Abstract. The terrestrial ring current is an electric current flowing toroidally around the Earth, centered at the equatorial plane and at altitudes of-10,000-60,000 km. Changes in this current are responsible for global decreases in the Earth's surface magnetic field, which are known as geomagnetic storms. Intense geomagnetic storms have severe effects on technological systems, such as disturbances or even permanent damage to telecommunication and navigation satellites, telecommunication cables, and power grids. The main carriers of the storm ring current are positive ions, with energies from -1 keV to a few hundred keV, which are trapped by the geomagnetic field and undergo an azimuthal drift. The ring current is formed by the injection of ions originating in the solar wind and the terrestrial ionosphere. The injection process involves electric fields, associated with enhanced magnetospheric convection and/or magnetospheric substorms. The quiescent ring current is carried mainly by protons of predominantly solar wind origin, while geospace activity tends to increase the abundance (both absolute and relative) of O + ions, which are of ionospheric origin. During intense magnetic storms, the O + abundance increases dramatically, resulting in a rapid intensification of the ring current and an O + dominance around storm maximum. This compositional change affects, among other processes, the decay of the ring current through the species-and energy-dependent charge exchange and wave-particle scattering loss. Energetic neutral atoms, products of charge exchange, enable global imaging of the ring current and are the most promising diagnostic tool of ring current evolution. This review will cover the origin of ring current particles, their transport and acceleration, the effects of compositional variations in the ring current, the effects of substorms on ring current growth, and the dynamics of ring current decay with an emphasis on the process of charge exchange and the potential for wave scattering loss.

Abstract. We examine the circulation of heavy ions of planetary origin within Mercury's magnetosphere. Using single particle trajectory calculations, we focus on the dynamics of sodium ions, one of the main species that are ejected from the planet's surface. The numerical simulations reveal a significant population in the near-Mercury environment in the nightside sector, with energetic (several keV) Na + densities that reach several tenths cm −3 at planetary perihelion. At aphelion, a lesser (by about one order of magnitude) density contribution is obtained, due to weaker photon flux and solar wind flux. The numerical simulations also display several features of interest that follow from the small spatial scales of Mercury's magnetosphere. First, in contrast to the situation prevailing at Earth, ions in the magnetospheric lobes are found to be relatively energetic (a few hundreds of eV), despite the low-energy character of the exospheric source. This results from enhanced centrifugal acceleration during E × B transport over the polar cap. Second, the large Larmor radii in the mid-tail result in the loss of most Na + into the dusk flank at radial distances greater than a few planetary radii. Because gyroradii are comparable to, or larger than, the magnetic field variation length scale, the Na + motion is also found to be non-adiabatic throughout most of Mercury's equatorial magnetosphere, leading to chaotic scattering into the loss cone or meandering (Speiser-type) motion in the near-tail. As a direct consequence, a localized region of energetic Na + precipitation develops at the planet's surface. In this region which extends over a wide range of longitudes at mid-latitudes (∼30 • -40 • ), one may expect additional sputtering of planetary material.

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.

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The general scientific objective of the ASPERA-3 experiment is to study the solar windatmosphere interaction and to characterize the plasma and neutral gas environment within the space near Mars through the use of energetic neutral atom (ENA) imaging and measuring local ion and electron plasma. The ASPERA-3 instrument comprises four sensors: two ENA sensors, one electron spectrometer, and one ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux (0.1-60 keV) with no mass and energy resolution, but high angular resolution. The measurement principle is based on registering products (secondary ions, sputtered neutrals, reflected neutrals) of the ENA interaction with a graphite-coated surface. The Neutral Particle Detector (NPD) provides measurements of the ENA flux, resolving velocity (the hydrogen energy range is 0.1-10 keV) and mass (H and O) with a coarse angular resolution. The measurement principle is based on the surface reflection technique. The Electron Spectrometer (ELS) is a standard top-hat electrostatic analyzer in a very compact design which covers the energy range 0.01-20 keV. These three sensors are located on a scanning platform which provides scanning through 180 • of rotation. The instrument also contains an ion mass analyzer (IMA). Mechanically IMA is a separate unit connected by a cable to

The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on board the Mars Express spacecraft found that solar wind plasma and accelerated ionospheric ions may be observed all the way down to the Mars Express pericenter of 270 kilometers above the dayside planetary surface. This is very deep in the ionosphere, implying direct exposure of the martian topside atmosphere to solar wind plasma forcing. The low-altitude penetration of solar wind plasma and the energization of ionospheric plasma may be due to solar wind irregularities or perturbations, to magnetic anomalies at Mars, or both.

The general scientific objective of the ASPERA-3 experiment is to study the solar windatmosphere interaction and to characterize the plasma and neutral gas environment within the space near Mars through the use of energetic neutral atom (ENA) imaging and measuring local ion and electron plasma. The ASPERA-3 instrument comprises four sensors: two ENA sensors, one electron spectrometer, and one ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux (0.1-60 keV) with no mass and energy resolution, but high angular resolution. The measurement principle is based on registering products (secondary ions, sputtered neutrals, reflected neutrals) of the ENA interaction with a graphite-coated surface. The Neutral Particle Detector (NPD) provides measurements of the ENA flux, resolving velocity (the hydrogen energy range is 0.1-10 keV) and mass (H and O) with a coarse angular resolution. The measurement principle is based on the surface reflection technique. The Electron Spectrometer (ELS) is a standard top-hat electrostatic analyzer in a very compact design which covers the energy range 0.01-20 keV. These three sensors are located on a scanning platform which provides scanning through 180 • of rotation. The instrument also contains an ion mass analyzer (IMA). Mechanically IMA is a separate unit connected by a cable to Space Science Reviews (2006) 126: 113-164

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