GEOTAIL observations of anomalously low density plasma in the magnetosheath

Abstract

During the anomalously low density solar wind interval of May 1999. GEOTAIL was in the magnetosheath for Ο37 hours after making an inbound crossing of the expanding bow shock at Ο8 RE upstream of its nominal position. Comparison among data sets obtained from GEOTAIL (magnetosheath), WIND (near upstream - bow shock), and ACE (far upstream) reveals several unique features: Firstly, during the interval of 1430-1530 UT on 11 May, we observed both in the solar wind and magnetosheath double-peaked protons with a peak separation of 250-300 km/s, which was close to the local Alfvén velocity during the event. Secondly, we observed extremely strong strahl electrons both in the solar wind and magnetosheath during the interval of 0600-2100 UT on 11 May 1999. We present an overview of the GEOTAIL observations, and discuss their physical significance.