Studies of the sharply defined L dependent energy threshold for isotropy at the midnight trapping boundary

Abstract

It has previously been reported (Imhof et al., 1977) that near the midnight trapping boundary the onset of isotropy for electrons and protons often occurs at a sharply defined energy threshold which decreases rapidly with increasing L value. In this paper the addition of energy spectrum measurements to the pitch angle distributions provides new information on the magnetic field configuration in the tail region during these cases. The energy dependent characteristics presented here include the positions and total latitude widths of the transition region from locally trapped to precipitating pitch angle distributions, the intensities and locations of near‐coincident enhancements in the trapped fluxes, and the intensities of electron precipitation at energies below the threshold for isotropy. During geomagnetically disturbed times, isotropy at a given energy tends to occur on lower L shells than it does at quiet times, but the widths of the transition region from locally trapped to isotropic pitch angle distributions show no well‐defined dependence upon geomagnetic activity. No evidence is found that unusually broad transition regions occur at any one characteristic phase of a substorm. At energies below the isotropy threshold the ratios of precipitating to trapped electron fluxes display large variations ranging from less than 10−3 to more than 10−1. Near the trapping boundary, enhancements are often observed in the fluxes of locally trapped electrons having minimum drift altitudes of ∼400 km or less. The L shells at which the maximum fluxes occur often display a rigidity dependence which is similar to that associated with the isotropy phenomenon.

Polar cap and cusp boundaries at day and night.

“…This mechanism acts on particles of both solar and magnetospheric origin. The morphology of the magnetospheric energetic particle precipitation as well as direct comparison between their observed characteristics and the modelled ones (IMHOF et al, 1979;SERGEEV et al,1983;IMHOF, 1988;SERGEEV and MALKOV, 1988) were found in a good agreement with the model predictions.…”

“…Such dependence was well documented for the precipitation of magnetospheric energetic particles by IMHOF et al (1979) andIMHOF (1988) but it remains to be supported for the solar particles. (Note, however, VAMPOLA (1971) mentioned, although not supporting documentally, that the SEE boundary position is energy dependent on the nightside but not on the dayside.)…”

Energy–latitude dispersion patterns near the isotropy boundaries of energetic protons

“…The distinctive feature of the CSS mechanism is a specific energy-latitude dispersion pattern near the equatorward boundary of proton IB, with a sharp isotropic precipitation boundary occurring at higher (lower) latitude for protons having lower (higher) energy. Such dispersions have been previously demonstrated in the magnetosphere (West et al, 1978), at low altitudes (Imhof, et al 1977(Imhof, et al , 1979Sergeev et al, 2012;Yue et al, 2013) and at middle altitudes (a few R e ) (e.g., Ganushkina et al, 2005). This forms the basic frame of energy-latitude dispersion, which does not change between quiet and disturbed conditions (see, e.g., Fig.…”

Butterfly pitch angle distribution of relativistic electrons in the outer radiation belt: Evidence of nonadiabatic scattering

“…One of the brightest manifestations of the charged particle nonadiabatic scattering in the nightside inner magnetosphere is observation by high-latitude spacecraft of the so-called boundary of isotropic precipitations [Imhof et al, 1977[Imhof et al, , 1979Sergeev and Tsyganenko, 1982]. For a given particle energy, this boundary determines that such L shell region that conjugated observations of high-latitude spacecraft demonstrate almost identical values of fluxes for trapped and precipitating particles.…”