STEEP ELECTRON-DENSITY GRADIENTS ASSOCIATED WITH DISCRETE AURORAE EXPLAINING THE DIVERSITY OF AURORAL VHF DOPPLER SPECTRA

At least two new types of spectra (types 3 and 4) from coherent VHF sc atter radar spectra of auroral E-region irregularities have been ident ified whose origins are poorly understood. They are often correlated w ith optical aurorae. The data obtained by a 50-MHz bistatic CW (contin uous wave) radar and an all-sky camera suggest that steep electron den sity gradients (SEDG) associated with discrete auroral arcs can explai n the diversity of the 50-MHz radar Doppler spectra, the narrow nonion -acoustic spectra in particular. Depending on the direction of the ele ctric field perpendicular to the arc, such gradients would either rais e or lower the threshold of two-stream instabilities that generate the irregularities. The increase in the electron collision rate with the electron temperature tends to decrease the importance of the gradient term in the dispersion relation. The consequence of electron heating i s that the gradients must be steeper to affect the threshold condition s significantly. Similarly, as previous studies demonstrated, the thre shold drift velocity at which the spectra tend to saturate can also be raised by an increase in the ambient ion-acoustic speed due to elevat ed electron temperatures (EET) in regions adjacent to auroral arcs. Du ring strongly turbulent conditions the spectral width is likely to inc rease and the mean velocities can exceed the nominal ion-acoustic spee d in the E-region. Several backscatter events from different times dur ing eastward and westward electrojet are presented to illustrate the d iversity of auroral spectra. It is shown that the EET and SEDG theorie s complement each other and can explain some of the composite spectra.