Dependence of the distant tail magnetopause position on the solar wind and IMF Original Research Article

Utilizing the GEOTAIL data obtained in the distant (X < −110 Re) magnetotail and the simultaneous solar wind and interplanetary magnetic field data from IMP 8, we investigate how the solar wind plasma and magnetic field affect the location of the distant tail magnetopause. Our analyses are performed in a coordinate system whose x axis is parallel to the incident direction of the upstream solar wind. The distant tail magnetopause is found to respond to the changes in the solar wind static (magnetic plus thermal) pressure rather than the dynamic pressure, in contrast with the past near-tail observations. This result confirms that in the distant region, the flaring of the magnetotail almost ceases and the solar wind dynamic pressure contribution to the pressure balance at the magnetopause is small. For northward interplanetary magnetic field, the power law index of the tail radius dependence on the solar wind static pressure is consistent with the −14 power law that is theoretically expected when the total magnetic flux in the tail does not depend on the solar wind pressure. On the other hand, during southward interplanetary magnetic field, the magnetic flux is often much larger than that during northward case. A likely cause of this feature is the pile up of the dayside-eroded magnetic flux.