Determination of the charge state of energetic magnetospheric ions by the observation of drift echoes

The mean charge states of heavy ions trapped in the earth's magnetosphere are important parameters for distinguishing among magnetospheric theories. No direct measurements have been made at energies greater than approx.50 keV. A technique, which uses the E/q dependence of ion drift velocity, is described in this letter. A drift-echo event, which followed a substorm that occurred at 2100-2200 UT at 25 February 1979, is discussed to illustrate the application of this technique. In this event the charge state of helium ions was +2 and CNO ions was >+2 and probably > or =+5. This result implies that the drift-echo particles were from freshly injected solar-wind plasma