Evidence for 50‐MHz bistatic radio observations of electrostatic ion cyclotron waves in the auroral plasma

Abstract

Crossed‐beam spectral measurements, made with two bistatic CW Doppler systems operating at 50 MHz, often reveal a type of radio auroral spectrum characterized by narrow peaks near 30 and 55 Hz. The experiment was designed so that spectra could be measured simultaneously from a common scattering region along two directions which differed in azimuth by 65° and which were at magnetic aspect angles of ∼5° and ∼12° from perpendicularity with the earth's field at 110 km altitude. During two magnetically active periods of continuous backscatter, this spectral type was found to occur regularly, especially in the few hours prior to magnetic midnight. Usually, the echoes were strong and lasted several minutes. The resonant peak in the spectrum occurs at either positive or negative shifts, or sometimes both. Mostly, the power lies in a narrow Doppler frequency band centered at either 55 or 30 Hz, but sometimes the power is in both bands, particularly for the path of larger magnetic aspect angle. The spectra can be almost identical in both observing directions but also can show systematic differences. From the relatively fixed locations of the spectral peaks, and from the occurrence of the echoes at angles well away from perpendicularity with the magnetic field, one infers that the echoes are due to primary 3‐m plasma waves having a finite wave vector component k∥ along the magnetic field lines and with ratio k∥ /k⊥ possibly as large as 1/3.5. The spectral evidence suggests the existence of electrostatic ion cyclotron waves near the gyrofrequencies of the main E region ions (i.e., NO+, O2+, O+). The existing kinetic and fluid theories of the ion cyclotron instability explain some, but not all, of the observed properties. Kinetic theory cannot explain the direct excitation of short (3 m) waves in the E region at such large magnetic aspect angles. Yet the frequent observation of these waves shows that the theories must be modified so as to relax the conditions under which the waves occur and to allow for Doppler shifts at almost exactly the gyrofrequencies of the main ions.