Perspectives gained from a combination of polar, cluster and ISEE energetic particle measurements in the dayside cusp

Abstract

Energetic particles are a consistent and common feature of the high-altitude dayside cusp. Observing these particles in a region where they cannot be stably trapped is one of the most striking findings of the Polar and Cluster satellites. The source of these cusp energetic particles (CEP) has centered on the possible role of the bow shock, leakage from the magnetosphere, and local acceleration within the cusp itself. The Polar satellite has documented that the shocked solar wind plasma enters the weak geomagnetic field of the polar region and produces cusp diamagnetic cavities (CDC) of apparent tremendous size (~6 RE) well within the traditional magnetosphere. Within these cavities the local magnetic field is depressed and very turbulent. The intensities of the energetic ions are observed to increase by many orders of the magnitudes during the CDC encounters. The four Cluster spacecraft have typically not observed such a large cusp and extended diamagnetic cavity. The search for evidence to resolve this inconsistency led to a revisiting of ISEE 1 and 2 satellite measurements during two encounters with the high altitude cusp. In each of these cases the very good energy and angular resolution of the ISEE energetic particle experiment revealed that energetic ions within the cusp located at GSM Z ranging from 4 RE to 5 RE appeared from closer to the Earth and streamed outward in very close association with the diamagnetic cavities in the measured magnetic field. The electrons demonstrated a peaked at 90° pitch angle distribution indicative of being confined within a cusp minimum field trap. In one of these cases the electron fluxes peak near the cusp boundary and in the other case they sharply define the CDC boundaries varying in a strictly anti-correlated manner during a large geomagnetic storm. The charge state distribution of these cusp cavity ions is indicative of their seed populations being a mixture of ionospheric and solar wind particles in many cases. Taken together these facts argue for a local acceleration of plasma within the cusp to many 10s and 100s of keV. By their geometry cusp magnetic field lines are connected to all of the magnetopause boundary layers and these cusp charged particles will form an energetic particle layer on the magnetopause. A source of energetic particles in the dayside high-altitude CDC will be effective in transferring the solar wind energy, mass, and momentum into the Earth’s magnetosphere and could provide the source of the ring current ions.