The Spatial Development of the Magnetospheric Low-Latitude Boundary Layer

The low-latitude boundary layer (LLBL) comprises a large fraction of the magnetospheric boundary layer making it a potentially important site for transport of mass, momentum and energy from the high-speed magnetosheath plasma into the magnetosphere. I have examined, by computer simulation, the processes involved in the spatial development of a 6.4 R_{rm E} (Earth radii) long section of the dayside LLBL from a thin and laminar boundary layer to a broad and turbulent one capable of significant transport. The computer simulation developed for this purpose is based on the full set of ideal magnetohydrodynamic (MHD) equations that govern the dynamics of most magnetospheric plasmas and uses a two-dimensional nonperiodic simulation geometry to permit the realistic downstream development of the boundary layer. Simulations started from several realistic initial conditions all exhibit the formation of a LLBL that broadens with downstream distance, from an upstream thickness of 0.12 R_{E} to as much as ~0.7 R_{E } downstream, and reproduces many of the observed boundary layer characteristics. The broadening occurs through the action of Reynold and Maxwell stresses generated by the Kelvin-Helmholtz (KH) instability in the boundary layer which deposit momentum and energy into the LLBL. The KH instability also transports mass into the LLBL by mixing plasma across the boundary layer through continuous vortex roll-ups and mergings and also appears capable of aiding diffusive transport processes by steepening density gradients at the magnetopause enough to trigger any of a number of possible diffusion processes. Simulations have also shown that the downstream development of the boundary layer may be slowed and possibly stopped in the presence of a flow-aligned component of the magnetosheath magnetic field. For example, for a magnetosheath magnetic field which is initialized to tilt 30 ^circ away from perpendicular to the flow, the KH instability still develops, but fails to generate the large, merging vortices necessary to the success of the transport processes because the vortices lose kinetic energy to magnetic field distortions as they wind up the magnetic field. The magnetosheath magnetic field is thus capable of nonlinearly stabilizing the KH instability in the LLBL even though the LLBL is linearly unstable to the KH instability.