Electron anisotropy constraint in the magnetosheath: Cluster observations

Abstract

The whistler anisotropy instability is driven by the condition T_$\perp$e/T_$\parallel$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_$\perp$e/T_$\parallel$e - 1 = S_e/β_$\parallel$e^α_e with fitting parameters 0.1 $\lesssim$ S_e $\lesssim$ 1 and 0.5 $\lesssim$ α_e < 0.7 over 0.10 <= β_$\parallel$e <= 1.0 where β_$\parallel$e = 8πn_eT_$\parallel$e/B_o². 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 $\simeq$ 0.2 and α_e $\simeq$ 0.6. This is the first reported observation of this constraint in a space plasma.