Elliptical model of cutoff boundaries for the solar energetic particles measured by POES satellites in December 2006

Abstract

Experimental data from a constellation of five NOAA Polar Orbiting Environmental Satellites (POES), satellites were used for studying the penetration of solar energetic particles (SEP) to high latitudes during long‐lasting SEP events on 5–15 December 2006. We determined cutoff latitudes for electrons with energies >100 keV and >300 keV and for protons with energies from 240 keV to >140 MeV. The large number of satellites allowed us to derive snapshots of the cutoff boundaries with 1‐hour time resolution. The boundaries were fitted well by ellipses. On the basis of the elliptical approach, we developed a model of cutoff latitudes for protons and electrons in the northern and southern hemispheres. The cutoff latitude is represented as a function of rigidity, R , of particles; MLT, geomagnetic indices Dst , Kp , and AE ; and dipole tilt angle PS . The model predicts tailward and duskward shifting of the cutoff boundaries in relation to intensification of the cross‐tail current, field‐aligned currents, and symmetrical and asymmetrical parts of the ring current. The model was applied for prediction of polar cap absorption (PCA) effects observed at high latitudes by the Canadian Advanced Digital Ionosonde network of ionosondes. It was found that the PCA effects are related mainly to intense fluxes of >2.5 MeV protons and >100 keV electrons, which contribute mostly to the ionization of ionospheric D ‐layer at altitudes of ∼75 to 85 km. This finding was confirmed independently by FORMOSAT‐3/COSMIC observations of the SEP‐associated enhancements of electron content at altitudes of ∼80 km.