Loss of plasmaspheric ions during a storm observed by the EUV scanner onboard Planet-B

The first measurement of He II (304 Angstrom) emission in the dayside magne tosphere has been done by the EUV scanner onboard Planet-B. A magnetic stor m with the minimum Dst of -70 nT occurred during the observation. Before th e initial phase of the magnetic storm, the average He+ density of similar t o1 cm(-3) in the dayside magnetosphere was observed. During the main phase, the average He+ density rapidly increased up to 4-20 cm(-3). During the re covery phase, the average density smoothly decreased to the lowest density, which is due to the daily ionospheric filling. The total loss of the ions to the outer magnetosphere is estimated by 3.6x10(30) ions. This is suffici ently large to populate the plasma sheet.