Observations of Lyman-alpha Emission in Solar Flares

Our nearest star, the Sun, has a profound influence on life on Earth. As we as a society become evermore reliant on space-based technology, the Sun's influence can become disruptive. Every so often, the Sun's radiative output increases due to a colossal release of energy known as a solar flare. These explosive events emit radiation across the entire electromagnetic spectrum, from radio waves to X-rays, and in extreme cases, gamma-rays. Of particular importance to Earth's atmosphere are the ultraviolet and X-ray emission, which can lead to chemical and dynamic changes in the ionosphere. This can have knock-on effects on satellite drag, radio communication, and GPS accuracy. Understanding the cause and effect of these increases in radiation is a key component of space weather research. Outside our solar system, knowledge of stellar flare emission is also crucial as the search for potentially habitable exoplanets intensifies. However, this emission from other stars can be difficult to detect due to attenuation by the interstellar medium. The study of solar flare emission therefore extends to a range of astrophysical research areas.

One component of solar flare radiation is known as Lyman-alpha emission, which is generated by neutral hydrogen atoms. As the Sun is comprised almost entirely of hydrogen, the emission line of Lyman-alpha is one of the brightest in the entire solar spectrum. However despite decades of solar observations at this wavelength, we have not had the capability to characterise the rapid variability in the Sun's output in Lyman-alpha light during solar flares until relatively recently. While the few studies that were previously reported were either inconclusive or contradicted one another, a recent statistical study has now provided a baseline of Lyman-alpha flare measurements, as well as confirming the link between Lyman-alpha emission and ionospheric disturbances. However, these measurements were broadband and integrated over the entire solar disc ("Sun-as-a-star" observations), and therefore provided little or no spatial or spectral information. The principle objective of this project is therefore to follow up this previous study by using recently released Lyman-alpha spectral data recorded during flares to understand changes in the line profile itself. This information is vital in understanding where in the solar atmosphere the emission is generated, and under what conditions. This will be aided by detailed comparison with predictions made by state-of-the-art numerical simulations on flare heating. This modelling effort is being carried out in parallel with collaborators at NASA's Goddard Space Flight Center.

The second element of this project is to understand why solar flares that occur close the limb of the Sun produce less of an enhancement in Lyman-alpha emission compared to those on the solar disc. This may be due to increased scattering of Lyman-alpha photons within the Sun's atmosphere along the line of sight as viewed from the Earth, or it may be an optical effect brought about by the change in viewing angle. To address this question, stereoscopic observations of solar flares will be investigated from both Earth and Mars at different separation angles simultaneously as there are currently Lyman-alpha instruments in orbit around both planets.

This project is also timely as it coincides with the dawn of a new solar cycle which will bring a flurry of solar activity over the coming years. The upcoming solar cycle will also be observed by the next generation of solar satellites. These included recently and soon-to-be launched missions by NASA, ESA, Jaxa and China, all of which will feature Lyman-alpha instruments.