First simultaneous and co-located measurements of the overshoot effect in the Polar Mesosphere Summer Echoes at 56 and 224 MHz

Abstract:

We report the first observations at 56 MHz (MORRO radar) of the overshoot effect in the polar mesosphere summer echoes (PMSE) when they are subject to artificial high power HF pulsed waves (Heating). Statistics indicate that there is an overall overshoot at this frequency despite of the high fluctuation of the backscatter. Simultaneous and co-located PMSE measurements at 224 MHz (EISCAT VHF radar) show also the overshoot effect. This experimental campaign was done around the peak of the PMSE season in 2013. The overall effect of the active modification of the PMSE strength is studied through the overshoot characteristic curve (OCC). At 224 MHz, available PMSE OCC measurements and modeling results indicate that during Heating the time scale of electron diffusion is shorter than the charging of dust particles. In this way, the free electron Bragg scatter structures surrounding dust particles are dispersed right after the Heating is turned on and leading to a decrease of the PMSE strength which can be followed by a recovery due to a slight increase of dust particle charging while the pulse is still active. Once the Heating is turned-off, the electrons cool down almost instantaneously and adopt the spatial distribution defined by dust particles, which at this time they are expected to be more charged due to influx of electrons. A highlight in this study is that we found some particular cases at 56MHz and 224MHz indicating that dust charging may overcome the diffusion process. This condition is known as the onset overshoot in which the backscatter increases after the heater was switched on. We have resorted to available models of the PMSE OCC at these two frequencies for finding similarities with our observations; especially those related to the onset overshoot. Through this evaluation we provide discussions about the differences between present measurements and model results, and plausible interpretation of physical conditions of particles and processes constituting PMSE observations included in this work.