Abstract
We report ground-based measurements of the polar middle atmosphere made using a 230-250GHz passive microwave radiometer deployed at Troll station (72 degrees 01S 02 degrees 32E, L shell of L=4.8), Antarctica. Our observations show enhanced mesospheric nitric oxide (NO) volume mixing ratio (VMR) during a series of small recurrent geomagnetic storms in the 2008 austral winter, reaching 1.2 ppmv on day 200 (18 July). The Lomb normalized periodogram of the NO VMR time series averaged over 65-80km for days 130 to 220 of 2008 (9 May to 7 August) shows a peak exceeding the 95% confidence limit at 25.8days, close to the synodic rotation period for low-latitude solar coronal holes. The highest correlations between the radiometer NO VMR data and trapped and quasi-trapped electron count rates for L=3.5-5.5 from the Polar Orbiting Environment Satellites 90 degrees telescope are for the >30keV (90e1) channel (r(max)=0.56, lag time of 5.1days) and >100keV (90e2) channel (r(max)=0.57, lag time of 4.4days). Maximum correlation between NO VMR and the >700keV (90P6) channel data is lower but lag times are close to zero. Superposed epoch analyses for the eight most significant geomagnetic storm periods and three Carrington rotations (2070-2072) within the 90 day observation period indicate that significant NO abundance observed at 65-80km in the Antarctic mesosphere may be produced directly by >200keV electron precipitation or originate from a source at higher altitudes, e.g., production by >30keV electrons followed by downward transport.