D. R. Linder

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Cassini Plasma Spectrometer Electron Spectrometer measurements during the Earth swing‐by on August 18, 1999

On August 18, 1999, Cassini flew by the Earth on its way to Saturn. The Cassini Earth swing-by was the fastest traversal of the Earths magnetosphere to date. The spacecraft was traveling at 9.1 R-E hr(-1) (16.1 km s(-1)) and made rapid traversals of several regions of the terrestrial magnetosphere. During the Cassini Earth swing-by the Electron Spectrometer (ELS) collected almost 9 hours of data in the Earths magnetosphere and almost 10 hours of solar wind data upstream of the Earth. During the pass, Cassini ELS sampled electrons in the solar wind, bow shock, magnetosheath, magnetopause, radiation belts, plasmasphere, plasma sheet, lobes, and crossings of the tail magnetopause. The purpose of this paper is (1) to give a summary of electron observations including the locations of magnetosphere boundary crossings and (2) to assess how the ELS is functioning as it takes measurements in the greatly differing plasma regimes encountered. Results are shown to be mainly consistent with previous observations with a few exceptions. In addition to anticipated results we present evidence of a low-energy field-aligned beam in the plasma sheet and evidence of asymmetry on the dayside and nightside plasmapause position. Preliminary calculations of density and temperature for the solar wind, magnetosheath, and plasma sheet are also presented.

Cassini Plasma Spectrometer Investigation

Cassini/Huygens is a joint project of NASA and the European Space Agency designed to explore the Saturnian system in depth during its four-year mission. Cassini, the orbiter spacecraft, will carry twelve hardware investigations while Huygens, the Titan atmospheric probe, will carry an additional six. The Cassini Plasma Spectrometer (CAPS), one of 12 orbiter investigations, includes 3 plasma sensors designed to cover the broadest possible range of plasma energy, composition, and temporal variation. It is conservatively estimated that CAPS will provide a factor of ten or more improvement in measurement capabilities over those of the Voyager spacecraft at Saturn.