Identifying the plasmapause in IMAGE EUV data using IMAGE RPI in situ steep density gradients

[1] Remote sensing of the entire plasmasphere is routinely accomplished by the Extreme Ultraviolet (EUV) imager on the IMAGE satellite. EUV observes the helium distribution in the plasmasphere by detecting resonantly scattered solar 30.4-nm ultraviolet radiation. In EUV images the plasmapause is assumed to be the He+ edge,'' i.e., the outermost sharp edge where the brightness of 30.4-nm He+ emissions drops abruptly. This assumption is verified by comparing the L-shell of steep electron density gradients, extracted from passive mode dynamic spectrograms recorded by the IMAGE Radio Plasma Imager (RPI) when the satellite is at low magnetic latitude, with the L-shell of EUV He+ edges obtained when the satellite is outside the plasmasphere near apogee. A statistical study of all inbound ( dawn sector) plasmapause crossings was performed for the month of June 2001. When the plasmapause location observed by RPI is compared to the location of the He+ edge extracted from the closest-in-time EUV image, a correlation coefficient of 0.83 is obtained. When the EUV He+ edge location is taken as the average of two EUV measurements ( one before and one after the RPI measurement), the correlation coefficient increases to 0.87. The high degree of correlation justifies the assumption that the He+ edge coincides with the plasmapause. For eighteen cases in which the plasmasphere has no sharp outer boundary the intensity of the uncalibrated EUV images is compared with the electron number density extracted from the RPI data, and the lower sensitivity threshold of the EUV instrument is estimated to be 40 +/- 10 electrons cm(-3).