FAST observations of solar illumination and solar cycle dependence of the acceleration of upflowing ion beams on auroral field lines

We present the magnetic local time, invariant latitude, and altitude distribution of upflowing ion beams and the dependence of their occurrence on whether the ionospheric foot point of the satellite is illuminated or dark and on solar cycle, as indicated by F10.7. The effects of illumination and solar cycle are additive, consistent with the fact that the solar EUV depends on solar cycle and that the ionospheric conditions are dependent on total flux illuminating the foot point. Consistent with previous studies, the occurrence of upflowing ion beams peaks in the premidnight local time sector, and the occurrence increases dramatically with altitude over the altitude range of FAST (∼700 km to ∼4000 km). Solar illumination both reduces the occurrence of ion beams observed by more than a factor of 10 and increases the altitude where the acceleration occurs so that beams are rare below ∼4000 km altitude. The effect of the increased solar EUV near solar maximum is almost as large. The inferred potential drops at altitudes below 4000 km, even during dark conditions, are usually less than 1 keV; during sunlit conditions and/or high F10.7, the potentials are usually less than 500 eV. These results place constraints on the altitude of the auroral parallel potential drop and on the relative importance of ionospheric conductivity and plasma density on the acceleration processes. There is no evidence that the statistical results are due to motion of large parallel potential drops to lower latitudes near solar maximum. Key Points Ion beams at altitudes below 4000 km suppressed by enhanced EUV at solar maximum Solar maximum effect comparable to but smaller than solar zenith angle effect Potential drop indicated by ion beam energy lower at solar maximum