Observations of the ionospheric projection of the plasmapause

We demonstrate a method for extracting the ionospheric projection of the plasmapause (PP) from DMSP measurements of the H + density in the topside ionosphere. The results agree well with PP locations derived from images of the plasmasphere acquired by the IMAGE spacecraft during a 72‐day period in 2001. The DMSP‐derived PP locations are usually earthward of the IMAGE EUV PP locations, on average about 0.5 L. The locations are also usually earthward of PP locations derived from the O'Brien and Moldwin [2003] empirical PP model, on average about 1 L. Nearly all of the cases when the DMSP‐derived PP location is more that about 0.5 L different than the IMAGE‐derived PP location occur during periods when the plasmasphere and PP are showing significant structure. Examination of these periods suggests that the DMSP measurements are often identifying physical structures in the plasmasphere.