Where's my donut?
posted: July 11, 2013

The plasmasphere is a region of "cold" (low-energetic) particles that extends from about 1600 km to over 30000 km above Earth's surface. It is an extension of the ionosphere, and overlaps the inner Van Allen radiation belt and a good part of the outer one. The difference between the plasmasphere and the Van Allen belts is that the latter contain "hot" (high-energetic) particles that behave according to a whole different set of rules.

The plasmasphere has the shape of an asymmetric torus (donut). It was a surprise when scientists discovered in the early 60's that the outer boundary of the plasmasphere had a sharp edge, where the density of the particles drops a ten- to hundredfold. This is called the plasmapause. The extent and density of the plasmasphere varies a lot more than the Van Allen radiation belts, and thus is also highly sensitive to geomagnetic disturbances.

In the next years, it was found that during strong geomagnetic storms, the plasmasphere loses particles to the Earth's magnetosphere. This process is also associated to the formation of "plumes" (see EUV-image by the Image satellite underneath). Additionally, in 1992, Joseph Lemaire from the Belgian Institute of Space Aeronomy (BISA) and his collaborators theorized there should also be a steady transport of cold plasmaspheric particles outwards across the geomagnetic field lines, even during prolonged periods of quiet geomagnetic conditions. This plasmaspheric wind is the result of an imbalance between gravitational, centrifugal and pressure gradient forces driving particles away from the plasmasphere.

This "space wind" is very subtle, and thus difficult to measure. However, recent analysis of measurements by the Cluster spacecraft indicated an imbalance between particles moving outwards and particles moving inwards (blue resp. red in simplified figure underneath). These observations were made during (magnetic) equatorial crossings of the outer plasmasphere during quiet geomagnetic conditions, and were recorded for all energy levels of particles. Calculations by Iannis Dandouras (IRAP) indicate that the plasmasphere loses about 1 kg of particles every second to Earth’s outer magnetosphere, thus directly confirming Lemaire's initial predictions. Though 90 tons a day seems like a lot of matter, it is continuously being replenished by the ionosphere and constitutes only a fraction of the total mass of the Earth's atmosphere. Hence, the plasmaspheric donut will not be depleted anytime soon.

Because of the low energies, particles in the plasmasphere pose no hazard to spaceflight. However, they do play a role in GPS as they add delays to the GPS signals travelling through the plasmasphere. More importantly, this plasma reservoir plays a crucial role in the dynamics of the Earth's radiation belts that in turn do pose a radiation risk to satellites and astronauts travelling through it. The confirmation of Lemaire's theory will now have to be taken into account in any future model of the Earth’s magnetosphere.

Credits and further reading - Information and images were taken from the following websites:
  1. Cluster discovers steady leak in Earth's plasmasphere, ESA press release
  2. Cluster spacecraft detects elusive space wind, EGU press release
  3. Detection of a plasmaspheric wind in the Earth's magnetosphere by the Cluster spacecraft, I. Dandouras, scientific paper in Annales Geophysicae
  4. Plasmasphere and radiation belts, BISA
  5. Image EUV, University of Arizona, mission page
  6. Cluster, ESA, mission page