On the distribution of By in the geomagnetic tail

Abstract

The distribution of B y in the geomagnetic tail associated with a net cross-tail magnetic flux, recently experimentally discovered, is here investigated within the framework of two-dimensional but non-planar field adiabatic time-independent equilbria. It is found that the flux distribution is controlled by the pressure anisotropy of the plasma, B y being enhanced at the current sheet centre relative to that in the lobes for P ∥> P ⊥ and vice-versa for P ⊥> P ∥. For P ⊥> P ∥ a broad region of depressed field strength is found across the centre plane of the current sheet, terminated at its outer boundaries by spikes in the perpendicular current, across which B y and B x are “switched on” and rapidly increase towards their values in the low-β lobes. For P ∥> P ⊥ a thin high-current density layer forms at the sheet centre if the marginal firehose condition is approached, across which the B x field reverses by rotation at nearly constant magnitude about the z-axis. The field magnitude in this thin layer depends upon the pressure anisotropy, such that the plasma remains just firehose stable within it, and may approach an appreciable fraction of the lobe field strength even for moderate anisotropies. Such structures have been observed in the geomagnetic tail, but do not appear to be a common feature of the quiet-time plasmasheet, where the field strength at the centre plane can reach small values with little obvious enhancement of B y. In terms of the present model these observations require that either P ⊥> P ∥ in the quiet-time tail or that the plasma is within one or two per cent of isotropy if P ∥> P ⊥. These results then indicate that the production of plasma pressure anisotropy during adiabatic inward transport towards the Earth, which is generally expected to lead to P ∥> P ⊥ and its destruction by either macroscopic or microscopic processes, requires further study.