Investigating the relationship between nonthermal electron properties and geoeffective Lyman-alpha emission in solar flares

Solar flares are intense eruptive events that involve the rapid release of large quantities of stored energy in the solar atmosphere, promoting the emission of excess radiation in multiple wavelengths including X-ray and Extreme Ultraviolet (EUV). The Lyman-alpha line of neutral hydrogen is the most intense emission line in the solar spectrum and is known to constitute a considerable portion of the total radiated energy in solar flares. Interest in the study of Lyman-alpha has increased in recent years, due to the improved availability of observations at this wavelength on flare timescales. In quiescent solar conditions, Lyman-alpha radiation is known to form and maintain the D-region of Earth's ionosphere through ionisation of constituent elements and recent studies have suggested that Lyman-alpha emitted during solar flares has a significant role in the formation of currents in the E-region. Perturbations to the ionosphere can have adverse effects on navigation and communication systems, thus understanding the radiation responsible for these effects is crucial to space weather and atmospheric research. Furthermore, in understanding the effects of excess radiation on Earth, extension can be made to other planetary atmospheres, aiding the rapidly developing search for habitable exoplanets.

It is generally accepted that accelerated nonthermal electrons colliding with the chromosphere are responsible for the emission of excess radiation during solar flares. However, flares of identical x-ray magnitudes, occurring at similar locations on the solar disk, can show significantly different corresponding Lyman-alpha responses. Thus, the primary objective of this research is to study how the properties of nonthermal electrons influence the corresponding Lyman-alpha (and other EUV line and continua) emission profiles in solar flares. To do this, multi-wavelength spectroscopic observations of solar flares at similar locations on the solar disk taken simultaneously from RHESSI, GOES and SDO will be analysed using bespoke computational software, developed by teams at NASA Goddard Space Flight Centre. This will allow for the diagnosis of flare-related nonthermal electron properties, the estimation of deposited energy in the chromosphere and comparison to the radiative losses in Lyman-alpha and other EUV components of flare radiation. This data will also act as observational input for state-of-the-art radiative-hydrodynamic and atmospheric models, which will allow investigation into the discrepancies between predicted Lyman-alpha emission profiles and those observed by space-based instruments. Ultimately, this research will provide a better understanding of how fundamental properties of accelerated electrons in solar flares can dictate their levels of geoeffectiveness.

This research coincides with a rise in solar activity as Solar Cycle 25 commences. In future, this research will guide the interpretation of future Lyman-alpha data sets from the new generation of X-ray and Lyman alpha instruments on board solar missions from NASA, ESA, and JAXA, as well as provide a tool for future developments to radiative-hydrodynamic and atmospheric models.