Lyman continuum observations of solar flares

Abstract

A study is made of Lyman continuum observations of solar flares, using data obtained by the Harvard College Observatory EUV spectroheliometer on the Apollo Telescope Mount. We find that there are two main types of flare regions: an overall ‘mean’ flare coincident with the Hα flare region, and transient Lyman continuum kernels which can be identified with the Hα and X-ray kernels observed by other authors. It is found that the ground level hydrogen population in flares is closer to LTE than in the quiet Sun and active regions, and that the level of Lyman continuum formation is lowered in the atmosphere from a mass column density m ≈ 5/sx 10⁻⁶ g cm⁻² in the quiet Sun to m ≈ 3/sx 10⁻⁴ g cm⁻² in the mean flare, and to m ∼ 10⁻³g cm⁻² in kernels. From these results we derive the amount of chromospheric material ‘evaporated’ into the high temperature region, which is found to be ∼- 10¹⁵g, in agreement with observations of X-ray emission measures. A comparison is made between kernel observations and the theoretical predictions made by model heating calculations, available in the literature; significant discrepancies are found between observation and current particle-heating models.