Particle Distributions in the Magnetotail

17 August 2011

Artists impression of the plasma regions of the magnetosphere. (c) UCL 2011

For the first time, Walsh et al. have examined, in detail, the particle distributions in the magnetotail to determine the average pitch angle distributions. 

The magnetotail on the nightside of the Earth's magnetosphere is generally considered to be divided into four regions that have different plasma characteristics. The plasma sheet is found at the centre of the magnetotail and consists of hot plasma with a temperature of around 12 million degrees celsius. Outside the plasma sheet is a region called the plasma sheet boundary layer, which is a transition region between the plasma sheet and the north and south magnetotail lobes, which are comparatively empty, not containing much plasma at all. 

It is generally thought that the ions and electrons that make up the plasma sheet are isotropic - there are equal numbers of ions and electrons moving in all directions. In a recent paper, however, Walsh et al. (2011) used data from one of the European Space Agency's four Cluster spacecraft to prove that,  on average, while this is true for the ions, it is not true for the electrons. They looked at three months of proton and electron data measured when the spacecraft was located in the plasma sheet or the plasma sheet boundary layer and compared the fluxes of particles travelling perpendicular to the Earth's magnetic field with the fluxes of those particles travelling parallel or antiparallel to the magnetic field. In an isotropic plasma these numbers should be the same. For the electrons they found that there were higher fluxes of particles travelling (anti)parallel to the magnetic field than perpendicular to it.

For more details see:

Walsh, AP and Owen, CJ and Fazakerley, AN and Forsyth, C and Dandouras, I (2011),

Average magnetotail electron and proton pitch angle distributions from Cluster PEACE and CIS observations. GEOPHYS RES LETT , 38, Article L06103, DOI:10.1029/2011GL046770.

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