Low‐latitude dusk flank magnetosheath, magnetopause, and boundary layer for low magnetic shear: Wind observations

Abstract

We have studied in detail a Wind spacecraft crossing of the low‐latitude dusk flank magnetosheath, magnetopause (MP), and the low‐latitude boundary layer (LLBL) when the local magnetic shear across the MP was low (<30°) and the interplanetary magnetic field (IMF) was northward. We find that the magnetosheath flow tangential to the MP slows down initially as one moves from the bow shock toward the MP. However, close to the MP this flow speeds up as the MP is approached. The source of flow acceleration is likely to be the magnetic force associated with draping of the field lines around the MP. Magnetic flux pile‐up and a plasma depletion layer are also observed next to the flank MP indicating that the level of magnetic flux transfer across the entire dayside low‐latitude MP via reconnection is low. The MP is characterized by changes in the plasma properties. The electron parallel temperature is enhanced across the MP and continues to increase across the LLBL, while the perpendicular temperature is constant across the MP. This constancy of the perpendicular temperature suggests that the transfer of plasma takes place across the local MP. In the LLBL, the ion and electron temperatures are well correlated with the density. In addition, the flow direction in a substantial portion of the LLBL is nearly aligned with that in the magnetosheath, and the flow speed tangential to the MP decreases gradually with decreasing LLBL density. The behavior of the particle distributions suggests that the entire LLBL was on closed field lines. In essence, our findings on the topology and on the LLBL plasma characteristics suggest that even in the absence of reconnection at the local low‐shear MP, the LLBL is locally coupled to the adjacent magnetosheath. The smooth variations of the plasma parameters with the density are consistent with the LLBL spatial profiles being gradual. This may suggest that diffusion processes play a role in the formation and dynamics of the LLBL. Finally, the magnetic field and the state of the plasma in the plasma sheet adjacent to the flank MP/LLBL appear to be functions of the IMF direction. Thus the IMF may control both the external (magnetosheath) and the internal (plasma sheet) boundary conditions for the flank MP processes.