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Plasma sheet pressure changes during the substorm growth phase

This paper reports clear evidence for increases in plasma pressure in the plasma sheet at 20–50 Re during the growth phase of substorms. The spacecraft WIND has provided the IMF Bz and other solar wind conditions, while the spacecraft GEOTAIL has observed plasma and magnetic field in the plasma sheet. We have examined 19 isolated substorms after clear southward turnings of the IMF Bz in steady solar wind conditions. Pressure increases start immediately after the southward turnings of the IMF Bz. Increases in plasma pressure are caused mainly by increases in plasma number density. Temperature does not show any significant changes. These observations suggest that the near‐Earth plasma sheet is compressed by enhanced magnetic field intensity in the tail lobes during the substorm growth phase.