C. Haldoupis et al., LOCALIZED AND STRONGLY UNSTABLE PLASMA REGIONS IN THE AURORAL E-REGION IONOSPHERE AND IMPLICATIONS FOR RADAR EXPERIMENTS, J GEO R-S P, 100(A5), 1995, pp. 7771-7782
The results in this paper were obtained with SAPPHIRE, a new auroral D
oppler radar experiment designed to study meter-scale E region irregul
arities. SAPPHIRE is a dual 50-MHz continuous wave, phased array, mult
ibeam, bistatic system which is capable of performing cross-beam measu
rements from two widely different directions. There are two transmitte
rs, each of which probes the auroral electrojet plasma over a large sp
atial target grid area of multiple intersections that determine 16 sca
ttering regions of cells. Initial observations using untapered antenna
arrays showed a class of scatter characterized by a narrow power spec
trum peaking at the same Doppler shift in all, of several, observing c
ells simultaneously. These are strong echoes ranging in lifetime from
a few tens of seconds to a few minutes and occurring preferentially in
the midnight and morning magnetic time sectors. The analysis showed t
hat this scatter is strongly anisotropic in azimuth and comes from loc
alized regions of spatially coherent large-amplitude plasma waves that
produce mostly type III, but also type I and the rare type IV, radar
auroras. By using many events and analyzing a large number of Doppler
spectra, we found that type III echoes are the strongest observed, hav
ing on the average relative intensities at least 15 dB higher than the
type I echoes. The observations relate to the ''short discrete radar
auroras'' which are known to originate in spatially confined, dynamic
plasma regions. The possibility exists that the large free energy for
instability in these active regions is provided from intense electric
fields and/or very sharp electron density gradients, both expected to
occur at times near the edges of discrete auroral arcs. Finally, the p
resent results confirm that, because of the large dynamic range of rad
io auroral echoes, strong scattering regions lead to the complete domi
nation, at times, by backscatter through antenna sidelobes. For the lo
calized regions of strong type III and type I echoes, this means that
the conventional 3-dB antenna beam width scale size of the scattering
region is unrealistic. Obviously, this has important implications for
the radar auroral experiments and the interpretation of observations.