Presentasjon av masteroppgave: Helle Merethe Bakke
Acceleration of High-Energy Electrons by Magnetic Reconnection in Coronal Nanoflares
The heating of the solar corona is an open issue in solar physics. It is established that processes related to the magnetic field contributes to the energetics of the corona, but the details of the physical mechanism is still debated. In this thesis we investigate a heating mechanism by acceleration of high-energy electrons by magnetic reconnection in coronal nanoflares. Because nanoflares are difficult to observe directly, we approach the problem using numerical simulations. We present an analytical method of finding magnetic reconnection sites by studying magnetohydrodynamic descriptions and magnetic field topology. The developed method is implemented into a module created for the numerical MHD code Bifrost. The module locates electron acceleration sites based on the calculation of reconnection sites. The electron energies are assumed to be nonthermal, and can not be described by a Maxwell-Boltzmann energy distribution. We present a power-law distribution as a proxy for the spectrum of non-thermal energies, where the inital energy is calculated by considering the total distribution with both thermal and non-thermal components. We compare two Bifrost simulations, with and without the effect of accelerated electrons. Vertical velocity variations at locations of elevated electron beam heating are investigated, where we find evidence of chromospheric evaporation. This finding is further supported by line asymmetries in the Mg II h&k synthetic spectra, investigated due to their formation in the chromopshere. In addition, we find brightenings in the line peaks, indicating heating of the chromospheric plasma. The developed method of finding reconnection sites is sufficient to describe locations of electron acceleration, and the beam heating simulation suggests evidence of heating signatures in the chromosphere.
Veileder: Førsteamanuensis Boris Vilhelm Gudiksen, Institutt for teoretisk astrofysikk, UiO
Intern sensor: Professor Øystein Elgarøy, Institutt for teoretisk astrofysikk, UiO
Ekstern sensor: Øivind Wikstøl, Forsvaret