EQUATORIAL SPACECRAFT-PLASMA INTERACTION PHENOMENON OBSERVED WITH DE 2

Data from the retarding potential analyzer and ion drift meter on Dyna mics Explorer 2 indicate that an unusual spacecraft-plasma interaction phenomenon occurs at times when the spacecraft velocity vector become s nearly aligned with the local geomagnetic field lines. The primary s ignature of the interaction is a transient increase in the ion collect ion currents obtained with these instruments, up to approximately 15% in magnitude and typically a few tens of seconds duration. This signat ure is indicative of an increase in the net ion and electron currents to the satellite and is accompanied by a small positive increase in th e spacecraft potential relative to the plasma. We present here a case study covering six of the strongest such events observed with DE 2 and discuss a possible physical mechanism. We suggest in particular that what might be called a collisional electron ''snowplow'' effect may be occurring, and we derive a simple numerical model based on this scena rio. Least squares fitting is employed to test the model and to derive new estimates of the ambient ion concentration at times when the meas urements are being perturbed by the interaction.