The Bn problem: Determination of local magnetic parameters of interplanetary shocks from in situ IMF data

Abstract

The angle èBn is the angle between the upstream magnetic field and the shock normal direction and is important for many phenomena in interplanetary physics. We show that for most shock observations, èBn cannot be determined without addressing its associated uncertainty. We propose a simple computational technique to infer a more reliable value for èBn and its associated uncertainty. This technique is based on magnetic field data only, and can be useful when we cannot apply the iterative techniques involving magnetic and plasma parameters. This method also can be applied to study other MHD discontinuities. To infer the shock local parameters, it is necessary to define upstream and downstream regions in the Rankine-Hugoniot relations. Fluctua- tions of the interplanetary magnetic field, particularly its direction, restrict the duration of these regions to just a few minutes before and after the shock. Quasi-parallel shocks have larger associated uncertainties than quasi-perpendicular shocks. When èBn has a large uncertainty, this angle becomes time dependent, i.e., does not have a well-defined value but varies within an angular range.