Reetika Joshi

• Faber, Jonas Thoen; Joshi, Reetika; Voort, Luc Rouppe van der; Wedemeyer, Sven; Øyre, Eilif Sommer & Poquet, Ignasi J. Soler [Vis alle 7 forfattere av denne artikkelen] (2026). Fine details in solar flare ribbons: Statistical insights from observations with the Swedish 1-m Solar Telescope. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 705. doi: 10.1051/0004-6361/202556738. Fulltekst i vitenarkiv Vis sammendrag

  Context. Flare ribbons serve as chromospheric footprints of energy deposition resulting from particle acceleration during magnetic reconnection. Their fine-scale structure provides a valuable tool for probing the dynamics of the flare reconnection process. Aims. Our goal is to investigate the fine-scale structure of flare ribbons through multiple observations of flares, utilising data obtained from the Atmospheric Imaging Assembly (AIA) and the Swedish 1-m Solar Telescope (SST). Methods. The aligned AIA and SST datasets for the three solar flares were used to examine their overall morphology. The SST datasets were specifically used to identify fine-scale structures within the flare ribbons. For spectroscopic analysis of these fine-scale structures, we applied machine-learning methods ( k -means clustering) and Gaussian fitting. Results. Using k -means, we identified elongated features in the flare ribbons, termed as ‘riblets’, which are short-lived and jet-like small-scale structures that extend as plasma columns from the flare ribbons. Riblets are more prominent near the solar limb and represent the ribbon front. Riblet widths are consistent across observations, ranging from 110−310 km (0.15−0.41), while vertical lengths span 620−1220 km (0.83−1.66), with a potential maximum of 2000 km (2.67), after accounting for projection effects. Detailed H β spectral analysis reveals that riblets exhibit a single, redshifted emission component, with velocities of 16−21 km s −1 , independent of the viewing angle. Conclusions. Our high-resolution observations of the three flare ribbons show that they are not continuous structures, but rather are composed of vertically extended, fine-scale substructures. These irregular features indicate that the reconnection region is not a smooth, laminar current sheet, but rather a fragmented zone filled with magnetic islands (blobs or riblets), consistent with the theory of patchy reconnection within the coronal current sheet.

• Siverio, Daniel Elias Nóbrega; Joshi, Reetika; Sola-Viladesau, Eva; Berghmans, David & Lim, Daye (2025). Thin coronal jets and plasmoid-mediated reconnection: Insights from Solar Orbiter observations and Bifrost simulations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 702. doi: 10.1051/0004-6361/202555357. Fulltekst i vitenarkiv Vis sammendrag

  Context. Coronal jets are ubiquitous, collimated million-degree ejections that contribute to the energy and mass supply of the upper solar atmosphere and the solar wind. Solar Orbiter observations provide an unprecedented opportunity to study fine-scale jets from a unique vantage point close to the Sun. Aims. We aim to uncover thin jets originating from coronal bright points (CBPs) and investigate observable features of plasmoid-mediated reconnection. Methods. We analyzed eleven datasets from the High Resolution Imager 174 Å of the Extreme Ultraviolet Imager (HRI EUV ) on board Solar Orbiter, focusing on narrow jets from CBPs and signatures of magnetic reconnection within current sheets and outflow regions. To aid in the interpretation, we compared the observations with radiative-magnetohydrodynamic simulations of a CBP conducted with the Bifrost code. Results. We identified thin coronal jets originating from CBPs with widths ranging from 253 km to 706 km. These are scales that could not be resolved with previous EUV imaging instruments. Remarkably, these jets are 30−85% brighter than their surroundings and can extend up to 22 Mm, while maintaining their narrow form. For one of the datasets, we directly identified plasmoid-mediated reconnection through the development within the current sheet of a small-scale plasmoid that reaches a length of 332 km and propagates at 40 km s −1 . For another dataset, we inferred indirect traces of plasmoid-mediated reconnection through the intermittent boomerang-like pattern that appears in the outflow region. The simulation self-consistently produces a current sheet and small-scale plasmoids similar to those observed, whose synthetic HRI EUV emission reproduces both direct imprints within the current sheet and intermittent patterns in the outflow region associated with their ejection. Conclusions. Our findings highlight Solar Orbiter’s unique capability to capture narrow jets and sub-megameter-scale plasmoid-mediated reconnection signatures in the corona. These results motivate future statistical studies aimed at assessing the role of such fine-scale phenomena in coronal dynamics and solar wind formation.

• Karki, Garima; Schmieder, Brigitte; Démoulin, Pascal; Devi, Pooja; Chandra, Ramesh & Joshi, Reetika (2025). Counter-Streaming Velocities in a Quiescent Filament. Solar Physics. ISSN 0038-0938. 300(7). doi: 10.1007/s11207-025-02509-w. Fulltekst i vitenarkiv Vis sammendrag

  Filaments/prominences are cold plasma (~10^4 K) embedded in the solar corona, two orders of magnitude hotter. Filament plasma is structured by the magnetic field in thin elongated threads. Counter-streaming flows have been observed. The aim of this paper is to characterize these flows. For that, we use high spatial resolution observations of spectral data obtained with THEMIS in Halpha and with IRIS in Mg II k lines on 29 September 2023. We best detect counter-streaming flows in both the blue and red wings of these spectral lines. They are forming long Doppler shifted strands slightly inclined on the filament axis. The blue/red shift alternates across the strands at the arc second scale. H spectral profiles with large widths are interpreted as formed by multi-strands with opposite velocity directions. The absorption in the core of Mg II k line is also broader than in the chromosphere. This corresponds also to counter-streaming velocities. We derive that a fraction of the filament plasma is moving at supersonic speed (of the order of 20 km s−1) with the assumption that the filament is optically thick. We conclude that the counter-directed Doppler shifts might not be magnetic field aligned flows but rather correspond to kink transverse oscillations of the magnetic field with independent motions in nearby strands.

• Joshi, Reetika; Dudík, Jaroslav; Schmieder, Brigitte; Aulanier, Guillaume & Chandra, Ramesh (2025). Spectroscopic investigations of a filament reconnecting with coronal loops during a two-ribbon solar flare. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 698. doi: 10.1051/0004-6361/202554011. Fulltekst i vitenarkiv Vis sammendrag

  Context. In the standard 2D model of eruption, the eruption of a magnetic flux rope is associated with magnetic reconnection occurring beneath it. However, in 3D, additional reconnection geometries are possible, in particular the AR – RF , where external reconnection involving the overlying arcades ( A ) and erupting flux rope ( R ) turns into another arcade and a flare loop ( F ). This process results in the drifting of the legs of the erupting flux rope. Aims. We investigate spectroscopic signatures of such AR – RF reconnection occurring in an erupting filament reconnecting with coronal arcades during a weak B3.2-class two-ribbon flare. Methods. We examined the evolution of the erupting filament eruption using imaging observations by the Atmospheric Imaging Assembly (AIA) as well as both imaging and spectroscopic observations by the Interface Region Imaging Spectrograph (IRIS). Results. As the filament rises into the corona, it reconnects with the surrounding arcade of coronal loops with localized brightenings, resulting in the disappearance of the coronal loops and formation of a hot flux rope, showing a slipping motion of its footpoints that extends to the previous footpoints of the coronal loops ( AR – RF reconnection), as was predicted by the 3D extensions to the standard solar flare model. These brightenings are accompanied by the presence of strong blueshifts in both the IRIS Si  IV and Mg  II lines, up to ≈200 km s −1 . The lines are also extremely wide, with nonthermal widths above 100 km s −1 . Furthermore, a strongly non-Gaussian profile of the most blueshifted component is detected at the start of the AR – RF reconnection, indicating the presence of accelerated particles and magnetohydrodynamic turbulence, and associated with the appearance of hot plasma in the AIA 94 Å passband. Results. For the first time, an observation has been reported in which the IRIS slit successfully captures AR – RF reconnection between a filament and overlying arcades, resulting in strong blueshifts and very broad line profiles.

• Ashfield, William; Polito, Vanessa; Lörinčík, Juraj; Pontieu, Bart Walter De; Chintzoglou, Georgios & Bose, Souvik [Vis alle 10 forfattere av denne artikkelen] (2025). Spectroscopic observations of solar flare pulsations driven by oscillatory magnetic reconnection. Nature Astronomy. 10(1), s. 54–63. doi: 10.1038/s41550-025-02706-4. Fulltekst i vitenarkiv Vis sammendrag

  Solar and stellar flares often exhibit oscillations, or quasi-periodic pulsations (QPPs), across the electromagnetic spectrum. While magnetic-field reconnection drives these events, it remains to be determined whether oscillatory reconnection causes the quasi-periodicity, or whether waves drive or mediate this process. Exploiting coordinated observations from NASA’s Interface Region Imaging Spectrograph and the Swedish 1 m Solar Telescope, here we present spectroscopic observations of QPPs in a solar flare at high-temporal (<1 s) and high-spatial (~60 km) resolution. Downwards velocities in the flare ribbon show synchronized oscillations at different atmospheric layers with a period of ~32 s. These velocities correlate with hard X-ray emissions, indicating a modulated deposition of accelerated electrons in the chromosphere as the driver. By negating magnetohydrodynamic sausage modes as the modulator, we demonstrate that repeated reconnection drives the QPPs. The QPP–reconnection relationship established here provides observational benchmarks for reconnection models and diagnostics for probing energy release across astrophysical environments.

• Wilde, M. De; Pietrow, A. G. M.; Druett, M. K.; Yabar, A. Pastor; Koza, J. & Kontogiannis, I. [Vis alle 25 forfattere av denne artikkelen] (2025). Synthesizing Sun-as-a-star flare spectra from high-resolution solar observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 700. doi: 10.1051/0004-6361/202554870. Fulltekst i vitenarkiv Vis sammendrag

  Context. Spatially resolved observations of the Sun and the astronomical sample size of stellar bodies are the respective key strengths of solar and stellar observations. However, the large difference in object brightness between the Sun and other stars has led to distinctly different instrumentation and methodologies between the two fields. Aims. We produced and analyzed synthetic full-disk spectra derived from 19 small area field-of-view optical observations of solar flares acquired by the Swedish 1-m Solar Telescope (SST) between 2011 and 2024. These were used to investigate what can and cannot be inferred about physical processes on the Sun from Sun-as-a-star observations. Methods. The recently released Numerical Empirical Sun-as-a-Star Integrator (NESSI) code provides synthetic full-disk integrated spectral line emission based on smaller field-of-view input while accounting for center-to-limb variations and differential rotation. We used this code to generate pseudo-Sun-as-a-star spectra from the SST observations. Results. We show that limited-area solar observations can be extrapolated to represent the full disk accurately in a manner close to what is achievable with Sun-as-a-star telescopes. Additionally, we identify nine spectral features, four of which are caused by instrumental effects. Most notably, we find a relation between the heliocentric angle of flares and the width of the excess emission left by them as well as a source of false positive coronal mass ejections-like signatures, and we defined an energy scaling law based on chromospheric line intensities that shows that the peak flare contrast roughly scales with the square root of the bolometric energy. Conclusions. The presented method of creating pseudo-Sun-as-a-star observations from limited field-of-view solar observations allows for the accurate comparison of solar flare spectra with their stellar counterparts while allowing for the detection of signals at otherwise unachievable noise levels.

• Guo, J.H.; Ni, Y.W.; Schmieder, B.; Guo, Y.; Xia, C. & Devi, P. [Vis alle 12 forfattere av denne artikkelen] (2025). The Birth of a Major Coronal Mass Ejection with Intricate Magnetic Structure from Multiple Active Regions. The Astrophysical Journal (ApJ). ISSN 0004-637X. 983(1). doi: 10.3847/1538-4357/adb976. Fulltekst i vitenarkiv Vis sammendrag

  Coronal mass ejections (CMEs) are the eruptions of magnetized plasma from the Sun and are considered the main driver of adverse space weather events. Hence, understanding their formation process, particularly the magnetic topology, is critical for accurate space weather prediction. Here, based on imaging observations and three-dimensional (3D) data-constrained thermodynamic magnetohydrodynamic (MHD) simulation in spherical coordinates, we exhibit the birth of a CME with intricate magnetic structure from multiple active regions (ARs) due to 3D magnetic reconnection. It is observed as a coronal jet between ARs, accompanied by the back-flowing of filament materials along the jet spine after the passage of the eruptive filament. This jet connects two dimming regions within different ARs. This is an observational proxy of 3D magnetic reconnection between the CME flux rope and the null-point magnetic field lines crossing ARs. Hereafter, the thermodynamic data-constrained MHD simulation successfully reproduces the observed jet and the reconnection process that flux ropes partake in, leading to a CME flux rope with a complex magnetic structure distinct from its progenitor. The generality of this scenario is then validated by data-inspired MHD simulations in a simple multipolar magnetic configuration. This work demonstrates the role of multiple ARs in forming CMEs with intricate magnetic structures. On the one hand, a noncoherent flux rope where not all twisted magnetic field lines wind around one common axis is naturally formed. On the other hand, our findings suggest that the topology of a real CME flux rope may not be solely determined by a single AR, particularly during periods of solar maximum.

• Karki, Garima; Schmieder, Birgitte; Devi, Pooja; Chandra, Ramesh; Labrosse, Nicolas & Joshi, Reetika [Vis alle 7 forfattere av denne artikkelen] (2025). Magnetic Reconnection between a Solar Jet and a Filament Channel. The Astrophysical Journal (ApJ). ISSN 0004-637X. 979(2). doi: 10.3847/1538-4357/ada28e. Fulltekst i vitenarkiv Vis sammendrag

  The solar corona is highly structured by bunches of magnetic field lines forming either loops, or twisted flux ropes representing prominences/filaments, or very dynamic structures such as jets. The aim of this paper is to understand the interaction between filament channels and jets. We use high-resolution Hα spectra obtained by the ground-based telescope Télescope Héliographique pour l'Etude du Magnétisme et des Instabilités Solaires on the Canary Islands and data from Helioseismic Magnetic Imager and Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. In this paper we present a multiwavelength study of the interaction of filaments and jets. They both consist of cool plasma embedded in magnetic structures. A jet is particularly well studied in all the AIA channels with a flow reaching 100–180 km s−1. Its origin is linked to cancelling flux at the edge of the active region. Large Doppler shifts in Hα are derived in a typical area for a short time (order of minute). They correspond to flows around 140 km s−1. In conclusion we conjecture that these flows correspond to some interchange of magnetic field lines between the filament channel and the jets leading to cool plasmoid ejections or reconnection jets perpendicularly to the jet trajectory.

• Faber, Jonas Thoen; Joshi, Reetika; Voort, Luc Rouppe van der; Wedemeyer, Sven; Fletcher, Lyndsay & Aulanier, Guillaume [Vis alle 7 forfattere av denne artikkelen] (2024). High-resolution observational analysis of flare ribbon fine structures. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202452370. Fulltekst i vitenarkiv Vis sammendrag

  Context. Since the mechanism of energy release from solar flares is still not fully understood, the study of fine-scale features developing during flares becomes important for progressing towards a consistent picture of the essential physical mechanisms. Aims. Solar flares release most of their luminous energy in the chromosphere. These luminous signatures, known as flare ribbons, act as the footpoints of the released energy and are crucial for the interpretation of reconnection processes causing these events. We aim to probe the fine structures in flare ribbons at the chromospheric level using high-resolution observations with imaging and spectral techniques. Methods. We present a GOES C2.4 class solar flare (SOL2022-06-26T08:12) observed with the Swedish 1-m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS), and the Atmospheric Imaging Assembly (AIA). Utilising imaging data from SST, IRIS, and AIA, we detail both the global and fine-structure evolution of the flare. The high-resolution SST observations offer spectroscopic data in the Hα, Ca II 8542 Å, and Hβ lines, which we use to analyse the flare ribbon. Results. The flare was associated with a filament eruption. Fibrils and coronal loops were connected from a negative polarity region to two positive polarity regions. Within the eastern flare ribbon, chromospheric bright blobs were detected and analysed in Ca II 8542 Å, Hα, and Hβ wavelengths. A comparison of blobs in Hβ observations and Si IV 1400 Å has also been performed. These blobs are observed as almost circular structures having widths from 140 km–200 km. The intensity profiles of the blobs show a red wing asymmetry. Conclusions. From the high spatial and temporal resolution Hβ observations, we conclude that the periodicity of the blobs in the flare ribbon, which are near-equally spaced in the range 330–550 km, is likely due to fragmented reconnection processes within a flare current sheet. This supports the theory of a direct link between fine-structure flare ribbons and current sheet tearing. We believe our observations represent the highest resolution evidence of fine-structure flare ribbons to date.

• Luna, Manuel; Joshi, Reetika; Schmieder, Birgitte; Moreno-Insertis, Fernando; Liakh, Valeriia & Terradas, Jaume (2024). Study of the excitation of large-amplitude oscillations in a prominence by nearby flares. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 691. doi: 10.1051/0004-6361/202450869. Fulltekst i vitenarkiv Vis sammendrag

  Context. Large-amplitude oscillations are a common occurrence in solar prominences. These oscillations are triggered by energetic phenomena such as jets and flares. On March 14–15, 2015, a filament partially erupted in two stages, leading to oscillations in different parts of it. Aims. In this study, we aim to explore the longitudinal oscillations resulting from the eruption, with special focus on unravelling the underlying mechanisms responsible for their initiation. We pay special attention to the huge oscillation on March 15. Methods. The oscillations and jets were analysed using the time-distance technique. For the study of flares and their interaction with the filament, we analysed the different AIA channels in detail and used the differential-emission-measure (DEM) technique. Results. In the initial phase of the event, a jet induces the fragmentation of the filament, which causes it to split into two segments. One of the segments remains in the same position, while the other is detached and moves to a different location. This causes oscillations in both segments: (a) the change of position apparently causes the detached segment to oscillate longitudinally with a period of 69 ± 3 minutes; (b) the jet flows reach the remaining filament also producing longitudinal oscillations with a period of 62 ± 2 minutes. In the second phase, on March 15, another jet seemingly activates the detached filament eruption. After the eruption, there is an associated flare. A large longitudinal oscillation is produced in the remnant segment with a period of 72 ± 2 minutes and a velocity amplitude of 73 ± 16. During the triggering of the oscillation, bright field lines connect the flare with the filament. These only appear in the AIA 131 Å and 94 Å channels, indicating that they contain very hot plasma. The DEM analysis also confirms this result. Both indicate that a plasma of around 10 MK pushes the prominence from its south-eastern side, displacing it along the field lines and initiating the oscillation. From this evidence, the flare and not the preceding jet initiates the oscillation. The hot plasma from the flare escapes and flows into the filament channel structure. Conclusions. In this paper, we shed light on how flares can initiate the huge oscillations in filaments. We propose an explanation in which part of the post-flare loops reconnect with the filament channel’s magnetic-field lines that host the prominence.

• Siverio, Daniel Elias Nóbrega; Cabello, I.; Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Reetika & Froment, C. [Vis alle 7 forfattere av denne artikkelen] (2024). Small-scale magnetic flux emergence preceding a chain of energetic solar atmospheric events. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 686. doi: 10.1051/0004-6361/202348894. Fulltekst i vitenarkiv Vis sammendrag

  Context. Advancements in instrumentation have revealed a multitude of small-scale extreme-ultraviolet (EUV) events in the solar atmosphere and considerable effort is currently undergoing to unravel them. Aims. Our aim is to employ high-resolution and high-sensitivity magnetograms to gain a detailed understanding of the magnetic origin of such phenomena. Methods. We used coordinated observations from the Swedish 1-m Solar Telescope (SST), the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO) to analyze an ephemeral magnetic flux emergence episode and the following chain of small-scale energetic events. These unique observations clearly link these phenomena together. Results. The high-resolution (0.057 pixel−1) magnetograms obtained with SST/CRISP allowed us to reliably measure the magnetic field at the photosphere and to detect the emerging bipole that caused the subsequent eruptive atmospheric events. Notably, this small-scale emergence episode remains indiscernible in the lower resolution SDO/HMI magnetograms (0.5 pixel−1). We report the appearance of a dark bubble in Ca II K 3933 Å related to the emerging bipole, a sign of the canonical expanding magnetic dome predicted in flux emergence simulations. Evidence of reconnection are also found, first through an Ellerman bomb and later by the launch of a surge next to a UV burst. The UV burst exhibits a weak EUV counterpart in the coronal SDO/AIA channels. By calculating the differential emission measure (DEM), its plasma is shown to reach a temperature beyond 1 MK and to have densities between the upper chromosphere and transition region. Conclusions. Our study showcases the importance of high-resolution magnetograms in revealing the mechanisms that trigger phenomena such as EBs, UV bursts, and surges. This could hold implications for small-scale events similar to those recently reported in the EUV using Solar Orbiter. The finding of temperatures beyond 1 MK in the UV burst plasma strongly suggests that we are examining analogous features. Therefore, we recommend caution when drawing conclusions from full-disk magnetograms that lack the necessary resolution to reveal their true magnetic origin.

• Joshi, Reetika; Aulanier, Guillaume; Radcliffe, Alice; Voort, Luc Rouppe van der; Pariat, Etienne & Siverio, Daniel Elias Nóbrega [Vis alle 7 forfattere av denne artikkelen] (2024). Generic low-atmosphere signatures of swirled-anemone jets. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202449553. Fulltekst i vitenarkiv Vis sammendrag

  Context. Solar jets are collimated plasma flows moving along magnetic field lines and are accelerated at low altitude following magnetic reconnection. Several of them originate from anemone-shaped low-lying arcades, and the most impulsive ones tend to be relatively wider and display untwisting motions. Aims. We aim to establish typical behaviours and observational signatures in the low atmosphere that can occur in response to the coronal development of such impulsive jets. Methods. We analysed an observed solar jet associated with a circular flare ribbon using high-resolution observations from SST coordinated with IRIS and SDO. We related specifically identified features with those developing in a generic 3D line-tied numerical simulation of reconnection-driven jets performed with the ARMS code. Results. We identified three features in the SST observations: the formation of a hook along the circular ribbon, the gradual widening of the jet through the apparent displacement of its kinked edge towards (and not away) from the presumed reconnection site, and the falling back of some of the jet plasma towards a footpoint offset from that of the jet itself. The 3D numerical simulation naturally accounts for these features, which were not imposed a priori. Our analyses allowed us to interpret them in the context of the 3D geometry of the asymmetric swirled-anemone loops and their sequences of reconnection with ambient coronal loops. Conclusions. Given the relatively simple conditions in which the observed jet occurred, together with the generic nature of the simulation that comprised minimum assumptions, we predict that the specific features that we identified and interpreted are probably typical of every impulsive jet.

• Joshi, Reetika; Voort, Luc Rouppe van der; Schmieder, Birgitte; Moreno-Insertis, Fernando; Prasad, Avijeet & Aulanier, Guillaume [Vis alle 7 forfattere av denne artikkelen] (2024). High-resolution observations of recurrent jets from an arch filament system. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 691. doi: 10.1051/0004-6361/202449715. Fulltekst i vitenarkiv Vis sammendrag

  Context. Solar jets are collimated plasma ejections along magnetic field lines observed in hot (extreme-ultraviolet (EUV) jets) and cool (chromospheric surges) temperature diagnostics. Their trigger mechanisms and the relationship between hot and cool jets are still not completely understood. Aims. We aim to investigate the generation of a sequence of active-region solar jets and their evolution from the photospheric to the coronal heights using multithermal observations from ground-based and space-borne instruments. Methods. Using the synergy of high-spatial-resolution and high-temporal-resolution observations by the Swedish 1-m Solar Telescope (SST), along with the Solar Dynamics Observatory (SDO), we analyzed a sequence of solar jets originating in a mixed-polarity region between the leading and following sunspots of an active region. We investigated the kinematics of these jets using the spectra from the SST observations. We used a non-force-free field (NFFF) extrapolation technique to derive the magnetic field topology of the active region. Results. A mixed-polarity region is formed over a long period (24 hours) with persistent magnetic flux emergence. This region has been observed as an arch filament system (AFS) in chromospheric SST observations. In this region, negative polarities surrounded by positive polarities create a fan surface with a null point at a height of 6 Mm detected in the NFFF extrapolation. SST observations in the Hβ spectral line reveal a large flux rope over the AFS moving from north to south, causing successive EUV and cool jets to move in the east–west direction and later towards the south along the long open loops. Conclusions. The high-resolution SST observations (0.038 per pixel) resolve the dark area observed at the jet base and reveal the existence of an AFS with an extended cool jet, which may be the result of a peeling-like mechanism of the AFS. Based on the combined analysis of SST and AIA observations along with extrapolated magnetic topology, it is suggested that the magnetic reconnection site may move southward by approximately 20 Mm until it reaches a region where the open magnetic field lines are oriented north–south.

• Siverio, Daniel Elias Nóbrega; Moreno-Insertis, F.; Galsgaard, K.; Krikova, Kilian; Voort, Luc Rouppe van der & Joshi, Reetika [Vis alle 7 forfattere av denne artikkelen] (2023). Deciphering Solar Coronal Heating: Energizing Small-scale Loops through Surface Convection. Astrophysical Journal Letters. ISSN 2041-8205. 958(2). doi: 10.3847/2041-8213/ad0df0. Fulltekst i vitenarkiv Vis sammendrag

  Abstract The solar atmosphere is filled with clusters of hot small-scale loops commonly known as coronal bright points (CBPs). These ubiquitous structures stand out in the Sun by their strong X-ray and/or extreme-ultraviolet (EUV) emission for hours to days, which makes them a crucial piece when solving the solar coronal heating puzzle. In addition, they can be the source of coronal jets and small-scale filament eruptions. Here we present a novel 3D numerical model using the Bifrost code that explains the sustained CBP heating for several hours. We find that stochastic photospheric convective motions alone significantly stress the CBP magnetic field topology, leading to important Joule and viscous heating concentrated around the CBP’s inner spine at a few megameters above the solar surface. We also detect continuous upflows with faint EUV signals resembling observational dark coronal jets and small-scale eruptions when H α fibrils interact with the reconnection site. We validate our model by comparing simultaneous CBP observations from the Solar Dynamics Observatory (SDO) and the Swedish 1‐m Solar Telescope (SST) with observable diagnostics calculated from the numerical results for EUV wavelengths as well as for the H α line using the Multi3D synthesis code. Additionally, we provide synthetic observables to be compared with Hinode, Solar Orbiter, and the Interface Region Imaging Spectrograph (IRIS). Our results constitute a step forward in the understanding of the many different facets of the solar coronal heating problem.

• Joshi, Reetika; Luna, Manuel; Schmieder, Birgitte; Moreno-Insertis, Fernando & Chandra, Ramesh (2023). Interaction of solar jets with filaments: Triggering of large-amplitude filament oscillations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 672. doi: 10.1051/0004-6361/202245647. Fulltekst i vitenarkiv Vis sammendrag

  Context. Large-amplitude oscillations (LAOs) are often detected in filaments. Using multi-wavelength observations, their origin can be traced back to the interaction with eruptions and jets. Aims. We present two different case studies as observational evidence in support of 2.5D numerical magnetohydrodynamics (MHD) experiments that show that the LAOs in the filament channels can be initiated by solar jets. Methods. We use longitudinal magnetic field observations using the Helioseismic Magnetic Imager to study the evolution of the filament channels. The LAOs in the filaments are analysed using two techniques. The first is time-distance diagnostics with extreme-ultraviolet (EUV) and H α datasets. In the second method, the oscillations in different parts of the filaments are examined using Fourier analysis of the brightness variations of all pixels in H α observations. Results. In the two studied events, we can identify a quadrupolar configuration with an X-point at the top of the parasitic region suggestive of a classical null-point. The X-point evolves into a flat structure suggestive of a breakout current sheet. A reconnection flow emanates from this structure, leading to a jet that propagates along the filament channel. In both cases, we can identify the quiescent and eruptive phases of the jet. The triggered LAOs have periods of around 70–80 min and are damped after a few oscillations. The minimum magnetic field intensity inferred with seismology for the filament turns out to be around 30 Gauss. Conclusions. We conclude that the two case studies are consistent with a recently published numerical model in which the LAOs are initiated by jets. The relationship between the onset of the jet and filament oscillations is straightforward for the first case but is less clear for the second case. In the second event, although there is some evidence for a relationship, we cannot rule out other possibilities such as activity unrelated to the null-point or changes in the magnetic structure of the filament. Both jets are associated with very weak flares that did not launch any EUV waves. Therefore, a role of EUV waves in triggering the filament oscillations can be eliminated for these two cases.

• Joshi, Reetika; Mandrini, Cristina H.; Chandra, Ramesh; Schmieder, Birgitte; Cristiani, Germán D. & Cormack, Cecilia Mac [Vis alle 8 forfattere av denne artikkelen] (2022). Analysis of the Evolution of a Multi-Ribbon Flare and Failed Filament Eruption. Solar Physics. ISSN 0038-0938. 297(7). doi: 10.1007/s11207-022-02021-5. Fulltekst i vitenarkiv Vis sammendrag

  Abstract How filaments form and erupt are topics about which solar researchers have wondered for more than a century and they are still open to debate. We present observations of a filament formation, its failed eruption, and the associated flare (SOL2019-05-09T05:51) that occurred in active region (AR) 12740 using data from the Solar Dynamics Observatory (SDO), the Solar-Terrestrial Relations Observatory A (STEREO-A), the Interface Region Imaging Spectrograph (IRIS) and the Learmonth Solar Observatory (LSO) of the National Solar Observatory/Global Oscillation Network Group (NSO/GONG). AR 12740 was a decaying region formed by a very disperse following polarity and a strong leading spot, surrounded by a highly dynamic zone where moving magnetic features (MMFs) were seen constantly diverging from the spot. Our analysis indicates that the filament was formed by the convergence of fibrils at a location where magnetic flux cancellation was observed. Furthermore, we conclude that its destabilisation was also related to flux cancellation associated with the constant shuffling of the MMFs. A two-ribbon flare occurred associated with the filament eruption; however, because the large-scale magnetic configuration of the AR was quadrupolar, two additional flare ribbons developed far from the two main ones. We model the magnetic configuration of the AR using a force-free field approach at the AR scale size. This local model is complemented by a global potential-field source-surface one. Based on the local model, we propose a scenario in which the filament failed eruption and the flare are due to two reconnection processes, one occurring below the erupting filament, leading to the two-ribbon flare, and another one above it between the filament flux-rope configuration and the large-scale closed loops. Our computation of the reconnected magnetic flux added to the erupting flux rope, compared to that of the large-scale field overlying it, allows us to conclude that the latter was large enough to prevent the filament eruption. A similar conjecture can be drawn from the computation of the magnetic tension derived from the global field model.

• Devi, Pooja; Chandra, Ramesh; Awasthi, Arun Kumar; Schmieder, Birgitte & Joshi, Reetika (2022). Extreme-Ultraviolet Wave and Accompanying Loop Oscillations. Solar Physics. ISSN 0038-0938. 297(12). doi: 10.1007/s11207-022-02082-6. Fulltekst i vitenarkiv

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• Joshi, Reetika (2022). Coordinated high-resolution observations of flare-related surges. Fulltekst i vitenarkiv
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