aditi

• Bhatnagar, Aditi; Prasad, Avijeet; Siverio, Daniel Elias Nóbrega; Voort, Luc Rouppe van der & Joshi, Jayant (2025). Small-scale dynamic phenomena associated with interacting fan-spine topologies: Quiet-Sun Ellerman bombs, UV brightenings, and chromospheric inverted-Y-shaped jets. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 698. doi: 10.1051/0004-6361/202453344. Fulltekst i vitenarkiv Vis sammendrag

  Context. Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower solar atmosphere. Sometimes, they exhibit transition region counterparts, known as ultraviolet (UV) brightenings. Magnetic field extrapolations suggest that QSEBs can occur at various locations of a fan-spine topology, with UV brightening occurring at the magnetic null point through a common reconnection process. Aims. We aim to understand how more complex magnetic field configurations such as interacting fan-spine topologies can cause small-scale dynamic phenomena in the lower atmosphere. Methods. QSEBs were detected using k -means clustering on H β observations from the Swedish 1-m Solar Telescope (SST). Further, chromospheric inverted-Y-shaped jets were identified in the H β blue wing. Magnetic field topologies were determined through potential field extrapolations from photospheric magnetograms derived from spectro-polarimetric observations in the Fe  I 6173 Å line. UV brightenings were detected in IRIS 1400 Å slit-jaw images. Results. We identify two distinct magnetic configurations associated with QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets. The first involves a nested fan-spine structure where, due to flux emergence, an inner 3D null forms inside the fan surface of an outer 3D null with some overlap. The QSEBs occur at two footpoints along the shared fan surface, with the UV brightening located near the outer 3D null point. The jet originates close to the two QSEBs and follows the path of high squashing factor, Q . We discuss a comparable scenario using a 2D numerical experiment with the Bifrost code. In the second case, two adjacent fan-spine topologies share fan footpoints at a common positive polarity patch, with the QSEB, along with a chromospheric inverted-Y-shaped jet, occurring at the intersection having high Q values. The width of the jets in our examples is about 0 . 3, and the height varies between 1–2. The width of the cusp measures between 1–2. Conclusions. This study demonstrates through observational and modelling support that small-scale dynamic phenomena, such as associated QSEBs, UV brightenings, and chromospheric inverted-Y-shaped jets, share a common origin driven by magnetic reconnection between interacting fan-spine topologies.

• Bhatnagar, Aditi; Prasad, Avijeet; Voort, Luc Rouppe van der; Siverio, Daniel Elias Nóbrega & Joshi, Jayant (2025). Magnetic topology of quiet-Sun Ellerman bombs and associated ultraviolet brightenings. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 693. doi: 10.1051/0004-6361/202452822. Fulltekst i vitenarkiv Vis sammendrag

  Context. Quiet-Sun Ellerman bombs (QSEBs) are small-scale magnetic reconnection events in the lower atmosphere of the quiet Sun. Recent work has shown that a small percentage of them can occur co-spatially and co-temporally with ultraviolet (UV) brightenings in the transition region. Aims. We aim to understand how the magnetic topologies associated with closely occurring QSEBs and UV brightenings can facilitate energy transport and connect these events. Methods. We used high-resolution Hβ observations from the Swedish 1-m Solar Telescope (SST) and detected QSEBs using k-means clustering. We obtained the magnetic field topology from potential field extrapolations using spectro-polarimetric data in the photospheric Fe I 6173 Å line. To detect UV brightenings, we used coordinated and co-aligned data from the Interface Region Imaging Spectrograph (IRIS) and imposed a threshold of 5σ above the median background on the (IRIS) 1400 Å slit-jaw image channel. Results. We identify four distinct magnetic configurations that associate QSEBs with UV brightenings, including a simple dipole configuration and more complex fan-spine topologies with a 3D magnetic null point. In the fan-spine topology, the UV brightenings occur near the 3D null point, while QSEBs can be found close to the footpoints of the outer spine, the inner spine, and the fan surface. The height of the 3D null varies between 0.2 Mm and 2.6 Mm, depending on the magnetic field strength in the region. Some QSEBs and UV brightenings, though occurring close to each other, are not topologically connected with the same reconnection process. The energy released during QSEBs falls in the range 1023–1024 ergs. Conclusions. This study shows that magnetic connectivity and topological features, such as 3D null points, are crucial in linking QSEBs in the lower atmosphere with UV brightenings in the transition region.

• Bhatnagar, Aditi; Voort, Luc Rouppe van der & Joshi, Jayant (2024). Transition region response to quiet-Sun Ellerman bombs. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 689. doi: 10.1051/0004-6361/202450070. Fulltekst i vitenarkiv Vis sammendrag

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

Se alle arbeider i NVA

• Bhatnagar, Aditi (2023). Chromosphere and Transition region response to Quiet Sun Ellerman Bombs. Fulltekst i vitenarkiv
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• Bhatnagar, Aditi (2021). Formation of the OI and CI lines in a flare. Fulltekst i vitenarkiv
• Bhatnagar, Aditi (2021). Formation of the IRIS OI and CI lines in a flare. Fulltekst i vitenarkiv
• Bhatnagar, Aditi & Voort, Luc Rouppe van der (2025). Spectral and topological study of small-scale energetic events in the solar atmosphere. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Small-scale magnetic reconnection in the quiet Sun contributes to the heating of the solar atmosphere, but the extent of this contribution remains unclear. In this thesis, I investigated the role of quiet-Sun Ellerman bombs (QSEBs) in heating the upper solar atmosphere using high-resolution observations from the Swedish 1-m Solar Telescope (SST) and coordinated observations from the Interface Region Imaging Spectrograph (IRIS). The SST Hβ spectral data and Fe i magnetograms were used along with IRIS Slit-Jaw Images and raster data to detect and analyse QSEBs and their transition region counterparts. The automatic detection of QSEBs using k-means clustering shows that they are abundant on the Sun, but only about 15% exhibit spatially- and temporally- correlated brightenings in ultraviolet diagnostics. This suggests that while QSEBs can locally heat the plasma to upper-atmospheric temperatures, they do not play a dominant role in the global heating of the upper atmosphere. Through potential field extrapolations of the photospheric magnetic field, we further explored three-dimensional magnetic field topologies that link QSEBs to Ultraviolet (UV) brightenings in the transition region. When these events occur in close proximity, the QSEBs are found to occur either at the polarity inversion line of a dipole or at different footpoints of a 3D null having a fan-spine structure. In the fan-spine topology, the QSEBs usually occur at the inner spine, outer spine or fan-surface footpoints based on the strength of the magnetic field at those locations. Meanwhile, the UV brightening is observed close to the 3D null point, whose height can vary based on the magnetic field strength at the inner spine. In addition to identifying QSEBs topologically connected to UV brightenings, we also found a case where a QSEB, while occurring nearby, was not connected to the UV brightening. This suggests that proximity alone is not sufficient to infer causal relationships between QSEBs and UV brightenings, and detailed topological analysis is required to link them to the same magnetic reconnection process. Building upon this analysis, we investigated more complex magnetic field configurations which involved interactions between multiple fan-spine topologies. In one scenario, a smaller fan-spine structure was found nested within a larger one, with QSEBs occurring at the footpoints of the fan-surfaces and a UV brightening near the outer 3D null. The interaction, due to flux emergence, between the emerging fan and the overlying fan surface likely caused the formation of current sheets, facilitating magnetic reconnection. In another case, a similar interaction between fan-surfaces of two adjacent 3D nulls caused the occurrence of a QSEB at their shared footpoint. In both cases, chromospheric inverted-Y-shaped jets were observed originating near the QSEBs, with the first scenario showing the jet closely following regions of high squashing factor. Comparisons with numerical experiment (using the Bifrost code) were performed to validate such a scenario where flux emergence-driven magnetic reconnection can lead to eruption of smallscale jets.

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