Hot, diamagnetic cavities upstream from the Earth's bow shock

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Abstract
An unusual type of plasma structure was observed by ISEE‐1 and ‐2 upstream from the earth's bow shock on eight occasions during the first 2 years of the mission. These events typically last ∼1–2 minutes and have very striking magnetic field and plasma signatures. The magnitude of the field is strongly reduced within the events, with the low‐field region (the magnetic cavity) flanked on one or both sides by strong enhancements. Within the cavity the plasma density is reduced compared to the ambient solar wind, and the flow velocity is much smaller and strongly deflected, with the direction of deflection correlated with the spacecraft location relative to the sun‐earth line. In addition, the ion and electron temperatures are very high (Ti ∼2×107 °K and Te ∼106 °K). Within the low‐field region the total pressure is an order of magnitude higher than in the ambient solar wind, and this pressure imbalance appears to be only partially offset by magnetic tension. These structures are interpreted as hot, probably expanding, diamagnetic cavities with a scale size on the order of a few RE, essentially convecting with the solar wind. A number of possible origins for the cavities have been explored, but no clear and completely satisfying explanation has been found. The most promising possibility considered is that the observed structures represent the solar wind signature of magnetopause flux transfer events.