Daniel Elias Nóbrega Siverio

• Bura, Annu; Siverio, Daniel Elias Nóbrega; Samanta, Tanmoy & Joshi, Jayant (2026). On the Origin of Coronal Picoflare Jets. The Astrophysical Journal (ApJ). ISSN 0004-637X. 1000(1). doi: 10.3847/1538-4357/ae48e6. Full text in Research Archive Show summary

  Small-scale jet-like eruptions, such as picoflare jets and jetlets, are recognized as potential contributors to coronal heating and solar wind acceleration, yet their physical origin is still not fully established. Using ultra-high-resolution extreme ultraviolet imaging datasets from the Extreme Ultraviolet Imager on board the Solar Orbiter mission, we investigate tiny coronal jets observed off-limb in the Sun’s polar regions. Visual inspection reveals that the majority of these jets exhibit distinct morphological features, including a bright spire accompanied by a dark, eruptive jet component. We analyzed 11 of these jets in detail and found that their spatial and temporal scales are comparable to previously reported jetlets, while their kinetic energies are two to three orders of magnitude lower, placing them in the picoflare regime. The bright and dark components show distinct dynamics, with the dark structures generally displaying lower speeds. A comparison with coordinated Interface Region Imaging Spectrograph data and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory data, together with 2.5D radiative-MHD simulations performed with the Bifrost code, reveals a one-to-one morphological correspondence between the dark counterparts and cool chromospheric surges accompanying the bright jet spire. This association suggests that flux emergence and magnetic reconnection at low atmospheric heights may produce coupled bright–dark structures, providing a plausible mechanism for the generation of picoflare jets. Our results demonstrate Solar Orbiter’s ability to resolve the dynamics of small-scale jets and place new constraints on their origin.

• 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. Full text in Research Archive Show summary

  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.

• 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.

• 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.

• Siverio, Daniel Elias Nóbrega; Moreno-Insertis, F.; Galsgaard, K.; Krikova, Kilian; Voort, Luc Rouppe van der & Joshi, Reetika [Show all 7 contributors for this article] (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. Full text in Research Archive Show summary

  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.

• Færder, Øystein Håvard; Siverio, Daniel Elias Nóbrega & Carlsson, Mats (2023). A comparative study of resistivity models for simulations of magnetic reconnection in the solar atmosphere. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 675. doi: 10.1051/0004-6361/202346447. Full text in Research Archive Show summary

  Context. Magnetic reconnection is a fundamental mechanism in astrophysics. A common challenge in mimicking this process numerically in particular for the Sun is that the solar electrical resistivity is small compared to the diffusive effects caused by the discrete nature of codes. Aims. We aim to study different anomalous resistivity models and their respective effects on simulations related to magnetic reconnection in the Sun. Methods. We used the Bifrost code to perform a 2D numerical reconnection experiment in the corona that is driven by converging opposite polarities at the solar surface. This experiment was run with three different commonly used resistivity models: 1) the hyper-diffusion model originally implemented in Bifrost, 2) a resistivity proportional to the current density, and 3) a resistivity proportional to the square of the electron drift velocity. The study was complemented with a 1D experiment of a Harris current sheet with the same resistivity models. Results. The 2D experiment shows that the three resistivity models are capable of producing results in satisfactory agreement with each other in terms of the current sheet length, inflow velocity, and Poynting influx. Even though Petschek-like reconnection occurred with the current density-proportional resistivity while the other two cases mainly followed plasmoid-mediated reconnection, the large-scale evolution of thermodynamical quantities such as temperature and density are quite similar between the three cases. For the 1D experiment, some recalibration of the diffusion parameters is needed to obtain comparable results. Specifically the hyper-diffusion and the drift velocity-dependent resistivity model needed only minor adjustments, while the current density-proportional model needed a rescaling of several orders of magnitude. Conclusions. The Bifrost hyper-diffusion model is as suitable for simulations of magnetic reconnection as other common resistivity models and has the advantage of being applicable to any region in the solar atmosphere without the need for significant recalibration.

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• Siverio, Daniel Elias Nóbrega (2023). Modeling Coronal Bright Points. Full text in Research Archive
• Siverio, Daniel Elias Nóbrega (2023). Unraveling the heating of Coronal Bright Points: a fundamental block in the solar atmosphere. Full text in Research Archive
• Siverio, Daniel Elias Nóbrega (2023). An observational study of magnetic flux emergence leading to a dark bubble, EB, surge, and a UV burst reaching 1 MK. Full text in Research Archive
• Siverio, Daniel Elias Nóbrega (2023). An overview of numerical experiments about ephemeral magnetic flux emergence: comparison with observations. Full text in Research Archive

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