Photospheric Flare Diagnositics

Lead Research Organisation: Queen's University Belfast
Department Name: Sch of Mathematics and Physics

Abstract

The prevailing model for flare initiation is based on an electron beam that penetrates the lower atmosphere, producing explosive evaporation and heating to high temperatures. Despite numerous studies of evaporation processes in the chromosphere and corona, the impact of the flare beams to the photospheric velocity profiles has remained unexplored. The project will combine state-of-the-art simulations with observations to address important questions on the impact of flares on solar/stellar atmospheres. This project's focus on the photosphere's response to a solar flare sets it apart from previous works which have focused on the more dramatic response of the chromosphere. At this much lower height in the solar atmosphere, the effects of a flare are greatly reduced, but the observational signatures of the processes at work can be identified in spectral lines and continua across the electromagnetic spectrum. It is therefore the purpose of this project to investigate through radiative-hydrodynamic (RHD) modelling if there may be observable shifts in line spectra formed in the photosphere. Such shifts, even if on the order of m/s, are important to establish when considering exoplanet detection around distant stars to avoid false attribution of line shifts that are actually due to stellar flares.

The main objectives of the project are:
Use the F-CHROMA grid of flare models to identify the effects of electron beams at the deepest layers of the solar atmosphere.

Expand the existing grid by including protons beams and direct heating.
Perform line synthesis to identify the photospheric lines that show the strongest response to the heating.
Compare the simulated spectra with high resolution spectroscopic observations.
Use the simulations to define the science goals of future ground-based and space-borne observatories. These observatories include the Daniel K Inouye Solar Telescope, European Solar Telescope, Solar Orbiter and others.
Other information:

QUB has been a member of F-CHROMA (www.fchroma.org), a research consortium funded by the European Commission, that focused on space-based and ground-based multi-mode, multi-wavelength studies of solar flares. The student will work in collaboration with an international team of researchers at QUB, Europe and the US.

Facilities to be used
RADYN & RH (Radiative Hydrodynamic Codes), Swedish Solar Telescope (La Palma), Daniel K Inouye Solar Telescope (Maui)

Publications

10 25 50

Studentship Projects

Project Reference Relationship Related To Start End Student Name
ST/T506369/1 30/09/2019 29/09/2023
2302544 Studentship ST/T506369/1 30/09/2019 30/03/2023 Aaron Monson
 
Description Flare-induced Doppler shifts of photospheric spectral lines have been commonly observed for decades and in recent years been a focus of increased radiative-hydrodynamic (RHD) modelling investigations. The intensity enhancement and Doppler shifting of these lines indicate significant energy transfer to the deepest regions of the solar atmosphere and the generation of a photospheric velocity field as a result of flare heating. The purpose of the work presented in this thesis was to investigate how the formation of three Fe I spectral lines was altered due to flare-accelerated non-thermal electron beams and diagnose the retrievability of accurate line-of-sight velocity information about the photospheric velocity field. This was conducted using the F-CHROMA grid of RHD flare models calculated from the RADYN code to investigate a range of electron beam parameter heating distributions. Discernable differences in the induced photospheric velocity field were found, dependent on the combination of electron beam parameters used, ranging from near-zero induced line-of-sight velocities and up to 1 km/s. The physical characteristics of these models were then used as inputs to the RH radiative transfer code to synthesise flare line profiles of the 617.3 nm, 630.1 nm, and 630.2 nm Fe I lines.

The inferred velocities from the Doppler shifts and velocity bisectors of the flare line profiles were compared against the true velocity field of the lower photosphere and evaluated if they accurately represented the lower photosphere line-of-sight velocities. The primary finding of this study was that changes to the contribution functions of the three spectral lines induced by flare heating resulted in increased emission from non-photospheric regions of the solar atmosphere, namely the lower chromosphere and in several cases chromospheric condensations. The influence of these higher regions' emission was the misleading Doppler shift of the flaring Fe I line profiles to apparent redshifts (downflows) or blueshifts (upflows) that were not representative of the true photospheric velocities. In the most extreme cases, for over 40% of several model's evolution the "observed" Doppler shifts directly contradicted the nature of the photospheric velocity field, e.g. indicating a downflow while the photosphere is purely upflowing. These periods of misleading Doppler shifts in observables were generally, but not exclusively, during the period of beam heating of a model. After the electron beam heating ended and the solar atmosphere was allowed to evolve further the observables generally showed greater agreement with the photospheric velocity field.

This analysis was expanded to investigate two more extreme cases of a high-energy flux electron beam heated solar plage initial atmosphere, and the flare from an M dwarf star. A breakdown of the total emergent intensity profiles into the three dominant components that define the line profile shapes through the use of the contribution functions revealed that photospheric velocity field information was generally irretrievable in these models. In both cases, the emission from regions higher in the atmosphere was so significant to the total profile shape that the traditional assumption that these three lines are solely photosphere-formed could provide drastically incorrect interpretations of the photospheric conditions. In particular, the stellar flare case line profiles were so dominated by emission from non-photosphere regions that the total profile appears in emission while the photospheric emitted absorption profile containing the desired line-of-sight velocity information is completely lost.
Exploitation Route Future studies investigating spectral lines formed deep in the solar and stellar atmospheres during flares should be cautious about the effects of emission from higher regions in the atmosphere giving misleading Doppler, and other, information about the state of the atmosphere.
Sectors Other

URL https://pure.qub.ac.uk/en/studentTheses/photospheric-flare-line-diagnostics-and-the-associated-velocity-f