STRONGLY ENHANCED INCOHERENT-SCATTER PLASMA LINES IN AURORA

Observations of incoherent-scatter plasma-line intensities, measured i n the E-region with the European Incoherent Scatter UHF radar, during auroral precipitation, are presented. Intensities up to 200 times the thermal level were observed in a restricted frequency interval from 5. 5 to 6.5 MHz. Intensities were lower at both higher and lower frequenc ies. The intensities are compared with quantitative estimates based on model suprathermal electron fluxes for the prevailing conditions. The minimum in suprathermal electron fluxes between 2 and 4 eV, which is caused by the excitation of vibrational levels in N-2, is found to res ult in a minimum in expected plasma-line damping and a maximum in inte nsity between 5.5 and 6.5 MHz, for the EISCAT UHF wavelength. Good agr eement is found between the calculated and observed intensities.