Enhanced Pitch Angle Scattering of Protons at Mid-Latitudes During Geomagnetic Storms

摘要

During geomagnetic storms highly localized regions of enhanced proton (ion) precipitation in the tens to several hundred keV energy range can appear at mid-latitudes. The particle pitch angle distribution in these enhanced regions is anisotropic with maximum intensity perpendicular to the magnetic field. In a few cases, however, the distribution can approach isotropy. These regions typically have widths of a few degrees invariant latitude, but can be as narrow as 0.25 deg. The intensity peak is most often concentrated in a specific particle energy range, although in many cases the intensity peak at a given location is distributed over a broader energy range. During the main phase of the storms the ion enhancement is mostly observed in the highest energy protons and only in the midnight/evening MLT sector. Coincident with the ion enhancement there was often an enhancement in electrons with energies > 300 keV. In the recovery phase of the storms the ion enhancement can be observed at all local times covered by our observations and there was not any coincident enhancement in the high energy electrons. Overall the observations seem to support a picture where scattering of protons into the loss cone by cyclotron resonant wave-particle interaction occurs, while high energy electrons are parasitically scattered into the loss cone by the same ion cyclotron waves. Throughout the storm the L-dependence of the enhancements in proton fluxes is similar to the K_p dependence of the location of the plasmapause. Whenever a direct comparison could be made, the SAR arc and the ion enhancement overlap. Thus the ion enhancement and SAR arc are associated, but not necessarily on a cause-effect basis.