Modulation of Polar Mesospheric Summer Echoes (PMSE) during HF Heating

The formation of Polar Mesospheric Summer Echoes (PMSE) is linked to the presence of charged dust/ice particles in the mesosphere and the PMSE modulation is a possible way to observe the effect of dust charging. We investigate the modulation of PMSE by HF radio waves based on measurements carried out with the EISCAT Heating facility and the EISCAT VHF radar in August 2018 and August 2020 toward the end of the PMSE season. The measurements were made during the night with reduced solar illumination. The EISCAT Heating was operated in subsequent 48s on and 168s off intervals. In our observations, the PMSE modulation by the HF heating disappears during ionospheric conditions of energetic particle precipitation. We observe more than half of the cycles being influenced by the heating with a reduced PMSE power when the heater is on and a similar amount of cycles showing an increase in power when the heater is turned off. In less than half of the observed cycles we see an overshoot and it seems to be largely influenced by the overall PMSE power; with smaller or nonexistent overshoot when the PMSE power is high. We observe instances of very large overshoots, where background PMSE power seems to be reduced. During periods when we observe modulation, they often vary strongly from one cycle to the next; they are highly variable on spatial scales smaller km and time scales of minutes that are shorter than the scales assumed for the variation of dust parameters. Averaged curves over several heating cycles are similar to the overshoot curves predicted by theory and observed previously. Some individual curves however, show a stronger overshoot than observed in previous studies. A possible explanation for this difference can lie in the dust charging conditions that are different during the night or other conditions might be at play. We observe two possible instances of sporadic E-layers that are influenced by heating but do not show overshoots, as is to be expected.