Observations and modelling of the chromosphere during solar flares

Kerr, Graham Stewart (2017) Observations and modelling of the chromosphere during solar flares. PhD thesis, University of Glasgow.

Abstract

Solar flares release an enormous amount of energy (up to 10^32 erg) which is transported through the Sun`s atmosphere until it is deposited in the chromosphere, resulting in a broadband enhancements to the solar radiative output. The bulk of the flare radiative output originates from the chromosphere. Despite the importance of the chromosphere we do not yet have a comprehensive understanding of the radiation produced there following flare energy deposition, and the diagnostic potential of radiation from this layer of the atmosphere has not been fully exploited. Additionally, there is evidence that the standard model of flare energy transport via non-thermal electron beams might not be the complete scenario. Chromospheric radiation will be crucial in discriminating between the standard model and alternative energy transport mechanisms. Through near-UV spectroscopy, optical imaging, and radiation hydrodynamic modelling using both the electron beam model and energy transport via Alfven waves, the chromospheric response to flare energy input was investigated.

One of the first detailed analyses of the response of the Mg II h & k spectral lines to flare energy input is presented. These are strong, optically thick, lines formed in multiple locations of the chromosphere. In addition to showing a strong intensity enhancement, the lines were redshifted, showed a blue wing asymmetry in the most intense sources, and were substantially broadened. The lines were also single peaked during the flare, in contrast to their double peaked, centrally-reversed structure in the non-flaring Sun. Despite this, the analysis suggested they remained optically thick during the flare.

Using snapshots from radiation hydrodynamic flare simulations in combination with a radiation transfer code capable of modelling partial redistribution effects, the Mg II h & k line formation properties during flares were analysed. These simulations showed the same qualitative behaviour as observations, but instead of being single peaked they contained a shallow central reversal. Additionally the lines were too narrow, suggesting the lower chromosphere was too cool in the simulations. Line core Doppler shifts were well-correlated with atmospheric velocity. The lines were formed lower than in the quiet Sun, with source functions (and therefore emergent intensities) that were more strongly coupled to the Planck function during the flare - that is, they reflected the local conditions to a greater degree. While the lines did indeed remain optically thick during the flare, some optically thin contributions resulted in asymmetries. However, the strongest blue wing asymmetries were the result of a stationary component to the line profile when the line core was redshifted.

Optical continuum enhancements are amongst the strongest emission during solar flares, though are relatively rare to observe. Understanding the emission mechanism responsible is important for models of flare energy transport, but there remains debate as to the dominant mechanism. This emission may originate from the heated photosphere, or from an overionised region of the chromosphere. Imaging in three optical passbands during a strong flare was used to analyse the temperature enhancement and luminosity of optical sources were under the assumption of two simple models. This was in an effort to determine the most likely emission mechanism. The models were a photospheric (blackbody) model and a chromospheric model with enhanced recombination radiation. Observations were most consistent with the photospheric origin, although some evidence that both mechanisms play a role is discussed. Additionally, initial analysis of observations of a flare in which both the optical continuum and near-UV continuum were observed is presented.

Finally, a radiation hydrodynamic numerical model was adapted to include flare energy transport via the dissipation of Alfven waves. Some representative simulations surveying the parameter space are discussed. Additionally, a detailed comparison is presented between a simulation using the standard model of energy transport via non-thermal electron beams, and a simulation using Alfv\'en wave dissipation. Both the hydrodynamic response is compared, as well as the radiative response of the Ca II 8542 and Mg II k-line. It was found that Alfven waves are able to sufficiently heat the chromosphere during flares, making them a viable candidate for energy transport, and that there is the potential for discriminating between energy transport models using observations of chromospheric radiation.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Solar physics, solar flares, solar chromosphere, astrophysics, radiation transfer, radiation hydrodynamics.
• Subjects: Q Science > Q Science (General); Q Science > QB Astronomy; Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Supervisor's Name: Fletcher, Professor Lyndsay
• Date of Award: 2017
• Depositing User: Mr Graham S. Kerr
• Unique ID: glathesis:2017-7895
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 26 Jan 2017 16:20
• Last Modified: 24 Feb 2017 09:17
• URI: https://theses.gla.ac.uk/id/eprint/7895