Magnetosheath and plasma sheet observations by GEOTAIL during the low-density solar wind event on 24-25 May 2002

During the low-density solar wind interval in May 2002, when the density in the solar wind was extremely low (0.1-0.3/cc), GEOTAIL was mostly in the dusk magnetosheath but went into the plasma sheet several times. Of particular interest are the plasma/field conditions in these regions when the solar wind seemed to become sub-Alfvénic intermittently (12 UT on 24 May - 12 UT on 25 May). We will report that (1) the magnitude of the magnetosheath magnetic field at GEOTAIL was lower than that in the upstream solar wind, that (2) the magnetosheath plasma density at GEOTAIL was comparable or even lower than that in the solar wind, and that (3) the plasma density in the plasma sheet became comparable to that of the magnetosheath solar wind. While the observations (1) and (2) give important information on the flow structure around the magnetosphere during such intervals, (3) extends our knowledge on the magnetosheath-plasma sheet density relationship.