Souvik Bose

• Bose, Souvik; Joshi, Jayant; Testa, Paola & Pontieu, Bart Walter De (2025). On the Million-degree Signature of Spicules. Astrophysical Journal Letters. ISSN 2041-8205. 983(1). doi: 10.3847/2041-8213/adc30d. Full text in Research Archive Show summary

  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 multithermal 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 (∼1 MK) emission associated with spicules. We perform differential emission measure 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 emission measure (EM) maps at 1 MK reveal the presence of ubiquitous, small-scale jets with a clear spatiotemporal coherence with the spicules observed in the IRIS/TR passband. Detailed spacetime 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.

• Ashfield, William; Polito, Vanessa; Lörinčík, Juraj; Pontieu, Bart Walter De; Chintzoglou, Georgios & Bose, Souvik [Show all 10 contributors for this article] (2025). Spectroscopic observations of solar flare pulsations driven by oscillatory magnetic reconnection. Nature Astronomy. 10(1), p. 54–63. doi: 10.1038/s41550-025-02706-4. Full text in Research Archive Show summary

  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.

• Sand, Mats Ola; Voort, Luc Rouppe van der; Joshi, Jayant; Bose, Souvik; Siverio, Daniel Elias Nóbrega & Griñón-Marín, Ana Belén (2025). Quiet Sun Ellerman bombs as a possible proxy for reconnection-driven spicules. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 697. doi: 10.1051/0004-6361/202453032. Full text in Research Archive Show summary

  Context. Spicules are elongated, jet-like structures that populate the solar chromosphere and are rooted in the lower solar atmosphere. 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. Aims. 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. Methods. We analysed a high-quality 40-minute time sequence acquired with the Swedish 1-m Solar Telescope. Hβ data were used to observe QSEBs and spicules, while spectropolarimetric measurements in the photospheric Fe I 6173 Å line provided line-of-sight magnetic field information. We employed k-means clustering to automatically detect QSEBs and explored their potential association with spicules. Results. We identified 80 clear cases in which spicules occurred soon after the QSEB onset and not later than 30 s after the ending of the QSEBs. In all these instances, the 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. In addition to these clear cases, 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. Conclusions. Our findings suggest that a fraction of the 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. We suggest that an observational programme including multiple Balmer lines would likely detect more unambiguous connections between QSEBs and spicules.

• Kuniyoshi, Hidetaka; Bose, Souvik & Yokoyama, Takaaki (2024). Comprehensive Synthesis of Magnetic Tornado: Cospatial Incidence of Chromospheric Swirls and Extreme-ultraviolet Brightening. Astrophysical Journal Letters. ISSN 2041-8205. 969(2). doi: 10.3847/2041-8213/ad5a0e. Full text in Research Archive Show summary

  Magnetic tornadoes, characterized as impulsive Alfvén waves initiated by photospheric vortices in intergranular lanes, are considered efficient energy channels to the corona. Despite their acknowledged importance for solar coronal heating, their observational counterparts from the corona have not been well understood. To address this issue, we use a radiative magnetohydrodynamic simulation of a coronal loop with footpoints rooted in the upper convection zone and synthesize the chromospheric and coronal emissions corresponding to a magnetic tornado. Considering Solar Dynamics Observatory/Atmospheric Imaging Assembly 171 Å and Solar Orbiter/Extreme Ultraviolet Imager 174 Å channels, our synthesis reveals that the coronal response to magnetic tornadoes can be observed as an EUV brightening of which the width is ∼2 Mm. This brightening is located above the synthesized chromospheric swirl observed in the Ca ii 8542 Å, Ca ii K, and Mg ii k lines, which can be detected by instruments such as the Swedish 1 m Solar Telescope/CRisp Imaging SpectroPolarimeter, Goode Solar Telescope/Fast Imaging Solar Spectrograph, and Interface Region Imaging Spectrograph. Considering the height correspondence of the synthesized brightening, magnetic tornadoes can be a mechanism for the small-scale EUV brightenings such as the solar "campfires." Our findings indicate that coordinated observations encompassing the chromosphere to the corona are indispensable for comprehending the origin of coronal EUV brightenings.

• Bose, Souvik; Pontieu, Bart Walter De; Hansteen, Viggo Haraldson; Dalda, Alberto Sainz; Savage, Sabrina & Winebarger, Amy (2024). Chromospheric and coronal heating in an active region plage by dissipation of currents from braiding. Nature Astronomy. doi: 10.1038/s41550-024-02241-8. Full text in Research Archive Show summary

  Abstract The question of what heats the outer solar atmosphere remains one of the longstanding mysteries in astrophysics. Statistical studies of Sun-like stars reveal a correlation between global chromospheric and coronal emissions, constraining theoretical models of potential heating mechanisms. However, spatially resolved observations of the Sun have surprisingly failed to show a similar correlation on small spatial scales. Here we use unique coordinated observations of the chromosphere (from the IRIS satellite) and the low corona (from the Hi-C 2.1 sounding rocket), and machine-learning-based inversion techniques, to show a strong correlation on spatial scales of a few hundred kilometres between heating in the chromosphere and emission in the upper transition region in strong magnetic field regions (‘plage’). Our observations are compatible with an advanced three-dimensional magnetohydrodynamic simulation in which the dissipation of current sheets caused by magnetic field braiding is responsible for heating the plasma simultaneously to chromospheric and coronal temperatures. Our results provide deep insight into the nature of the heating mechanism in solar active regions.

• Siverio, Daniel Elias Nóbrega; Cabello, I.; Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Reetika & Froment, C. [Show all 7 contributors for this article] (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. Full text in Research Archive Show summary

  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.

• Herde, Vicki L.; Chamberlin, Phillip C.; Schmit, Don; Daw, Adrian; Milligan, Ryan O. & Polito, Vanessa [Show all 20 contributors for this article] (2024). The Solar EruptioN Integral Field Spectrograph. Solar Physics. ISSN 0038-0938. 299(8). doi: 10.1007/s11207-024-02367-y. Full text in Research Archive Show summary

  The Solar eruptioN Integral Field Spectrograph (SNIFS) is a solar-gazing spectrograph scheduled to fly in the summer of 2025 on a NASA sounding rocket. Its goal is to view the solar chromosphere and transition region at a high cadence (1 s) both spatially (0.5") and spectrally (33 mÅ) viewing wavelengths around Lyman alpha (1216 Å), Si iii (1206 Å), and O v (1218 Å) to observe spicules, nanoflares, and possibly a solar flare. This time cadence will provide yet-unobserved detail about fast-changing features of the Sun. The instrument is comprised of a Gregorian-style reflecting telescope combined with a spectrograph via a specialized mirrorlet array that focuses the light from each spatial location in the image so that it may be spectrally dispersed without overlap from neighboring locations. This paper discusses the driving science, detailed instrument and subsystem design, and preintegration testing of the SNIFS instrument.

• Herde, Vicki L.; Chamberlin, Phillip C.; Schmit, Don; Bose, Souvik; Daw, Adrian & Milligan, Ryan O. [Show all 7 contributors for this article] (2023). Spicules in IRIS Mg II Observations: Automated Identification. The Astrophysical Journal (ApJ). ISSN 0004-637X. 946(2). doi: 10.3847/1538-4357/acbe3f. Full text in Research Archive Show summary

  Abstract We have developed an algorithm to identify solar spicules in the first ever systematic survey of on-disk spicules exclusively using Mg ii spectral observations. Using this algorithm we identify 2021 events in three Interface Region Imaging Spectrograph (IRIS) data sets with unique solar feature targets spanning a total of 300 minutes: (1) active region, (2) decayed active region/active network, and (3) coronal hole. We present event statistics and relate occurrence rates to the underlying photospheric magnetic field strength. This method identifies spicule event densities and occurrence rates similar to previous studies performed using H α and Ca ii observations of active regions. Additionally, this study identifies spicule-like events at very low rates at magnetic field intensities below 20 G, and increasing significantly between 100 and 200 G in active regions and above 20 G in coronal holes, which can be used to inform future observation campaigns. This information can be be used to help characterize spicules over their full lifetimes, and compliments existing H α spectral capabilities and upcoming Ly α spectral observations with the Solar eruptioN Integral Field Spectrograph (SNIFS) sounding rocket. In total, this study presents a method for detecting solar spicules exclusively using Mg ii spectra, and provides statistics for spicule occurrences in the Mg ii h line with respect to the magnetic field strength for the purpose of predicting spicule occurrences.

• Bose, Souvik; Siverio, Daniel Elias Nóbrega; Pontieu, Bart Walter De & Voort, Luc Rouppe van der (2023). The Chromosphere Underneath a Coronal Bright Point. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb544. Full text in Research Archive Show summary

  Abstract Coronal bright points (CBPs) are sets of small-scale coronal loops, connecting opposite magnetic polarities, primarily characterized by their enhanced extreme-ultraviolet (EUV) and X-ray emission. Being ubiquitous, they are thought to play an important role in heating the solar corona. We aim at characterizing the barely explored chromosphere underneath CBPs, focusing on the related spicular activity and on the effects of small-scale magnetic flux emergence on CBPs. We used high-resolution observations of a CBP in H β and Fe i 617.3 nm from the Swedish 1 m Solar Telescope in coordination with the Solar Dynamics Observatory. This work presents the first high-resolution observation of spicules imaged in H β . The spicules were automatically detected using advanced image processing techniques, which were applied to the Dopplergrams derived from H β . Here we report their abundant occurrence close to the CBP “footpoints” and find that the orientation of such spicules is aligned along the EUV loops, indicating that they constitute a fundamental part of the whole CBP magnetic structure. Spatiotemporal analysis across multiple channels indicates that there are coronal propagating disturbances associated with the studied spicules, producing transient EUV intensity variations of the individual CBP loops. Two small-scale flux emergence episodes appearing below the CBP were analyzed, one of them leading to quiet-Sun Ellerman bombs and enhancing the nearby spicular activity. This paper presents unique evidence of the tight coupling between the lower and upper atmosphere of a CBP, thus helping to unravel the dynamic phenomena underneath CBPs and their impact on the latter.

• Hansteen, Viggo Haraldson; Martinez-Sykora, Juan; Carlsson, Mats; Pontieu, Bart Walter De; Gošić, Milan & Bose, Souvik (2023). Numerical Simulations and Observations of Mg II in the Solar Chromosphere. The Astrophysical Journal (ApJ). ISSN 0004-637X. 944(2). doi: 10.3847/1538-4357/acb33c. Full text in Research Archive Show summary

  The Mg ii h and k lines are among the best diagnostic tools of the upper solar chromosphere. This region of the atmosphere is of particular interest, as it is the lowest region of the Sun's atmosphere where the magnetic field is dominant in the energetics and dynamics, defining its structure. While highly successful in the photosphere and lower to mid-chromosphere, numerical models have produced synthetic Mg ii lines that do not match the observations well. We present a number of large-scale models with magnetic field topologies representative of the quiet Sun, ephemeral flux regions and plage, and also models where the numerical resolution is high and where we go beyond the MHD paradigm. The results of this study show models with a much improved correspondence with IRIS observations in terms of both intensities and widths, especially underscoring the importance of chromospheric mass loading and of capturing the magnetic field topology and evolution in simulations. This comes in addition to the importance of capturing the generation of small-scale velocity fields and including nonequilibrium ionization and ion−neutral interaction effects. However, it should be noted that difficulties in achieving a good correspondence remain, especially when considering the width of Mg ii h and k lines in plage. Understanding and modeling all these effects and their relative importance is necessary in order to reproduce observed spectral features and in isolating the missing pieces necessary to fully comprehend Mg ii formation.

• Mathur, Harsh; Joshi, Jayant; Nagaraju, K.; Voort, Luc Rouppe van der & Bose, Souvik (2022). Properties of shock waves in the quiet-Sun chromosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 668. doi: 10.1051/0004-6361/202244332. Full text in Research Archive Show summary

  Context. Short-lived (100 s or less), sub-arcsec to a couple of arcsec sized features of enhanced brightenings in the narrowband images at the H 2V and K 2V positions of the Ca  II H&amp;K lines in the quiet Sun are known as bright grains. These bright grains are interpreted as manifestations of acoustic shock waves in the chromosphere. Aims. We aim to study time-varying stratified atmospheric properties, such as the temperature, line-of-sight (LOS) velocity, and microturbulence inferred from observations of the bright grains during such acoustic shock events. Methods. With simultaneous observations of a quiet-Sun internetwork region in the Fe  I 6173 Å, Ca  II 8542 Å, 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. Results. The Ca  II K profiles of bright grains show enhancement in the K 2V peak intensities with the absence of the K 2R features. At the time of maximum enhancement in the K 2V peak intensities, we found average enhancements in temperature at lower chromospheric layers (at log  τ 500 ≃ −4.2) of about 1.1 kK, with a maximum enhancement of ∼4.5 kK. These temperature enhancements are co-located with upflows, as strong as −6 km s −1 , in the direction of the LOS. The LOS velocities at upper chromospheric layers at log  τ 500 &lt; −4.2 show consistent downflows greater than +8 km s −1 . The retrieved value of microturbulence in the atmosphere of bright grains is negligible at chromospheric layers. Conclusions. This study provides observational evidence to support the interpretation that the bright grains observed in narrowband images at the H 2V and K 2V positions of the Ca  II H&amp;K lines are manifestations of upward propagating acoustic shocks against a background of downflowing atmospheres.

• Salvatelli, Valentina; Santos, Luiz F. G. Dos; Bose, Souvik; Neuberg, Brad; Cheung, Mark C. M. & Janvier, Miho [Show all 9 contributors for this article] (2022). Exploring the Limits of Synthetic Creation of Solar EUV Images via Image-to-image Translation. The Astrophysical Journal (ApJ). ISSN 0004-637X. 937(2). doi: 10.3847/1538-4357/ac867b. Full text in Research Archive Show summary

  Abstract The Solar Dynamics Observatory (SDO), a NASA multispectral decade-long mission that has been daily producing terabytes of observational data from the Sun, has been recently used as a use case to demonstrate the potential of machine-learning methodologies and to pave the way for future deep space mission planning. In particular, the idea of using image-to-image translation to virtually produce extreme ultraviolet channels has been proposed in several recent studies, as a way to both enhance missions with fewer available channels and to alleviate the challenges due to the low downlink rate in deep space. This paper investigates the potential and the limitations of such a deep learning approach by focusing on the permutation of four channels and an encoder–decoder based architecture, with particular attention to how morphological traits and brightness of the solar surface affect the neural network predictions. In this work we want to answer the question: can synthetic images of the solar corona produced via image-to-image translation be used for scientific studies of the Sun? The analysis highlights that the neural network produces high-quality images over 3 orders of magnitude in count rate (pixel intensity) and can generally reproduce the covariance across channels within a 1% error. However, the model performance drastically diminishes in correspondence to extremely high energetic events like flares, and we argue that the reason is related to the rareness of such events posing a challenge to model training.

• Santos, Luiz F.G. Dos; Bose, Souvik; Salvatelli, Valentina; Neuberg, Brad; Cheung, Mark C. M. & Janvier, Miho [Show all 10 contributors for this article] (2021). Multichannel autocalibration for the Atmospheric Imaging Assembly using machine learning. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 648. doi: 10.1051/0004-6361/202040051. Full text in Research Archive Show summary

  Context. Solar activity plays a quintessential role in affecting the interplanetary medium and space weather around Earth. Remote-sensing instruments on board heliophysics space missions provide a pool of information about solar activity by measuring the solar magnetic field and the emission of light from the multilayered, multithermal, and dynamic solar atmosphere. Extreme-UV (EUV) wavelength observations from space help in understanding the subtleties of the outer layers of the Sun, that is, the chromosphere and the corona. Unfortunately, instruments such as the Atmospheric Imaging Assembly (AIA) on board the NASA Solar Dynamics Observatory (SDO), suffer from time-dependent degradation that reduces their sensitivity. The current best calibration techniques rely on flights of sounding rockets to maintain absolute calibration. These flights are infrequent, complex, and limited to a single vantage point, however. Aims. We aim to develop a novel method based on machine learning (ML) that exploits spatial patterns on the solar surface across multiwavelength observations to autocalibrate the instrument degradation. Methods. We established two convolutional neural network (CNN) architectures that take either single-channel or multichannel input and trained the models using the SDOML dataset. The dataset was further augmented by randomly degrading images at each epoch, with the training dataset spanning nonoverlapping months with the test dataset. We also developed a non-ML baseline model to assess the gain of the CNN models. With the best trained models, we reconstructed the AIA multichannel degradation curves of 2010–2020 and compared them with the degradation curves based on sounding-rocket data. Results. Our results indicate that the CNN-based models significantly outperform the non-ML baseline model in calibrating instrument degradation. Moreover, multichannel CNN outperforms the single-channel CNN, which suggests that cross-channel relations between different EUV channels are important to recover the degradation profiles. The CNN-based models reproduce the degradation corrections derived from the sounding-rocket cross-calibration measurements within the experimental measurement uncertainty, indicating that it performs equally well as current techniques. Conclusions. Our approach establishes the framework for a novel technique based on CNNs to calibrate EUV instruments. We envision that this technique can be adapted to other imaging or spectral instruments operating at other wavelengths.

• Bose, Souvik; Joshi, Jayant; Henriques, Vasco Manuel de Jorge & Voort, Luc Rouppe van der (2021). Spicules and downflows in the solar chromosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 647. doi: 10.1051/0004-6361/202040014. Full text in Research Archive Show summary

  Context. High-speed downflows have been observed in the solar transition region (TR) and lower corona for many decades. Despite their abundance, it has been hard to find signatures of such downflows in the solar chromosphere. Aims. In this work, we target an enhanced network region which shows ample occurrences of rapid spicular downflows in the H α spectral line, which could potentially be linked to high-speed TR downflowing counterparts. Methods. We used the k -means algorithm to classify the spectral profiles of on-disk spicules in H α and Ca  II K data observed from the Swedish 1 m Solar Telescope and employed an automated detection method based on advanced morphological image processing operations to detect such downflowing features, in conjunction with rapid blue-shifted and red-shifted excursions (RBEs and RREs). Results. We report the existence of a new category of RREs (termed as downflowing RRE) for the first time that, contrary to earlier interpretation, are associated with chromospheric field aligned downflows moving toward the strong magnetic field regions. Statistical analysis performed on nearly 20 000 RBEs and 15 000 RREs (including the downflowing counterparts), which were detected in our 97 min long dataset, shows that the downflowing RREs are very similar to RBEs and RREs except for their oppositely directed plane-of-sky motion. Furthermore, we also find that RBEs, RREs, and downflowing RREs can be represented by a wide range of spectral profiles with varying Doppler offsets, and H α line core widths, both along and perpendicular to the spicule axis, that causes them to be associated with multiple substructures which evolve together. Conclusions. We speculate that these rapid plasma downflows could well be the chromospheric counterparts of the commonly observed TR downflows.

• Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge & Bose, Souvik (2021). Signatures of ubiquitous magnetic reconnection in the deep atmosphere of sunspot penumbrae. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 648. doi: 10.1051/0004-6361/202040171. Full text in Research Archive Show summary

  Context. Ellerman bombs are regions with enhanced Balmer line wing emission and mark magnetic reconnection in the deep solar atmosphere in active regions and the quiet Sun. They are often found in regions where opposite magnetic polarities are in close proximity. Recent high-resolution observations suggest that Ellerman bombs are more prevalent than previously thought. Aims. We aim to determine the occurrence of Ellerman bombs in the penumbra of sunspots. Methods. We analyzed high spatial resolution observations of sunspots in the Balmer H α and H β lines as well as auxiliary continuum channels obtained with the Swedish 1-m Solar Telescope and applied the k -means clustering technique to systematically detect and characterize Ellerman Bombs. Results. Features with all the defining characteristics of Ellerman bombs are found in large numbers over the entire penumbra. The true prevalence of these events is only fully appreciated in the H β line due to the highest spatial resolution and lower chromospheric opacity. We find that the penumbra hosts some of the highest Ellerman bomb densities, surpassed only by the moat in the immediate surroundings of the sunspot. Some penumbral Ellerman bombs show flame morphology and rapid dynamical evolution. Many penumbral Ellerman bombs are fast moving with typical speed of 3.7 km s −1 and sometimes more than 10 km s −1 . Many penumbral Ellerman bombs migrate from the inner to the outer penumbra over hundreds of km, and some continue moving beyond the outer penumbral boundary into the moat. Many penumbral Ellerman bombs are found in the vicinity of regions with opposite magnetic polarity. Conclusions. We conclude that reconnection is a near continuous process in the low atmosphere of the penumbra of sunspots that manifest in the form of penumbral Ellerman bombs. These are so prevalent that they may be a major sink of sunspot magnetic energy.

• Bose, Souvik; Voort, Luc Rouppe van der; Joshi, Jayant; Henriques, Vasco Manuel de Jorge; Siverio, Daniel Elias Nóbrega & Martinez-Sykora, Juan [Show all 7 contributors for this article] (2021). Evidence of the multi-thermal nature of spicular downflows. Impact on solar atmospheric heating. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 654(51). doi: 10.1051/0004-6361/202141404. Full text in Research Archive Show summary

  Context. Spectroscopic observations of the emission lines formed in the solar transition region commonly show persistent downflows on the order of 10−15 km s −1 . The cause of such downflows, however, is still not fully clear and has remained a matter of debate. Aims. We aim to understand the cause of such downflows by studying the coronal and transition region responses to the recently reported chromospheric downflowing rapid redshifted excursions (RREs) and their impact on the heating of the solar atmosphere. Methods. We have used two sets of coordinated data from the Swedish 1 m Solar Telescope, the Interface Region Imaging Spectrograph, and the Solar Dynamics Observatory for analyzing the response of the downflowing RREs in the transition region and corona. To provide theoretical support, we use an already existing 2.5D magnetohydrodynamic simulation of spicules performed with the Bifrost code. Results. 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 the hotter coronal channels. These downflowing features are thought to be likely associated with the returning components of the previously heated spicular plasma. Furthermore, the transition region Doppler shifts associated with them are close to the average redshifts observed in this region, which further implies that these flows could (partly) be responsible for the persistent downflows observed in the transition region. We also propose two mechanisms – (i) a typical upflow followed by a downflow and (ii) downflows along a loop –from the perspective of a 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. Conclusions. We present evidence that suggests that at least some of the downflowing RREs are the chromospheric counterparts of the transition region and lower coronal downflows.

• Voort, Luc Rouppe van der; Pontieu, Bart Walter De; Carlsson, Mats; Rodriguez, Jaime de la Cruz; Bose, Souvik & Chintzoglou, Georgios [Show all 33 contributors for this article] (2020). High-resolution observations of the solar photosphere, chromosphere, and transition region: A database of coordinated IRIS and SST observations. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 641. doi: 10.1051/0004-6361/202038732. Full text in Research Archive Show summary

  NASA’s Interface Region Imaging Spectrograph (IRIS) provides high-resolution observations of the solar atmosphere through ultraviolet spectroscopy and imaging. Since the launch of IRIS in June 2013, we have conducted systematic observation campaigns in coordination with the Swedish 1 m Solar Telescope (SST) on La Palma. The SST provides complementary high-resolution observations of the photosphere and chromosphere. The SST observations include spectropolarimetric imaging in photospheric Fe  I lines and spectrally resolved imaging in the chromospheric Ca  II 8542 Å, H α , and Ca  II K lines. We present a database of co-aligned IRIS and SST datasets that is open for analysis to the scientific community. The database covers a variety of targets including active regions, sunspots, plages, the quiet Sun, and coronal holes.

• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2019). Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 631. doi: 10.1051/0004-6361/201936617. Full text in Research Archive Show summary

  We characterize, for the first time, type-II spicules in Ca II K 3934 Å using the CHROMIS instrument at the Swedish 1 m Solar Telescope. We find that their line formation is dominated by opacity shifts with the K3 minimum best representing the velocity of the spicules. The K2 features are either suppressed by the Doppler-shifted K3 or enhanced via increased contribution from the lower layers, leading to strongly enhanced but unshifted K2 peaks, with widening towards the line core as consistent with upper-layer opacity removal via Doppler-shift. We identify spicule spectra in concurrent IRIS Mg II k 2796Å observations with very similar properties. Using our interpretation of spicule chromospheric line formation, we produce synthetic profiles that match observations.

• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Voort, Luc Rouppe van der & Pereira, Tiago Mendes Domingos (2019). Semi-empirical model atmospheres for the chromosphere of the sunspot penumbra and umbral flashes. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 627. doi: 10.1051/0004-6361/201935289. Full text in Research Archive Show summary

  Context: The solar chromosphere and the lower transition region are believed to play a crucial role in the heating of the solar corona. Models that describe the chromosphere (and the lower transition region), accounting for its highly dynamic and structured character are, so far, found to be lacking. This is partly due to the breakdown of complete frequency redistribution (CRD) in the chromospheric layers and also because of the difficulty in obtaining complete sets of observations that adequately constrain the solar atmosphere at all relevant heights. Aims: We aim to obtain semi-empirical model atmospheres that reproduce the features of the Mg II h&k line profiles that sample the middle chromosphere with focus on a sunspot. Methods: We used spectropolarimetric observations of the Ca II 8542 Å spectra obtained with the Swedish 1 m Solar Telescope and used NICOLE inversions to obtain semi-empirical model atmospheres for different features in and around a sunspot. These were used to synthesize Mg II h&k spectra using the RH1.5D code, which we compared with observations taken with the Interface Region Imaging Spectrograph (IRIS). Results: Comparison of the synthetic profiles with IRIS observations reveals that there are several areas, especially in the penumbra of the sunspot, where most of the observed Mg II h&k profiles are very well reproduced. In addition, we find that supersonic hot down-flows, present in our collection of models in the umbra, lead to synthetic profiles that agree well with the IRIS Mg II h&k profiles, with the exception of the line core. Conclusions: We put forward and make available four semi-empirical model atmospheres. Two for the penumbra, reflecting the range of temperatures obtained for the chromosphere, one for umbral flashes, and a model representative of the quiet surroundings of a sunspot.

• Bose, Souvik & Nagaraju, K (2018). Role of the background regimes towards the Solar Mean Magnetic Field (SMMF). Proceedings of the International Astronomical Union. ISSN 1743-9213. 13, p. 85–86. doi: 10.1017/S1743921318001758. Full text in Research Archive
• Bose, Souvik & Nagaraju, K (2018). On the Variability of the Solar Mean Magnetic Field: Contributions from Various Magnetic Features on the Surface of the Sun. The Astrophysical Journal (ApJ). ISSN 0004-637X. 862(1). doi: 10.3847/1538-4357/aaccf1. Full text in Research Archive Show summary

  The solar mean magnetic field (SMMF) is referred to as the disk-averaged line-of-sight (LOS) magnetic field that also reflects the polarity imbalance of the magnetic field on the Sun. The origin of the SMMF has been debated over the past few decades, with one school of thought suggesting that the contribution to the SMMF is mostly due to the large-scale magnetic field structure, also called the background magnetic field, whereas other and more recent studies have indicated that active regions have a major contribution to the observed SMMF. In this paper, we re-investigate the issue of the origin of the SMMF by decomposing the solar disk into plages, networks, sunspots, and background regions, thereby calculating the variation in the observed SMMF due to each of these features. We have used full-disk images from Solar Dynamics Observatory (SDO)/AIA recorded at 1600 Å for earmarking plages, networks, and background regions and 4500 Å images for separating the sunspots. The LOS fields corresponding to each of these regions are estimated from the co-temporal SDO/Helioseismic and Magnetic Imager full-disk magnetograms. The temporal variation of the SMMF shows a near one-to-one correspondence with that of the background field regions, suggesting that they constitute the major component of the observed SMMF. A linear regression analysis based on the coefficient of determination shows that the background field dominates and accounts for 89% of the variation in the SMMF, whereas the magnetic field from the other features accounts for the rest 11%.

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• Bose, Souvik (2026). Heating of the chromosphere and corona in the active regions of the Sun. Full text in Research Archive Show summary

  The solar chromosphere serves as the critical interface through which magnetoconvective energy must pass to heat the million-degree corona. While 3D radiative-MHD simulations and high-resolution observations have revealed a host of candidate heating mechanisms--ranging from waves and shocks to flux emergence and reconnection, identifying which processes dominate remains a major challenge. In this talk, I will present recent progress in sharpening this picture by confronting coordinated ground- and space-based observations with state-of-the-art numerical models. Our results demonstrate that heating in active-region plage is tightly coupled across atmospheric layers down to scales of a few hundred kilometers, driven by the dissipation of currents from magnetic braiding. Crucially, our recent work pinpoints the locus of this energy release: it occurs not within the strong-field cores of magnetic flux tubes, but along their weaker-field edges. This strongly implicates magnetic geometry, rather than field strength alone, as the governing factor in atmospheric heating. I will conclude by discussing how upcoming capabilities with DKIST, SOLAR-C, and MUSE will allow us to put these geometric heating models decisively to the test.

• Bose, Souvik (2026). Magnetic edges dominate heating in the solar atmosphere. Full text in Research Archive Show summary

  At the interface between the Sun’s million-degree corona and its surface lies the chromosphere: cooler and far denser than the corona, yet it processes essentially all of the magnetoconvective energy that ultimately powers the outer atmosphere. How and where that energy is converted into heat remains a central open question in astrophysics. Here we combine high-resolution observations from IRIS, Hinode and SDO with a state-of-the-art 3D radiative-MHD simulation to show that the key sites of energy release in active-region plage are not the familiar strong-field cores, but their magnetic edges. Machine-learning-assisted chromospheric inversions (IRIS2+) reveal that radiative losses per unit magnetic flux sharply peak in weak-to-intermediate fields at the expanding rims of plage flux tubes, whereas kG cores are comparatively “dark”, carrying most of the flux yet contributing only a minor fraction of the losses. The simulation reproduces this rim-bright, core-dim pattern and indicates that magnetic geometry—rather than field strength alone—governs atmospheric heating.These results identify flux-tube boundaries as primary sites of energy conversion in active regions and provide a new observational constraint for models of chromospheric and coronal heating, with clear, testable predictions for upcoming facilities such as MUSE, Solar-C and next-generation ground-based observatories.

• Bose, Souvik (2025). Active region heating in the chromosphere and the corona: insights from high resolution observations and simulations. Full text in Research Archive Show summary

  At the interface between the Sun’s million-degree corona and its surface lies the chromosphere. At roughly 10,000 K it is much cooler than the corona, but also several orders of magnitude denser. Understanding what heats the corona is indirectly related to understanding the heating mechanisms in the chromosphere since it processes all the magnetoconvective energy that ultimately drives the heating of the corona. Yet many questions pertaining to the coupling of these layers remains a topic of significant research. In this talk, I will focus on recent progresses in high-resolution ground and space-based observations and state-of-the-art numerical simulations of the dynamic solar atmosphere, in particular of the active regions, in an attempt to understand how the atmosphere above the active regions is heated from tens of thousands to millions of degrees. Moreover, this talk will also highlight how the synergy between observations and modeling is key to make advances in our understanding of the chromosphere.

• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2020). Rapid Downflows in the solar chromosphere. Full text in Research Archive
• Bose, Souvik (2020). Characterization of spicules in high resolution observations and simulations. Full text in Research Archive
• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2020). Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines (Corrigendum). Astronomy and Astrophysics (A & A). ISSN 0004-6361. 637. doi: 10.1051/0004-6361/201936617e. Full text in Research Archive
• Bose, Souvik (2019). How might we expand the capabilities of NASA’s Solar Dynamics Observatory using Machine Learning? Full text in Research Archive
• Bose, Souvik; Henriques, Vasco Manuel de Jorge; Joshi, Jayant & Voort, Luc Rouppe van der (2019). Characterization and formation of Ca II K and Mg II k spectra of spicules. Full text in Research Archive
• Bose, Souvik (2019). Expanding the Capabilities of NASA's Solar Dynamics Observatory. Full text in Research Archive
• Bose, Souvik; Henriques, V. M. J.; Carlsson, Mats & Pereira, Tiago Mendes Domingos (2018). Semi-empirical model atmospheres for the upper chromosphere/lower transition region of a sunspot and its immediate surroundings. Full text in Research Archive
• Bose, Souvik (2018). Role of the background regimes towards the Solar Mean Magnetic Field (SMMF). Full text in Research Archive
• Bose, Souvik & Voort, Luc Rouppe van der (2021). On the dynamics of spicules and mass flows in the solar atmosphere. Universitetet i Oslo. Full text in Research Archive Show summary

  The atmosphere of the Sun is envisioned as composed of inherently complex, non-homogeneous, and dynamic layers. A detailed understanding of the physical processes involved in these layers is still lacking. For example, it is largely unknown why the outermost layer of the Sun’s atmosphere (corona) is so much hotter than the photosphere by millions of degrees. Astrophysicists think that the layer sandwiched between the photosphere and the corona, known as the interface region, may hold the key to a better understanding of the nature of this controversy. With the help of coordinated high-resolution ground and space-based observations from the Swedish 1-m Solar Telescope (SST) on La Palma, Spain, and NASA’s Interface Region Imaging Spectrograph (IRIS), I aim to unlock some of the mysteries surrounding the dynamics of the interface region with a focus on small-scale jets, known as “spicules”. Spicules are found almost everywhere on the Sun’s surface and at any given moment there can be as many as 10 million of them rapidly shooting outwards. Because of their “omnipresence”, it is suggested that they play a major role in energizing the outer atmospheric layers of the Sun. This thesis focuses on their physical characteristics, dynamics, and their role in the mass-balance and heating of the solar atmosphere.

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