Sudden impulses at subauroral latitudes: Response for northward interplanetary magnetic field

<jats:p>The response of subauroral <jats:bold>H</jats:bold> component magnetograms at the time of passage of interplanetary shocks under northward interplanetary magnetic field (IMF) conditions is used to examine the behavior of the magnetosphere when it is suddenly compressed. At subauroral latitudes, near 55° or <jats:italic>L</jats:italic> = 3, the response has some similarities and some important differences from the low‐latitude response. The first effect is a preliminary impulse which appears to be due to the Hall current driven in the ionosphere by the arrival of the first Alfven waves along the magnetic field lines from the distant equatorial magnetosphere that has been set in motion by the penetration of the shock wave into the magnetosphere. Next there is a general increase of the magnetic field everywhere as the magnetosphere is compressed to its new size and the information of this compression propagates to the ground. This increase takes about 5 min followed by a 10‐min relaxation to the final asymptotic value. This general increase is accompanied by two more localized current systems: one transient and one steady. The transient current appears to be a dual‐vortex current system launched from noon toward the nightside. The other steady system is a fixed double‐cell convection system. We identify the former with the propagation of the magnetopause indentation associated with the enhanced pressure in the solar wind. We attribute the latter, steady system to high‐latitude reconnection with northward IMF.</jats:p>