The radiation belt boundary near solar cycle maximum as determined from the trapping of energetic electrons

Abstract

The termination of the radiation belt, as defined by the trapping boundary for electrons of energy greater than 500 kev, is studied during the solar cycle maximum period of August 8 to September 10, 1959, using Explorer 6 scintillation counter data. The region investigated is on the evening side with respect to the earth-sun direction. The satellite traversed this region approximately twice each day, and the daily fluctuation in the position of the radiation belt boundary is presented for magnetically stormy and nonstormy periods. The boundary is compressed to low L values during magnetic storms and expands to high L values during periods of low magnetic activity. Evidence is presented to show that the expansion of the boundary is due to the addition of new particles at high L values. It is concluded that the mechanism active in injecting electrons of E > 500 kev (either by local acceleration or diffusion) into the radiation belt is effective during periods of low magnetic activity. Thus the local acceleration, diffusion, or injection of particles into the radiation belt is a more or less continuous process. Furthermore, to the extent that the boundary drops to low L values for electrons of E > 500 kev, magnetic storms generally impede rather than aid this process. The following results are discussed: (a) A well-defined trapping boundary for electrons (E > 500 kev) exists on the evening side of the magnetosphere. A comparison of the boundary data with the Explorer 6 magnetometer data indicates that on the night side the radiation belt trapping boundary is not representative of a coincident magnetospheric boundary. The trapping boundary is regarded as the demarcation between a relatively orderly magnetic field, effective in trapping particles, and a region that is ineffective in trapping energetic electrons, (b) During periods of moderate magnetic activity (KP > 1.2) at approximately 2100 local time the boundary is at L values less than 10Re, with daily fluctuations in position ranging from 8 to 10Re. (c) During magnetic storms the boundary is compressed to L values of 5.1 to 7Re. (d) During magnetically quiet periods (Kp < 1) the boundary expands to higher L values; for example, on August 28, when the Kp index approached its minimum value (Kp ∼ 0.5), the boundary was at an L value of 12.8 Re. (e) The presence of large fluxes of trapped, bunched, or untrapped electrons (E > 500 kev) past the boundary in the night side transition region is also correlated with solar activity. (ƒ) It is clear from these data that the concept of a fixed and well-defined electron trapping region is not a useful one. The fluctuation in the position of the boundary and the appearance of large fluxes of energetic electrons for short periods of time imply gross changes in the characteristics of the region.