Model studies of photoionization and photoelectron production in response to solar flares

The solar flux, shortward of 102.6 nm deposits energy into the Earth's thermosphere and initiates chemical processes that affect the composition and structure of the ionospheric D and E regions. One of the primary processes is the photoionization of the major neutral constituents N₂, O₂ and O. The photoionization of the major species leads to the formation of energetic photoelectrons. These primary photoelectrons create secondary electrons that can cause further ionization, dissociation, and excitation of particles. We use a photoelectron model to study the effects of variability of the solar flux in the production of the primary and secondary photoelectrons in the D and E regions of the thermosphere. Using a detailed hydrodynamic model of a solar flare arcade, we have synthesized the spectral irradiance from a large solar flare, extending to energies in the hard X-rays. We use this synthetic spectrum to study the effects of solar flares at altitudes lower than 90 km, i.e. the D region. We have also revised the ionization and absorption cross-sections of the neutral species, including wavelengths based on new laboratory data. The new cross-sections are significantly different in the neighborhood of the Lyman beta emission. In this presentation, we examine the role of ionization by both photons and photoelectrons due to Lyman beta in the context of the revised cross sections.

The improved cross-sections and extension of the input solar flux to higher energies and therefore to lower altitudes, give us a better understanding of the effects of solar flares on the Earth's ionosphere.