First VHF auroral radarinterferometer observations

The radar interferometer technique on two separate antennas. The coherence first used at the magnetic equator in Peru is (amplitude) of the cross spectrum at each also a very powerful means for studying auroral frequency is a measure of how localized the plasma instabilities. We present here the first scattering target corresponding to that results, obtained with a 49.92 MHz, 20-25 KW peak particular Doppler shift is within the radar power pulsed radar located in Ithaca, NY (42.5 ø scattering volume. The corresponding phase of N, 76.4 ø W). Strong auroral echoes were obtained the cross spectrum is determined by the angular during several highly active periods. Phase position of this scattering center. Changes in differences between the signals received on the this phase with time are a measure of the two antennas accurately determine the E-W velocity normal to the radar beam. Combining the position, within the scattering volume, of Doppler and phase information gives the full localized scattering centers, and changes in this horizontal velocity vector at each range. The phase determine the corresponding velocity. The purpose of this paper is primarily to describe signal Doppler shift describes radial and illustrate the technique; the observations (essentially N-S) motion. The data provide will be discussed in detail elsewhere. detailed information on the turbulent structure of the echoing region and show clearly that Experimental Technique different features in the Doppler power spectrum often represent signals coming from different The results presented here were obtained with locations. If we assume that the radial and a 49.92 MHz pulsed radar located in Ithaca, NY transverse phase velocities represent real drift (42.5 o N and 76.4 o W ). Two 26 element arrays of velocities, we can determine full horizontal co-linear half-wave dipole antennas placed side velocity vectors from the data and hence the by side and 1.5 wavelengths above the ground were horizontal electric field, usually with a time directed 20 ø east of geomagnetic north. The resolution of the order of 15-30s. beamwidth of each antenna was about 5 degrees. Figure 1 shows the location and direction of the Introduction radar beam. The west antenna was used for transmission and both antennas were used Recent work in Peru has shown that a radar separately for reception. The distance between interferometer can provide considerable the phase centers of the two antennas is 51.6 m. information about equatorial plasma processes The transmitter was operated with a pulse length such as drifts and turbulence œFarley et al., of 50 Us at a peak power of 20 to 25 XW with an 1981; Kudeki et al., 1981]. This work motivated interpulse period (IPP) of 2 ms for the spectral us to try the same technique on auroral E-region mode and 10 ms for the RTI (Range-Time-Intensity) irregularities, and we have found that it works mode. The two quadrature base band outputs from at least as well as, if not better than, at the equator. Other investigators [Balsley and Ecklund, 1972; Greenwald et al., 197õ; Haldoupis and Sofko, 1976, 1979; Moorcroft and Tsunoda, 1978] have obtained detailed Doppler spectra with either single or multibeam radars, but the spectra suffered from poor vertical and horizontal spatial resolution. Since the Ithaca auroral radar interferometer can obtain Doppler spectra with good temporal and two dimensional spatial resolution, we hope to learn much about the plasma instabilities which generate the irregularities. The structure of the irregularities is resolved in the north-south direction with the usual range gating of the pulsed radar and in the east-west direction by the interferometer technique. ( The vertical structure can also be studied with a properly oriented interferometer, but that topic will not be discussed here.) This technique entails first finding the normalized cross spectrum of the signals received Copyright 1983 by the American Geophysical Union. Paper number 3L0330. 0094-8 276 / 83 / 003L-033 053.00 / . _ __1•(} "•0 • _ •' 900 km uJ I ...... Girardville • •. r ou ,m +:1. Rol:;erv•l l 4o _ /00m _1 _ t 450 km •.Ottawa