ION-BEAMS OBSERVED IN THE NEAR-EARTH PLASMA SHEET REGION ON MAY 10, 1996

This Letter reports observations of ion distributions made by the Wind 3D plasma experiment on May 10, 1996 during the 0400 UT substorm. The observations come from the 3D ion analyzer with a geometrical factor two orders of magnitude larger than most of the similar instruments fl own through the near-earth geomagnetic tail. This has permitted observ ations of detailed ion beam characteristics during the passage of the spacecraft across the plasma sheet boundary into the lobe where the de nsity is very low. The plasma initially consists of cold and warm comp onents, with an additional hot component observed as the spacecraft ap proaches the plasma sheet-lobe interface. The beams originate from the warm component, which is the most dynamic. The Wind detector was even able to detect a weak beam inside the lobe where the density is less than or similar to 0.01 cm(-3). Some of these observations are new and are not completely explained by current theories.