Generic low-atmosphere signatures of swirled-anemone jets (2024)

2.1 Instruments

We use the chromospheric H$\alpha$ observations from the CRisp Imaging SpectroPolarimeter (CRISP; Scharmer etal., 2008) instrument at the SST.The H$\alpha$ line was sampled at 15 spectral line positions at a temporal cadence of 11.4s and with a pixel size of 0$\aas@@fstack{\prime\prime}$058.The CRISP data was processed with the Multi-Object Multi-Frame Blind Deconvolution (MOMFBD; Van Noort etal., 2005) image restoration technique.In addition to the chromospheric observations from CRISP, we include Si iv transition region observations from the 1400Åslit-jaw (SJI) channel from IRIS. The SJI 1400 Si iv images were recorded at an average temporal cadence of 17s and a pixel size of 0$\aas@@fstack{\prime\prime}$166.The aligned SST and IRIS data were earlier analysed byCarlsson etal. (2015) and Skogsrud etal. (2016), and are publicly available (Rouppe van der Voort etal., 2020). The SST data were processed following an early version of the CRISPRED(de la Cruz Rodríguez etal., 2015) reduction pipeline which includes Multi-Object Multi-Frame Blind Deconvolution (MOMFBD, Van Noort etal., 2005) image restoration.For more details on the SST data processing and SST to IRIS alignment, we refer to Rouppe van der Voort etal. (2020). These chromospheric and transition region observations were further combined with the AIA 304Åand 171Åchannels that provide details of the hotter components of the jet eruption.We use one HMI map to show the magnetic field configuration at the jet region, where a small positive polarity patch is embedded inside an extensive area with negative magnetic polarity.The jet region is also scanned through the IRIS slit, however, we are not using the spectral information, as this is out of scope of this paper.

2.2 Quiet phase

This AR was a target for coordinated observation with SST and IRIS on June 11, 2014 starting from 07:36 UT, so we examined it for more than an hour before the jet eruption along with the circular solar flare started $\sim$09:06 UT.At 08:03 UT, we observed a very clear swirled anemone structure in SST H$\alpha$ line core observations, and a circular brightening at the same location in transition region temperature in IRIS Si iv SJIs. Examining the hot temperature EUV channel, interestingly, we observed a straight spine in AIA 171 Åoriginating from the swirled anemone base. These observational features are presented in Fig. 2 at 08:03 UT observed with SDO/AIA, IRIS, and SST instruments. As this straight and narrow spine in hot EUV channel is observed one hour before the main jet activity, we named it as the quiet phase jet.In the hot EUV channels this quite jetappears at 07:40 UT disappears soon after ten minutes it disappears and reappears at 08:02 UT.So this long loop present in hot EUV channels probably corresponds to some pre-eruptive quasi-steady reconnection ongoing at that location. Its labelling as a jet may be subject to caution, although its relative intermittency may be regarded as being reminiscent of previously-reported hot narrow jets (e.g. some of those seen in Cirtain etal., 2007).

2.3 Impulsive phase

The swirled anemone structure observed in the quiet phase started to develop a solar flare $\sim$09:05 UT and a clear circular flare ribbon formed at $\sim$ 09:16 UT around the anemone-shaped base. Along with the solar flare a wide solar jet was observed starting at $\sim$ 09:14 UT travelling towards the South-West. The circular flare ribbon formation is evident in the high resolution IRIS observations (Fig.3a–c). The jet plasma shows an upward flow in the beginning and followed by falling back jet material. From an analysis of the jet H$\alpha$ profiles, we infer Doppler offsets of about $-1.2$Åfrom the line core, indicating upward velocities around 55kms${}^{-1}$ along the line of sight. Typically, velocities measured in cool jets (surges) are of this magnitude or even smaller, both from observations (e.g., Schmieder etal., 1983, 1995; Joshi etal., 2020a) and simulations (e.g, Moreno-Insertis & Galsgaard, 2013; Nóbrega-Siverio etal., 2016).

H$\alpha$ observations in the red wing (6563 +0.4Å) are used to probe the downflowing of some of the jet material. With these high resolution observations, it has been found that a “hook” like structure is developed in the bright flare ribbon in the North-East of the anemone region (see Fig. 3). This hook structure corresponds to a portion of the flare ribbon. In the standard flare model, the ribbons would be magnetically connected to the reconnection site, and would map the footpoint of separatrix field lines (e.g. Savcheva etal., 2015, 2016).A minute or two after, this hook-shaped structure starts to deform with time and fade away afterwards (Fig. 3 middle row). We highlight this deformation of hook in Fig.3 with curved cyan arrows (panels d–f) in H$\alpha$ red wing observations. The arrow of the curve is fixed at the North-East side of the hook shape, while the tail of the curve is at the head of the hook. With time, the shorter length of the cyan curve shows the deformation of this hook shape structure.

After the deformation of the hook structure, we observed the widening of the jet towards the South in a clockwise direction. An elbow-shaped dark curtain of absorbed plasma material appeared in H$\alpha$ observations, with a sharp boundary in the South at $\sim$ 09:18 UT (see Fig. 3g). It appears to be moving towards the South in the clockwise direction from $\sim$ 09:18–09:23 UT. To reflect the southward motion of this dark plasma (curtain like structure), we put a straight arrow starting from the elbow of the curtain and heading towards South in Fig.3g-i. The arrow head is fixed in all three panels and the shortening of the cyan arrows depicts the widening of the jet towards the South with the elbow at the front.

2.4 Recovery phase

Inspecting the three different spectral lines of the SST H$\alpha$ observations: blue wing (6563 $-$0.4Å), line core (6563Å), and red wing (6563 +0.4Å), we observed that a part of the up-going jet material is falling back during 09:16–09:25 UT. The origin of the jet from the North of the anemone-shaped base is shown as “jet footpoint” and the wide jet in the blue wing is shown in Fig.4. Strong absorption in the blue and red wing of the H$\alpha$ wavelength reveals that the plasma material is being propelled upwards (blue-shifted) as well as downwards (red-shifted). It is not necessarily the same plasma material which is flowing up and down. It is interesting to notice that the jet material is falling back at an offset location in the anticlockwise direction from the jet footpoint. From the observations, it gives an idea that the falling back jet material follows a different path than the upflow plasma material. Figure4 illustrates this scenario with a time evolution of the jet flow in three different spectral lines next to each other, panel (i) presents the offset between jet footpoint and return footpoint.

3.1 ARMS simulation and QSL calculation

The model used to obtain insights on the dynamics of the observed jet is based on the numerical experiments of Pariat etal. (2015) performed with the ARMS (Adaptively Refined MHD Solver) numerical code (DeVore, 1991). Complete information about the ARMS code and the setup of the numerical experiment can be found in Pariat etal. (2015, and reference therein). The concepts for this 3D model for solar jets were initially proposed by Pariat etal. (2009) and have been developed since then in multiple directions (e.g. Pariat etal., 2010, 2015, 2016; Dalmasse etal., 2012; Wyper etal., 2016, 2018, 2019; Karpen etal., 2017). Comparisons between this model and observations have already been carried out by Patsourakos etal. (2008) in order to understand the 3D properties of coronal jets observed stereoscopically by the EUV imagers of the STEREO mission (Howard etal., 2008).

The fundamental basis of this jet model centers around the presence of a 3D null point, which partitions the coronal volume into two connectivity domains. One domain is closed, situated below the dome-like fan separatrix surface of the null point, while the other domain is open and positioned above it. In the simulations, this standard null point configuration is the result of a vertically oriented magnetic dipole, embedded slightly below the bottom boundary, which generated a photospheric magnetic field concentration, and a large scale nearly uniform and spatially slowly varying background vertical magnetic field of a direction opposite to the dipole. As can be seen in Fig.5 (a), the resulting configuration contains two distinct flux systems: a circular patch of strong closed magnetic flux surrounded by weaker open flux. Such magnetic field configuration is very commonly observed with coronal jets and coronal bright points (Moreno-Insertis etal., 2008; Zhang etal., 2012; Joshi etal., 2017; Nóbrega-Siverio etal., 2023).

The numerical simulation analysed in the present study is a variation the parametric simulations presented in Pariat etal. (2015), for which the uniform background vertical field is inclined by $30^{\circ}$. More precisely, the magnetic field is defined by Eq. (2) of Pariat etal. (2015) with the angle $\theta$ defined by the inclination of the open field with respect to the vertical direction such as $\theta=30^{\circ}$ ($\theta=0$ would correspond to a vertical field). Such null point magnetic topological system first permits the very efficient storage of magnetic energy in the closed domain either thanks to shear (e.g Pariat etal., 2009) or the formation of a twisted flux rope (e.g Wyper etal., 2018). In the present study, the injection of magnetic energy and helicity in the system follows Pariat etal. (2015) : very slow (with respect to the local Alfvén speed) horizontal motions are applied at the line-tied photospheric boundary, only in the main central positive of the closed magnetic polarity. The system evolves quasi-steadily, with field lines being slowly sheared above the PIL (see Fig.5, panels b and c). As the magnetic system acquires poloidal flux, the magnetic system slowly bulges and the fan dome and 3D null point rise. Similarly to Pariat etal. (2009), the driving motions are along the isocontours of the magnetic field. However unlike with the axisymetric configuration of Pariat etal. (2009), because of the inclination of the vertical field, the driving motions also become asymmetric. This naturally results in asymmetrically sheared field lines, as observed in Fig. 5c. This is reminiscent of the observed “swirled anemone” feature of the present event and justify the choice of driving used here for the comparison between the observation and the numerical model (cf. also 3.2).

Finally, the null point configuration enables a very efficient energy release, thanks tomagnetic reconnection (e.g Pariat etal., 2009, 2010), inducing the self-consistent generation of complex impulsive solar jet-like eruption, presenting both bulk flows and wave dynamics, being multi-velocity and multi-thermal, similarly to observed jets (Patsourakos etal., 2008; Raouafi etal., 2016; Joshi etal., 2020b). The adiabatic energy equation used in the model of (Pariat etal., 2015) only permits a limited understanding of the plasma emission and absorption in comparison to more sophisticated numerical experiments (Fang etal., 2014; González-Avilés etal., 2020; Chen etal., 2022). Nonetheless in the low beta coronal environment, the model is fully capable of capturing the essential dynamics of magnetic field lines. Of significant importance is the implementation of an adaptive mesh refinement strategy, as detailed in the Karpen etal., 2012, Appendix of, which enhances grid resolution at the sites of reconnecting current sheets. This enhancement allows for the precise identification of diffusion regions and the reconnection site. Such an approach enables a confident comparison with observational data, as elaborated in Sect.3.2

In addition, in order to analyze and visualize the magnetic topology and the evolution of the magnetic connectivity, we compute the squashing factor Q (Titov etal., 2002; Titov, 2007). The squashing factor is a measure of the gradients of the connectivity mapping between two planes. Numerically, the computation was done following the Aslanyan etal. (2021) implementation of the GPU-compatible QSL Squasher code (Tassev & Savcheva, 2017). More specifically, following Aslanyan etal. (2021), we here plot a modified version of the signed log Q (slog Q Titov etal., 2011) initially introduced by Titov etal. (2011), in which positive and negative values of Q are computed between different couples of reference planes. Applying the red–blue palette to the slog Q distributions, we are able to simultaneously visualize (quasi-) separatrix footprints for both open (negative/blue) and closed (positive/red) magnetic field. One shall keep in mind that since the Q values are computed between different planes for the open and closed field, the absolute Q values do not have the same significance. However since we are here only interested in the morphology of the Q distribution as well as the dynamics of the separatrix between open and closed field, the slog Q map are very instructive, in particular to understand and interpret the observed dynamics of the flare ribbons (see Sect.3.4).

3.2 Early reconnection and straight jet

In the relatively earlier phase of the simulation, the magnetic shear of the coronal arcades (as induced by the slow rotational boundary motion prescribed in the parasitic positive polarity) has resulted in a gradual bulging of the shearing field lines. Given the asymmetric nature of the initial magnetic field configuration, the bulging is actually not axisymmetric around in the inner spine. Indeed, simple geometrical arguments point to the fact that relatively-larger shear angles are induced in relatively-shorter field lines for the same shearing footpoint-displacements. These different shear angles generate relatively-stronger Lorentz force imbalances in relatively-shorter and more-sheared field lines, which therefore tend to expand relatively more to find their sheared equilibrium. This eventually results in one section of the fan surface (the one on the left in panel (a), being on the right in panel [b], in both Fig.5 and Fig.6) along with its corresponding ‘set a’ of underlying field lines, to actually bulge more than any other part of the fan and any other set of field lines. This differential bulging is responsible for the gradual formation of a current sheet around the null point (as manifested by the acute angle made between the fan surface and the spine field line seen in Fig.6a). This gradual current-sheet formation process due to field-line bulging is actually similar to what happens in simulations of flux emergence on one side of a null point (e.g. Moreno-Insertis & Galsgaard, 2013; Nóbrega-Siverio etal., 2016)

In the current model, at time t = 500, the asymmetric swirled-anemone has evolved into a configuration where its sheared-most field lines (illustrated in cyan in Fig.5 and Fig.6) are now almost aligned with the polarity-inversion line, as depicted in Fig.6a-b. This occurs nearly simultaneously with the current sheet reaching the scale of the mesh in the vicinity of the coronal null-point, approximately at $t\simeq 550$. This event signals the initiation of magnetic reconnection and the beginning of the jet.Nevertheless, the resulting modeled jet is yet in a relatively quiet stage. Indeed, the reconnection and the plasma dynamics are not very impulsive. They do neither involve any large-scale restructuring of the magnetic field connectivities, nor do they lead to bulk plasma acceleration at the scale of the whole system. Instead, field lines reconnect gradually, in a quasi two dimensional way, with little total flux-transfer per unit-time from below the fan surface to above it, along the outer-spine field line. The latter can be seen qualitatively from Fig.6. Indeed, panels (e,f) show one of the pair ‘set a’ field lines (drawn in pink) that has reconnected between $t=500$ and $t=550$, while all other represented field lines, including the other very close-by ‘set a’ field line, have hardly evolved between these two times. Also, panels (c,g) show how the footpoints of the quasi-separatrix layer (QSL) have almost-rigidly and only-slightly shifted (toward the left) during this reconnection, sweeping a relatively small area that contains the footpoint of the single ‘set a’ reconnecting field line.

These signatures are typical of a two-dimensional-like reconnection pattern that does not involve the whole magnetic field configuration. Instead, these manifestations are, as expected, reminiscent of what was already extensively described in Pariat etal. (2015), i.e. what they named the “straight jet” phase, as illustrated in the left panels of their Fig.1. Interestingly, it is also at the same time that the swirled anemone and that the straight/quiet jet are seen together in the SST and SDO observations of the event studied in this paper (see Fig.6d,h). So far, all these behaviors are typical of non-helical jets (as reviewed in Raouafi etal., 2016) and of some circular-ribbon and null-point related confined-flares (see e.g. Masson etal., 2009; Guglielmino etal., 2010; Zuccarello etal., 2017; Prasad etal., 2020; Devi etal., 2020; Cheng etal., 2023). But in the present model, they only represent the precursor of a much more dynamic phase, whose related jet was extensively studied by Pariat etal. (2015), and which we analyze hereafter the low-altitude manifestations.

3.3 Fast widening of the jet

By time $t=650-700$, the system endures a global ideal kink-instability and produces a “helical jet”, as described in Pariat etal. (2015). Firstly, this kink instability pushes against the fan surface from below, resulting in an extension of the current sheet over a significant fraction of the fan, not only in the vicinity of the null point. Secondly, this extended current sheet provides a widespread region over which magnetic reconnection occurs, involving sheared field lines located at various places under the fan, and eventually allowing the widening of the jet over a transverse scale-length comparable to the size of the fan. This widening of the model jet is illustrated in Fig.7a–f where the the second field line of ‘set a’ (in pink) has reconnected in the time interval $t=550-650$ between and both field lines of ‘set b’ (in orange) are reconnecting in the time interval $t=650-700$. The distance between these field lines, as well as with the outer spine (in yellow) shows the width of the modeled jet.

Upon revisiting our SST observation, it becomes evident that the studied jet not only widened resembling a dark curtain but also extended clockwise towards the south, with its leading edge forming an elbow-like structure (see the cyan arrow in Fig. 3g). This may lead to think that the jet was extending toward the null point, being located at the top of the elbow. Such a behavior would be puzzling, since it would imply that the jet did not extend away from the null point, as post-reconnected relaxing field lines should normally behave. Instead we conjecture that the elbow of the dark curtain actually marks the position of the null point (or almost). In this line, what SST shows would be a fast shift in position of the null toward the south during the impulsive phase of the jet. However, this shift does not happen in the MHD simulation. Oppositely, a shift in the opposite direction would even have been expected. Indeed, both the magnetic reconnection and the jet itself are removing free magnetic energy from below the fan. This energy decrease should force the magnetic configuration back towards the potential field, for which the null point is offset counterclockwise as compared to the jet state (compare Fig.5 and Fig.7). However, the counterclockwise rotation of the null point does not manifest in the simulation either. We propose that the observed stagnation of the modeled null point may be attributed to the natural counterclockwise rotation resulting from energy decrease, which must be counterbalanced by a clockwise rotation. This clockwise rotation could potentially be induced by the coupling between the kink instability and reconnection, leading to large-scale rotational motions at the helical jet’s base. However, this conjecture requires further quantitative investigation in future studies.

3.4 A hook moving along the circular ribbon

Following the usual association between QSL footpoints in models and flare ribbons in observations, the simulation qualitatively retrieves the occurrence and the displacement of the observed hook like structure in the circular flare ribbon, located at the north-west of the parasitic polarity (see Fig.3 and Fig.7,d,h) for the SST observation). This match is only qualitative, however. Indeed the hook in the model is rather located at north-east of the parasitic positive polarity, and even evolves toward the east (see Figure 7,c,g). Nevertheless, it is interesting to note that this evolution of the circular ribbon is not only very different from the earlier quiet/straight-jet phase described above, but also that it was not prescribed a priori in the simulation setup. Instead, this is a behavior that spontaneously emerged from the free evolution of the system, once the photospheric driving was stopped.

This hook-shape structure in the circular ribbon actually traces the edges of “a channel of locally open field forms, penetrating deep into the previously closed field region as far as the polarity inversion line”, as written by Wyper etal. (2016) who obtained the very same behavior in their jet simulation, as plotted in their Figure 8. This origin and the shift in position of this finite-size inclusion of reconnected field are difficult to firmly establish. But both seem to be associated with the deformation of the flux tube that surrounds the inner spine, while it endures the ideal kink instability. This ongoing coronal deformation of the flux tube would naturally be responsible for a gradual shift in position of the bulging of the field lines below the fan, even after the photospheric drivers have been switched off, as these field lines are pushed by the kinking inner-spine. This relates in a straightforward way to to the sequential shifting of reconnection starting with the field lines of ‘set a’ (Fig.6), moving to those of ‘set b’ (Fig.7), and eventually involving those of ‘set c’ (Fig.8).

In this context, the presence and displacement of a hook along the circular flare ribbon illustrates the gradual shift in the positions of coronal kinking, bulging, and reconnection onto the surface. These phenomena are inherent in a swirled-anemone configuration, even in its most basic, generic, initially-symmetric form in Pariat etal. (2009).Furthermore, it is important to highlight that while the moving hooked-ribbon correlates with the overarching large-scale swirling/twisting pattern of the system, it does not signify (atleast in our simulation) the existence of a drifting low-lying flux rope positioned along the PIL, as hooked ribbons typically do in the context of two-ribbon flares (see Aulanier & Dudík, 2019). Therefore, while the hook remains associated with the presence of some twist, it does not serve as a definitive signature of a flux rope.

3.5 Downflow of the jet plasma offset from its launch site

The last peculiar behavior that we noticed in this jet was the falling back of some of the jet material at a ‘return footpoint’ located at a different position (counterclockwise) than the ‘jet footpoint’, the latter corresponding to the launch site of the material from the chromospheric layer (see Fig.4). On one hand, the simulation does not include gravity, and it uses an initially-uniform atmosphere. So it cannot be used to model, plot, and follow the lift-off followed by the fall of chromospheric/jet material. But on the other hand, one can still use the simulation to follow the spatio-temporal evolution of its field lines, and to use them to infer how some material would flow along the field, according to some gravitational pull toward the line-tied plane. This is how we proceed hereafter.

The field line analysis is reported in Fig.8. There it can be seen that the field lines from ‘set b’ (in orange) that have reconnected and opened into the jet at $t=750$ have actually reconnected a second time and have been closing down below the fan surface by $t=800$. As a consequence, any chromospheric material that could have been accelerated along these ‘set b’ field lines, and having reached out into the jet at high altitude above the fan by $t=700$, have no way of falling back to their initial position along the same ‘set b’ field lines for $t\leq 800$, because those lines are not connected to the jet anymore. Meanwhile, field lines from ‘set c’ (in red) are reconnecting and opening into the jet at time $t\simeq 800$. These ‘set c’ field lines now provide new channels along which previously-accelerated material now has the possibility to fall back to the line-tied plane in the model, i.e. the chromosphere in the real Sun. And these new ‘set c’ channels have their footpoints that are offset from those of ‘set b’ field lines. When the model is oriented with angles corresponding to projection of the event as observed from Earth (see Fig.8,b,e,h), one can see that this offset of the ‘set c return footpoints’ is counterclockwise from the position if the ‘set b jet footpoint’.

Interestingly, similar to the observation regarding the hook, this generic and idealized model qualitatively corresponds to the observed behavior, particularly concerning the offset of the downflowing jet material. Notably, no specific configuration was imposed in the simulation to account for this behavior. It is also noteworthy that this observed offset, which may be perceived as complex, differs from the straightforward fallback of material to its point of origin, as the latter is an inherent property of a relatively simplified model.

Finally, it should be emphasized that the model allows to make a causal association between the moving hook and the offset of the downflowing material away from the jet footpoint. The fact that both are moving counterclockwise along the circular ribbon already provided a first observational hint that they were related. Thanks to the model, we can see that they are actually coupled through the magnetic reconnection in the corona: the moving hook (see Section 3.4) is located on the edge of the an equally-moving open-field penetrating-region (as first noticed in Wyper etal., 2016), and that is the signature at the low atmospheric level to the coronal reconnection dynamics that closes some previously-open fields (along which jet-plasma was accelerated earlier) and that opens previously-closed and distant field-lines (along which the same plasma will later fall back, remotely from its original location), leading to forming a hook and an open-field penetrating-region that shift in position.