The radio plasma imaging technique is based on the reflecting properties of plasma cutoffs [Stix,1992]. As a freely propagating electromagnetic wave, of frequency f, enters a plasma or magnetic gradient, its propagation velocity in the direction of the gradient slows down because of the dielectric properties of the magneto-active plasma. The slowing down of the wave continues until it encounters a plasma cutoff where the wave frequency f equals the local cutoff frequency fc defined by the local plasma parameters: the electro gyro- and plasma frequencies, fce and fpe, respectively. At the cutoff, the wave refractive index vanishes and the wave energy is specularly reflected. For the ordinary mode wave, the cutoff frequency is fo=fpe; and for the extraordinary wave, the cutoff is fx=(fce/2){1+[1+4(fpe/fce)2]1/2}.
By sweeping in the sounding frequency, ionospheric physicists have been successful in using the radio sounding technique to determine the plasma structure of the topside [Jackson, 1969] and bottomside [Reinisch, 1986] ionosphere. Similarly, this technique can be applied to studying the magnetosphere [Franklin and MacLean, 1969; Green and Fung, 1994; Reiff et al., 1994]. With advanced digital sounding techniques and instrumentation, magnetospheric radio sounding has been shown to be feasible [Calvert et al., 1995; Green et al., 1996].
Figure 1 shows a simulated plasmagram where the radio echo time delays are plotted against the sounding frequencies. Two echo traces, one from the boundary layer and one from the plasmasphere, are shown simultaneously to indicate the probable radio plasma imaging observations from IMAGE when it is located within the magnetospheric cavity. Although the boundary layer and plasmasphere echo traces are likely to be observed simultaneously, they are distinguishable by their different angles of arrival, time delays, and power levels as well as characteristic shapes.
Figure 1. A simulated plasmagram showing the echo delays as a function of the sounding frequencies for both the boundary layer and plasmasphere simultaneously.