Comments on ‘The energy selective precipitation of inner zone electrons’ by W. L. Imhof, J. B. Reagan, and E. E. Gaines

Abstract

The events presented exhibit peaks which have a half width consistent with a monoenergetic spectrum and have a peakto-continuum ratio of up to 10, which is relatively good resolution for a plastic scintillator on a spacecraft immersed in an energetic electron flux producing bremsstrahlung background. The energy versus L characteristics of these peaks indentfly them as probably being the same phenomenon as observed by VK, although the latter study discussed events which appeared as monoenergetic spectra with peaks 3-4 orders of Copyright ¸ 1980 by the American Geophysical Union. magnitude above background with no continuum present. The difference in apparent background is due to the difference in detection method. However, this difference may cause a bias in the 1971-89A data base in the following manner. If a precipitation event of short duration (by precipitation we mean a lowering of mirror points to altitudes within either the drift loss cone or the local bounce loss cone) occurs over a narrow longitude interval, drift dispersion will produce a narrowing of the energy peak and a consequent reduction in the integral of the flux above the continuum, reducing the probability of identifying the event in the IRG data. Energy spectra from precipitation occuring over a wide longitude region or for a long time duration (tens of minutes) would not be degraded by this effect and could then predominate in the data base even if they were not the dominant precipitation pattern. Identification of events in the VK data is not affected by drift dispersion, since events are readily identified in a single channel. Some events appeared in only one or two channels in the VK data. The drift dispersion effects will be discussed in more detail later.The VK analysis utilized pitch angle distribution information to determine the probable longitude at which the particle distribution last interacted with the atmosphere and, by inference, the longitude and local time at which the precipitation occurred. By contrast, the 1971-89A data set contains no information about the pitch angle distribution other than that particles are in the local bounce loss cone or in that portion of the drift loss cone locally observable. The circular orbit of the 1971-89A, along with the lack of pitch angle information, restricts the data set to a single B value at a given longitude and hemisphere for each L value. Measurements obtained at different longitudes are, in general, also obtained at different B values and hence are not directly comparable. Intensity measurements, to be valid as an indicator of the longitudinal extent of precipitation, must of the particle distributions on the same drift shell and mirror B. No events were observed in the local bounce loss cone (presumably indicating that either the distribution of strong precipitation events is isolated in time and longitude and the probability of observing them is low, or continuous slow diffusion is responsible for the appearance of electrons in the drift loss cone and the local intensity is too l...

Reply [to “Comments on ‘The energy selective precipitation of inner zone electrons’ by W. L. Imhof, J. B. Reagan, and E. E. Gaines”]

“…For example, in the 175ø-205øE longitude interval, data were processed from only five passes across the L --1.65 shell in the southern hemisphere, all at local times for which peaks were less frequently observed, so the absence of L dependent peaks for B > 0.325 has little statistical significance. At comparable B values in the three different longitude intervals there is a clear trend for L dependent peaks to be observed more often at the more easterly longitudes Vampola and Kuck [1979]. discuss the possibility that drift dispersion might, in some fashion, account for the less frequent observation of peaks in the 53ø-96øE interval.…”

“…However, since the frequency of occurrence of peaked spectra and the average fluxes were significantly lower in this longitude interval than at comparable B values to the east, we conclude that the precipitation events were not confined exclusively to longitudes near 50øE. Let us now consider in turn the points raised by Vampola and Kuck [1979]. It is undeniably true that the nearly circular orbit of the satellite restricts the data set to a single B value at a given longitude and hemisphere for each L value.…”

“…In the foregoing commentary, Vampola and Kuck [1979] have questioned the validity of the interpretation presented in Imhofet al [1978] as to the source location of energy selective electron precipitation. Before addressing the differences in interpretation it is important to note that it is mutually agreed, as discussed in Vamoeola and Kuck [1979], that the energy versus L characteristics of the peaks originally discovered by Imhof et al [1973] and studied in greater detail in Imhof et al [1978] are probably the same phenomenon observed by Vamoeola and Kuck [1978].…”