A radio sounder outside the plasmasphere could provide nearly continuo
us remote density measurements of the magnetopause and plasmasphere, a
s well as other important density features elsewhere in this region. U
sing digital integration and tuned reception at frequencies from a few
kilohertz to a few megahertz with 400-m to 500-m tip-to-tip dipole an
tennas and 10 W transmitter power, such a sounder would be capable of
10% density resolution and 500 to 1300 km spatial resolution in only a
few minutes at distances of up to 4 R(E). By providing such detailed
observations of its principal density structures, such a sounder would
then clearly revolutionize magnetospheric research.