The Energetic Particles Detector (EPD) Aboard the Galileo Spacecraft

Abstract

In December 1995 the Galileo spacecraft reached the Jovian magnetosphere after more than 6 years travelling through the inner heliosphere. We report on first results of the Energetic Particles Detector (EPD) which is one of the scientific instruments aboard the spacecraft. EPD is designed to measure three-dimensional distributions of energetic electrons (15 keV–11 MeV) and ions (20 keV–55 MeV). Measurements in the Io-torus show “conic” distributions for sulfur and trapped distributions for protons which can be explained by charge exchange processes which are more effective for heavier ions than for protons. EPD discovered highly collimated field-aligned beams of electrons at closest approach to Io, which are related to the Io flux tube connecting the moon and the ionosphere of Jupiter. During the second encounter with Ganymede EPD could identify, together with plasma wave and magnetometer data, crossings of Ganymede’s magnetopause, a further proof for the first discovery of a magnetosphere within a magnetosphere. In addition EPD made measurements during the so-called first minitour, the first full orbit inside the Jovian magnetosphere with apojove near 115 Jovian radii (R J ) in the predawn sector. In this region, besides the predominantly corotating particles, bursts of ions moving radially outward from the planet were found.