Onset of Severe Amplitude Scintillations of Satellite Radio Signals during Sunrise

Abstract

WHEN considering ionospheric sunrise effects one must remember that the light of sunrise at a specific ionospheric height has been filtered through the entire atmosphere before reaching a point of ground sunrise; also the light has had the filtering action of the air between the point of ground sunrise and the westward point at that ionospheric altitude. Thus the ionization power of solar radiation at high altitudes is reduced to mere heat effects during the time before ground sunrise. In this respect, vibrational excitation¹ may have to be considered, but specifically at lower altitudes. Furthermore, existing drifts in the ionosphere, such as winds and waves in the neutral gas as well as electron and ion fluxes, have some influence. A downward electron flux is typical for a period after midnight or before dawn². During winter, an arrival of electrons from the conjugate ionosphere (which is in summer and has an earlier sunrise) may play a part by increasing the pre-sunrise ion and electron temperatures especially at European mid-latitudes³. Finally, the diurnal cycle of the ionospheric motions, that is the downward move of the F-region maximum from an altitude of 400 km before sunrise to an altitude of 250 km after sunrise, has to be considered (H. F. Bates in a paper read to the US URSI spring meeting in 1968). Together, these effects are likely to produce a turbulent motion within the ionosphere at a specific point of first illumination.