Low‐energy particles at the bow shock, magnetopause, and outer magnetosphere of Saturn

Abstract

Low‐energy electrons (>22 keV) and protons (≳30 keV) measured by the Low‐Energy Charged Particle Experiment (LECP) during the encounters of the two Voyager spacecraft with Saturn are described. The characteristics of the dayside bow shock, magnetopause, and outer magnetosphere are emphasized. Only one crossing of the Saturnian bow shock was observed inbound during the Voyager 1 encounter, whereas five crossings of the bow shock were identified during the Voyager 2 approach to the planet. During several of these bow shock crossings, low energy protons were observed to be streaming from the direction of the dawnside of the magnetosphere. In the magnetosheath the protons were observed to be oriented primarily with pitch angles of ∼90°. Prior to the inbound magnetopause crossings (as defined by the magnetometer experiment on Voyager), the low‐energy protons and electrons were observed to increase in intensity. Further, during Voyager 2 encounter, an increase in the proton and electron fluxes accompanied a change in orientation of the magnetosheath magnetic field from one with a vertical component opposite to the planetary field to one with a vertical component in the direction of the planetary field. Examination of the flux distributions of the protons suggests that the magnetopause was moving inward with a lower limit speed of ∼10 km/s during the Voyager 2 approach to the planet. The observed average subsolar magnetopause position at the time of Voyager 2 encounter was 18.5 RS, whereas during the Voyager 1 encounter it was considerably more extended, at 23.5 RS. While the ratios of the low‐energy proton energy densities to the magnetic field energy densities (βh) during both encounters were ≳0.5 in the outer magnetosphere, the Voyager 2 measurements were made at higher latitudes; reasonable extrapolation suggests that βh was >1 in the equatorial plane at this time, suggesting that a significant hot plasma sheet probably existed at Saturn. Intervals of diamagnetic depression observed in the magnetic field energy density during the Voyager 2 encounter coincided very well with enhancements in the low‐energy proton energy density; during one 10‐min interval the ratio of the proton to magnetic field energy density βh was significantly greater than 1, even at the higher latitudes where the measurement was made.