Plasma sheet pressure anisotropies

Abstract

The ecliptic plane components of the pressure tensors for low‐energy (<30 keV) electrons and protons have been examined for more than 400 hours of quiet time plasma sheet data from Imp 6 and Imp 8. Individual high time resolution (≲100 s) measurements show that while at times the tensors may be clearly anisotropic with the pressure parallel to the magnetic field (P ∥ ) being as much as 1.5–2.0 times the pressure perpendicular (P ⊥ ), the anisotropies are not usually of this magnitude and are typically below the value thought necessary to balance the tension of the magnetic field. One‐hour averages of the particle counts have also been calculated. While these averages mask more rapid changes due to variations in the distributions and the magnetic field and can yield only a lower limit to the true anisotropy, examinations of the individual measurements indicate that they do provide a reasonable summary. On this time scale, the protons are nearly isotropic: the ratio of the maximum ecliptic component of the proton pressure tensor to the minimum is typically ≲1.1, and there is no apparent tendency for the distributions to be field aligned. On the same time scale, the electron distributions do show a tendency to be field aligned with P ∥ /P ⊥ ≥ 1.2 approximately 25% of the time. Due to the low energy density of the electrons, however, this anisotropy is not itself sufficient to balance the tension of the magnetic field.