ENHANCED INCOHERENT-SCATTER PLASMA LINES

Detailed model calculations of auroral secondary and photoelectron dis tributions for varying conditions have been used to calculate the theo retical enhancement of incoherent scatter plasma lines. These calculat ions are compared with EISCAT UHF radar measurements of enhanced plasm a lines from both the E and F regions, and published EISCAT VHF radar measurements. The agreement between the calculated and observed plasma line enhancements is good. The enhancement from the superthermal dist ribution can explain even the very strong enhancements observed in the auroral E region during aurora, as previously shown by Kirkwood et al . The model calculations are used to predict the range of conditions w hen enhanced plasma lines will be seen with the existing high-latitude incoherent scatter radars, including the new EISCAT Svalbard radar. I t is found that the detailed structure, i.e. the gradients in the supr athermal distribution, are most important for the plasma line enhancem ent. The level of superthermal flux affects the enhancement only in th e region of low phase energy where the number of thermal electrons is comparable to the number of suprathermal electrons and in the region o f high phase energy where the suprathermal fluxes fall to such low lev els that their effect becomes small compared to the collision term. To facilitate the use of the predictions for the different radars, the e xpected signal-to-noise ratios (SNRs) for typical plasma line enhancem ents have been calculated. It is found that the high-frequency radars (Sondre Stromfjord, EISCAT UHF) should observe the highest SNR, but on ly for rather high plasma frequencies. The VHF radars (EISCAT VHF and Svalbard) will detect enhanced plasma lines over a wider range of freq uencies, but with lower SNR.