Interpretation of the He II (304 Å) EUV image of the inner magnetosphere by using empirical models

The visualization of the inner magnetosphere is a long‐cherished goal in magnetospheric physics. This goal was recently achieved with the He II (304 Å) image of the inner magnetosphere by using the EUV scanner on board the Planet‐B spacecraft (Nakamura et al., 2000). However, our understanding of the inner magnetosphere in relation to these images has not been assessed. The two‐dimensional (2‐D) EUV image is difficult to interpret, as it is necessary to infer the three‐dimensional (3‐D) spatial information of the inner magnetosphere from the 2‐D remote sensing data. We examine our present knowledge of the inner magnetosphere and compare simulated 2‐D images with the real EUV image. We conclude that an empirical model that assumes conventional E × B flows and combines the diffusive equilibrium model of Chiu et al. (1979) and the Weimer (1995) electric potential and Tsyganenko (1989) magnetic field models is in good agreement with the EUV image only within the Earth‐dominated flow regime. Furthermore, the model gives a better fit to the EUV image data, by assuming the convection‐dominated flow regime to be filled with ionospheric He + at a filling rate of 1.2·(6.6/ L ) −3.0 (He + ions cm −3 d −1 ), instead of the well‐known L −4 density dependence on flux tube volume.