Study of the high-latitude, outer-zone boundary region for ≥40-kev electrons with satellite Injun 3

Abstract

The ϕ boundary was introduced in 1968 as a more meaningful concept of the high-latitude limit to durably trapped ≥40-kev electrons than the usual intensity cutoff. In the present study, this boundary is defined more precisely by using four different features of the latitude profile of the ϕ parameter. Two additional high-latitude boundaries, the intensity cutoff (Λc) and the background boundary (Λbkg) for ≥40-kev-electrons are defined, and the average positions of each of these six ‘boundaries’ are investigated as a function of invariant latitude, magnetic local time, and magnetic activity. A frequency of occurrence of isotropic and near-isotropic pitch-angle distributions for ≥40-kev electrons is also investigated as a function of magnetic local time and magnetic activity. The results of this study suggest that at least two separate mechanisms control the position of the high-latitude boundary as a function of magnetic local time. One mechanism operates continuously near midnight, but its effectiveness decreases monotonically with increasing local time through dawn and noon, then reaches a minimum in effectiveness in the late afternoon and dusk hours. A second mechanism operates only near the noon meridian (0800 to 1400 hours) and is the dominant mechanism in this local-time interval. There is also an indication that the orientation and symmetry axis of the boundary may change with time.