Radar Measurement of Ionosphere Motion in the Presence of Current‐Induced Spectral Asymmetries

The technique of measuring ionosphere motion from the shift in the incoherent‐scatter ion line has been known for some time. The underlying assumption in the adaptations of the technique applied so far is that there are no detectable current‐induced asymmetries in the spectrum. In the auroral zone this assumption can be violated during magnetospheric storms. To cope with this possibility we use an estimate of the first moment of the spectrum together with a matched‐filter‐like estimate. The first moment is the sum of the first moment of the unshifted spectrum plus the Doppler shift. The matched‐filter estimate, while not unaffected by asymmetries, tends to give an estimate closer to the Doppler shift. Hence, a difference between the two estimates is an indication of a current flowing in the range gate where the spectrum is measured. We assume that no other sources of asymmetry are present, although a large temperature gradient would have a similar effect. In principle the difference can be calibrated to give the magnitude of the current. This step is not yet completed owing to uncertainties in the composition of the ionosphere at low altitudes and a loss of frequency resolution due to the finite pulse length. The detectability of the currents by our method, however, is not affected by poor frequency resolution. Preliminary results from the Chatanika Auroral‐Zone Incoherent‐Scatter Radar are presented which show large velocities correlated in detail with simultaneous magnetometer data. Also an indication of a current flow has been observed in premidnight data taken on a very active night. An azimuth scan showed the sense of the current to be predominantly westward.