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- Title
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. Plain Language Summary: The Earth's upper atmosphere is slowly escaping to space, either in the form of light electrically neutral atoms (mainly hydrogen) or in the form of ions (all species). The Cluster mission, launched in year 2000 and consisting of four spacecraft orbiting the Earth, has greatly advanced our understanding of how the ions are outflowing from the upper atmosphere, can be accelerated by the electric and magnetic fields present there, and can then eventually escape to outer space. The results show how this escape is highly sensitive to the level of solar activity and geomagnetic activity. Although escape occurs under all activity conditions, during space storms it dramatically increases, and during extreme space weather events this increase in the escape rate can be by almost two orders of magnitude. This escape in the long‐term (few billion years) is adequate to change the composition of the Earth's atmosphere. Key Points: Cluster has advanced our understanding of how the atmosphere slowly escapes to space and has revealed previously hidden escaping populationsThe outflowing ion populations cover a very wide energy domain, from cold plasma to energetic ions, and contain both light and heavy ionsIon outflow occurs during all activity levels, but it can increase by two orders of magnitude during extreme space weather events
- Subjects
ATMOSPHERIC ion precipitation; MAGNETOSPHERE; ELECTRON precipitation; SPACE vehicles; PLASMA waves
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 10, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2021JA029753