Electron anisotropy constraint in the magnetosheath: Cluster observations

The whistler anisotropy instability is driven by the condition T-perpendicular to e/T-parallel to e>1, where the subscript e denotes electrons and the other subscripts denote directions relative to the background magnetic field B-o. 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-perpendicular to e/T-parallel to e-1=S-e/beta(alpha e)(parallel to e) with fitting parameters 0.1 less than or similar to S-e less than or similar to 1 and 0.5 less than or similar to alpha(e)<0.7 over 0.10 <=beta(parallel to e)<= 1.0 where (beta parallel to)e=8 pi n(e)T(parallel to e)/B-o(2). 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 S-e similar or equal to 0.2 and alpha(e)similar or equal to 0.6. This is the first reported observation of this constraint in a space plasma.