Foreshock Cavities: Direct Transmission Through the Bow Shock

Global hybrid code simulations predict enhanced densities and magnetic field strengths not only in foreshock compressional boundaries on the flanks of the steady‐state dayside foreshock under near‐radial IMF conditions but also on the edges of the traveling foreshock cavities formed when slabs of interplanetary magnetic field (IMF) lines connect to the bow shock. The simulations predict only modest velocity or temperature perturbations attending the foreshock compressional/cavity boundaries. However, they predict pronounced flow enhancements and depressed temperatures accompanying the enhanced densities and magnetic field strengths on foreshock compressional/cavity boundaries transmitted into the magnetosheath. Many of these properties are similar to those of magnetosheath jets, previously attributed to processes occurring at locations where the solar wind flow makes a grazing encounter with a locally oblique bow shock. Simultaneous THEMIS observations upstream and downstream from the Earth’s bow shock from 1430 to 1630 UT on June 14, 2007 confirm simulation predictions for the direct transmission of foreshock cavities into the magnetosheath. Foreshock cavity density and magnetic field strength perturbations diminish very rapidly with distance upstream from the bow shock. Plain Language Summary Simulations predict that solar wind ions energized by their interaction with the Earth’s bow shock excavate cavities bounded by enhanced densities and magnetic field strengths, but no significant flow and temperature variations, in the solar wind. The same simulations indicate that transmission through the bow shock causes these boundaries to exhibit enhanced flows and depressed temperatures. A case study of multipoint THEMIS spacecraft observations provides evidence for cavity transmission through the bow shock. The regions of enhanced density, flow, magnetic field strength, but depressed temperature on the edges of magnetosheath cavities have properties similar to events previously described as “jets.” Key Points Foreshock cavities and their transmitted counterparts in the magnetosheath exhibit similar correlated density and magnetic field signatures Flow enhancements and depressed temperatures bound transmitted cavities The boundaries of transmitted cavities have properties similar to magnetosheath jets but do not lie behind the oblique portions of rippled bow shocks