Ion Outflow and Escape in the Terrestrial Magnetosphere: Cluster Advances

Abstract

Cluster was the first mission in the terrestrial magnetosphere to involve four spacecraft in a tetrahedral configuration, providing three-dimensional measurements of the space plasma parameters. Cluster was also equipped with a very comprehensive instrumentation, allowing the measurement of the ion populations outflowing from the ionosphere, their circulation in the magnetosphere, and their eventual escape to outer space. The observations of the outflowing and escaping ion populations performed by Cluster are reviewed and the most prominent results highlighted. These show the dominance in the magnetotail lobes of cold plasma outflows originating from the polar caps. For the energetic heavy ion outflow, the cusps constitute the main source. Their transport and acceleration through the polar cap into the lobes and then into the plasma sheet has been characterized. The dependence of the polar outflow on the solar wind parameters and on the geomagnetic activity has been evaluated for both cold ion populations and heavy energetic ions. For the latter, outflow has been observed during all periods but an increase by two orders of magnitude has been shown during extreme space weather conditions. This outflow is adequate to change the composition of the atmosphere over geological timescales. At lower latitudes, the existence of a plasmaspheric wind, providing a continuous leak from the plasmasphere, has been demonstrated. The general scheme of the outflowing ion circulation in the magnetosphere or escape, and its dependence on the IMF conditions, has been outlined. However, several questions remain open, waiting for a future space mission to address them.