Measuring the Plasma Density Along the Magnetic Field: A Radio Sounding Technique

Song, P,  Huang, X,  Reinisch, B W,  Green, J L,  Fung, S F,  Gallagher, D

A new technique is introduced that remotely measures the plasma density profile
in the plasmasphere. The Radio Plasma Imager (RPI) on board the IMAGE satellite
transmits coded signals in the magnetosphere and receives the echoes. The
sounding frequency and the time delay of an echo carry the information about the
plasma density and distance at the reflection point of an echo. Echoes of the
same mode from a given transmission sweep and arriving from the same (Poynting
flux) direction should form a distinct trace on a plasmagram of echo amplitude as
a function of frequency and time delay. From such a trace and a given dispersion
relation, a density profile can be derived using a best-fit inversion algorithm.
Sometimes, multiple echo traces appear in a single plasmagram of a complete
frequency scan. We demonstrate how these traces may result from echoes of
different wave modes propagating along the field into both hemispheres. We will
show specific cases in which the inversion calculations are likely to yield the
electron density profile along the field from one hemisphere to the other.