), during (e), and one after (f) the surge
extension.  Figure 5.6: Height-time plots for the jet in different AIA
wavelengths. The surge is erupted with a maximum speed of $\approx$ 100 km s-1
and the dark material came back with a speed of $\approx$ 40 km s-1.

At the same time (02:04 UT), a jet with two branches inserting a surge is also
observed. Dark absorbing material is visible at $\approx$ 02:02 UT, resembling
a blob with no really defined shape in the southern part of the arch-shaped
brightening (Figure 5.5 panel c). It then extends to the north and, finally,
goes along the jet direction. The surge appears as a dark area in the images
because of the absorption of the UV emission. Therefore, the dark part
observed in 171 Å and in the other AIA filters, that is, (211, 193, 94 Å)
should correspond to cool plasma as seen in Hα (Schmieder2004). It is why we
call it a ‘surge’ (Figure 5.3 and 5.4). The surge appears as a bright
structure in the NVST images (Figure 5.5 e). However, the surge consists of
cool plasma because Hα formation temperature is lower than 1.5 $\times$ 104 K.
The jet base on the east side of the EMF1 is extended along a more or less
north-south direction, along the PIL between EMF1 and EMF2 (P1 and N2; Figure
5.4 g) and the jet top on the west side of the EMF1 is limited at the location
of N1. At 02:11 UT and over a few minutes up until 02:18 UT, we can observe
long bright and dark AFS striding over the two EMFs (right columns of Figure
5.3 and Figure 5.4). The characteristics of the jet are the following: length
around 50 Mm, the base width between 15-20 Mm. The jet lifetime is between
02:02 UT to 02:11 UT.

With AIA 304 Å data, we made several observations of a mini-flare,
approximately at the same location, nearly one every hour and generally not
accompanied by such a wide jet. The detail of the recurrent mini-flares is as
follows: at the beginning of the movie, a mini-flare is visible at 20:00 UT,
then at 20:27 UT, 21:28 UT, 22:51 on March 21 and at 00:39 UT, 01:25 UT, 01:39
UT, 02:03 UT on March 22. Regularly, before each burst, we can clearly see two
AFSs: one over EMF1 in the west and one over EMF2 in the east. After the
burst, long-arch filaments connect the extreme eastern polarity P2 to the
extreme western polarity N1. Prior to our mini-flare and jet (around 01:59
UT), the two AFSs were separated by an area with mixed bright and dark
patches. Then at 02:09 UT, there is a long system of arch filaments. At 02:28
UT, when the phase of the activity is over, the initial configuration with the
two distinguished AFSs, just as before the jet, is restored. This chain of
mini-flare and ejection is recurrent.

To analyse the evolution of the observed jet, we created the projected height-
time plots of the jet in different AIA wavebands (171 Å, 211 Å, and 304 Å),
which are presented in Figure 5.6. To obtain this height-time plot, we chose a
broad slit (width = 8 pixels) to cover the plasma outflow. The average jet
speed along the slit direction shows two different slopes: a steep slope in
the starting phase (till 02:05 UT) of the jet eruption with an average speed
of 350 km s-1 and a slow phase of 80 km s-1 in the later stage from 02:05 UT
to 02:08 UT. The slow phase may be due to the presence of loop system in the
path of the jet. It seems that when the jet material is passed through this
loop system, it decelerated and, finally, it stopped. The cool material
visible as an absorbing feature in AIA channels is detected about one minute
after the hot jet with no well defined speed. The cool material appears to be
present along the LOS in small patches but it is not moving towards the west
as the jet is doing. Later, the cool material (or surge) is escaping in
slightly different directions than the jet. Among it, we could identify some
blobs with different projected speeds ($\approx$ 100 and 30 km s-1). Then the
cool material came back at a speed of $\approx$ \- 40 km s-1 (Fig 5.6 (d)).
According to the location of the AR (W60), these velocities are underestimated
by a factor of cos(60). The positive velocities correspond to the material
that is heading away from the observer’s view.

Figure 5.7: Solar jet observed in co-aligned images of EUV AIA 304 Å (top), of
IRIS C II SJI (middle), and of IRIS Mg II SJI (bottom). The four positions of
the slit in the raster mode is shown with vertical cyan lines in panel (g). In
IRIS CII SJI images (middle row) the reconnection point is indicated.

####  5.3.2 Comparison between IRIS SJIs and AIA 304 Å observations

The analysis of IRIS data, all along the evolution of the jet, shows a good
correspondence between the structures visible in AIA 304 Å and in IRIS CII
SJIs. This correspondence is summarised in Figure 5.7. We note that the
nominal coordinates of IRIS in the file headers do not correspond to the
nominal coordinates of AIA. Therefore, we had to shift the FOV of AIA by
4$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ in x-axis and
3$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ in y-axis to obtain a good co-
alignment.The IRIS slit, with its four positions, crosses the bright zone
corresponding to the jet base, namely, the dome top, which is supposed to be
the reconnection site along a few pixels between around pixels 60 to 120 in
the left slit position (Figure 5.7 (e)). Around 02:00 UT, in the 304 Å image
as well as in CII and Mg II IRIS SJIs, small bright threads along two vertical
paths that are mixed with tiny round-shape darker areas are visible in the
middle of the FOV where the reconnection occurred (Figure 5.7 a, d, and g). It
is clear that in this small zone, there is no north-south filament along the
PIL (N2-P1) which would be visible by absorption in AIA 193 Å. The very light-
dark filament-type structure with a vague sigmoidal shape in the NVST images
that is localised at this place is, in fact, part of the AFS (Figure 5.5 b)
because there is no sigmoid visible in the hot channels of AIA, where plasma
should be heated due to high electric currents along a sigmoid
(Barczynski2020). In the north of this zone, long-lying, more or less east-
west AFS, as well as, on both sides of the zone (pixels 60-120), the short
AFS-overlying EMF1 and EMF2 are visible. The short AFS-overlying EMF2 have a
dome shape like the asymmetrical anemone formed by the fibrils visible in the
NVST images (Figure 5.5). The location of the onset of the mini-flare is
indicated by the point ‘X’ at the crossing location between the arch-shape QSL
and an east-west bright line in Figure 5.7 (d). The location of the ‘X’ point
in IRIS observation is at 709$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$,
218$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$ and in AIA it is at
705$\leavevmode\nobreak\ \\!\\!{}^{\prime\prime}$, 215$\leavevmode\nobreak\
\\!\\!{}^{\prime\prime}$. In Figure 5.7 (top panels), we translated the AIA
images to obtain a good co-alignment with IRIS images. Around 02:03 - 02:04
UT, the arch-shape QSL was brightening and the flare started with the onset of
the jet ejection visible in the CII and MgII SJIs (Figure 5.7 b,e). The bright
jet was obscured by dark material in front of it, which is the surge; both the
jet and the surge were extending at the same time around 02:04 UT. From 02:05
UT to 02:07 UT, the jet extended along two bright branches with a dark area
between them (Figure 5.7 (c),(f)). At 02:07 UT, AIA 304 Å image shows the
extension of the surge covering all the bright jet. The dark area is due to
the absorption of the 304 Å emission by He continua (Anzer2005) (Figure 5.7
(c)). In the CII and MgII filters, it is not so pronounced because of the
large band-pass of IRIS filters relative to the width of the lines and the low
emission of the lines in the jets (Figure 5.7 (f),(i)). Nevertheless, we can
still distinguish a bright EW-elongated jet in the south and some dark area
above it that might correspond to the surge. This is confirmed in the NVST
images (Figure 5.5).

###  5.4 Magnetic environment of the AR

In Section 5.3, we follow the birth of the AR using HMI longitudinal magnetic
field. Here, we analyse the magnetic topology of the AR using the full vector
magnetic field to understand the orientation of the magnetic field lines
inside the two bipoles (P1-N2 and JP1-JN2) involved in the mini-flare and the
jet to confirm the existence of a BP.

####  5.4.1 HMI Magnetic field vector maps

Figure 5.8: Panel (a): The jet base appeared as brightening with an arch-shape
in between the positive (green contours) and negative (blue contours) in AIA
94 Å. Panel (b): HMI Vector magnetic field map with the same FOV as of the
(a). The white square is the FOV for the Figure 5.9 (a-b).

The HMI SHARP longitudinal magnetic field movie, with its high cadence, shows
the fast evolution of the EMF2. The negative polarities N2-JN2 were
continuously sliding along the positive polarity P1 and initiating bright
points from time to time. We used the HMI vector magnetic field map at the
closest time of the reconnection at 02:00 UT. Figure 5.8 presents in the right
panel the magnetic field vector map computed with the UNNOFIT inversion code
at 02:00 UT and the corresponding full AR as a contextual image meant to show
the brightening at the base of the jet in AIA 94 Å. The vector magnetic field
maps represent the full magnetic field vectors with their three components in
the solar local reference frame, generally referred to as the heliospheric
reference frame. The vertical component in this reference frame is represented
via a colour table. The two horizontal components are associated to form an
horizontal vector, which is represented by arrows. However, the pixel
dimension is viewed along the LOS in order to be able to co-align the FOV with
the AIA images. A box indicates the small FOV encircling the region which
contains the brightening at the jet base corresponding to the QSL at the
reconnection site. We carried out a zoom analysis to probe the nature of
magnetic field vectors in this jet region (Figure 5.9 a). The length of the
arrows represent the strength of the horizontal magnetic field.

####  5.4.2 FR vector pattern and formation of small bipole

In the long region between P1 and N2, we make note of a characteristic pattern
of the magnetic field vectors that suggests the presence of a twisted FR with
vectors converging together in the PIL in the middle part (between P1 and N2)
and vectors turning at both ends, in the top and bottom parts of the FR,
resembling the hooks of a FR (Figure 5.9 (a)). In the vicinity of the FR,
there is an interface that separates the regions of turning and returning of
the vectors, which represent the boundary between the FR and the arcades over
the FR and its surrounding area. This pattern is relatively stable according
with the 22 maps computed around the jet time. On March 21 at 20:00 UT, the FR
was already created and was continuously observed until March 22 at 05:00 UT.
At first glance, the FR does not seem to participate to the formation of the
jet. A very detailed study shows that, in fact, this was a very particular
case involving a transfer of twist from the FR to the jet during the FR
extension towards the south before the reconnection.

Figure 5.9: Panel (a): Vector magnetic field configuration of a part of AR
NOAA 12736. Panel (b): LOS magnetic configuration at the AR including the two
bipoles P1-N2, JP1-JP2. Color bar indicates the vertical magnetic field
strength and the arrow shows the strength of the horizontal magnetic field.

The relationship of the FR and the jet is detected in the HMI movie of the
longitudinal magnetic field where the formation of the small bipole (where the
jet was initiated) is observed. The longitudinal HMI movie shows a stress
created by the sliding of the two opposite polarities (P1 and N2). These two
polarities come from the two opposite magnetic emerging regions (EMF1 and
EMF2). On March 21 at 23:00 UT, a few hours before the jet, a negative
polarity part of N2 detaches and moves towards the south, sliding along the
positive polarity P1 to form the small bipole JN2-JP1 (Figure 5.9 g-j). The
new bipole is formed with the small positive (JP1) and the negative (JN2)
polarity encircled in Figure 5.9 (e). This small bipole is formed by collision
of two opposite sign polarities coming from two different magnetic systems and
not by direct magnetic flux emergence.

####  5.4.3 BP magnetic configuration and twist transfer

Looking at the direction of the magnetic field vectors between JN2 and JP1, we
find that they are oriented from the negative polarity to the positive
polarity, which is evidence that it is a BP region, more generally, that it is
a region with magnetic field lines that exhibit a dip grazing the surface at
the PIL (Figure 5.9 e). We note that the BP is observed only at this precise
time (02:00 UT) – and not before and not after (Figure 5.9 c, d, f).

We arrive at the conclusion that with the extension of the FR towards the
south, it is possible that the arcades of FR interact with the overlying
magnetic field. Some part of the twist of the FR could be transferred to the
jet, however, there is still a remnant component in the small bipole, as we
see in Figure 5.9 (f). The rotation of the structure at the base could explain
this transfer of twist. To make certain of the existence of the FR, we compare
this finding with MHD simulations.

####  5.4.4 Comparison between MHD models and observations

We used the MHD simulations of Zuccarello2015, where the physical conditions
are used to create a FR in an AR. Starting from an asymmetric, bipolar AR, as
in Aulanier2010, they investigated different classes of photospheric motions
that are capable of forming a FR. Here, we consider the results of the
simulations with regard to converging motions towards the PIL of the AR with
magnetic flux cancellation. Progressively twisted magnetic field lines were
globally wrapping around an axis and, eventually, formed a FR. The dynamics of
the FR is modelled by using a version of the Observationally driven High-order
scheme Magnetohydrodynamic (OHM) code (Aulanier2005OHM; Aulanier2005;
Aulanier2010). It is a $\beta$ = 0 simulation, so the plasma conditions are
not studied. The OHM code solves the standard zero-$\beta$ MHD equations in
the Cartesian coordinates with line-tied and open boundary conditions. The
line-tied reflective boundary conditions ensure that the foot-point of
magnetic field line can only perform the horizontal motion on to the boundary.
The pressureless ($\beta$ = 0) time-dependent MHD equations for an ionized and
collisional plasma are given as (Aulanier2005OHM):

$\frac{\partial\rho}{\partial t}=-\vec{\nabla}\cdot(\rho\leavevmode\nobreak\
\vec{u})$ (5.1)

$\rho\frac{\partial\vec{u}}{\partial
t}=-\rho(\vec{u}\cdot\vec{\nabla})\leavevmode\nobreak\
\vec{u}+\vec{J}\times\vec{B}+\rho\leavevmode\nobreak\
\mathcal{D}\leavevmode\nobreak\ \vec{u}$ (5.2)

$\frac{\partial\vec{B}}{\partial
t}=\vec{\nabla}\times(\vec{u}\times\vec{B})+\mathcal{R}\leavevmode\nobreak\
\vec{B}$ (5.3)

$\vec{\nabla}\times\vec{B}=\mu\vec{J}$ (5.4)

$\vec{\nabla}\cdot\vec{B}=0$ (5.5)

where, $\rho$ is the mass density, $\vec{u}$ is the plasma velocity, $\vec{J}$
is the electric current density, $\vec{B}$ is the magnetic field, $\mu$ is the
magnetic permeability, $\mathcal{D}$ is the diffusion operator for the
velocity, and $\mathcal{R}$ is the diffusion operator for the magnetic field.
OHM code solves these equations using Einstein’s notation for special
derivatives (Aulanier2005OHM; Aulanier2005). The detail on the numerical
method for OHM code is well explained in Aulanier2005OHM. The results from
these MHD simulations have already been validated by testing different flare
activities, such as sigmoid currents of FR (Aulanier2010), electric current
density increase in flare ribbons (Janvier2014), and electric current density
decrease at CME footpoints (Barczynski2020). In this study, we want to test if
the footprints of the FR in the HMI magnetic vector (vec B) maps have a
similar pattern as the footprints of the theoretical FR in these MHD
simulations.

The comparison between our observations (panels a-b) and MHD simulations
(panels c-d) is presented in Figure 5.10. We rotated our observation in panel
(a-b) by 30∘ in the clockwise direction for an improved comparison with the
MHD simulations. It is very clear that a sheared magnetic field is generated
along the PIL and the vectors are strongly inclined along with PIL. We have
also evidence for swirling of the magnetic field in the top and bottom part of
the FR.

Figure 5.10: FR evidenced in the HMI observations (a,b) and comparison with
MHD simulations (c,d). Panel (a-b): Vector magnetic field and current density
maps computed with the UNNOFIT code. Panel (c): From MHD simulations the iso-
contours of vertical magnetic field with vectors. Panel (d): The magnetic
field lines are plotted with grey color and red/blue contours are electric
currents.

The sheared vec B that converges towards the PIL is a characteristic motion to
create a BP. This pattern can also be seen in Barczynski2019. It is due to the
summed effects of: (i) the shear that creates a BP with vec B in the negative
polarity pointing towards the positive polarity; and (ii) the asymmetry of the
photospheric flux concentration with a stronger positive polarity (in the
model and observations) which is due to the magnetic pressure pushing all the
fields towards the (weaker) negative polarity. Hence it is leading to some
sheared vector within the positive polarity to point towards the negative
polarity. These two effects lead to the convergence. polarity. These two
effects lead to the convergence. The swirling motions visible at both ends of
FR are well-represented by vec B, which display similar angles and similar
spatial gradients at the edges of the swirlings, which separate the swirling
vec B from the surrounding magnetic field that has more potential, that is,
exhibiting more radial from the center of the magnetic polarity. For example,
this separation is visible at the top right in Figure 5.10 (a and c) in the
positive polarity, where radial vectors are close to turning vectors to the
left, and at the bottom of the negative polarity, there is a similar
separation between radial vectors and vectors turning to the left. This kind
of separation is reminiscent of a QSL, just as in the MHD models (Janvier2013;
Aulanier2019). Moreover, those swirls correspond exactly to the footpoints of
sigmoidal field lines, even though they are not visible in the extreme
ultraviolet (EUV). Finally, the similarity of all these characteristics
structures (e.g. BP, QSL, sigmoidal field line) between the MHD models and our
observations leads us to infer the existence of a FR in the immediate vicinity
of the jet.

In addition to the pattern of the photospheric horizontal fields, a relatively
good match is also found for the vertical current densities. Both the HMI
observation and the MHD simulation display a dominance of the Jz and Bz of the
same signs in each polarity of the bipole, with an elongated double-peaked Jz
pattern all along the PIL, as well as more extended patches at the ends of the
sheared PIL. In the model, those extended patches correspond to the footpoints
of the FR field lines (Figure 5.10 d). One difference, however, between the
observation and the model is that with HMI, the extended patches in the
negative polarity is more clearly visible than in the positive polarity
(Figure 5.10 b). We argue that this difference is minor since it may be due to
that fact that in the positive polarity, the swirling patterns of the vector
fields are located in relatively weaker vertical fields than in the negative
one (i.e. 500G in the former vs. 1500G in the later, Figure 5.10 a). With the
same twist in both polarities, the weaker fields result in weaker current
densities in the positive polarity. Another difference is that in the MHD
simulation, some strong QSL-related current sheets surround the FR footpoint
related extended patches (Janvier2013; Aulanier2019). These are not visible
with HMI and we argue that this is due to the limitations of the HMI data,
from which current sheets can only be extracted during flares and with some
processing of the data, as in Janvier2014; Barczynski2020.

Comparing the magnitudes of current densities between models and observations
requires us to scale the model to physics (solar) units. The reason behind
this is that the model was calculated with dimensionless units, resulting in
maximum current densities on the order of five units. Such a scaling has
already been done for the estimation of flare energies (Aulanier2013). Here,
they need to be adjusted to this specific observed bipole. Still, we should
bear in mind that this can only be done approximately given the differences in
shape between the observed and modelled flux concentrations. To convert the
non-denationalized j MHD-units to real j solar-units, we calculate as
(Aulanier2005OHM):

$J_{solar}(Am^{-2})=j_{0}\times\frac{1}{\mu}\times\frac{B_{solar}}{B_{0}}\times\frac{L_{0}}{L_{solar}}$
(5.6)

Thus, using HMI as a reference (Figure 5.10 a), we attributed a magnetic field
amplitude of $\pm 1200G$ to the Bz isocontour $B_{z}=\pm 1.7$ and a bipole
size of 15 Mm to the width of 5.2 space units as displayed in Figure 5.10 c.
Then we scaled the OHM model using a magnetic unit $B_{0}$ = 700G and a
spatial unit $L_{0}=2.9\times 10^{6}$ m. We reset the magnetic permeability
from unity in the simulation to its real value $\mu$ = 4 $\pi\times 10^{-7}$ N
$A^{-2}$. As a result, the dimensionless current-densities that we model here
have to be multiplied by $B_{0}/(\mu L_{0})$ to be expressed in A/m2 as in
equation 5.6. With these settings, the currents reached up to 100 mA/m2 at the
FR footpoints. This value is only half of what is measured with HMI, so the
modelled currents are in qualitative agreement with the observed ones. The
difference in magnitude may be attributed to the existence of a stronger twist
in the observed bipole than the twist in the model. Yet it is arguably more
likely due the different aspect ratios of the observed and modelled bipoles,
the latter being less elongated than the former (Figure 5.10 a and Figure 5.10
c).

####  5.4.5 Magnetic shear: consequence of the jet

When the twisted FR fieldline (the one initially rooted in JN2 and stretched
by the footpoint motion) eventually reconnects with the large western
untwisted loop (rooted in JP1 and far west in the negative polarity) then two
new field lines are formed (Figure 5.11):

  1. 1.

The first one is the untwisting jet field line. Composed of a long truncated-
FR part at the east and a long truncated-loop at the west.

  2. 2.

The second one is like a flare-loop forming below the ‘X’-point. It is rooted
between JN2 and JP1. It is composed at the east of the truncated leg of the FR
field line, and at the west it is the former leg of the loop. That’s the
JN2-JP1 structure, shown by a small loop in Figure 5.11 (d).

Just after these two lines have reconnected, they are composed of a eastern
part where field aligned currents are present (which are associated to twist
in the FR), and of a western part where no (or weak) electric currents exist
(i.e. a potential field). After reconnection these lines are not in force-free
condition. A force-free field satisfies (Demoulin1997; Aulanier2010;
Aulanier2014):

$(\vec{B}\cdot\vec{\nabla})\alpha=0$ (5.7)

where, $\vec{j}$ is the electric current density and $\vec{B}$ is the magnetic
field. $\alpha$ is a constant related with the intensity of the coronal
electric current density (Demoulin1997; Chandra2011). These equations justify
that in force free fields currents are co-linear with magnetic fields (i.e.
field-aligned currents) and the ratio between currents and magnetic field is
constant along a field line. In the present case the products of reconnection
have a finite $\alpha$ at the east, and zero $\alpha$ at the west. So they are
not force free. Therefore, some (torsional) Alfvén waves must be launched to
reach a new balance, in which twist is eventually redistributed all along the
previously-reconnected field-lines. After the relaxation, $\alpha$ must has
been redistributed along the loop (shown in panel d), so the whole reconnected
JN2-JP1 loop must be current-carrying. These currents are either associated to
shear or twist. The shear was not present before the jet onset, but develops
during and after the jet onset. So the shear is not the source of the jet, but
it is a consequence of the jet.

The clues of our interpretation are the identification of a non-eruptive FR,
from which some twist is carried away and eventually reconnects into the jet
at ‘X’-point current-sheet. The transport of twist away from the FR towards a
BP is supported by the HMI observations of a moving negative flux-
concentration whose transverse fields point towards a positive one. The twist
is transported at a long distance of the FR which remains non-eruptive. The
tilt observed in the IRIS spectra in the four positions of the slit which by
chance are exactly at the site reconnection confirmed the transfer of twist at
the jet base.

###  5.5 Results and conclusion

In this chapter, the observations of a twisted jet, a surge, and a mini-flare
which occurred in the AR 12736 on March 22, 2019 at 02:05 UT are studied. The
event was observed in multi-wavelengths with AIA and IRIS instruments and
detailed in the magnetic field vector maps obtained by HMI and computed with
the UNNOFIT code. The MHD simulations were used to validate the vec B
observations (Aulanier2010; Zuccarello2015). The main results and conclusions
are following :

The AR consisted of the collapse of two EMF regions, each of them overlaid by
an AFS. The jet and surge reconnection site is along the PIL between these two
AFS. The AFS over the east side evolved rapidly due to photospheric surface
motions. Prior to the reconnection, the AFS exhibit a dome shape. After the
reconnection, long AFS overlying both EMFs are observed. This is confirmed in
the NVST Hα images. A large FR in the vicinity of the jet region is detected.
The patterns of transverse fields and vertical current densities, as observed
by HMI and appearing without being constrained a priori in an MHD simulation
of non-eruptive FR formation with flux-cancellation of sheared loops, show a
good accordance. The location of the FR is fully supported by HMI vec B and
electric currents Jz maps. The magnetic topology of the AR demonstrates a BP
region due to the particular formation of the bipole by collision of opposite
polarities, which is dynamically transformed to an ‘X’-point current sheet.
The fast extension of the FR towards the site of reconnection due to
photospheric surface motions offers the possibility for the FR arcades to
reconnect with magnetic pre-existing field lines at the ‘X’-point current
sheet without the eruption of the FR. The extension of the FR may transmit
twist to the jet.

Figure 5.11: Sketch of the formation of the jet and transfer of the twist from
the FR to the jet during reconnection. Panel (a) magnetic configuration before
the reconnection, panel (b) formation of the BP current sheet, panel (c)
X-point current sheet, panel (d) the untwisting jet after the reconnection and
the remnant twist in the bipole JP1 and JN2.

A cartoon is proposed where the FR between P1 and N2 is represented by the
solid twisted line (Figure 5.11, a). It is extended to the south, creating the
bipole JN2-JP1. The BP current sheet is generated between the overlying arcade
of FR and the magnetic field line of the west emerging flux P1-N1 (panel b).
At this time, a first reconnection occurs at a localised point that is very
deep in the atmosphere. The Mg II profiles resemble those found in IRIS bombs
(IB) with extended wings (Peter2014), which are proposed to have been formed
during BP current sheet reconnection (Zhao2017). Such chromospheric wide
profiles have been modelled in MHD simulations (Hansteen2019). It has also
been shown that a BP could be transformed immediately at a null point. We
propose in panel (c) that the reconnection occurs in the null point
(‘X’-point) that is formed dynamically along a current sheet or a flat spine-
surface above a dome that is not depicted in the cartoon panel (c). Cool
material trapped in the BP during its formation is expelled with a large
blueshift, as revealed in IRIS Mg II line profiles with extended blue wings.
The spectra shows an evident tilt, which indicates the presence of helical
motions. The reconnection site is heated at all the temperatures and the hot
jet is expelled towards the west side in twisted field lines (panel d). The
cool material follows different paths than the hot and acts as a wall in front
of the hot jet. It resembles the surges that accompany jets in the MHD
simulations of Moreno2008; Nobrega2016; Nobrega2018.

Our magnetic analysis benefit from the treatment of the HMI vector magnetic
field by the UNNOFIT code which uses a filling factor which takes into account
the non resolved structures. In each pixel there is an equilibrium between
magnetized regions and non-magnetized regions which implies a better
determination of the magnetic field inclination (Bommier2016). This is an
important aspect for regions with weak magnetic field. It is the case in the
small bipole where our jet reconnection takes place and where we have detected
the BP. A more important aspect for this case is the chance to have the IRIS
spectra exactly at the reconnection site. IRIS spectra shows directly the
transfer of twist between two stable systems at the reconnection point by
unveiling a helical structure. The spectral analysis of IRIS C II, Si IV, and
Mg II lines will probe the multi-thermal atmosphere of solar flares and will
provide a precise calibration of magnetic reconnection height.

##  Chapter 6 Spectroscopic analysis of the multi thermal atmosphere of a
flare and jet