Steep electron density gradients associated with discrete aurorae explaining the diversity of auroral VHF Doppler spectra

Details Steep electron density gradients associated with discrete aurorae explaining the diversity of auroral VHF Doppler spectra Steep electron density gradients associated with discrete aurorae explaining the diversity of auroral VHF Doppler spectra

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995angeo..13...25p&link_type=abstract

Annales Geophysicae (ISSN 0992-7689), vol. 13, no. 1, p. 25-37

Other

4

Auroral Arcs, Auroral Electrojets, Auroral Spectroscopy, E Region, Electron Density Profiles, Acoustic Velocity, Backscattering, Doppler Radar, Electron Energy, Ion Acoustic Waves, Spectral Signatures, Very High Frequencies

Scientific paper

At least two new types of spectra (types 3 and 4) from coherent VHF scatter radar spectra of auroral E-region irregularities have been identified whose origins are poorly understood. They are often correlated with optical aurorae. The data obtained by a 50-MHz bistatic CW (continuous wave) radar and an all-sky camera suggest that steep electron density gradients (SEDG) associated with discrete auroral arcs can explain the diversity of the 50-MHz radar Doppler spectra, the narrow nonion-acoustic spectra in particular. Depending on the direction of the electric field perpendicular to the arc, such gradients would either raise 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 is that the gradients must be steeper to affect the threshold conditions significantly. Similarly, as previous studies demonstrated, the threshold drift velocity at which the spectra tend to saturate can also be raised by an increase in the ambient ion-acoustic speed due to elevated electron temperatures (EET) in regions adjacent to auroral arcs. During strongly turbulent conditions the spectral width is likely to increase and the mean velocities can exceed the nominal ion-acoustic speed in the E-region. Several backscatter events from different times during eastward and westward electrojet are presented to illustrate the diversity of auroral spectra. It is shown that the EET and SEDG theories complement each other and can explain some of the composite spectra.

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