Steady state slow shock inside the Earth's magnetosheath: To be or not to be? 1. The original observations revisited

International audienceThe original data that led to the slow mode transition (SMT) scenario for the diversion of the solar wind flow, in front of the dayside magnetoseath, are reexamined. It is shown that a number of SMT cases on the original list should be rejected because the corresponding solar wind data observed upstream from the magnetosheath display numerous density data gaps or have low time resolution inconsistent with accurate correlation study, or the SMT occurs between two successive crossings of the magnetopause. Therefore the SMT scenario is not established at a statistical level because of the limited number of well-identified cases. Temporal variations of the interplanetary magnetic field (IMF) are shown to play a crucial role in the origin, duration and magnetic field topology of SMTs through the introduction of multiple time shifts in any correlations of magnetic field data obtained on two spacecraft whose distance in a plane perpendicular to the Sun-Earth line is more than a few of tens Earth radius. These temporal IMF variations, correlated with solar wind density enhancements, induce a compressed magnetopause during SMT observations. The exogeneous nature of SMTs is established. Case studies reveal the different processes at the origin of the large enhancement of the density in SMTs with respect to the density upstream of the SMTs. These processes include: (1) an increase of the solar wind density, (2) variations in the density in the magnetosheath linked to increases of the Alfvén Mach number and to a new orientation of the IMF, and (3) a density gradient effect induced in the magnetosheath depletion layer by motion of the magnetopause. Other cases display proeminent density peaks linked to interplanetary magnetic field discontinuities imbedded in the SMTs. We also show that a previous analysis about stationarity of a SMT's front as well as of identification of slow modes propagating against the flow are not confirmed. The exogeneous scenario based on IMF and solar wind density variations for the origin and nature of the SMT phenomenon answers all the questions so far asked about this process