Numerical simulations of chromospheric physics: sunspot resonating cavity, 2D magnetic reconnection and forward modelling

Snow, Ben (2017) Numerical simulations of chromospheric physics: sunspot resonating cavity, 2D magnetic reconnection and forward modelling. Doctoral thesis, Northumbria University.

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Abstract

There are three branches to this thesis: chromospheric resonator, forward modelling, and magnetic reconnection at different atmospheric heights.

The first branch investigates the behaviour of resonant waves in the chromosphere above sunspot umbrae. The steep temperature gradients at the solar photosphere and transition region allow waves to be partially transmitted and partially reflected creating the boundaries of a resonating cavity. These resonant waves can be used to explain the existence of three-minute oscillations frequently observed above sunspot umbrae. 1D numerical simulations have been performed of wave propagation along a magnetic field line perpendicular to the umbra to analyse the behaviour of this resonating cavity driven by continuous random noise. It was found that the gradient of the velocity spectra present in the corona is directly correlated with the size of the chromospheric cavity. This provides a potentially useful diagnostic for indirectly measuring chromospheric cavity size.

The second branch uses line-of-sight integration to compare simulation data with observations. LOS integration has been performed on two cases: 1D resonances above sunspot umbrae, and a 3D simulation of a coronal kink instability. The LOS integration of resonant waves is an extension to the first branch of this thesis. The intensity was calculated using several coronal lines. It was found that the broadband of excited frequencies is correlated with chromospheric cavity size, providing a potentially useful diagnostic for chromospheric cavity size. For the kink-unstable loop, several Hinode/EIS lines were used to investigate the observational intensity and Doppler velocities. It was found that the intensity maps overestimate the loop width due to temporal and spatial degradation. Synthetic intensity maps were also generated using spectral lines from the DKIST/DL-NIRSP instrument.

The final branch consists of a numerical investigation into the onset of 2D magnetic reconnection in the solar photosphere, chromosphere and corona. The initial state is an equilibrium Harris current sheet. A reconnection event is triggered by applying a velocity driver perpendicular to the magnetic field. This external velocity driver allows us to study the early behaviour of reconnection in a naturalistic manner. The heating manifests differently across the different atmospheric layers. The photospheric heating is guided by the slow-mode shocks. The chromospheric heating is confined to the reconnection region. The ambipolar diffusion in the chromospheric case alters the distribution of the current on the inflow region in line with theoretical models.

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