IMAGE and DMSP observations of a density trough inside the plasmasphere

We report coordinated observations of a density trough within the plasmasphere using the measurements from the radio plasma imager (RPI) and extreme ultraviolet imager (EUV) on the IMAGE satellite and the measurements from DMSP F-15. The density trough inside the plasmasphere with a width of ∼0.7 R_E in terms of L shell (from L ∼ 2.3 to L ∼ 3.0) was observed in situ by RPI when IMAGE traversed the plasmasphere in ∼2130 magnetic local time (MLT) sector. The plasmasphere images taken by the IMAGE EUV instrument confirm that the density trough is inside the plasmasphere. A 2-D electron density image constructed from the RPI active sounding measurements reveals that the density trough extends along the magnetic field from the IMAGE orbit to at least 41 magnetic latitude. Meanwhile, the DMSP-F15 satellite, circling the Earth at about 850 km altitude, detected a light ion density trough at the same time on the same L shells and similar MLT sector. The coordinated observations with the IMAGE and DMSP-F15 satellite demonstrate, for the first time, that the density trough is a low-density plasmaspheric structure extending from the plasmasphere to the topside ionosphere along the geomagnetic field lines.