Abstract
Abstract. A small, 54 MHz wind-profiler radar, MARA, was operated at Troll, Antarctica (72° S, 2.5° E), continuously from November 2011 to January 2014, covering two complete Antarctic winters. Despite very low power, MARA observed echoes from heights of 55–80 km (polar mesosphere winter echoes, PMWE) on 60% of all winter days (from March to October). This contrasts with previous reports from radars at high northern latitudes, where PWME have been reported only by very high power radars or during rare periods of unusually high electron density at PMWE heights, such as during solar proton events. Analysis shows that PWME at Troll were not related to solar proton events but were often closely related to the arrival of high-speed solar wind streams (HSS) at the Earth, with PWME appearing at heights as low as 56 km and persisting for up to 15 days following HSS arrival. This demonstrates that HSS effects penetrate directly to below 60 km height in the polar atmosphere. Using local observations of cosmic-noise absorption (CNA), a theoretical ionization/ion-chemistry model and a statistical model of precipitating energetic electrons associated with HSS, the electron density conditions during the HSS events are estimated. We find that PMWE detectability cannot be explained by these variations in electron density and molecular-ion chemistry alone. PWME become detectable at different thresholds depending on solar illumination and height. In darkness, PWME are detected only when the modelled electron density is above a threshold of about 1000 cm−3, and only above 75 km height, where negative ions are few. In daylight, the electron density threshold falls by at least 2 orders of magnitude and PWME are found primarily below 75 km height, even in conditions when a large proportion of negative ions is expected. There is also a strong dawn–dusk asymmetry with PWME detected very rarely during morning twilight but often during evening twilight. This behaviour cannot be explained if PMWE are caused by small-scale structure in the neutral/molecular-ion gas alone but may be explained by the presence of charged meteoric dust.
Highlights
Polar mesosphere winter echoes (PWME) are VHF radar echoes from layered structures at heights 50–90 km during the winter months at high latitudes
Analysis shows that PWME at Troll were not related to solar proton events but were often closely related to the arrival of high-speed solar wind streams (HSS) at the Earth, with PWME appearing at heights as low as 56 km and persisting for up to 15 days following HSS arrival
Our results show that PWME are detected by Moveable Atmospheric Radar for Antarctica (MARA) in conditions when ionization rates are increased by the precipitation of high-energy electrons from the radiation belts, which is triggered by the arrival of high-speed solar wind streams
Summary
Polar mesosphere winter echoes (PWME) are VHF radar echoes from layered structures at heights 50–90 km during the winter months at high latitudes. A noticeable feature of PWME is the short interval each day when they are detected – below 75 km height they are confined to a few hours around noon (whereas PMSE are observed 24 h a day) This is apparent in all of the examples, which show PWME concentrated in the hours around noon and is further illustrated, which shows occurrence rates for all of our Troll PWME as a function of solar zenith angle – very few echoes are detected at solar zenith angles larger than 98◦, i.e. in darkness. PWME have often been reported in association with solar proton events, or during conditions of high geomagnetic www.ann-geophys.net/33/609/2015/
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