Dust charging and density conditions deduced from observations of PMWE modulated by artificial electron heating

Abstract

We present an analysis of relatively strong Polar Mesospheric Winter Echoes (PMWE) under artificial electron heating that changes the PMWE intensity. A major purpose is to find reliable estimates of the relaxation time of the heater modified PMWE to their undisturbed state during the heater switch-off phase; the implications regarding charge/discharge mechanisms; and to exploit the diagnostic potential of artificial electron heating. The relaxation time is between 60 to 70 s for the regions with strong PMWE layers and substantial electron heating. This short relaxation time, related to the variation of charges on the nanometer dust which most likely is present in PMWE, rules out ion attachment as the mechanism to bring the dust charges to their equilibrium state. Neutral winds, sweeping the heated electrons out of the radar beam, are unlikely to be the cause of the observed relaxation, since this requires winds of around 100 m s−1. The most probable cause is photo detachment by which negatively charged dust can lose excess electrons by photon absorption with energies less than the dust material's work function. By comparing the observed heating with heating model profiles, the electron density at 65 km height must have been of the order of 3 × 109 m−3. This agrees with PMWE occurring mainly during disturbed conditions with high electron densities. Our results also indicate that in the strongest PMWE layers, electron bite-outs exist consistent with the role of charged dust particles in the mechanism of PMWE and implying larger dust densities.