Effect of λ < 10 Å Solar X-Rays on the Ionosphere between 60 and 100 km

Abstract

Computations have been made in order to evaluate the ionospheric effects caused by the observed λ < 10 Å X-ray quiescent flux and flare flux. Of seventy-two solar X-ray flares having F (2–12 Å) greater than 3 × 10⁻³ erg/cm.² sec as recorded by the University of Iowa detector on Explorer 33, sixty-seven are found to be accompanied by reported SID's in which the electron density of the ionosphere between 60 and 100 km. undergoes a rapid increase during a few minutes. These rapid increases in electron density are presumably caused by the enhanced X-ray flux in the wavelength range 1 < λ < 10 A because softer X-rays would be completely absorbed above 100 km. and harder X-rays would penetrate below 60 km. without significant attenuation at higher altitudes. The altitudes for peak electron production range from 99 km. for 10 A X-rays down to 58 km. for 1 Å X-rays. The electron-production rate equations have been solved using a quiescent X-ray energy spectrum and a mean flare X-ray energy spectrum derived from the work of various observers during the past nine years. The physical and chemical processes involved in the mechanism of ionization of the local atmospheric constituents according to existing models are discussed.

Distribution of the computed electron production rate and the resulting electron density with height have been plotted for quiescent and flare conditions. These plots have been compared with available experimental data. It is confirmed that the SID's can be explained quantitatively assuming flare X-rays as the only ionizing agent. The magnitude of the SID's depends on 0·10 Å peak flux and flare spectrum. It is further shown that under 1966 quiescent conditions X-ray ionization in the region under study is comparable to ionization by solar Lyman alpha. Solar Lyman α flux is known to be very steady varying by a factor not more than 2 between solar minima and solar maxima; while 1–10 Å solar X-ray flux varies greatly with solar activity (see text). Solar X-rays of wavelength below 10 Å are thus responsible for both rapid variations and day-to-day variations of the D region electron density. The variation of the effective height of the D region with solar activity may also be traced to the variation of 0–10 Å solar X-ray flux.