On ion acoustic plasma waves at large magnetic aspect angles in the high‐latitude E region of the ionosphere

Abstract

A series of 50 MHz Doppler experiments has been performed during 3 consecutive summers, to investigate auroral backscatter properties at different magnetic aspect angles ranging from about 1° to 14°. In this paper the large data base is used to study a particular class of echoes, namely the ion acoustic‐type echoes. In the present understanding of radio aurora, this category of scatter, with characteristics reminiscent of equatorial type 1 echoes, is attributed to electrostatic plasma waves generated by the Farley‐Buneman instability. Linear theory predicts that the propagation vectors of such waves are limited to a range of angles within 0 to 1.5 degrees of perpendicularity to the magnetic field. Our observations, however, show that type 1 auroral echoes at 50 MHz occur regularly in the entire aspect angle range from about 0° to 14°. Therefore, the corresponding 3‐m irregularities could have finite wavevector components along the magnetic field, with ratios k∥/k⊥ as large as 0.25. On the average, the echoes appear to be weakly aspect sensitive at a rate near −1.5 dB/degree. The observed phase velocities decrease slowly with aspect angle at a rate of about 7 m/s per degree and, in general, have values in the nominal ion acoustic speed range of the plasma. The observations might be explained within the framework of the two‐stream instability, by invoking a very high anomalous electron collision frequency, since that would have a strong destabilizing effect at the larger aspect angles. At present, however, there is no theoretical justification for this interpretation and the aspect sensitivity of radio aurora remains an open and challenging problem.