Plasmaspheric wind in the Earth’s magnetosphere: contribution to the early evolution of the terrestrial atmosphere

The Earth’s plasmasphere dominates the mass content of the inner magnetosphere. During extended periods of relatively quiet geomagnetic conditions the outer plasmasphere can become diffuse, with a gradual fall-off of plasma density. During increasing magnetospheric activity, however, the plasmasphere is eroded and plumes, forming at the plasmapause and released outwards, constitute a well-established mode for plasmaspheric material release to the Earth’s magnetosphere. These plumes are associated to active periods and the related electric field change. In 1992, Lemaire and Shunk proposed the existence of an additional mode for plasmaspheric material release to the Earth’s magnetosphere: a plasmaspheric wind, steadily transporting cold plasmaspheric plasma outwards across the geomagnetic field lines, even during prolonged periods of quiet geomagnetic conditions. This has been proposed on a theoretical basis. The Cluster spacecraft, that cross the plasmasphere from south to north during their perigee passes, provided for the first time an experimental confirmation of the plasmaspheric wind. This is based on the analysis of ion measurements acquired by the CIS experiment onboard these spacecraft, which allows also to study the plasmaspheric dynamics under various geomagnetic activity conditions. The plasmaspheric wind has been systematically detected in the outer plasmasphere during quiet and moderately active periods, and provides a contribution to the magnetospheric plasma populations outside the Earth’s plasmasphere.

During the early terrestrial evolution (around 2 to 4 billion years ago), when the Earth’s rotation period around its axis was much shorter, the imbalance between gravitational, centrifugal and pressure gradient forces, giving rise to the plasmaspheric wind, should generate much stronger outflows. The plasmapheric wind should then have played an important role in the early evolution of the terrestrial atmosphere, through enhanced atmospheric escape, including the escape of heavy elements (C⁺, N⁺, O⁺).