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.