Anisotropy characteristics of geomagnetically trapped ions

Abstract

Radiation belt ions (protons plus heavier ions) are found to exhibit quiet time pitch angle anisotropies that vary significantly and nonmonotonically with L shell and energy in the range 20 keV to several MeV. Data from two spacecraft are reported' Explorer 45 measurements obtained in June 1972 and ISEE 1 measurements obtained from November 1977 through December 1978. Both data sets show that the radial distribution of pitch angle anisotropy contains one major minimum for most energies at some midrange L shell. The 1972 data collected over a 15-day period in a localized region of the magnetosphere (between L = 2 and L = 5 in the afternoon local time sector) revealed an anisotropy profile for most energies that increased with lower L shells below L ~ 2.5, minimized near L = 3, then attained a local maximum around L ~ 4, and evolved to more isotropic distributions near the Explorer 45 orbit apogee at L = 5.2. The 1977-1978 pitch angle anisotropies observed by ISEE 1 were obtained over all local times from approximately L = 2.5 to the outer limit of trapping. These data in general tended to confirm the 1972 finding of nonmonotonic anisotropy distributions, but with some important modifications. The later quiet time distributions showed a broader region of low anisotropy and less regular L shell dependence than the 1972 data, and showed systematic trends in the anisotropy that were clearly dependent not only on L shell but also on energy. A single, major anisotropy minimum was observed at virtually every local time, the location of this minimum occurred at lower L shells with progressively increasing energy. Comparison of the two quiet time data sets separated by about 6 years suggests that there may also be an anisotropy dependence on the phase of the solar cycle, possible resulting from ion composition differences or from different wave-particle interactions.

Comparison of TWINS and THEMIS observations of proton pitch angle distributions in the ring current during the 29 May 2010 geomagnetic storm

“…It depends only on the shape of the pitch angle distribution (see the definition of A in section 3 above). It is also to be noted that the TWINS results show an increase in the anisotropy along the path; i.e., anisotropy increases with smaller L. This is consistent with the results of Garcia and Spjeldvik [], Chen et al . [], and Fritz et al .…”

Crucial role of ring current H+ in electromagnetic ion cyclotron wave dispersion relation: Results from global simulations

“…We use the flux measurements from the Magnetospheric Plasma Analyzer, and the Syn- (1) and (2), the simulation was started at 0000 UT on 1 May 1998 using a background noise level for the He + -mode EMIC waves [e.g., Akhiezer et al, 1975b], the statistically derived quiet time RC H + energy distribution of Sheldon and Hamilton [1993], and the initial pitch angle characteristics of Garcia and Spjeldvik [1985]. The initial RC and EMIC wave distributions are derived independently and, moreover, they have nothing to do with a particular state of the magnetosphere during a simulated event.…”

Long-term evolution of magnetospheric current systems during storms

“…The σ R =0.8 gives the rate of increase (decrease) in current density distribution so that all the notable current would be included in between 2 and 6 R E . A/2=1 was set following the Garcia and Spjeldvik (1985) study on anisotropy of particle distribution, and it means that current density is concentrated near the equator and decreases away from the equator. For the additional thin current sheet we set x 1ntc =−2.0 R E , x 2ntc =−10.0 R E , and D 0 =0.2 R E .…”