Diagnostics of solar tornado-like prominences

Levens, Peter James (2018) Diagnostics of solar tornado-like prominences. PhD thesis, University of Glasgow.

Abstract

Solar tornado-like prominences have been observed for over 90 years, but their true nature has recently been one of the most hotly debated aspects of prominence research. They have been linked to prominence eruptions, so understanding their stability and the plasma motions seen could provide a link between these dynamic features and the Sun-Earth space weather, which is important to fully understand in modern-day society. This thesis aims to answer some of the open questions on solar ‘tornadoes’, specifically on the plasma behaviour at different temperatures and the magnetic field structure of these apparently-rotating phenomena. Using a range of spectral diagnostic techniques and data from space-based and ground-based instruments, a more complete picture of solar tornadoes is built here.
Optically thin emission at coronal temperatures (∼ 1.5 MK) has previously been shown to give anti-symmetric Doppler velocity patterns in a tornado, indicative of rotation. Using the same data set, from 14 September 2013, it has been possible to show that the Doppler pattern is visible in all spectral lines formed above 1 MK, but at lower plasma temperatures the pattern is not present. Electron densities are calculated from density-sensitive line pairs, and it is found that the electron density is lower in the tornado than the surrounding corona. Non-thermal line widths are calculated, showing that there is some additional broadening at the tornado compared to the surrounding corona, which could be due to a turbulent magnetic field component or the presence of unresolved Kelvin-Helmholtz instabilities at the tornado-corona boundary. The temperature structure along an observed line of sight is calculated using the technique of Differential Emission Measure, and this indicates that tornadoes are part of the larger prominence structure that is seen in some wavebands.
A dedicated coordinated observation was designed to study tornadoes at lower plasma temperatures (< 1 MK), and to investigate their unknown magnetic structure.
An observation of two tornadoes from 15 July 2014 is presented, from which the Mg II h and k lines and the magnetic field are analysed in detail. The optically thick Mg II lines, formed at chromospheric temperatures, show no velocity patterns similar to those seen at higher temperatures. The Mg II lines show a mix of reversed and non-reversed profiles in the prominence. This is the first report of strong central reversal of the h and k lines in a prominence. Comparing to a grid of isothermal isobaric Mg II models reveals that the large central reversals seen in the 15 July 2014 prominence indicates high optical thicknesses and pressures in the prominence slab.
The magnetic field in the tornadoes on July has been measured using spectropolarimetry of the He I D3 line, which gives the magnetic field strength and orientation. Field strengths of up to 60 G are found in places, but the average field strength is around 20 − 30 G. The inclination of the magnetic field indicates that it is horizontal, parallel to the solar surface. These observations suggest that the tornado magnetic field is not twisted, but instead horizontal with plasma suspended in dips.
An attempt has been made to find correlations between plasma parameters and the observed magnetic field parameters. No correlations are found, but this study has allowed a clearer, statistical investigation into the parameters available from this coordinated observation. These statistics are useful for comparing observations to models, in order to better understand the physical conditions that created the observed line profiles.
Finally, this thesis contains an update to a radiative transfer prominence modelling code, PROM, to include calculations of the emergent intensities of Mg II lines. This step was taken to have the ability to freely explore models for larger ranges of model parameters than presented by previous authors, with the scope to investigate more complex (2D, 3D) multi-thread models. The output of the updated code is compared to the results of another Mg ii modelling code, finding good agreement in the recovered optical thickness, but integrated intensities are found to vary by 30−40% for some models. An extended grid of isothermal isobaric and PCTR models is then explored in order to understand the links between observable Mg II h and k line parameters and model parameters. A number of correlations are found, meaning that observed Mg II h and k lines can be used to identify physical parameters in a prominence. These models are compared to observations from 15 July 2014, finding that they can explain some of the observed line profiles, but more complex models are required to fully explain the observations.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Solar physics, solar prominences, the Sun, spectral diagnostics, observational astrophysics, radiative transfer.
• Subjects: Q Science > QB Astronomy; Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Labrosse, Dr. Nicolas and Fletcher, Professor Lyndsay
• Date of Award: 2018
• Depositing User: Mr Peter James Levens
• Unique ID: glathesis:2018-8684
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 22 Jan 2018 10:26
• Last Modified: 30 Jan 2018 09:20
• URI: https://theses.gla.ac.uk/id/eprint/8684