PhD Defense: 3D numerical simulations of oscillations in solar prominences and gravitational instability in threads.

The 14th of July 2021 at 11.00 Andres Adrover will defend his PhD thesis in the Gaspar Melchor de Jovellanos Videoconference room : EDAVCJO1. You can also attend the defense through this link.

Title: 3D numerical simulations of oscillations in solar prominences and gravitational instability in threads.
Author: Andrés Adrover 
Director: Jaume Terradas
Summary:
Solar promineces are plasma structures that can raise up to 100 Mm above the solar surface. Prominences are about 100-fold denser and cooler than the surrounding solar corona and they are seen as bright cloud-like structures beyond the solar limb or as dark filamentous bodies laying on the solar disk. To be suspended in the tenuous corona, prominences are supported against the gravity force mainly by magnetic fields. These spectacular structures have awaken the curiosity of many observers and theorists who carried out a tremendous progress in understanding the highly dynamic nature of prominences. However, several outstanding issues still require an answer.

By means of numerical simulations we carried out a study of vertical, transverse and longitudinal oscillations by solving the ideal magnetohydrodynamic (MHD) equations for a wide range of parameters. We studied the periodicity and attenuation of the induced oscillations for a curtain-shaped model of a prominence initially permeated by an unsheared magnetic arcade with dips.

We extended the simulations of our initial prominence model by including a strong shear in the magnetic arcade. The redistribution of the magnetic dips due to shear makes the prominence unstable to displacements along the magnetic field lines. We investigated other types of magnetic structures but we found that might be unstable. For this reason, we decided to investigate the gravitational instability of prominences using a basic model.

We analysed the stability of individual plasma threads in a very simple configuration. First, we considered a circular magnetic flux tube where no magnetic dips exist and only gas pressure gradient provides the restoring force against gravity. Finally, the effect of magnetic dips at the curved magnetic structure is incorporated into the analytical expressions.

It is shown that longitudinal oscillations can be fit with the pendulum model, whose restoring force is the field-aligned component of gravity, but other mechanisms such as pressure gradients may contribute to the movement. On the other hand, transverse oscillations are mostly subject to magnetic forces. The attenuation of transverse oscillations was investigated by analysing the velocity distribution and computing the Alfvén continuum modes. We conclude that resonant absorption is the main cause.

In the study of gravitational instability, we derived analytical expressions for the different feasible equilibria and the corresponding frequencies of oscillation. It is found that prominences may have diverse stable or unstable equilibrium states subject to the initial position of the thread, its density contrast and length, and the total length of the magnetic field lines. The transition between the two types of solutions is produced at specific bifurcation points that have been determined analytically in some particular cases.

Finally, the effect of magnetic dips at the curved magnetic structure is incorporated into the analytical expressions. In a well formed prominence where the cold plasma is hosted by the magnetic dip, the system develops a stable solution at the bottom of the dip plus two pairs of stable/unstable fixed points at the lateral edges of the tube. In this sense, two qualitative different stable states coexist, namely the central solution, stable to relatively small perturbations, and the most external lateral fixed points. On the other hand, prominences initially located around short and shallow dips develop a central unstable equilibrium point that makes the prominence to fall down when it is laterally displaced.