First detection of a plasmaspheric wind in the Earth’s magnetosphere

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A study carried out at the Institut de Recherche en Astrophysique et Planétologie (OMP-IRAP-CNRS/Université Paul Sabatier Toulouse III) provides the first experimental confirmation of the presence of a plasmaspheric wind in the Earth’s magnetosphere. The existence of this wind, which continuously carries cold plasma from the plasmasphere to the outer magnetosphere, was postulated in 19921.2 from theoretical calculations related to the plasma stability. However, it had not yet been detected. This study, based upon the analysis of ion measurements by the CIS3 experience aboard the  Cluster4 satellites, confirmed its presence and led to assess its role in the mass loss of the ionized upper atmosphere. It is published on July 2 in the journal Annales Geophysicae.

The plasmasphere is a toroidal region around the Earth above the ionosphere, which is the largest reservoir of plasma inside the magnetosphere (Figure 1). Its configuration is very dynamic and is related to the level of geomagnetic activity. During periods of high activity, such as magnetic storms, whole blobs of plasma detach from the plasmasphere and are ejected to the outer magnetosphere, constituting “plasmaspheric plumes.”

Figure 1: Schematic view of the terrestrial plasmasphere. The plasmasphere is populated by a dense and cold plasma (a few eV) of ionospheric origin. It occupies the inner part of the Earth’s magnetosphere.
Copyright Windows to the Universe (http://www.windows2universe.org/)

Are these plumes the only way to eject the plasma to the outer magnetosphere? In 1992, Lemaire and Schunk had proposed an additional mode: the existence of a plasmaspheric wind continually carrying cold plasma from the plasmasphere outwards, even during periods of low magnetic activity (Figure 2). This wind, accross the lines of force of the geomagnetic field, was postulated from theoretical calculations related to the stability of the plasma1, 2. It is similar to the equatorial expansion of the solar corona, which gives birth to the solar wind.

Figure 2 : Animation showing the transport of cold plasma from the plasmasphere to the outside

The analysis of the ionic measurements obtained through the CIS3 experiment aboard the Cluster4 satellites allowed, for the first time, to confirm the presence of this plasmaspheric wind and to measure its speed, which is of the order of 1 km per second. These measurements were obtained during the perigee passage of the satellites and required for the instrument the use of a special operating mode, as well as the development of a suitable filtering and data analysis technique.

The plasmaspheric wind is an additional mechanism of plasma loss for the plasmasphere, but it is also a source of plasma for the outer magnetosphere. The mass loss due to this wind is of the order of 1 kg per second. This is an important element in the mass balance of the plasmasphere (source-losses balance) and has implications for the time “filling” of the plasmasphere, after a strong erosion resulting from a severe geomagnetic storm. Because of the plasmaspheric wind, refilling the plasmasphere with plasma (from the ionosphere below) is like filling with a liquid a container with a hole. Note also that the plasmasphere, as the main reservoir of plasma in the inner magnetosphere, interacts with the radiation belts and plays a key role in the dynamics of these belts.

Similar winds should also exist around other planets, or astrophysical objects rotating around their axis and having an ionized atmosphere and an intrinsic magnetic field. These winds would be, for these planets, an additional mode of atmospheric escape into space.

Note(s) :

  1. Plasmaspheric wind,J. F. Lemaire et R. W. Schunk, J. Atmos. Terr. Phys., 1992
  2. Convective instabilities in the plasmasphere, N. André et J. F. Lemaire, J. Atmosph. Sol.-Terr. Phys., 2006
  3. Cluster Ion Spectrometry: ion spectrometry experience prepared by an international consortium under the primary responsibility of IRAP (ex-CESR) and with the support of the CNES (http://cluster.irap.omp.eu/).
  4. Cluster : constellation of four satellites in tetrahedral configuration on a very eccentric orbit around the Earth (http://sci.esa.int/cluster/). ESA project with NASA. The mission was recently extended until 2016 by ESA : http://smsc.cnes.fr/CLUSTER/Fr/

Further Resources

IRAP Contact

  • Iannis Dandouras, IRAP-OMP (CNRS/Université Paul Sabatier-Toulouse III) : Iannis.Dandouras@irap.omp.eu, 05 61 55 83 20

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