IMAGE Observations of "Patchy" Specularly Reflected Whistler Mode Echoes: A New Diagnostic Tool for Large-scale Field-aligned Irregularities

Field-aligned irregularities (FAIs) are one of the key indicators of space weather and physical processes that occur due to changing space weather conditions. Whistler mode (WM) radio sounding from the IMAGE satellite has led to observations of "Patchy" specularly reflected (SR) echoes, which provide a new way to measure FAI. Patchy SR echoes result from IMAGE signals (transmitted at different frequencies) propagating in the presence of largescale FAI and returning to the satellite after reflecting specularly at the Earth's ionosphere boundary. The presence of FAI along the ray paths leads to the deflection of signals over a range of transmitted frequencies, resulting in a "patchy" echo pattern. Using patchy SR echoes observed on 22 October 2005, we demonstrate a raytracing method to determine FAI parameters. Our results indicate that irregularities of 10-100 km scale size at 200 – 1600 km and enhancement of 50-70% along L ~ 2.185 explain observed patchy SR echoes. The method demonstrated here for FAI measurements applies to the lightning-generated whistlers (that display a patchy pattern) observed on spacecraft such as Van Allen Probes. Application of this method to patchy echoes/whistlers observed on spacecraft (e.g., IMAGE, DSX, Van Allen Probes) should provide data on ionospheric and magnetospheric plasma density structure not available previously and could be used to build empirical models of FAI that can potentially serve as an indicator of space weather.