arXiv subjects

Jayant Joshi

Publications and source records attributed to Jayant Joshi.

At least 19 recordsLinked to original sources

arXiv2026

Comparative Analysis of Ellerman and Quiet Sun Ellerman Bombs in the Solar Atmosphere

Ellerman Bombs (EBs) and Quiet-Sun Ellerman Bombs (QSEBs) are small-scale signatures of magnetic reconnection in the lower solar atmosphere, observed in active regions and quiet-Sun areas, respectively. We investigate and compare some of their properties using coordinated multiwavelength observations from the Swedish 1-m Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory. We employ k-means clustering to identify EBs and QSEBs and perform a detailed analysis of a subset of these events. Our results show that EBs are frequently associated with opposite magnetic polarities, whereas QSEBs generally lack clear polarity signatures, likely due to limited spatial resolution. Spectral inversions using the STiC code reveal temperature enhancements of up to 1700 K in the lower chromosphere for EBs. In contrast, no clear temperature enhancement is detected for QSEBs, which may be attributed to the limited spatial resolution or insufficient wavelength sampling of the Ca II 8542 A. We further find that some EBs exhibit signatures extending to transition-region temperatures. An analysis of EBs temporal evolution reveals episodic heating, with a range of periodicities, most commonly around 6-7 minutes. In addition, we identify spatial associations between the footpoints of some spicules and EBs/QSEBs, suggesting that reconnection in these events may contribute to spicule formation. These results demonstrate similarities and differences between EBs and QSEBs and support the interpretation that small-scale magnetic reconnection contributes to heating and dynamics in EBs, while the underlying mechanism of QSEBs requires further investigation.

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arXiv2026

Apparent Transverse Motion of Light Bridges Coupled to Coronal Loop Dynamics

Light bridges are commonly observed in active regions and are interpreted as signatures of magnetoconvective processes in sunspots. Several studies have attempted to classify them in the past based on their morphological characteristics. Recent observations have revealed new dynamical properties of light bridges, including their signatures in the upper solar atmosphere, particularly in the chromosphere, and their coupling with coronal features. In this study, we observed two cases of rare and unusual dynamics as light bridges evolve. Using data from the Solar Dynamics Observatory, the evolution of the light bridges is analysed, and the results are reported here. Based on our findings, we propose that the unique movements of the light bridges in the observed sunspot and earlier studies could be an apparent view of the umbral core dynamics. Investigation into these dynamics through signatures in the higher atmosphere reveals a clear coupling to coronal loops and their dynamics.

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arXiv2026

On the Origin of Coronal Picoflare Jets

Small-scale jet-like eruptions, such as picoflare jets and jetlets, are recognized as potential contributors to coronal heating and solar wind acceleration, yet their physical origin is still not fully established. Using ultra-high-resolution extreme ultraviolet imaging datasets from the Extreme Ultraviolet Imager on board the Solar Orbiter mission, we investigate tiny coronal jets observed off-limb in the Sun's polar regions. Visual inspection reveals that the majority of these jets exhibit distinct morphological features, including a bright spire accompanied by a dark eruptive jet component. We analyzed eleven of these jets in detail and found that their spatial and temporal scales are comparable to previously reported jetlets, while their kinetic energies are two to three orders of magnitude lower, placing them in the picoflare regime. The bright and dark components show distinct dynamics, with the dark structures generally displaying lower speeds. A comparison with coordinated Interface Region Imaging Spectrograph and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory data, together with 2.5D radiative-MHD simulations performed with the Bifrost code, reveals a one-to-one morphological correspondence between the dark counterparts and cool chromospheric surges accompanying the bright jet spire. This association suggests that flux emergence and magnetic reconnection at low atmospheric heights may produce coupled bright-dark structures, providing a plausible mechanism for the generation of picoflare jets. Our results demonstrate Solar Orbiter's ability to resolve the dynamics of small-scale jets and place new constraints on their origin.

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arXiv2026

Probing the CME Core--Prominence Relation Using Inner Coronal Observations

Coronal mass ejections (CMEs) often exhibit a three-part structure consisting of a bright inner core, an outer leading edge, and an intervening dark cavity. While the core has traditionally been attributed to prominence material, an alternative interpretation suggests it may arise from the projection effects of a twisted flux rope. We focused on limb CME events to reassess the connection between CME cores and their associated prominences in the inner corona. The CME cores were analyzed using white-light observations from the Mauna Loa Solar Observatory (MLSO) K-Coronagraph (K-Cor), while the corresponding prominence eruptions were examined using H$\alpha$ data from the Global Oscillation Network Group (GONG) and 304 \AA{} images from the Atmospheric Imaging Assembly (AIA). Our results show a strong spatial correspondence between H$\alpha$ prominences and CME cores in white light, with an average image correlation of $\sim$0.7, while correlations between white light and AIA 304 \AA{} are comparatively weaker ($\sim$0.5). Several events could be continuously traced into the Large Angle and Spectrometric Coronagraph Experiment (LASCO/C2) field of view, confirming the persistence of prominence material into the outer corona. We find back-extrapolating LASCO/C2 CME cores under constant-velocity, linear-trajectory assumptions can introduce large errors -- up to 40$^\circ$ in inferred position angle and $\sim$140 minutes in eruption time relative to their true values -- underscoring the importance of inner-coronal observations for accurately constraining CME dynamics. Overall, our findings suggest that in prominence-associated CMEs, the bright cores are predominantly composed of prominence material.

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arXiv2025

The H$\alpha$ line as a probe of chromospheric magnetic fields

We explore the diagnostic potential of the H$\alpha$ line for probing the chromospheric magnetic field using a realistic 3D radiative magnetohydrodynamic (rMHD) model. The Stokes profiles of the H$\alpha$ line are synthesized through full 3D radiative transfer under the field-free approximation, alongside the Ca II 8542 {\AA} and Fe I 6173 {\AA} lines for comparison. The line-of-sight (LOS) magnetic fields are inferred using the weak field approximation (WFA) for theH$\alpha$ and Ca II 8542 {\AA} lines, while the Fe I 6173 {\AA} line is analyzed through Milne-Eddington inversion techniques. The comparison between the inferred LOS magnetic field maps and the magnetic fields in the rMHD model revealed that the H$\alpha$ line core primarily probes the chromospheric magnetic field at log tau_500 = -5.7, which corresponds to higher layers than the Ca II 8542 {\AA} line core, which is most sensitive to conditions at log tau_500 = -5.1. On average, the Stokes V profiles of the H$\alpha$ line core form 500 km higher than those of the Ca II 8542 {\AA} line core. The H$\alpha$ polarization signals persist after adding noise, and with noise at the level of 10^-3 Ic, most simulated magnetic structures remain visible. These findings suggest that spectropolarimetric observations of the H$\alpha$ line can provide complementary insights into the stratification of the magnetic field at higher altitudes, especially when recorded simultaneously with widely used chromospheric diagnostics such as the Ca II 8542 {\AA} line.

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arXiv2025

Small-scale dynamic phenomena associated with interacting fan-spine topologies: quiet-Sun Ellerman bombs, UV brightenings, and chromospheric inverted-Y-shaped jets

QSEBs are small-scale magnetic reconnection events in lower solar atmosphere. Sometimes, they exhibit transition region counterparts, known as UV brightenings. Magnetic field extrapolations suggest that QSEBs can occur at various locations of a fan-spine topology, with UV brightening occurring at null point through a common reconnection process. We aim to understand how complex magnetic configurations like interacting fan-spine topologies can cause small-scale dynamic phenomena in lower atmosphere. QSEBs were detected using k-means clustering on Hbeta observations from Swedish 1-m Solar Telescope (SST). Further, chromospheric inverted-Y-shaped jets were identified in the Hbeta blue wing. Magnetic field topologies were determined through potential field extrapolations from photospheric magnetograms using the Fe I 6173 A line. UV brightenings were detected in IRIS 1400 A SJI. We identify two distinct magnetic configurations associated with QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets. The first involves a nested fan-spine structure where, due to flux emergence, an inner 3D null forms inside fan surface of an outer 3D null with some overlap. QSEBs occur at two footpoints along the shared fan surface, with UV brightening located near the outer 3D null point. The jet originates close to the two QSEBs and follows the path of high squashing factor Q. We discuss a comparable scenario using a numerical simulation. In second case, two adjacent fan-spine topologies share fan footpoints at a common positive polarity patch, with the QSEB, along with a chromospheric inverted-Y-shaped jet, occurring at the intersection having high Q values. This study demonstrates through observational and modelling support that associated QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets share a common origin driven by magnetic reconnection between interacting fan-spine topologies.

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arXiv2025

Quiet Sun Ellerman bombs as a possible proxy for reconnection-driven spicules

Spicules are elongated, jet-like structures that populate the solar chromosphere and are rooted in the photosphere. In recent years, high-resolution observations and advanced numerical simulations have provided insights into their properties, structures, and dynamics. However, the formation mechanism of spicules, particularly the more dynamic type II spicules, which are primarily found in the quiet Sun and coronal holes, remains elusive. This study explores whether quiet Sun Ellerman bombs (QSEBs), which are ubiquitous small-scale magnetic reconnection events in the lower atmosphere, are linked to the formation of type II spicules. We analysed a high-quality 40-minute time sequence acquired with the Swedish 1-m Solar Telescope. H-beta data were used to observe QSEBs and spicules, while spectropolarimetric measurements in the photospheric Fe i 6173 A line provided line-of-sight magnetic field information. We employed k-means clustering to automatically detect QSEBs and explored their potential association with spicules. We identified 80 clear cases where spicules occurred soon after the QSEB and not later than 30 s after the ending of the QSEBs. All events involved type II spicules, rapidly fading from the images. The footpoints of the spicules seemed to be rooted in QSEBs, where the onset of QSEBs often preceded the formation of the associated spicules. Additionally, we found around 500 other events that hinted at a connection but with some ambiguities. The combined clear and ambiguous cases constitute 34% of the total detected QSEBs and a smaller percentage of the spicules in our dataset. Our findings suggest that a fraction of type II spicules originate from QSEBs, supporting magnetic reconnection as a potential driving mechanism. In this context, QSEBs and spicules represent the conversion of magnetic energy into thermal and kinetic energy, respectively.

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arXiv2025

Unveiling the Dynamics and Genesis of Small-scale Fine Structure Loops in the Lower Solar Atmosphere

Recent high-resolution solar observations have unveiled the presence of small-scale loop-like structures in the lower solar atmosphere, often referred to as unresolved fine structures, low-lying loops, and miniature hot loops. These structures undergo rapid changes within minutes, and their formation mechanism has remained elusive. In this study, we conducted a comprehensive analysis of two small loops utilizing data from the Interface Region Imaging Spectrograph (IRIS), the Goode Solar Telescope (GST) at Big Bear Solar Observatory, and the Atmospheric Imaging Assembly (AIA) and the Helioseismic Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), aiming to elucidate the underlying process behind their formation. The GST observations revealed that these loops, with lengths of $\sim$3.5 Mm and heights of $\sim$1 Mm, manifest as bright emission structures in H$\alpha$ wing images, particularly prominent in the red wing. IRIS observations showcased these loops in 1330 angstrom slit-jaw images, with TR and chromospheric line spectra exhibiting significant enhancement and broadening above the loops, indicative of plasmoid-mediated reconnection during their formation. Additionally, we observed upward-erupting jets above these loops across various passbands. Furthermore, differential emission measurement analysis reveals an enhanced emission measure at the location of these loops, suggesting the presence of plasma exceeding 1 MK. Based on our observations, we propose that these loops and associated jets align with the minifilament eruption model. Our findings suggest a unified mechanism governing the formation of small-scale loops and jets akin to larger-scale X-ray jets.

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arXiv2025

On the million-degree signature of spicules

Spicules have often been proposed as substantial contributors toward the mass and energy balance of the solar corona. While their transition region (TR) counterpart has unequivocally been established over the past decade, the observations concerning the coronal contribution of spicules have often been contested. This is mainly attributed to the lack of adequate coordinated observations, their small spatial scales, highly dynamic nature, and complex multi-thermal evolution, which are often observed at the limit of our current observational facilities. Therefore, it remains unclear how much heating occurs in association with spicules to coronal temperatures. In this study, we use coordinated high-resolution observations of the solar chromosphere, TR, and corona of a quiet Sun region and a coronal hole with the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) to investigate the (lower) coronal ($\sim$1MK) emission associated with spicules. We perform differential emission measure (DEM) analysis on the AIA passbands using basis pursuit and a newly developed technique based on Tikhonov regularization to probe the thermal structure of the spicular environment at coronal temperatures. We find that the EM maps at 1 MK reveal the presence of ubiquitous, small-scale jets with a clear spatio-temporal coherence with the spicules observed in the IRIS/TR passband. Detailed space-time analysis of the chromospheric, TR, and EM maps show unambiguous evidence of rapidly outward propagating spicules with strong emission (2--3 times higher than the background) at 1 MK. Our findings are consistent with previously reported MHD simulations that show heating to coronal temperatures associated with spicules.

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arXiv2024

Magnetic Topology of quiet-Sun Ellerman bombs and associated Ultraviolet brightenings

Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower atmosphere of the quiet Sun. Recent work has shown that a small percentage of them can occur co-spatially and co-temporally to ultraviolet (UV) brightenings in the transition region. We aim to understand how the magnetic topologies associated with closely occurring QSEBs and UV brightenings can facilitate energy transport and connect these events. We used high-resolution H-beta observations from the Swedish 1-m Solar Telescope (SST) and detected QSEBs using k-means clustering. We obtained the magnetic field topology from potential field extrapolations using spectro-polarimetric data in the photospheric Fe I 6173 A line. To detect UV brightenings, we used coordinated and co-aligned data from the Interface Region Imaging Spectrograph (IRIS) and imposed a threshold of 5 sigma above the median background on the (IRIS) 1400 A slit-jaw image channel. We identify four distinct magnetic configurations that associate QSEBs with UV brightenings, including a simple dipole configuration and more complex fan-spine topologies with a three-dimensional (3D) magnetic null point. In the fan-spine topology, the UV brightenings occur near the 3D null point, while QSEBs can be found close to the footpoints of the outer spine, the inner spine, and the fan surface. We find that the height of the 3D null varies between 0.2 Mm to 2.6 Mm, depending on the magnetic field strength in the region. We note that some QSEBs and UV brightenings, though occurring close to each other, are not topologically connected with the same reconnection process. We find that the energy released during QSEBs falls in the range of 10^23 to 10^24 ergs. This study shows that magnetic connectivity and topological features, like 3D null points, are crucial in linking QSEBs in the lower atmosphere with UV brightenings in the transition region.

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arXiv2024

Transition Region Brightenings in a Moss Region and their Relation with Lower Atmospheric Dynamics

Small-scale Brightenings (SBs) are commonly observed in the transition region that separates the solar chromosphere from the corona. These brightenings, omnipresent in active region patches known as "moss" regions, could potentially contribute to the heating of active region plasma. In this study, we investigate the properties of SB events in a moss region and their associated chromospheric dynamics, which could provide insights into the underlying generation mechanisms of the SBs. We analyzed the data sets obtained by coordinated observations using the Interface Region Imaging Spectrograph and the Goode Solar Telescope at Big Bear Solar Observatory. We studied 131 SB events in our region of interest and found that 100 showed spatial and temporal matches with the dynamics observed in the chromospheric H$\alpha$ images. Among these SBs, 98 of them were associated with spicules that are observed in H$\alpha$ images. Furthermore, detailed analysis revealed that one intense SB event corresponded to an Ellerman Bomb (EB), while another SB event consisted of several recurring brightenings caused by a stream of falling plasma. We observed that H$\alpha$ far wings often showed flashes of strong brightening caused by the falling plasma, creating an H$\alpha$ spectral profile similar to an EB. However, 31 of the 131 investigated SB events showed no noticeable spatial and temporal matches with any apparent features in H$\alpha$ images. Our analysis indicated that the predominant TR SB events in moss regions are associated with chromospheric phenomena primarily caused by spicules. Most of these spicules display properties akin to dynamic fibrils.

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arXiv2024

Transition region response to Quiet Sun Ellerman Bombs

Quiet Sun Ellerman Bombs (QSEBs) are key indicators of small-scale photospheric magnetic reconnection events. Recent high-resolution observations have shown that they are ubiquitous and that large numbers of QSEBs can be found in the quiet Sun. We aim to understand the impact of QSEBs on the upper solar atmosphere by analysing their spatial and temporal relationship with the UV brightenings observed in transition region diagnostics. We analyse high-resolution H-beta observations from the Swedish 1-m Solar Telescope and utilise k-means clustering to detect 1423 QSEBs in a 51 min time series. We use coordinated and co-aligned observations from the Interface Region Imaging Spectrograph (IRIS) to search for corresponding signatures in the 1400 A slit-jaw image (SJI) channel and in the Si IV 1394 A and Mg II 2798.8 A triplet spectral lines. We identify UV brightenings from SJI 1400 using a threshold of 5$\sigma$ above the median background. We focused on 453 long-lived QSEBs ($>1$ min) and found 67 cases of UV brightenings from SJI 1400 occurring near the QSEBs, both temporally and spatially. Temporal analysis of these events indicates that QSEBs start before UV brightenings in 57 % of cases, while UV brightenings lead in 36 % of instances. The majority of the UV brightenings occur within 1000 km from the QSEBs in the direction of the solar limb. We also identify 21 QSEBs covered by the IRIS slit, with 4 of them showing emissions in both or one of the Si IV 1394 A and Mg II 2798.8 A triplet lines, at distances within 500 km from the QSEBs in the limb direction. We conclude that a small fraction (15 %) of the long-lived QSEBs contribute to localized heating observable in transition region diagnostics, indicating a minimal role in the global heating of the upper solar atmosphere.

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arXiv2024

Simultaneous spectropolarimetric observations in the H$\alpha$ and Ca II 8662 {\AA} lines of an active region

We present spectropolarimetric observations of an active region recorded simultaneously in the H$\alpha$ Ca II 8662 {\AA} lines. The sunspot exhibits multiple structures, including a lightbridge and a region where Ca II 8662 {\AA} line core is in emission. Correspondingly, the H$\alpha$ line core image displays brightening in the emission region, with the spectral profiles showing elevated line cores. The stratification of the line-of-sight magnetic field is inferred through non-LTE multiline inversions of the Ca II 8662 {\AA} line and the weak field approximation over the H$\alpha$ line. The field strength inferred from the H$\alpha$ line core is consistently smaller than that inferred from inversions at $\log \tau_{500}$ = $-$4.5. However, the study finds no correlation between the WFA over the core of the H$\alpha$ line and that inferred from inversions at $\log \tau_{500}$ = $-$4.5. In regions exhibiting emission features, the morphology of the magnetic field at $\log \tau_{500}$ = $-$4.5 resembles that at $\log \tau_{500}$ = $-$1, with slightly higher or comparable field strengths. The magnetic field morphology inferred from the core of the H$\alpha$ line is also similar to that inferred from the full spectral range of the H$\alpha$ line in the emission region. The field strength inferred in the lightbridge at $\log \tau_{500}$ = $-$1 is smaller than the surrounding umbral regions and comparable at $\log \tau_{500}$ = $-$4.5. Similarly, the field strength inferred in the lightbridge from the WFA over the H$\alpha$ line appears lower compared to the surrounding umbral regions.

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arXiv2023

Data processing of Visible Emission Line Coronagraph Onboard ADITYA L1

ADITYA-L1 is India's first dedicated mission to observe the sun and its atmosphere from a halo orbit around L1 point. Visible emission line coronagraph (VELC) is the prime payload on board at Aditya-L1 to observe the sun's corona. VELC is designed as an internally occulted reflective coronagraph to meet the observational requirements of wide wavelength band and close to the solar limb (1.05 Ro). Images of the solar corona in continuum and spectra in three emission lines 5303{\AA} [Fe xiv], 7892{\AA} [Fe xi] and 10747 [Fe xiii] obtained with high cadence to be analyzed using software algorithms automatically. A reasonable part of observations will be made in synoptic mode, those, need to be analyzed and results made available for public use. The procedure involves the calibration of instrument and detectors, converting the images into fits format, correcting the images and spectra for the instrumental effects, align the images etc. Then, develop image processing algorithms to detect the occurrence of energetic events using continuum images. Also derive physical parameters, such as temperature and velocity structure of solar corona using emission line observations. Here, we describe the calibration of detectors and the development of software algorithms to detect the occurrence of CMEs and analyze the spectroscopic data.

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arXiv2023

Does H$\alpha$ Stokes~$V$ profiles probe the chromospheric magnetic field? An observational perspective

We investigated the diagnostic potential of the Stokes $V$ profile of the H$\alpha$ line to probe the chromospheric line-of-sight (LOS) magnetic field ($B_{\mathrm{LOS}}$) by comparing the $B_{\mathrm{LOS}}$ inferred from the weak field approximation (WFA) with that of inferred from the multi-line inversions of the Ca II 8542 {\AA}, Si I 8536 {\AA} and Fe I 8538 {\AA} lines using the STiC inversion code. Simultaneous spectropolarimetric observations of a pore in the Ca II 8542 {\AA} and H$\alpha$ spectral lines obtained from the SPINOR at the Dunn Solar Telescope on the 4th of December, 2008 are used in this study. The WFA was applied on the Stokes $I$ and $V$ profiles of H$\alpha$ line over three wavelength ranges viz.: around line core ($\Delta\lambda=\pm0.35$ {\AA}), line wings ($\Delta\lambda=[-1.5, -0.6]$ and $[+0.6, +1.5]$ {\AA}) and full spectral range of the line ($\Delta\lambda=\pm1.5$ {\AA}) to derive the $B_{\mathrm{LOS}}$. We found the maximum $B_{\mathrm{LOS}}$ strengths of $\sim+800$ and $\sim+600$ G at $\log\tau_{\mathrm{500}}$ = $-$1 and $-$4.5, respectively in the pore. The morphological map of the $B_{\mathrm{LOS}}$ inferred from the H$\alpha$ line core is similar to the $B_{\mathrm{LOS}}$ map at $\log\tau_{\mathrm{500}}$ = $-$4.5 inferred from multi-line inversions. The $B_{\mathrm{LOS}}$ map inferred from the H$\alpha$ line wings and full spectral range have a similar morphological structure to the $B_{\mathrm{LOS}}$ map inferred at $\log\tau_{\mathrm{500}}$ = $-$1. The $B_{\mathrm{LOS}}$ estimated from H$\alpha$ using WFA is weaker by a factor of $\approx 0.53$ than that of inferred from the multi-line inversions.

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arXiv2022

Properties of shock waves in the quiet Sun chromosphere

Short-lived (100s or less), sub-arcsec to a couple of arcsec size features of enhanced brightenings in the narrowband images at the $\mathrm{H_{2V}}$ and $\mathrm{K_{2V}}$ positions of the Ca II H and K lines in the quiet Sun are known as bright grains. With simultaneous observations of a quiet Sun internetwork region in the Fe I 6173 {\AA}, Ca II 8542 {AA}, and Ca II K lines acquired by the CRisp Imaging Spectro-Polarimeter and the CHROMospheric Imaging Spectrometer instruments on the Swedish 1-m Solar Telescope, we performed multi-line non-local thermodynamic equilibrium inversions using the STockholm inversion Code to infer the time-varying stratified atmosphere's physical properties such as the temperature, line-of-sight (LOS) velocity, and microturbulence. The Ca II K profiles of bright grains show enhancement in the $\mathrm{K_{2V}}$ peak intensities with absence of the $\mathrm{K_{2R}}$ features. At the time of maximum enhancement in the $\mathrm{K_{2V}}$ peak intensities, we found average enhancements in temperature at lower chromospheric layers (at $\log\tau_{500}$ $\simeq$ $-$4.2) of about 1.1 kK with maximum enhancement of about 4.5 kK. These temperature enhancements are colocated with upflows, as strong as $-$6 $\mathrm{km\;s^{-1}}$, in the direction of the LOS. The LOS velocities at upper chromospheric layers at $\log\tau_{500}$ < $-$4.2 show consistent downflows greater than $+$8 $\mathrm{km\;s^{-1}}$. The retrieved value of microturbulence in the atmosphere of bright grains is negligible at chromospheric layers. The study provides observational evidence to support the interpretation that the bright grains observed in narrowband images at the $\mathrm{H_{2V}}$ and $\mathrm{K_{2V}}$ positions of the Ca II H and K lines are manifestations of upward propagating acoustic shocks against a background of downflowing atmospheres.

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arXiv2022

Properties of ubiquitous magnetic reconnection events in the lower solar atmosphere

Magnetic reconnection in the deep solar atmosphere can give rise to enhanced emission in the Balmer hydrogen lines, a phenomenon referred to as Ellerman bombs. Recent high quality H$\beta$ observations indicate that Ellerman bombs are more common than previously thought and it was estimated that at any time about half a million Ellerman bombs are present in the quiet Sun. We performed an extensive statistical characterization of the quiet Sun Ellerman bombs (QSEBs) in these new H$\beta$ observations. We analyzed a 1 h dataset of quiet Sun observed with the Swedish 1-m Solar Telescope that consists of spectral imaging in the H$\beta$ and H$\alpha$ lines, as well as spectropolarimetric imaging in Fe I 617.3 nm. We detected a total of 2809 QSEBs. The lifetime varies between 9 s and 20.5 min with a median of 1.14 min. The maximum area ranges between 0.0016 and 0.2603 Mm$^2$ with a median of 0.018 Mm$^2$. A subset (14%) of the QSEBs display enhancement of the H$\beta$ line core. On average, the line core brightening appears 0.88 min after the onset of brightening in the wings, and the distance between these brightenings is 243 km. This gives rise to an apparent propagation speed ranging between $-$14.3 and +23.5 km s$^{-1}$, with an average that is upward propagating at +4.4 km $^{-1}$. The average orientation is nearly parallel to the limbward direction. QSEBs are nearly uniformly distributed over the field of view but we find empty areas with the size of mesogranulation. QSEBs are located more frequent near the magnetic network where they are often bigger, longer lived and brighter. We conclude that QSEBs are ubiquitous in quiet Sun and appear everywhere except in areas of mesogranular size with weakest magnetic field ($B_{\rm{LOS}}\lesssim50$~G). Our observations support the interpretation of reconnection along vertically extended current sheets.

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arXiv2021

Evidence of multithermal nature of spicular downflows. Impact on solar atmospheric heating

Spectroscopic observations of the emission lines formed in the solar transition region (TR) commonly show persistent downflows of the order of 10--15 km/s. The cause of such downflows, however, is still not fully clear and has remained a matter of debate. We aim to understand the cause of such downflows by studying the coronal and TR responses to the recently reported chromospheric downflowing rapid red shifted excursions (RREs), and their impact on heating the solar atmosphere. We have used two sets of coordinated data from SST, IRIS, and SDO for analyzing the response of the downflowing RREs in the TR and corona. To provide theoretical support, we use an already existing 2.5D MHD simulation of spicules performed with the Bifrost code. We find ample occurrences of downflowing RREs and show several examples of their spatio-temporal evolution, sampling multiple wavelength channels ranging from the cooler chromospheric to hotter coronal channels. These downflowing features are thought to be likely associated with the returning components of the previously heated spicular plasma. Furthermore, the TR Doppler shifts associated with them are close to the average red shifts observed in this region which further implies that these flows could (partly) be responsible for the persistent downflows observed in the TR. We also propose two mechanisms (a typical upflow followed by a downflow and downflows along a loop), from the perspective of numerical simulation, that could explain the ubiquitous occurrence of such downflows. A detailed comparison between the synthetic and observed spectral characteristics, reveals a distinctive match, and further suggests an impact on the heating of the solar atmosphere. We present evidence that suggests that at least some of the downflowing RREs are the chromospheric counterparts of the TR and lower coronal downflows.

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