• Occurrence patterns of whistler mode (WM) echoes observed by RPI/IMAGE and their relation to geomagnetic activity

      Reddy, Amani (2007-12)
      This thesis presents an analysis of whistler mode (WM) echoes observed at altitudes less than 5,000 km by the Radio Plasma Imager (RPI) on the IMAGE satellite. WM echoes are generated either by specular reflection (SR) of RPI signals at the Earth-ionosphere boundary (~90 km) or by magnetospheric reflection of RPI signals [...] at altitudes greater than 1,000 km [Sonwalkar et al., 2004; Sonwalkar et al., 2006]. These echoes are further influenced by field aligned irregularities (FAI) and are categorized into discrete, multipath or diffuse SR- and MR- WM echoes, based on their characteristic spectral forms. A survey of WM echoes observed during January 2004- December 2005 showed that WM echoes occurred at all latitudes and under moderate geomagnetic conditions. Occurrence patterns of WM echoes observed in August-December 2005 during geomagnetically quiet and disturbed periods indicate that geomagnetic storms lead to significant changes in FAI that affect the propagation of WM echoes. Our results help (1) in better understanding propagation and generation mechanisms of naturally occurring WM waves, and (2) in planning future WM wave injection experiments in space.
    • Variation of the plasmaspheric field-aligned electron density and ion composition as a function of geomagnetic storm activity

      Reddy, Amani; Sonwalkar, Vikas S.; Watkins, Brenton; Hawkins, Joseph G.; Bogosyan, Seta (2015-08)
      Whistler mode (WM) radio sounding is a powerful new method that provides measurement of both field-aligned electron and ion densities from the satellite altitude (<5,000 km) down to 90 km. Using radio sounding data from the Radio Plasma Imager (RPI) onboard the IMAGE (Imager for Magnetosphere-to-Aurora Global Exploration) satellite, this thesis presents a systematic and efficient approach to implement the whistler mode radio sounding method and discusses the uncertainties in the measured plasma parameters. The sounding method is applied to obtain the first measurements of plasmaspheric field-aligned electron density and ion composition as a function of geomagnetic storm activity during the mid-August to September 2005 period. This period included several geomagnetic storms of varying strength that occurred in succession. The plasmapause was located at L~2.4 during the onset and main phases of the storms. The whistler mode sounding results were augmented by measurements from the CHAMP and DMSP satellites, and ground ionosonde stations during the same period. On the day-side, at L~2, as a function of storm activity the following general results were found: (1) The electron density, relative ion concentrations, and O⁺/H⁺ transition height underwent temporal changes as a function of geomagnetic storm activity, and each species had different temporal behavior thus indicating different recovery times. (2) O⁺/=H⁺ transition height increased by ~200-300 km during the onset, main and early recovery phases of the storms. (3) Variation in the electron density below the O⁺=H⁺ transition height was different than that above. (4) Electron density at F2 peak increased during the onset or main phase of storms followed by a decrease in the recovery phase. (5) Electron density above O+=H+ transition height increased either in the onset or on the first day of recovery phase followed by a decrease. (6) αH₊ decreased during the onset, main and/or early recovery phases of storms; αo₊ increased in the early recovery phases of the storms; αHe₊ varied in a complex manner but in general there was an increase in αHe₊ during the onset phases and decrease in αHe₊ during the recovery phases of the storms. (7) When storms occurred in succession in an interval of roughly less than a day, the latter storms had little or no effect on the electron density and/or ion composition. On the night-side, WM sounding data was sparse. In the case of one moderate storm, we found that 3 days after the storm, at L~2.3, electron density at F2 peak and relative ion concentrations (at all altitudes) were comparable to those before the storm, whereas electron density above O⁺=H⁺ transition height decreased. WM sounding results for the day-side and night-side were in agreement with measurements from CHAMP (~350 km) and DMSP (~850 km). Whistler mode sounding results coupled with physics-based models will allow: (a) investigation of the role of thermospheric winds, dynamo electric fields, and storm time electric fields in causing the variations in electron and ion densities and (b) testing of current theories and validating physics-based models of the thermosphere-ionosphere-magnetosphere.