Isee 1 charged particle observations indicative of open magnetospheric field lines near the subsolar region

On November 20, 1977, at 0230–0300 UT, Isee 1 encountered unusual charged particle distributions within the magnetosphere. The three‐dimensional distribution observations for energetic (>24 keV) ions and plasma show the development of field‐aligned asymmetries in the energetic ion distributions simultaneously with a marked change in plasma flow. We conclude that the most likely explanation for these observations is that Isee 1 encountered open magnetospheric field lines at its position within the magnetosphere (∼1030 LT and ∼1200 ±300 km from the magnetopause). Field lines were open near the geomagnetic equator, and the geometry was spatially or temporally variable. Other features of the field line topology are presented.