Magnetospheric Active Wave Measurements P. Song, B.W. Reinisch, and X. Huang The magnetospheric community has conducted active wave measurements, which are made by radiating known signals to space and measuring the dispersion of the signals when propagating through magnetospheric plasma, for more than three decades. Because of the advances in space electronics and signal processing technology, active wave experiments can now cover a large dynamic range in frequency and be made in a large spatial range for versatile applications. They are becoming a promising new technique to probe the space plasma conditions. In this review we demonstrate the capability of magnetospheric sounding techniques employed by the radio plasma imager (RPI) on board the IMAGE satellite. This new technique, combined with the mathematical density inversion algorithm, measures the plasma density in situ at the satellite location and remotely and instantaneously along the magnetic field line from one hemisphere to the other down to as low as half an Earth radius (Re) in altitude. The technique has been formally validated. The database from the RPI active measurements covers all local times from 1.5 Re to 5 Re under different geomagnetic activities. Empirical models that are being developed specify the density as functions of radial distance, latitude, local time, distance along the field line from the earth's surface, solar wind conditions, geomagnetic indices, and other possible variables that affect the density distribution. The models can describe the statistical behaviors of the plasma distribution and can also provide snapshots of the plasma conditions on occasions. Dynamical processes that cause variations from the average models, such as depletion/refilling processes and plasma convection tail formation, can be studied. When applied to multiple satellites, or a constellation of satellites, the active wave measurements also make it possible for magnetospheric tomography, in which transmissions and reception of various waves within the constellation are used to derive the plasma density and magnetic field component in the constellation plane. _______________ Frontiers of Magnetospheric Plasma Physics: Celebrating 10 Years of Geotail Operation, Elsevier, 235-246, 2005.