Magnetospheric Confinement for Energetic Particles

Propagation of energetic particles in the outer magnetosphere and magnetotail is non-adiabatic due to the scattering at magnetic inhomogeneities. Because of that, energetic particles can escape from the trapped region to interplanetary medium and, vice versa, solar energetic particles (SEP) are able to penetrate to lower latitudes than those predicted by a pure dipole model. It was found [1] that at nightside, SEP electrons precipitate in the region of closed magnetic field lines. The mean free path of SEP has an explicit velocity dependence that leads to larger values for electrons than for protons at the same rigidity below a certain threshold of several MV [2]. Low-energy protons of such rigidities have smaller mean free path than electrons. Hence, the protons are scattered more effectively and, thus, occupy larger spatial region and lower latitudes in the outer magnetosphere. Experimental data from five polar orbiting low-altitude (~950 km) NOAA POES satellites were used to study the magnetospheric confinement for SEP protons with energies>240 keV and electrons with energies>100 keV. Large number of satellites allowed deriving snap shots of the penetration boundaries with 1-hour time resolution. The boundaries are well fitted by ellipses in the coordinate system of invariant co-latitude versus magnetic local time (MLT). The SEP protons and electrons reveal different penetration patterns. The boundaries for low-energy SEP demonstrate a substantial dependence on geomagnetic indices Dst and Kp that corresponds to effects of the cross-tail and ring currents. The penetration of electrons reveals an additional dependence on the field-aligned currents represented by auroral electroject (AE) index. At nighttime, the SEP penetration boundaries intersect deeply with the region of auroral precipitation that makes possible a significant leakage of energetic particles from substorm acceleration region to the interplanetary medium