Overshoot effect and polar mesosphere winter echoes on October 24, 2006

Belova, E.1, Smirnova, M.1, Rietveld, M.2, Isham, B.3, Kirkwood, S.1 and Sergienko, T.1

1 Swedish Institute of Space Physics, Kiruna, Sweden
2 EISCAT Scientific Association, Tromsø, Norway
3 Interamerican University, BayamYon, USA

Polar Mesosphere Winter Echoes (PMWE) are strong radar backscatter from altitudes between 50 and 80 km observed by VHF radars in the polar latitudes during winter time. They are seen when there is extra D-region ionization due to e.g. energetic solar protons or magnetospheric electron precipitation. Because PMWE are somewhat similar to Polar Mesosphere Summer Echoes (PMSE) there were attempts to explain them in the same way as PMSE: by neutral turbulence with the high Schmidt numbers. This assumes the presence of charged aerosol particles at PMWE heights. So far convincing evidence for the presence such particles has not been found when using radar, lidar or rockets experiments.

Havnes (2004) proposed an original experimental method for detecting aerosol particles at the summer mesopause which was called the PMSE overshoot effect. It is based on artificial heating of electrons in the PMSE region with a particular scheme of heater modulation. We have applied the same method to test PMWE for the presence of aerosols.

On October 24, 2006 PMWE were initially detected with the Esrad MST radar located at Esrange, northern Sweden and then at about 11 UT with the EISCAT VHF radar located near Tromso, Norway. At the same time we started to run the EISCAT heating experiment using a modulation pattern of 20 s heater on and 160 s heater off. Preliminary analysis shows a modulation of PMWE at about 68 km altitude with heating such that their strength was decreased during intervals when the heater was turned on. We did not found any ‘recovery’ of PMWE strength during the 20 s of heating. This is not enough to prove or disprove the presence of charged particles. We will present the results of further analysis of this experiment.