Multi-mode Radio Sounding from IMAGE: Magnetospheric Electron Density (~90 km – 8000 km) Obtained from Simultaneous Nonducted Whistler and fast Z mode Echoes Observed on IMAGE

Abstract:

Using a recently developed ray tracing method [Sonwalkar et al., JGR, 116, A11211, 2011], we obtain magnetospheric electron density in the ~90 km – 8000 km altitude range from nonducted whistler mode (WM) and Z mode (ZM) echoes observed on the IMAGE satellite. The conventional topside radio sounding in free space R-X and L-O modes provides electron density profile from the satellite altitude down to ~F2 peak altitude. Radio sounding in a frequency range that covers both whistler mode and Z mode passbands, it is possible to obtain electron density profile down to Earth-ionosphere boundary near ~90 km. In addition, if the satellite is located within the Z mode cavity [Gurnett et al., JGR, 88, A1,1983], it is possible to receive fast Z mode echoes from reflections occurring both below and above the satellite altitudes [Carpenter et al., JGR 108, A12, 2003]. In such cases, the simultaneous occurrence of WM echoes reflecting from below and fast Z mode echoes reflecting from above and below allow us to determine electron density from altitudes several thousand km above the satellite altitude down to ~90 km. We apply ray tracing method to nonducted WM and fast ZM echoes observed by IMAGE on 03 Aug 2005 (ALT = 2216 km, L= 2.87, MLAT = 46.8°, MLT = 4.2). On this day, IMAGE observed obliquely incident specularly reflected (OI-SR), and normally incident specularly reflected (NI-SR) whistler mode echoes [Sonwalkar et al., JGR, 116, A11210, 2011] in ~20 – 250 kHz frequency range, and nonducted fast Z mode echoes from altitudes above and below the satellite in ~260-330 kHz frequency range. The measured time delay (tg) as a function of frequency (f) were matched with those calculated from ray tracing simulations to obtain electron density in ~90 - 8000 km altitude range. The electron density so obtained is in good agreement with in situ measurements from DMSP (850 km) and CHAMP (350 km) satellites and empirical models, including Carpenter and Anderson [JGR, 87, A2,1992], Gallagher et al. [JGR, 105, 18, 2000], Ozhogin et al. [JGR, 117, A06225, 2012], and IRI. We discuss the potential applications of combined WM and ZM radio sounding to better understand the physics of the inner magnetosphere, such as the problem of plasmasphere refilling after a geomagnetic storm.