Electron anisotropy constraint in the magnetosheath: Cluster observations

Abstract

The whistler anisotropy instability is driven by the condition T⊥e/T∥e > 1, where the subscript e denotes electrons and the other subscripts denote directions relative to the background magnetic field Bo. Instability growth leads to enhanced field fluctuations which scatter the electrons; theory and simulations show that this scattering imposes an upper bound on the electron anisotropy in the form T⊥e/T∥e - 1 = Se/β∥eαe with fitting parameters 0.1 ≲ Se ≲ 1 and 0.5 ≲ αe < 0.7 over 0.10 ≤ β∥e ≤ 1.0 where β∥e = 8πneT∥e /Bo2. Here measurements from the PEACE instrument on the Cluster 1 spacecraft show that electron anisotropies in two crossings of the dayside terrestrial magnetosheath are constrained statistically by this equation with Se ≃ 0.2 and αe ≃ 0.6. This is the first reported observation of this constraint in a space plasma. Copyright 2005 by the American Geophysical Union.