Energetic particle measurements during the Earth swing‐by of the Cassini spacecraft in August 1999

Abstract

We report on energetic particle measurements from the Low‐Energy Magnetospheric Measurement System (LEMMS) aboard the Cassini spacecraft during the Earth swing‐by maneuver in August 1999. LEMMS is capable of identifying the energy and incidence direction of energetic ions and electrons with energies of a few tens of keV to several tens of MeV. Cassini flew by Earth with a velocity of 16 kms−1 or 9 Earth radii (RE) per hour, which means that the entire day side magnetosphere was passed within 1 hour. This fast flyby trajectory of the spacecraft provided a snapshot of the Earth's magnetosphere where key regions were passed within a few hours instead of typically tens of hours or days for orbiting spacecraft. The measurements provide a means to show the performance of the instrument's capabilities as well as to test models of energetic particle distributions within the Earth's magnetosphere [e.g. Sibeck et al., 1987]. Particle pitch angle distributions (PADs) on the inbound pass along the 1300 magnetic local time (MLT) meridian gradually changed from normal distributions (maximum at 90° pitch angle) to butterfly pitch angle distributions (maximum at intermediate pitch angles). Close to the plasmapause, LEMMS observed field‐aligned bidirectional distributions. During the outbound pass through the inner magnetosphere at 0130 MLT, pitch angle distributions are generally close to isotropic. Shortly after a substorm onset, identified by ground‐based observations and aboard the Polar spacecraft [Khan et al., this issue], LEMMS observed an energy‐time dispersed enhancement in the differential flux of low‐energy electrons. We further address the question of whether signatures of the Earth magnetotail were observed at distances beyond 5000 RE when Cassini passed through the Earth's downstream region. LEMMS measured a series of particle increases during that time. The angular distributions during these enhancements show that most of these particles did indeed arrive from the Earth's direction. Nevertheless, the lack of singly charged heavy ions does not allow for a definite identification of this event as a deep‐tail encounter.