Variation of electron and ion density distribution along earth's magnetic field line deduced from whistler mode (wm) sounding of image/rpi satellite below altitude 5000 km

Abstract

This thesis provides a detailed survey and analysis of whistler mode (WM) echoes observed by IMAGE/RPI satellite during the years 2000-2005 below the altitude of 5000 km. Approximately 2500 WM echoes have been observed by IMAGE during this period. This includes mostly specularly reflected whistler mode (SRWM) echoes and ~400 magnetospherically reflected whistler mode (MRWM) echoes. Stanford 2D raytracing simulations and the diffusive equilibrium density model have been applied to 82 cases of MRWM echoes, observed during August-December of the year 2005 below 5000 km to determine electron and ion density measurements along Earth's magnetic field line. These are the first results of electron and ion density measurements from WM sounding covering L-shells ~1.6-4, a wide range of geomagnetic conditions (Kp 0+ to 7), and during solar minima (F10.2~70-120) in the altitude range 90 km to 4000 km. The electron and ion density profiles obtained from this analysis were compared with in situ measurements on IMAGE (passive recording; electron density (Ne)), DMSP (~850 km; Ne and ions), CHAMP (~350 km; Ne), Alouette (~500-2000 km; Ne and ions), ISIS-1, 2 (~600-3500 km; Ne, ions), AE (~130-2000 km; ions) satellites, bottom side sounding from nearby ionosonde stations (Ne), and those by GCPM (Global Core Plasma Model), IRI-2012 (International Reference Ionosphere). Based on this analysis it is found that: (1) Ne shows a decreasing trend from L-shell 1.6 to 4 on both the day and night sides of the plasmasphere up to altitude ~1000 km, which is also confirmed by the GCPM and IRI-2012 model. (2) Above ~2000 km altitude, GCPM underestimates Ne by ~30-90% relative to RPI passive measurements, WM sounding results. (3) Below 1500 km, the Ne is higher at day side than night side MLT (Magnetic Local Time). Above this altitude, significant MLT dependence of electron density is not seen. (4) Ion densities from WM sounding measurements are within 10-35% of those from the Alouette, AE, and DMSP satellites. (5) The effective ion mass in the day side is more than two times higher than night side below altitude ~500 km. (6) The O⁺/H⁺ and O⁺/(H⁺+H⁺+) transition heights at day side are ~300-500 km higher than night side; the transition heights from the IRI-2012 model lie within the uncertainty limit of WM sounding for night side, but for day side (L-shell>2.5) they are 200 km higher than WM uncertainty limits. (7) foF2 (F2 peak plasma densities) from ionosonde stations and the IRI-2012 model are ~1.5-3 MHz higher than those from WM sounding during daytime. These measurements are very important as the ion density profile along geomagnetic field lines is poorly known. They can lead to a better understanding of global cold plasma distribution inside the plasmasphere at low altitude and thereby bridge the gap between high topside ionosphere and plasmasphere measurements. These results will provide important guidance for the design of future space-borne sounders in terms of frequency and virtual range, in order to adequately cover ion density measurements at low altitudes and wide range of MLTs, solar and geophysical conditions.