Hybrid Simulation of the Formation of Dayside Low-Latitude Boundary Layer Under Northward IMF

A 3-D global hybrid simulation is carried out to study the formation of the dayside low-latitude boundary layer (LLBL) under a purely northward interplanetary magnetic field (IMF). The simulation shows that: (1) as a result of magnetic reconnection occurring in both the northern and southern hemispheres, a thick magnetopause boundary layer forms due to the capture of magnetosheath ions on the newly closed field lines and the transmitted magnetosheath ions near the high-latitude X lines, as suggested by a number of satellite observations and also found in some previous MHD simulations. (2) The newly closed field lines shorten as the kink of the field line propagates sunward from the X line, as predicted by Song and Russell [JGR, 1992]. (3) The formation of the LLBL is associated with the tailward spreading of the transmitted ions along the magnetopause, as the constantly formed closed flux tubes convect tailward and sideways with nearly the convection speed in the magnetosheath. The LLBL can form within the dayside convection time scale. (4) Meanwhile, compressional waves and transverse waves in magnetic field are generated near the equator and at the high-latitude reconnection sites. (5) The ion velocity distributions in the cusp and the boundary layer show the existence of multiple ion populations of the transmitted magnetosheath ions with different parallel bulk velocities, as well as ion heating.