Electric field and plasma density oscillations due to the high-frequency Hall current two-stream instability in the auroralEregion

Observations of oscillating electric fields and plasma density fluctuations have been made in the E region of the earth's auroral ionosphere during a major magnetospheric substorm. By means of double probe and barium cloud measurements of the ambient dc electric field and Langmuir probe measurements of the plasma density, the plasma was known to be unstable to the high-frequency Hall current two-stream instability. The altitude range, frequency dependence, polarization, and amplitude of the fluctuations were in excellent agreement with the theory for this instability. In addition, simultaneous measurements of the plasma density and electric field fluctuations allowed determination of the real part of the dispersion relation at low frequencies. The waves were found to propagate with no dispersion at a velocity of about 500 m/sec in a reference frame fixed to the earth, a result consistent with an acoustic wave explanation for the phenomenon and implying that nonlinear effects stabilize the wave at a speed less than the electron streaming velocity.