Comment on “Modeling the ion chemistry of the D region: A case study based upon the 1966 total solar eclipse” by Sears et al.

Abstract

In a recent paper, Sears et al. [1981] modeled the data set from the November 12, 1966, total solar eclipse campaign conducted in Casino, Brazil, by using a D region chemical code. One of the main conclusions from their analysis was that for adequate modeling of the D region electron density it was necessary to include in the chemical code an ionization source function arising from the precipitation of inner radiation belt energetic electrons into the lower ionosphere over the south 'Atlantic anomaly. They calculated ion production profile by using electron energy spectra based on the trapped flux in the energy range from 50 kev to 1. [1979] to infer possible ionization rate arising from the precipitation of energetic electrons into anomaly region. They carried out their calculations by comparing the ionospheric parameters measured during the full sun and total eclipse conditions and deducing a residual ion production rate profile, in the height region 92-108 km, that could not be attributed to the residual ionizing radiation coming from the eclipsed sun and to the scattered ultraviolet radiation. This residual ion production rate was attributed to precipitating energetic electrons in the south Atlantic anomaly. Precipitation of energetic neutral particles, resulting from the charge exchange chemistry of the outer radiation belt, is also believed to be an ionizing source over low latitude (see, for example, Mizera and Blake [1973], Tinsley [1976], and Lyons and Richmond [1978]). However, the ion production rate due to this source gets important only during a storm main phase, and the height of the maximum ionization occur usually between 125 and 180 km. Under magnetically moderate periods the maximum ion production rate from this source, as seen from the.calculations of Lyon and Richmond [1978], is less than 10 cm -3 s -•, which is significantly smaller than the ion production rate that we have inferred from our eclipse data analysis (to be presented below), which refers to a lower height region (90-110 km). The purpose of this comment is to point out that the ion pair production rate due to precipitating electrons considered by Sears et al. In Figure 1 we may note that near 95-100 km the ion production rate calculated by Sears et al. [1981] is less, by 3-4 orders of magnitude, than that deduced by Abdu et al. [1979] to represent the same ambient condition. This difference is so large that it raises the question as to which of these values should be representing closer the conditions that prevailed during the eclipse campaign. One of the key factors that could affect our determination of q•,A is the residual ion production rate ql• that we have considered for the totality of the eclipse that is also plotted in Figure 1 with the q!,n derived by us from rocket ion density data. However, the ion production rate obtained from the electron energy spectrum of Gledhill and Hoffman [1981] decreases with increasing altitude, whereas the height profile of q!,n is seen increasing with height, giving rise to large d...