Swedish Institute of Space Physics, Kiruna, Sweden
Polar Mesosphere Summer Echoes (PMSE) are very strong radar backscatters from 80-90 km altitude which are seen in the summer time at polar latitudes. They have been observed by radars operating over a wide variety of frequencies including the EISCAT VHF and UHF radars. All features of PMSE are far from being completely understood despite of many years of intensive studies. Chilson et al. (2000) introduced a new method for investigation of PMSE using ionospheric heating by powerful ground-based HF transmitter the EISCAT Heating facility. They showed that PMSE were modulated by heating impulses in the way that their power dropped immediately when the heater was turned on and returned to the initial level when the heater was switched off. From a series of similar experiments deductions were made about the role of ambipolar diffusion and charged aerosol/dust particles in PMSE generation. Then Havnes et al. (2003) proposed theoretically (and confirmed experimentally) an ‘overshoot’ effect on PMSE whereby they are influenced by specially modulated heating so that a strong enhancement in PMSE strength occurs at the moment of heater switch-off compared to its initial level prior to heating. The behavior of PMSE power during the course of the heating cycle can provide us with information about the size and concentration of charged aerosol particles, and about enhanced electron temperature.
Recently one more polar mesospheric phenomenon, similar to PMSE but occurring in wintertime (whenever D-region ionisation is enhanced ) has been re-examined and termed PMW(inter)E (e.g. Kirkwood et al., 2002; Kirkwood et al., 2006). PMWE have been observed by 50 MHz MST radars as well as by the EISCAT VHF radar (Belova et al., 2005). Different mechanisms such as very strong neutral turbulence with or without presence of charged dust particles, or highly-damped ion-acoustic waves generated by infrasonic waves have been suggested for explanation of the phenomenon. PMWE- heating experiments might serve as diagnostic tool for charged particles in the winter mesosphere.
I will present an overview of the recent results in PMSE/PMWE research using ionospheric heating experiments and models.