larsfrog

• Frogner, Lars & Gudiksen, Boris Vilhelm (2024). Accelerated particle beams in a 3D simulation of the quiet Sun: Effects of advanced beam propagation modelling. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 683. doi: 10.1051/0004-6361/202348457. Fulltekst i vitenarkiv Vis sammendrag

  Context. Charged particles are constantly accelerated to non-thermal energies by the reconnecting magnetic field in the solar atmosphere. Our understanding of the interactions between the accelerated particles and their environment can benefit considerably from three-dimensional atmospheric simulations that account for non-thermal particle beam generation and propagation. In a previous publication, we presented the first results from such a simulation, which considers quiet Sun conditions. However, the original treatment of beam propagation ignores potentially important phenomena such as the magnetic gradient forces associated with a converging or diverging magnetic field. Aims. Here we present a more general beam propagation model incorporating magnetic gradient forces, the return current, acceleration by the ambient electric field, corrected collision rates due to the ambient temperature, and collisions with heavier elements than hydrogen and the free electrons they contribute. Neglecting collisional velocity randomisation makes the model sufficiently lightweight to simulate millions of beams. We investigate how each new physical effect in the model changes the non-thermal energy transport in a realistic three-dimensional atmosphere. Methods. We applied the method of characteristics to the steady-state continuity equation for electron flux to derive ordinary differential equations for the mean evolution of energy, pitch angle, and flux with distance. For each beam, we solved these numerically for a range of initial energies to obtain the evolving flux spectrum, from which we computed the energy deposited into the ambient plasma. Results. Magnetic gradient forces significantly influence the spatial distribution of deposited beam energy. The magnetic field converges strongly with depth in the corona above loop footpoints. This convergence leads to a small coronal peak in deposited energy followed by a heavy dip caused by the onset of magnetic mirroring. Magnetically reflected electrons carry away 5 to 10% of the injected beam energy on average. The remaining electrons are relatively energetic and produce a peak in deposited energy below the transition region a few hundred kilometres deeper than they would in a uniform magnetic field. A diverging magnetic field at the beginning of the trajectory, which is common in the simulation, enhances the subsequent impact of magnetic mirroring. The other new physical effects do not qualitatively alter the picture of non-thermal energy transport for the atmospheric conditions under consideration.

• Bakke, Helle; Frogner, Lars; Voort, Luc Rouppe van der; Gudiksen, Boris Vilhelm & Carlsson, Mats (2023). Accelerated particle beams in a 3D simulation of the quiet Sun: Lower atmospheric spectral diagnostics. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 675. doi: 10.1051/0004-6361/202346765. Fulltekst i vitenarkiv Vis sammendrag

  Context. Nanoflare heating through small-scale magnetic reconnection events is one of the prime candidates to explain heating of the solar corona. However, direct signatures of nanoflares are difficult to determine, and unambiguous observational evidence is still lacking. Numerical models that include accelerated electrons and can reproduce flaring conditions are essential in understanding how low-energetic events act as a heating mechanism of the corona, and how such events are able to produce signatures in the spectral lines that can be detected through observations. Aims. We investigate the effects of accelerated electrons in synthetic spectra from a 3D radiative magnetohydrodynamics simulation to better understand small-scale heating events and their impact on the solar atmosphere. Methods. We synthesised the chromospheric Ca  II and Mg  II lines and the transition region Si  IV resonance lines from a quiet Sun numerical simulation that includes accelerated electrons. We calculated the contribution function to the intensity to better understand how the lines are formed, and what factors are contributing to the detailed shape of the spectral profiles. Results. The synthetic spectra are highly affected by variations in temperature and vertical velocity. Beam heating exceeds conductive heating at the heights where the spectral lines form, indicating that the electrons should contribute to the heating of the lower atmosphere and hence affect the line profiles. However, we find that it is difficult to determine specific signatures from the non-thermal electrons due to the complexity of the atmospheric response to the heating in combination with the relatively low energy output (∼10 21 erg s −1 ). Even so, our results contribute to understanding small-scale heating events in the solar atmosphere, and give further guidance to future observations.

• Frogner, Lars; Gudiksen, Boris Vilhelm & Bakke, Helle (2020). Accelerated particle beams in a 3D simulation of the quiet Sun. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 643. doi: 10.1051/0004-6361/202038529. Fulltekst i vitenarkiv Vis sammendrag

  Context. Observational and theoretical evidence suggest that beams of accelerated particles are produced in flaring events of all sizes in the solar atmosphere, from X-class flares to nanoflares. Current models of these types of particles in flaring loops assume an isolated 1D atmosphere. Aims. A more realistic environment for modelling accelerated particles can be provided by 3D radiative magnetohydrodynamics codes. Here, we present a simple model for particle acceleration and propagation in the context of a 3D simulation of the quiet solar atmosphere, spanning from the convection zone to the corona. We then examine the additional transport of energy introduced by the particle beams. Methods. The locations of particle acceleration associated with magnetic reconnection were identified by detecting changes in magnetic topology. At each location, the parameters of the accelerated particle distribution were estimated from local conditions. The particle distributions were then propagated along the magnetic field, and the energy deposition due to Coulomb collisions with the ambient plasma was computed. Results. We find that particle beams originate in extended acceleration regions that are distributed across the corona. Upon reaching the transition region, they converge and produce strands of intense heating that penetrate the chromosphere. Within these strands, beam heating consistently dominates conductive heating below the bottom of the transition region. This indicates that particle beams qualitatively alter the energy transport even outside of active regions.

• Bakke, Helle; Frogner, Lars & Gudiksen, Boris Vilhelm (2018). Non-thermal electrons from solar nanoflares In a 3D radiative MHD simulation. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 620. doi: 10.1051/0004-6361/201834129. Fulltekst i vitenarkiv Vis sammendrag

  Context: We introduce a model for including accelerated particles in pure magnetohydrodynamics (MHD) simulations of the solar atmosphere. Aims: We show that the method is viable and produces results that enhance the realism of MHD simulations of the solar atmosphere. Methods: The acceleration of high-energy electrons in solar flares is an accepted fact, but is not included in the most advanced 3D simulations of the solar atmosphere. The effect of the acceleration is not known, and here we introduce a simple method to account for the ability of the accelerated electrons to move energy from the reconnection sites and into the dense transition zone and chromosphere. Results: The method was only run for a short time and with low reconnection energies, but this showed that the reconnection process itself changes, and that there is a clear effect on the observables at the impact sites of the accelerated electrons. Further work will investigate the effect on the reconnection sites and the impact sites in detail.

Se alle arbeider i NVA

• Frogner, Lars & Gudiksen, Boris Vilhelm (2023). Accelerated particles in the solar atmosphere. Fulltekst i vitenarkiv
• Frogner, Lars & Gudiksen, Boris Vilhelm (2023). Accelerated particles in the solar atmosphere. Fulltekst i vitenarkiv
• Frogner, Lars (2022). Accelerated particle beams in a 3D simulation of the quiet Sun. Fulltekst i vitenarkiv
• Frogner, Lars (2020). Accelerated particles in the solar atmosphere. Fulltekst i vitenarkiv
• Gudiksen, Boris Vilhelm; Frogner, Lars; Bakke, Helle & Clemmensen, Frederik (2020). Accelerated particles in the solar atmosphere. Fulltekst i vitenarkiv
• Gudiksen, Boris Vilhelm; Frogner, Lars & Bakke, Helle (2020). Accelerated particles in the solar atmosphere. Fulltekst i vitenarkiv
• Frogner, Lars; Gudiksen, Boris Vilhelm & Carlsson, Mats (2024). Modelling accelerated particles in the solar atmosphere. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  This thesis explores the Sun’s dynamic atmosphere, focusing on the role of accelerated particles. These particles are propelled to extremely high speeds during solar flares, which are bright, explosive events in the Sun’s atmosphere. Accelerated particle beams can contain much of the energy released during a flare. Traditionally, particle beams have been simulated individually in simplified, one-dimensional atmospheres. By including accelerated particles in an advanced three-dimensional simulation of the Sun’s atmosphere, we could simulate many particle beams in a more realistic setting. Our simulations demonstrate how the Sun’s intricate magnetic field influences how the beams emerge and evolve. These results show how important it is to consider the presence of many beams in a complex magnetic environment when trying to understand the role of accelerated particles. In addition to providing this new perspective on energy transport in the solar atmosphere, our research lays the groundwork for more accurate models in the future.

Se alle arbeider i NVA