THE MAGNETOSPHERE  BOUNDARY LAYER UNDER NORTHWARD IMF CONDITIONS

D. H. Fairfield
Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center
Donald.H.Fairfield@nasa.gov

Steady northward Interplanetary Magnetic Field (IMF) enhances the density 
of the magnetotail plasma sheet, inhibits magnetospheric substorms and 
creates ideal conditions for investigating the effects of solar wind 
pressure pulses on the magnetosphere. Steady northward IMF also leads to 
aligned magnetosphere and magnetosheath magnetic fields, a condition 
conducive to the Kelvin-Helmholtz (KH) instability at the magnetopause. The 
trailing portion of an interplanetary magnetic cloud on January 10-11, 1997 
produced both aligned fields and pressure pulses, allowing study of the 
motion and structure of the boundary layer by the two closely spaced 
satellites, Geotail and Interball. The pressure pulses illustrate how a 
magnetopause wave is generated which propagates tailward at a speed 
exceeding the tailward convection of the pressure discontinuity in the 
magnetosheath. The simultaneous measurements show that this magnetosheath 
discontinuity travels more slowly near the magnetopause so that the front 
makes an acute angle with the downstream magnetopause. During intervals 
without pressure pulses, magnetopause wave trains with several minutes 
period are observed which are thought to be due to the KH 
instability.  Earlier MHD simulations have shown that vortices created by 
the instability may twist the magnetic field, producing the highly variable 
magnetic field that is observed.  This process may lead to reconnection 
within the vortex which can allow direct entry of magnetosheath particles 
into the magnetosphere that would otherwise not occur with a closed 
configuration. This process may help explain the higher-than-normal 
densities in the magnetosphere plasma sheet that tend to be associated with 
northward IMF.