Electron anisotropy constraint in the magnetosheath: Cluster observations

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 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 ⊥ e / T ∥ e − 1 = with fitting parameters 0.1 ≲ S e ≲ 1 and 0.5 ≲ α e < 0.7 over 0.10 ≤ β ∥ e ≤ 1.0 where β ∥ e = 8π n e T ∥ 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 ≃ 0.2 and α e ≃ 0.6. This is the first reported observation of this constraint in a space plasma.