Artificial Modification of Polar Mesospheric Winter Echoes (PMWE) With an RF Heater: Do Charged Dust Particles Play an Active Role?

An investigation of Polar Mesospheric Winter Echoes (PMWE) has been carried out with the MST MORRO radar operating at 56 MHz. MORRO has been deployed recently at the EISCAT installation near Tromsoe in northern Norway home of two other large incoherent scatter radars and an RF heating facility. The main object of the investigation is to examine whether, and if yes how, RF heating influences PMWE. In particular, the experimental observation of the overshoot effect would indicate the presence of charged dust particles. On 11 February 2008 we measured several weak and variable PMWE layers and we used the RF heater with an on period of 20 s and off period of 100 s to modulate the radar scatter from the layers. We chose one layer, which for 44 minutes was the strongest and most stable layer, for further analysis. The signal intensity variation during an averaged on/off heater period shows the expected weakening of the signal intensity when heating is turned on followed by a significant small recovery of the signal during the on phase and a corresponding small overshoot of the signal strength of about 13-15% over the background level when heating is switched off. The recovery and overshoot are attributable to charge accumulation on the dust particles due to electron heating. The overshoot characteristic curve shows that a considerable increase in the electron temperature did take place during heating and that charged dust particles should be present, probably with radii of some nanometers.