Drift Motions of Auroral Ionization

Abstract

The primary subject of this report is the drift motions of auroral ionization. The existence of rapid motions of the ionization has been demonstrated in previous radar studies of the aurora, but neither the nature of the motions nor their explanation has been established until now. The purpose of our experimental observations was to determine the direction and speeds of the motions. In doing so, we obtained additional information concerning the general nature of the auroral ionization. Measurements were taken at College, Alaska, during the winter and spring of 1956-57* using CW transmitters. By locating the transmitters at Sielson Air Force Base, it was possible to separate the transmitters and the receivers by bZ kilometers along a ge©magnetically east-west line. The basic technique used was to examine the frequency spectra of radio echoes from the aurora at 106 Mc/s and tyl.l5 Mc/s. A comparison of the results obtained at 106 Mc/s and WL.15 Mc/s showed that the frequency shifts are proportional to the transmitted frequency, as would be expected of Doppler shifts. By measuring the spectra of the echoes received from east and west of geomagnetic north at the same time, it was possible to determine the following: (i) That the motions are generally horizontal and in the geomagnetic east-west plane; and (ii) That the speeds of the motions vary from 350 meters per second to 2,000 meters per second. On the basis of our experimental results and the published literature, we show that the electron drift motions in the aurora are of the same order of magnitude and direction as the motions of the electrons in the ionospheric current system required to explain magnetic disturbances. These electron motions produce the Doppler shifts that are responsible for the well known rapid fading of auroral radio echoes. The fading of radar auroral echoes is therefore associated with the increased electric fields which drive the currents in auroral regions. Following a review of the available information concerning general motions in the ionosphere, motions of the visible aurora, and motions inferred from magnetic storms, we show that the drift motions of auroral ionization do not constitute a separate and distinct group. Instead, they are found at the upper end of a continuous curve of increasing speed of motions with increasing magnetic disturbance. The intense ionospheric currents that produce the magnetic disturbances are found to be associated with both increased electron density and increased speed of motion. In our examination of the amplitude of VHF radio auroral echoes, the basic premises of the theory of scattering by non-isotroplc irregularities produced by turbulence [Booker, 1956] are found to be satisfactory. However, the numerical values of the parameters suggested by Booker require revision. In particular, our results indicate that the mean square fractional deviation of electron density is much greater than Booker conjectured on the basis of the then available evidence; in fact, it is greater by two to three orders of ten.