arXiv subjects

L. P. Chitta

Publications and source records attributed to L. P. Chitta.

At least 19 recordsLinked to original sources

arXiv2026

Origin of small-scale evaporation flows deep in the chromosphere during a solar flare

Flares are caused by an abrupt release of magnetic energy in the solar atmosphere. Plasma heated to well over 10 MK filling the post-flare corona originates from a rapid heating and ablation of the cooler chromospheric material. This chromospheric evaporation is thought to be facilitated primarily by nonthermal electrons impinging on to the lower atmosphere. Questions on when and where in the chromosphere these upflows originate, however, are not fully resolved. Here we report on unprecedented high-resolution observations of an M-class flare recorded by the Sunrise Ultraviolet Spectropolarimeter and Imager on board the balloon-borne SUNRISE observatory, that reveal highly structured upflows on spatial scales of ~100 km originating deep in the chromosphere. The flows even precede the onset of nonthermal electrons by about 10 minutes and last through the impulsive phase of the flare. Our observations shed new light on the lower atmospheric heating and mass circulation in flares that are challenging to reconcile with the standard solar flare model.

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arXiv2026

Elemental Composition Evolution during the 2024 September 30 Solar Eruption: A Comparison of Hot and Cool Plasma Components with Solar Orbiter/SPICE, Hinode/EIS, and Chandrayaan-2/XSM

Solar plasma composition differs between the photosphere and corona over a range of timescales, with preferential enhancement of elements with low first ionization potential (FIP). However, the physical origin of the FIP fractionation remains incompletely understood. Furthermore, during flares, the FIP bias also exhibits rapid changes, associated with fast transport of material with different FIP biases. We present novel observations from Solar Orbiter SPICE and EUI, Hinode/EIS, and Chandrayaan-2 XSM instruments, finding rapid abundance changes in the emitting plasma, on timescales of minutes, during the eruptive M7.6-class solar flare observed on 2024 Sept 30. These instruments have wide temperature coverage and find contrasting abundance-evolution patterns between the hotter and cooler plasma components. 3D reconstruction of the active region and additional observations from the Solar Orbiter STIX X-ray telescope show how the hot and cool plasma components, emitting in different spectral regions and observed with the various instruments, sample the plasma composition evolution in distinct locations within the observed flaring plasma. The bright post-flare loop tops observed by SPICE show coronal FIP bias, while the hot plasma observed with XSM exhibits FIP-bias decreasing from coronal to photospheric during the impulsive phase. We interpret these observations as evidence of the X-ray diagnostics seeing hot coronal reconnection outflows mixing with chromospheric plasma as flare loops sequentially energize and relax, explaining why the FIP bias decreases from coronal to a hybrid; and the cool loop tops seen with SPICE show coronal abundances due to coronal material deposited near the looptops.

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arXiv2026

Solar flare ribbons structured by uncombed chromospheric loops

A part of the magnetic energy released during a flare is transported to the lower atmosphere. High-resolution observations show that flare ribbons, sites of energy deposition at the footpoints of flaring loops which appear bright in the chromosphere and transition region, are structured on small spatial scales on the order of 100 km. Based on idealized numerical models of flares it is suggested that the ribbon fine-structures could originate from a tearing instability and the development of plasmoids in current sheets. Here we report on Fe I 5250.6 Å and Mg I b2 5173 Å spectral observations of a solar flare from the Tunable Magnetograph onboard the SUNRISE III balloon-borne mission that reveal an intricate link between the flare ribbon structure and the ambient chromosphere. We identified uncombed chromospheric loops and non-flaring fine-structures that are interspersed among brighter flare ribbon threads. These loops remain stable on timescales of minutes. Spectral lines from these regions show reduced emission or self-reversal in the line core compared with the immediately adjacent flare ribbons. We discuss the potential role of these structures in the onset of a flare. Furthermore, we suggest that irrespective of the complexities in the flaring current sheet, uncombed chromospheric loops and nonflaring fine-structure might play a role in spatially modulating the flare energy deposition in the lower atmosphere.

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arXiv2026

Magnetic structure of coronal dark halos

At low coronal temperatures around or below 1 MK distinct areas in the surroundings of active regions (AR) show emission at a level significantly below the emission coming from the quiet Sun (QS). These areas are referred to as dark halos, dark canopies, or dark moats. To better understand the nature of dark halos we study the connection between the photospheric magnetic field and coronal emission at different temperatures. Combining Solar Orbiter data from the high-resolution Polarimetric and Helioseismic Imager (SO/PHI) and Extreme Ultraviolet Imager (EUI) instruments allows us to identify these areas that are dark in the extreme ultraviolet (EUV) in the immediate vicinity of an AR. We probe both the photospheric magnetic field as well as the coronal intensities as a function of distance to the AR NOAA 12893. The dark halo has an unsigned magnetic flux density similar to the QS, but shows a strong radial dependence with distance from the AR centre. It drops by 38 % from 6.1 G at the inner boundary to 3.8 G at the outer, shifting from above to below QS levels. Coronal emission $\leq$1 MK is $\sim$40 % below QS and shows no dependence on distance to the AR centre. In contrast, at $\geq$1.6 MK, emission exceeds QS levels, but declines outward toward QS values. A few hot loops extend from the AR periphery across the halo, while at lower temperatures no such loops appear and short loops dominate the corona. The reduced unsigned magnetic flux density in the outermost parts of the dark halo, below QS level, suggests that reduced coronal heating due to weak underlying magnetic flux heating could be partially responsible for the reduced emission around 1 MK. Closer to the AR, other mechanisms might lead to reduced heating. The different loops structures detected for hotter and cooler coronal temperatures likely play a crucial role in understanding coronal dark halos.

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arXiv2026

Small-scale impulsive EUV emission enhancements along network loops

Network loops are a common feature in the quiet Sun. The physical processes sustaining their energy budget is still under discussion. We rely on a multi-instrumental (Solar Orbiter/EUI, Solar Orbiter/PHI, IRIS) observation of a six hours quiet Sun region to measure the dynamics and the possible magnetic drivers of impulsive EUV emission enhancements along network loops. We report the detection of small-scale impulsive EUV emission enhancements with EUI/HRIEUV in three network loops. We selected four EUV emission enhancements to measure their plane-of-sky velocities in HRIEUV; their Doppler velocities in the line (log Si iv T = 4.8) with IRIS ; their possible relation to small-scale flux emergence and fluctuation in one of the loop footpoint. The plane-of-sky velocities of the four EUV emission enhancements have a component that seem to appear almost instantaneously along the loop (more than 220 km/s) ; and two of them had a co-temporal component with a PoS velocity of up to , starting near one of the loop footpoint. In one case, we measured with IRIS a co-temporal intensity increase in the line associated with Doppler velocities down to and up to along the line of sight. Finally, we found evidence of small-scale (8E16 Mx) mixed polarity field emergence and fluctuation near one of the loop footpoint. We concluded that the fast component on the plane-of-sky are consistent with a thermal transfer or supersonic plasma flows, while the slower component is consistent with plasma flows. A possible physical origin for these EUV emission enhancements would be magnetic reconnection driven by either photospheric motion of the loop footpoints or by the reconnection of the loop with small-scale magnetic bipoles.

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arXiv2026

High-resolution observations of small-scale activity in coronal hole plumes

Plumes have been proposed to channel MHD waves and the solar wind into the heliosphere. High-speed propagating disturbances (PDs), though well detected in plumes, cannot yet be clearly assigned to MHD waves or to mass flows. Additionally, plume bases as observed in the extreme ultraviolet are riddled with small-scale transients that could be related to the PDs. We study three plumes within an equatorial coronal hole observed by the EUV High Resolution Imager of the Extreme Ultraviolet Imager on board Solar Orbiter. The properties of the small-scale brightenings at the plume bases are investigated to interpret their nature and possible relation with PDs. We process images with the Difference of Gaussians method to highlight the target brightenings, which are further identified with two different approaches. In the 30-min observation, 50 brightenings are visually selected, which also help set thresholds for automatic detection, where we find 451 brightenings. Their properties, including velocities on the plane of sky (PoS), are analyzed statistically. Potential field extrapolation based on the magnetic field data from the Polarimetric and Helioseismic Imager on board Solar Orbiter is used for correcting the PoS velocity to the real velocity along the magnetic field. We observe that the majority of the base brightenings are small-scale, short-lived, and slightly elongated at the plume bases. They display intricate movements, with most exhibiting velocities in the PoS of less than 10 km/s. Their 3-dimensional velocities are found to be substantially lower than (and difficult to reconcile with) the speeds of PDs. A direct link between base brightenings and PDs remains inconclusive. We propose two possibilities for base brightenings: they may be related to wave-driven Type I spicules or originate from interchange reconnections. Further investigation is required to validate these hypotheses.

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arXiv2026

A magnetic avalanche as the central engine powering a solar flare

Solar flares are the most powerful, magnetically driven, explosions in the heliosphere. The nature of magnetic energy release in the solar corona that heats the plasma and accelerates particles in a flare, however, remains poorly understood. Here, we report high-resolution coronal observations of a flare by the Solar Orbiter mission that reveal initially weaker but rapid reconnection events, on timescales of a few seconds at most, leading to a more prominent activity of a similar nature that explosively causes a flare. Signatures of this process are further imprinted on the widespread raining plasma blobs with short lifetimes, giving rise to the characteristic ribbon-like emission pattern associated with the flare. Our observations unveil the central engine of a flare and emphasize the crucial role of an avalanche-like magnetic energy release mechanism at work.

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arXiv2025

The first out-of-ecliptic observations of the polar magnetic field of the Sun

Direct remote-sensing observations of the solar poles have been hindered by the restricted view obtained from the ecliptic plane. For the first time ever, Solar Orbiter with its remote-sensing instruments observed the poles of the Sun from out of the ecliptic in the Spring of 2025. Here we report the first measurements of the magnetic field of the solar poles taken when Solar Orbiter was at heliographic latitudes ranging between 14.9$^\circ$ and 16.7$^\circ$. The data-sets were collected by the High Resolution Telescope of the Polarimetric and Helioseismic Imager (SO/PHI-HRT) on board Solar Orbiter. Two sets of observations, approximately one month apart, for the south and north pole are considered in this work. The magnetic flux and flux density measured during these campaigns are reported as a function of the heliographic latitude observed by SO/PHI-HRT. The net fluxes show a different latitudinal distribution for the two polar caps. We also discuss the observed dependence of the measured fluxes on the viewing angle. These first results highlight the importance of high-resolution direct measurements of the polar field, paving the way to the high-latitude observations planned for SO/PHI-HRT in the coming years.

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arXiv2025

Supergranulation and Poleward Migration of the Magnetic Field at High Latitudes of the Sun

Magnetoconvection at the solar surface governs the dynamics in the upper solar atmosphere and sustains the heliosphere. Properties of this fundamental process are poorly described near the solar poles. Here we report the first out-of-ecliptic remote-sensing observations of the south pole of the Sun from a high-latitude campaign of the Solar Orbiter spacecraft which reveal spatial and temporal evolution of supergranular convective cells. The supergranular cells have spatial scales of 20--40 Mm. From eight days of observations starting on 2025 March 16, our analysis shows that the magnetic network migrates poleward, on average, at high latitudes (above 60\textdegree), with speeds in the range of 10--20 m s$^{-1}$, depending on the structures being tracked. These results shed light on the buildup of the polar magnetic field that is central to our understanding of the solar cycle and the heliospheric magnetic field.

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arXiv2025

Coronal hole picoflare jets are progenitors of both fast and Alfvénic slow solar wind

Solar wind, classified by its bulk speed and the Alfvénic nature of its fluctuations, generates the heliosphere. The elusive physical processes responsible for the generation of the different types of this wind are a topic of active debate. Recent observations reveal intermittent jets, with kinetic energy in the picoflare range, emerging from dark areas of a polar coronal hole threaded by open magnetic field lines. These could substantially contribute to solar wind. However, their ubiquity and direct links to solar wind have not been established. Here, we report a unique set of remote-sensing and in situ observations from the Solar Orbiter spacecraft that establish a unified picture of fast and Alfvénic slow wind, connected to the similar widespread picoflare jet activity in two coronal holes. Radial expansion of coronal holes ultimately regulates the speed of the emerging wind.

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arXiv2025

Long-lived coronal loops in solar active regions

Coronal loops are plasma structures in the solar atmosphere with temperatures reaching millions of Kelvin, shaped and sustained by the magnetic field. However, their morphology and fundamental nature remain subjects of debate. By studying their cross-sectional properties and how they change along the loop and in time, we can understand their magnetic structure and heating mechanisms. In this study, we investigated the cross-sectional intensity profiles, both spatially and temporally, of two unique coronal loops, observed in the periphery of two distinct active regions by the Extreme Ultraviolet Imager (EUI/HRI$_{\rm EUV}$) on board the Solar Orbiter spacecraft. The main results of this study are 1. The lifetimes of these two loops (loop1 > 120 min \& loop2 > 50 min) are longer than the typical timescales of radiative cooling and thermal conduction. 2. Their widths determined by the FWHM of the single Gaussian fit to the cross-axis intensity profiles are greater than 6-7 pixels of EUI/HRI$_{\rm EUV}$, indicating that the loop cross-section is uniformly filled on well-resolvable scales. 3. These loops exhibited an almost constant width, both spatially and temporally (width for loop1 is 2.1 $\pm$ 0.4 Mm and for loop2 is 1.3 $\pm$ 0.2 Mm), indicating that they are stable non-expanding structures. 4. We present observational evidence that the one of the loops (loop2) is not braided, which strongly suggests that the non-expanding nature of this multi-stranded loop along its length cannot be attributed to the twist of the magnetic field lines. In conclusion, we find that these coronal loops are long, stable, multi-stranded, non-expanding structures with a uniform cross-section that persist in the corona for an unusually extended duration. This not only challenges our current understanding of the structure of the coronal magnetic field but also raises critical questions about the mechanisms

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arXiv2025

Data-constrained 3D MHD Simulation of a Spiral Jet Caused by an Unstable Flux Rope Embedded in Fan-spine Configuration

Spiral jets are impulsive plasma ejections that typically show an apparent rotation motion. Their generation, however, is still nont understood thoroughly. Based on a high-resolution vector magnetogram form the Polarimetric and Helioseismic Imager onboard Solar Orbiter, we constrcut a data-constrained three-dimensional (3D) MHD model, aiming to disclose the eruption mechanism of a tiny spiral jet at a moss region observed on March 3 2022. The initial configuration of the simulation consists of an extrapolated coronal magnetic field based on the vector magnetogram and an inserted unstable flux rope constructed by the Regularized Biot-Savart Laws method. Our results highlight the critical role of the fan-spine configuration in forming the spiral jet and confirm the collapse of the pre-existing magnetic null to a curved 3D current sheet where external reconnection takes places. It is further disclosed that the flux rope quickly moves upward, reconnecting with the field lines near the outer spine, thereby enabling the transfer of twist and cool material from the flux rope to the open field, giving rise to the tiny spiral jet we observed. The notable similarities between these characteristics and those for larger-scale jets suggest that spiral jets, regardless of their scale, essentially share the same eruption mechanism.

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arXiv2024

Stereoscopic observations reveal coherent morphology and evolution of solar coronal loops

Coronal loops generally trace magnetic lines of force in the upper solar atmosphere. Understanding the loop morphology and its temporal evolution has implications for coronal heating models that rely on plasma heating due to reconnection at current sheets. Simultaneous observations of coronal loops from multiple vantage points are best suited for this purpose. Here, we report a stereoscopic analysis of coronal loops in an active region based on observations from the Extreme Ultraviolet Imager on board the Solar Orbiter Spacecraft and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Our stereoscopic analysis reveals that coronal loops have nearly circular cross-sectional widths and that they exhibit temporally coherent intensity variations along their lengths on timescales of around 30\,minutes. Results suggest that coronal loops can be best represented as three-dimensional monolithic or coherent plasma bundles that outline magnetic field lines. Therefore, at least on the scales resolved by Solar Orbiter, it is unlikely that coronal loops are manifestations of emission from the randomly aligned wrinkles in two-dimensional plasma sheets along the line-of-sight as proposed in the `coronal veil' hypothesis.

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arXiv2024

Photospheric Hot Spots at Solar Coronal Loop Footpoints Revealed by Hyperspectral Imaging Observations

Poynting flux generated by random shuffling of photospheric magnetic footpoints is transferred through the upper atmosphere of the Sun where the plasma is heated to over 1 MK in the corona. High spatiotemporal resolution observations of the lower atmosphere at the base of coronal magnetic loops are crucial to better understand the nature of the footpoint dynamics and the details of magnetic processes that eventually channel energy into the corona. Here we report high spatial resolution ($\sim$0.1\arcsec) and cadence (1.33 s) hyperspectral imaging of the solar H$α$ line, acquired by the Microlensed Hyperspectral Imager prototype installed at the Swedish 1-m Solar Telescope, that reveal photospheric hot spots at the base of solar coronal loops. These hot spots manifest themselves as H$α$ wing enhancements, occurring on small spatial scales of $\sim$0.2\arcsec, and timescales of less than 100 s. By assuming that the H$α$ wings and the continuum form under the local thermodynamic equilibrium condition, we inverted the H$α$ line profiles and found that the hot spots are compatible with a temperature increase of about 1000 K above the ambient quiet-Sun temperature. The H$α$ wing integrated Stokes $V/I$ maps indicate that hot spots are related to magnetic patches with field strengths comparable to or even stronger than the surrounding network elements. But they do not show the presence of parasitic polarity magnetic field that would support the interpretation that these hot spots are reconnection-driven Ellerman bombs. Therefore, we interpret these features as proxies of locations where convection-driven magnetic field intensification in the photosphere can lead to energy transfer into higher layers. We suggest that such hot spots at coronal loop footpoints may be indicative of the specific locations and onset of energy flux injection into the upper atmosphere.

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arXiv2024

High-resolution Observations of Clustered Dynamic Extreme-Ultraviolet Bright Tadpoles near the Footpoints of Corona Loops

An extreme ultraviolet (EUV) close-up view of the Sun offers unprecedented detail of heating events in the solar corona. Enhanced temporal and spatial images obtained by the Solar Orbiter during its first science perihelion enabled us to identify clustered EUV bright tadpoles (CEBTs) occurring near the footpoints of coronal loops. Combining SDO/AIA observations, we determine the altitudes of six distinct CEBTs by stereoscopy, ranging from $\sim$1300 to 3300 km. We then notice a substantial presence of dark, cooler filamentary structures seemingly beneath the CEBTs, displaying periodic up-and-down motions lasting 3 to 5 minutes. This periodic behavior suggests an association of the majority of CEBTs with Type I spicules. Out of the ten selected CEBTs with fast downward velocity, six exhibit corrected velocities close to or exceeding 50 km $s^{-1}$. These velocities notably surpass the typical speeds of Type I spicules. We explore the generation of such velocities. It indicates that due to the previous limited observations of spicules in the EUV wavelengths, they may reveal novel observational features beyond our current understanding. Gaining insights into these features contributes to a better comprehension of small-scale coronal heating dynamics.

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arXiv2024

Hard X-rays from the deep solar atmosphere. An unusual UV burst with flare properties

Explosive transient events occur throughout the solar atmosphere. The differing manifestations range from coronal mass ejections to Ellerman bombs. The former may have negligible signatures in the lower atmosphere, and the latter may have negligible nonthermal emissions such as hard X-radiation. A solar flare generally involves a broad range of emission signatures. Using a suite of four space-borne telescopes, we report a solar event that combines aspects of simple UV bursts and hard X-ray emitting flares at the same time. The event is a compact C-class flare in active region AR11861, SOL2013-10-12T00:30. By fitting a combined isothermal and nonthermal model to the hard X-ray spectrum, we inferred plasma temperatures in excess of 15\,MK and a nonthermal power of about $3\times10^{27}$\,erg\,s$^{-1}$ in this event. Despite these high temperatures and evidence for nonthermal particles, the flare was mostly confined to the chromosphere. However, the event lacked clear signatures of UV spectral lines, such as the Fe\,{\sc xii} 1349\,Å and Fe\,{\sc xxi} 1354\,Å emission lines, which are characteristic of emission from hotter plasma with a temperature over 1\,MK. Moreover, the event exhibited very limited signatures in the extreme-UV wavelengths. Our study indicates that a UV burst -- hard X-ray flare hybrid phenomenon exists in the low solar atmosphere. Plasma that heats to high temperatures coupled with particle acceleration by magnetic energy that is released directly in the lower atmosphere sheds light on the nature of active region core heating and on inferences of flare signatures.

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arXiv2024

Coronal Heating and Solar Wind Generation by Flux Cancellation Reconnection

In this paper we propose that flux cancellation on small granular scales ($\lesssim 1000$ km) ubiquitously drives reconnection at a multitude of sites in the low solar atmosphere, contributing to chromospheric/coronal heating and the generation of the solar wind. We analyse the energy conversion in these small-scale flux cancellation events using both analytical models and three-dimensional, resistive magnetohydrodynamic (MHD) simulations. The analytical models -- in combination with the latest estimates of flux cancellation rates -- allow us to estimate the energy release rates due to cancellation events, which are found to be of the order $10^6-10^7$ erg cm$^{-2}$sec$^{-1}$, sufficient to heat the chromosphere and corona of the Quiet Sun and active regions, and to power the solar wind. The MHD simulations confirm the conversion of energy in reconnecting current sheets, in a geometry representing a small-scale bipole being advected towards an intergranular lane. A ribbon-like jet of heated plasma is accelerated upwards, that could also escape the Sun as the solar wind in an open-field configuration. We conclude that through two phases of atmospheric energy release -- pre-cancellation and cancellation -- the cancellation of photospheric magnetic flux fragments and the associated magnetic reconnection may provide a substantial energy and mass flux contribution to coronal heating and solar wind generation.

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arXiv2023

Fleeting Small-scale Surface Magnetic Fields Build the Quiet-Sun Corona

Arch-like loop structures filled with million Kelvin hot plasma form the building blocks of the quiet-Sun corona. Both high-resolution observations and magnetoconvection simulations show the ubiquitous presence of magnetic fields on the solar surface on small spatial scales of $\sim$100\,km. However, the question of how exactly these quiet-Sun coronal loops originate from the photosphere and how the magnetic energy from the surface is channeled to heat the overlying atmosphere is a long-standing puzzle. Here we report high-resolution photospheric magnetic field and coronal data acquired during the second science perihelion of Solar Orbiter that reveal a highly dynamic magnetic landscape underlying the observed quiet-Sun corona. We found that coronal loops often connect to surface regions that harbor fleeting weaker, mixed-polarity magnetic field patches structured on small spatial scales, and that coronal disturbances could emerge from these areas. We suggest that weaker magnetic fields with fluxes as low as $10^{15}$\,Mx and/or those that evolve on timescales less than 5\,minutes, are crucial to understand the coronal structuring and dynamics.

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