Model of the low‐latitude boundary layer with finite field‐aligned potential drops and nonconstant mapping factors

Abstract

A 2½‐dimensional, steady state model of the low‐latitude boundary layer (LLBL) on closed field lines has been developed. Coupling to the dayside auroral ionosphere via region 1 field‐aligned currents and associated finite field‐aligned potential drops is included. The boundary layer (BL) flow is influenced by viscous forces, inertia forces, pressure forces, and j × B forces. The BL approximation and kinematic treatment of the plasma motion perpendicular to the equatorial plane greatly simplify the model. The model has three modules: an equatorial LLBL module, a force‐free coupling module, and an ionospheric module. It includes nonconstant magnetosphere‐ionosphere mapping factors, describing the increasing stretching of magnetic field lines with increasing distance away from the subsolar region, as well as variations in mapping factors across the BL. In this paper, both the BL height and the ionospheric conductivities are assumed constant. The effects of finite field‐aligned potential drops and nonconstant mapping factors on the BL flow, on the plasma density and temperature, on the magnetic field line bending, and on the field‐aligned current distribution are presented. The effects of varying the magnitude of the (constant) ionospheric Pedersen conductivity are also discussed. In addition, the effects of viscosity, including varying its magnitude and allowing it to be a function of location both along and across the LLBL, are examined. A number of qualitative features of the simulation results are found to be consistent with various observations.