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• Fuglestad, Anders Nødland; Marisaldi, Martino; Sarria, D.; Mezentsev, Andrey; Østgaard, Nikolai & Engeland, I. B. [Vis alle 16 forfattere av denne artikkelen] (2026). The Source Brightness Distribution of Terrestrial Gamma‐Ray Flashes From the ALOFT Flight Campaign. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 131(6). doi: 10.1029/2025jd045235. Fulltekst i vitenarkiv Vis sammendrag

  Terrestrial Gamma‐ray Flashes (TGFs) are naturally occurring phenomena that consist of bursts of gamma radiation associated with thunderclouds. Measurements of TGFs have been predominantly based on satellite instruments due to the significantly larger observation area compared to ground measurements. Due to atmospheric attenuation of the gamma rays between the TGF source deep in the atmosphere, there is a minimum brightness threshold TGFs must reach to be detectable from space. How many TGFs occur below this threshold and the minimum possible source brightness of a TGF is still not known. In this paper we show 43 TGFs with reliable locations detected by the Airborne Lightning Observatory for Fly's Eye Geostationary Lightning Mapper (GLM) Simulator (FEGS) and TGFs (ALOFT) flight campaign, which recorded observations using the NASA ER‐2 aircraft at an altitude of 20 km, targeting gamma‐ray glowing thunderclouds and TGFs. Monte Carlo simulations were used to estimate the source photon brightness of the TGFs. Based on the results for 37 of the 43 TGFs, it was found that the TGF source brightness distribution follows a power‐law with index α = −0.45 with min max α = (−0.29, −0.79) which hardens below 10¹⁴, >100 keV photons at source. Additionally, it was found that at least 97% of the ALOFT TGFs cannot be detected from space, and TGFs can have source brightnesses down to five orders of magnitude lower than what is detectable from space.

• Færder, Øystein Håvard; Lehtinen, Nikolai Grigorievich; Sarria, D.; Marisaldi, Martino; Østgaard, Nikolai & Bjørge-Engeland, Ingrid [Vis alle 7 forfattere av denne artikkelen] (2026). Numerical Parameter‐Space Studies of Various Types of Thundercloud Gamma‐Ray Emissions. Journal of Geophysical Research (JGR): Atmospheres. ISSN 2169-897X. 131(5). doi: 10.1029/2025jd045214. Fulltekst i vitenarkiv Vis sammendrag

  Until recently, mainly two types of hard radiation were observed in thunderstorms: intensive, short‐lived terrestrial gamma‐ray flashes (TGFs) and weak, long‐lasting gamma‐ray glows (GRGs). The Airborne Lightning Observatory for Fly's Eye GLM Simulator and TGFs (ALOFT, GLM: Geostationary Lightning Mapper) flight campaign revealed additional gamma‐ray phenomena of intermediate characteristics, including flickering gamma‐ray flashes (FGFs) consisting of multiple pulses with moderate intensity and duration. Here, we propose a model for reproducing hard radiation signals from thunderclouds, applying relativistic feedback discharge (RFD) theory and using a set of ordinary differential equations resembling a Lotka‐Volterra predator‐prey system. The model reproduces signals similar to the above‐mentioned emission types, and a deeper parameter‐space exploration reveals the following. Strong GRGs mainly result from an electric field building up slowly while remaining below the self‐sustained feedback threshold, whereas TGFs are caused by a rapid jump in the electric field well beyond that threshold. FGFs serve as a middle ground resulting from the field increasing moderately until self‐sustained feedback is barely attained. Compared with TGFs and GRGs, ALOFT's FGFs require a relatively large charge‐layer separation distance to be produced, in agreement with their current status of being exclusively observed in large tropical thunderclouds. The simulations also indicate that the minimum charge layer distance needed to reproduce those FGFs can be reduced by allowing the electric field to increase slowly toward the minimum feedback threshold before rising at moderate pace. Our results indicate that RFD could play a significant role in several hard radiation phenomena produced in thunderstorms.

• Færder, Øystein Håvard; Siverio, Daniel Elias Nóbrega & Carlsson, Mats (2024). A comparative study of resistivity models for simulations of magnetic reconnection in the solar atmosphere. II. Plasmoid formation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 683. doi: 10.1051/0004-6361/202348046. Fulltekst i vitenarkiv Vis sammendrag

  Context. Plasmoid-mediated reconnection plays a fundamental role in different solar atmospheric phenomena. Numerical reproduction of this process is therefore essential for developing robust solar models. Aims. Our goal is to assess plasmoid-mediated reconnection across various numerical resistivity models in order to investigate how plasmoid numbers and reconnection rates depend on the Lundquist number. Methods. We used the Bifrost code to drive magnetic reconnection in a 2D coronal fan-spine topology, carrying out a parametric study of several experiments with different numerical resolution and resistivity models. We employed three anomalous resistivity models: (1) the original hyper-diffusion from Bifrost, (2) a resistivity proportional to current density, and (3) a resistivity quadratically proportional to electron drift velocity. For comparisons, experiments with uniform resistivity were also run. Results. Plasmoid-mediated reconnection is obtained in most of the experiments. With uniform resistivity, increasing the resolution reveals higher plasmoid frequency with weaker scaling to the Lundquist number, obtaining 7.9–12 plasmoids per minute for S L  ∈ [1.8 × 10 4 , 2.6 × 10 5 ] with a scaling of S L 0.210 in the highest-resolution resistivity cases, transcending into Petschek reconnection in the high- S L limit (where the diffusive effects of the resistivity become small compared to the non-uniform viscosity) and Sweet-Parker reconnection in the low- S L limit. Anomalous resistivity leads to similar results even with lower resolution. The drift-velocity-dependent resistivity excellently reproduces Petschek reconnection for any Lundquist number, and similar results are seen with resistivity proportional to current-density though with slightly lower reconnection rates and plasmoid numbers. Among the different resistivity models applied on the given numerical resolution, the hyper-diffusion model reproduced plasmoid characteristics in closest resemblance to those obtained with uniform resistivity at a significantly higher resolution.

• Færder, Øystein Håvard; Siverio, Daniel Elias Nóbrega; Carlsson, Mats & Martinez-Sykora, Juan (2024). Extreme-ultraviolet (EUV) observables of simulated plasmoid-mediated reconnection in the solar corona. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 687. doi: 10.1051/0004-6361/202449357. Fulltekst i vitenarkiv Vis sammendrag

  Context. Understanding the role of magnetic reconnection in the heating and dynamics of the solar atmosphere requires detailed observational data of any observable aspect of the reconnection process, including small-scale features such as plasmoids. Aims. Here, we examine the capability of active and upcoming instruments to detect plasmoids generated by reconnection in the corona including low-density regimes. Methods. We used the Bifrost code to perform simulations of plasmoid-mediated reconnection in the corona with a 2D idealized setup: a fan-spine topology with uniform density including thermal conduction. Through a forward-modeling of extreme-ultraviolet (EUV) observables, we checked whether our simulated plasmoids could be detected with the instruments of Solar Dynamics Observatory (SDO) and Solar Orbiter (SO), as well as the upcoming Multi-Slit Solar Explorer (MUSE) and Solar-C missions. Results. Short-lived (∼10 − 20 s) small-scale (∼0.2 − 0.5 Mm) coronal plasmoids are not resolvable with the Atmospheric Imaging Assembly (AIA) on board SDO. In contrast, they could be captured with the EUV High-Resolution Imager at the Extreme Ultraviolet Imager (EUI-HRIEUV) of SO. The spatial and temporal high-resolution planned for the MUSE spectrograph (SG) is adequate to obtain full spectral information of these plasmoids. To achieve a sufficient signal-to-noise ratio (S/N) for ∼0.8 MK plasmoids in the MUSE/SG 171 Å channel, full-raster images are attainable for regions with electron densities above 109 cm−3, while sit-and-stare observations are recommended for lower-density regions. The future Solar-C mission could also capture these coronal plasmoids using the EUV High-Throughput Spectroscopic Telescope (EUVST), considering rapid changes in Doppler shift and line widths in different EUV lines caused by plasmoid motions along the current sheet. Conclusions. With the combined spectra of MUSE/SG and Solar-C/EUVST in multiple emission lines, along with high-resolution images from SO/EUI-HRIEUV and MUSE/CI, it should be possible to gain new insights about plasmoid formation in the corona.

• 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. Fulltekst i vitenarkiv Vis sammendrag

  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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• Færder, Øystein Håvard; Lehtinen, Nikolai Grigorievich; Sarria, David Alexandre Stephan; Marisaldi, Martino & Østgaard, Nikolai (2025). A parameter-space exploration of the Relativistic Discharge Model mapping for which conditions ALOFT’s Flickering Gamma-ray Flashes are produced. doi: 10.5194/egusphere-egu25-10378. Fulltekst i vitenarkiv Vis sammendrag

  During the ALOFT flight campaign, July 2023, a novel type of multi-pulse gamma-ray emission from thunderclouds was systematically recorded. Referred to as flickering gamma-ray flashes (FGFs), this type of emission is not linked with lightning leaders and does not coincide with detectable radio emissions [1]. A promising candidate theory for explaining this phenomenon is the relativistic feedback discharge (RFD) developed by Dr. J. Dwyer and his group [2]. Fully self-consistent 3D Monte-Carlo calculations of RFD [3], which take the field quenching by produced currents into account, are quite computationally intensive. In fact, the full physics of RFD has barely been explored outside Dwyer’s group. Therefore, we developed an independent numerical model especially made to evaluate the capability of the RFD theory to reproduce FGFs. Despite its simplification into a set of spatially-independent ordinary differential equations (ODEs), it applies the most relevant physics: ionisation, electron dynamics, attachment processes, relativistic runaway electron avalanche (RREA), and feedback akin to Dwyer’s theory. The ODEs that we end up solving are analogous to a complexified Lotka-Volterra model which describes a system with oscillations. In this presentation, we introduce a 0.5D model (i.e., with indirect account of the spatial size of the RREA region) and demonstrate its ability to reproduce emission light curves very alike FGFs under realistic conditions, given the right set of parameters (see below). Furthermore, we show that the same model also reproduces light curves alike terrestrial gamma-ray flashes (TGFs, both single and multiple pulses) and gamma-ray glows (GRGs) for different sets of parameters but still under realistic conditions, hence proving this model to be even more general than originally intended. With this, we performed a parameter-space exploration, using our model and systematically applying different values for 1) the initial (background) internal electric field strength of the cloud, 2) the characteristic growth time of the external electric field, 3) the vertical size of the high-field region in the cloud, and 4) the maximum change of the external field. The results, as shown in parameter-space diagrams, are qualitatively as expected. TGFs tend to occur for relatively small high-field regions. For larger high-field regions, the model reproduces GRGs in the case of slowly increasing external fields while FGFs and weak TGFs in the case of rapidly increasing external fields. The amplitude and the number of pulses typically scale with the maximum change of the external field. Finally, increasing the value of the initial internal electric field leads to a decrease in the minimum required change in the external field needed to reproduce FGFs, multi-pulse TGFs and GRGs.

• Færder, Øystein Håvard; Lehtinen, Nikolai Grigorievich; Sarria, David Alexandre Stephan; Marisaldi, Martino; Østgaard, Nikolai & Bjørge-Engeland, Ingrid [Vis alle 7 forfattere av denne artikkelen] (2025). Systematic reproduction of hard radiation from thunderclouds including ALOFT’s Flickering Gamma-ray Flashes using an ODE model resembling a Lotka-Volterra predator-prey system. Fulltekst i vitenarkiv Vis sammendrag

  For decades, hard radiation from thunderclouds has been divided into two clearly distinct classes: the highly intensive, short-lived terrestrial gamma-ray flashes (TGFs) and the weak, long-lasting gamma-ray glows (GRGs). The ALOFT flight campaign of July 2023 revealed additional gamma-ray phenomena of intermediate characteristics, including flickering gamma-ray flashes (FGFs) consisting of multiple pulses with moderate intensity and duration. In this presentation, we demonstrate our model for reproducing hard radiation signals from thunderclouds which applies the relativistic feedback discharge (RFD) theory, developed by Dr. Dwyer, using a set of ordinary differential equations which resemble a Lotka-Volterra predator-prey system. The model is capable of reproducing signals in resemblance of any of the above-mentioned emission types. We performed a deeper parameter-space exploration of the model, and our main findings are as follows. The GRGs are mainly related to an electric field building up slowly while remaining below the self-sustained feedback threshold, whereas the TGFs are triggered by a rapid jump in the electric field well beyond that threshold. FGFs serve as a middle ground as they result from the field being increased at moderate pace until self-sustained feedback is barely attained. Compared with TGFs and GRGs, the FGFs seen with ALOFT require a relatively large charge-layer separation distance to be produced, in agreement with their current status of being exclusively observed in large tropical thunderclouds. The simulations also indicate that the minimum charge layer distance needed to reproduce these FGFs may be reduced by letting the electric field increase slowly towards the minimum feedback threshold before rising at moderate pace. Our results indicate that RFD could play a significant role in several hard radiation phenomena produced in thunderstorms.

• Sarria, David Alexandre Stephan; Østgaard, Nikolai; Bjørge-Engeland, Ingrid; Marisaldi, Martino; Mezentsev, Andrey & Lehtinen, Nikolai Grigorievich [Vis alle 10 forfattere av denne artikkelen] (2025). Evidence of gamma-ray glows observed in the relativistic feedback regime during the ALOFT 2023 flight campaign. Fulltekst i vitenarkiv
• Færder, Øystein Håvard; Nóbrega-Siverio, Daniel & Carlsson, Mats (2023). Magnetic Reconnection Signatures in the Corona through MUSE Forward Modelling. Fulltekst i vitenarkiv
• Færder, Øystein Håvard (2023). Modelling the resistivity for magnetic reconnection on the Sun – a comparative study of anomalous resistivity models. Fulltekst i vitenarkiv
• Færder, Øystein Håvard (2022). Simulating reconnection in the Solar Corona with different resistivity models. Fulltekst i vitenarkiv
• Færder, Øystein Håvard; Carlsson, Mats; Hansteen, Viggo Haraldson & Siverio, Daniel Elias Nóbrega (2024). The role of resistivity in plasmoid-mediated reconnection in the solar corona and its observational imprints. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Magnetic reconnection is a fundamental mechanism in astrophysics and a promising candidate for solving the coronal heating problem. This process has been shown to produce a variety of different observable phenomena on the Sun, including Ellerman bombs, ultraviolet bursts, coronal bright points, surges, coronal jets, and flares. Achieving a better understanding on the role that reconnection plays in the heating and dynamics of the solar atmosphere requires observational data and numerical reproduction of any aspect related to reconnection, including small-scale features such as plasmoids. A common challenge in mimicking reconnection on the Sun numerically is how to model the electrical resistivity, since this quantity is small compared to the diffusive effects caused by the discrete nature of numerical codes. To address this challenge, we ran a series of magnetohydrodynamic (MHD) simulations using the Bifrost code with its inbuilt hyper-resistivity model along with other resistivity models, for which we developed new modules especially for our research. Our experiments proved that the hyper-resistivity model of Bifrost is at least as suitable for mimicking reconnection phenomena on the Sun as other widely used resistivity models and has the advantage of being applicable in any part of the solar atmosphere without the need for re-calibration. Furthermore, we found that when simulating plasmoid-mediated reconnection with a moderate resolution, another takeaway with the hyper-resistivity model is the capability of reproducing plasmoid characteristics in closer resemblance to those attained with uniform resistivity at a significantly higher resolution. Observing small-scale features of reconnection on the Sun, such as plasmoids, can also be demanding, due to the limited resolution provided by currently-active telescopes and spectrographs. Furthermore, plasmoids in the corona are quite dim because of the low densities, hence often difficult to detect due to an underwhelming signal-to-noise ratio. Through forward-modelling of synthetic extreme-ultraviolet (EUV) observables from simulations of plasmoid-mediated reconnection, we found that combined spectra from the upcoming Multi-slit Solar Explorer (MUSE) and Solar-C missions along with high-resolution images from the MUSE context imager and the recently-launched Solar Orbiter will together contribute strongly in providing new insights into plasmoid formation on the Sun. Those insights can be used to constrain our numerical models in terms of further development of state-of-the-art resistivity models, hence lead us one step forward in unravelling the mysteries of our life-giving star.

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