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Electron anisotropy constraint in the magnetosheath: Cluster observations
Author(s) -
Gary S. Peter,
Lavraud Benoit,
Thomsen Michelle F.,
Lefebvre Bertrand,
Schwartz Steven J.
Publication year - 2005
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2005gl023234
Subject(s) - magnetosheath , anisotropy , cluster (spacecraft) , constraint (computer aided design) , electron , geophysics , physics , geology , astrophysics , magnetosphere , plasma , nuclear physics , magnetopause , computer science , quantum mechanics , mathematics , geometry , programming language
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.