Radiative hydrodynamic modelling of the Lyman continuum during solar flares

Abstract

During quiescent periods on the Sun, the Lyman continuum (<911.2 Å) forms at the base of the transition region, the interface between the chromosphere and the corona. Therefore, the continuum is sensitive to chromospheric energy perturbations, particularly during solar flares, as the bulk of nonthermal energy in a flare is deposited at chromospheric altitudes. To investigate the chromospheric response to nonthermal energy injection during solar flares, synthetic Lyman continuum emissions were analysed using a grid of field-aligned radiative hydrodynamic models generated using the RADYN code. The temporal evolution of the continuum’s spectral profile, the colour temperature, 𝑇𝑐, and the NLTE departure coefficient of neutral hydrogen, 𝑏1, were investigated in response to various nonthermal electron distribution functions. The Lyman continuum intensity was found to increase by 4–5.5 orders of magnitude during solar flares, where the response of the continuum strongly depended on the flux of the nonthermal electron beam. During the solar flares, 𝑇𝑐 increased from 8–9 kK to 10–16 kK, while 𝑏1 generally decreased from 102–103 to closer to unity, indicating the LyC forms closer to LTE conditions during flares. When 𝑏1 was at a minimum, 𝑇𝑐 was found to be approximately equal to the electron temperature of the plasma. During quiescent periods, the Lyman continuum was found to be optically thick, forming at the base of the transition region, whereas, during flares, this layer shifted deeper into the solar atmosphere as the chromosphere was strongly compressed, forming close to the peak nonthermal electron heating. During flares, optically thin Lyman continuum layers formed at higher altitudes, where these layers enhance intensities near the continuum tail (<700 Å). These optically thin layers may have direct implications for SPICE observations, which provide partial coverage of the Lyman continuum over the 704–790 Å range. As solar cycle 25 continues, these findings provide a theoretical understanding of the Lyman continuum formation properties, allowing for an interpretation of current and future Lyman continuum observations.

This analysis was expanded by performing observationally motivated bespoke multithread modelling of the 15𝑡ℎ of February 2011 solar flare, comparing synthetic Lyman continuum emission to EVE observations over the flare impulsive phase. The multithread simulations overestimated the flaring Lyman continuum emission by a factor of ten, likely due to the multi-thread assumption (i.e. the emission from individual flare loops was scaled to the entire flaring area). The models failed to reproduce the structure of the observed lightcurve, where the observations peaked after the peak nonthermal electron flux, contrary to the models. While a filament eruption could not be ruled out due to the saturation of the AIA 1600 and 1700 Å channels, the models relaxed back to pre-flare conditions once beam heating ceased, while the observations maintained emissions into the flare gradual phase. This suggests there is a secondary heating source not categorised by the nonthermal electron distribution. Finally, this analysis was extended to spectral lines, finding that the ratio of Lyman beta and gamma to Lyman alpha emission over the flare impulsive phase was consistent with observations, indicating that RADYN correctly predicts the relative level populations of n=1, 2, and 3 for neutral hydrogen.

To test the effect the starting atmosphere had on the multi-thread simulations, the pre-flare starting atmosphere was changed from a VAL-C atmosphere to a radiative equilibrium (RE) atmosphere, finding that the starting atmosphere significantly altered the synthesised flaring emission. The RE atmosphere predicted that flaring continua radiate away 7–24% of the nonthermal energy, whereas the VAL-C atmosphere predicts continua radiate away 3–7 % of this energy. This is likely due to the different atmospheric density profiles of the pre-flare starting atmospheres, which alters the nonthermal electron energy deposition. Further investigation and constraints on the model starting atmosphere are required. There remains significant scope to improve upon the multi-thread modelling assumptions made in this analysis, allowing the fundamental physics of 1D field-aligned radiative hydrodynamic codes to be tested as well as understanding the formation properties of various lines and continua. Multi-thread modelling provides a next step in 1D flare modelling, where this work highlights the advantages and shortcomings of multi-thread frameworks.

Date of Award	Jul 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Science & Technology Facilities Council
Supervisor	Ryan Milligan (Supervisor) & Michail Mathioudakis (Supervisor)