Foreshock compressional boundary

Abstract

We employ 2.5-D electromagnetic, hybrid simulations that treat ions kinetically via particle-in-cell methods and electrons as a massless fluid to study the formation and properties of a newly discovered boundary named the foreshock compressional boundary (FCB). This boundary forms in the ion foreshock and is associated with enhanced densities and magnetic field strengths. At times, but not always, the FCB separates the pristine solar wind plasma from the ion foreshock. In this study, we investigate the dependence of FCB characteristics on solar wind Mach number and cone angle (the angle between flow velocity and interplanetary magnetic field). We show that the strength of the foreshock compressional boundary increases with the Mach number. This enhancement is in turn tied to the density and velocity of the backstreaming ions in the foreshock whose interaction with the solar wind results in ULF turbulence which is ultimately responsible for the formation of FCB. During small cone angles the foreshock compressional boundary is symmetric with respect to the radial direction. As the cone angle increases, the FCB becomes less symmetric and eventually is confined to one side of the foreshock. The strength of the FCB also decreases with increasing cone angle but depending on the Mach number can exist for cone angles of 40° and beyond. A recent study that compared data from a global hybrid simulation of the foreshock with Cluster spacecraft observations showed that encounters with foreshock cavities can be interpreted as back and forth motion of a FCB causing spacecraft to move from the solar wind through the FCB into the foreshock and back into solar wind. An example of a FCB observed by the Cluster spacecraft is presented and shown to be in general agreement with model predictions.