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Low Mach number bow shock locations during a magnetic cloud event: Observations and magnetohydrodynamic simulations

During the magnetic storm on October 18–19, 1998 a magnetic cloud encountered the Earth's magnetosphere. As the cloud passed, the solar wind density decreased dramatically, and as a consequence the solar wind magnetosonic Mach number dropped markedly, approaching 1 at times. Magnetohydrodynamic (MHD) simulations, driven by upstream interplanetary magnetic field (IMF) and solar wind parameters observed on WIND and ACE showed that during the interval of the lowest density, the subsolar distance of the bow shock (BS) reached as much as 50 RE. IMP‐8 spacecraft observations of the expansion and subsequent retreat of the BS support the finding that the shock reached a considerable distance upstream. Surprisingly, at times the BS moved sunward at a speed approaching the magnetosonic speed of the solar wind.