An ionospheric phenomenon associated with explosive solar flares

Abstract

The purpose of this letter is to draw attention to a close relationship between the occurrences of the ionospheric phenomenon f sudden frequency deviations (SFD's) and the explosive phases of visible Hα flares. A sudden frequency deviation is a sudden change in frequency of a high-frequency radio signal reflected from the ionosphere [Davies, 1965; pp. 113 and 262]. SFD's are associated with bursts of X rays and/or extreme ultraviolet radiation, usually at the times of Hα flares. An example of a simple SFD is shown in Figure 1 of the accompanying Letter [Baker and Davies, 1966]. Two phases are evident: a positive phase, during which the electron production processes exceed the electron losses, followed by a negative phase, in which the electron loss processes predominate. Davies [1963] has shown that, to a first approximation, the instantaneous frequency shift Δƒ is proportional to the electron production function and, hence, to the instantaneous value of the excess solar radiation. Thus, the shape of the positive phase represents (roughly) the time variation in the ionizing solar flux. A statistical study of sudden frequency deviations [Agy et al., 1965] has shown that the frequency deviations are rather short-lived, having a most probable duration of about 4 minutes (including the negative phase). This compares with a most probable duration of 11 to 15 minutes for the Hα flare [Smith and Smith, 1963; Table 13]. During long-lasting flares, the SFD occurs between the start of the Hα flare and its peak. The point to notice, therefore, is that SFD's are associated with a particular phase of the visible flare.