Field aligned electron and ion density distribution inside the plasmasphere below 4000 km using whistler mode radio sounding by RPI on IMAGE

The whistler mode (WM) radio sounding method developed by Sonwalkar et.al. [2011b] for determining electron and ion densities along the geomagnetic field line was applied to 60 cases observed within the plasmasphere, covering L-shells 1.6 to 3.8 at altitudes below 4000 km. These cases were observed during solar minima (F10.7=72-110) and under quiet to moderate geomagnetic conditions (K_p =1+ to 4+). Our analysis shows: (1) both on day and night side of the plasmasphere the electron density (N_e) shows a decreasing trend from L-shell 2 to 3.8, (2) above 1500 km altitude N_e does not show any pronounced MLT dependence, (3) below 1500 km altitude, the effective ion mass on day side is more than two times higher than on night side (4) both O⁺-H⁺and O⁺-(H⁺+ He⁺)transition heights on the dayside are nearly two times higher than that on the night side. The electron and ion density profiles obtained from the analysis of these cases were compared with in situ measurements on IMAGE (electrons), DMSP (~850 km; electrons and ions), and CHAMP (~350 km; electrons) satellites, bottom side sounding from nearby ionosonde stations (electrons), and those by GCPM and IRI models. The comparison shows that (1) both DMSP satellite data and WM sounding results show significant amount of He⁺ (~0.1 to 0.4) on dayside during solar minima, (2) above ~1500 km altitude, GCPM underestimates Ne by ~30-70 % relative to RPI passive and WM sounding results, (3) IRI overestimates transition heights by ~100-500 km and NmF2 (F2 peak electron density) by ~20-80% relative to those measured by WM sounding, (4) the HmF2 (F2 peak height) from IRI was found to be constant at ~250 km on dayside and ~350 km on nightside, whereas the HmF2 from ionosonde and WM did not show any distinct pattern on day or night sides, but varied widely from 200 km to 400 km. Our work will lead to improved measurements of electron and ion densities along the geomagnetic field line and hence an empirical model of plasma parameters at altitude <4000 km inside plasmasphere, a region important for ionosphere-magnetosphere coupling.