Interplanetary magnetic field variations and slow mode transitions in the Earth's magnetosheath

The event observed on September 17, 1978 on ISEE 1-2, which led to the conc ept of a stationary slow mode transition region (SMT) in the magnetosheath in front of the magnetopause, is revisited. We establish that the two edges of this SMT have an exogenous origin induced by two discontinuities of the interplanetary magnetic field. The key of our analysis is that the outer e dge of the SMT is built up by a tangential interplanetary discontinuity whi ch is observed on ISEE-3 at a large distance from the Sun-Earth line and wh ich has an unusual direction. In this SMT the subsolar magnetosheath is ent irely downstream of a quasi-parallel bow shock, while upstream this SMT the subsolar magnetosheath is downstream of a quasi-perpendicular shock. We id entify three effects at the origin of the density enhancement in this SMT. We extend this approach to the original statistical study and we find that any SMT is connected to interplanetary magnetic field variations. This corr oborates our hypothesis that SMTs have an exogeneous origin driven by inter planetary magnetic field variations.