Abstract
The Ku-band microwave frequencies (10.70-14.25 GHz) overlap emissions from ozone (O-3) at 11.072 GHz and hydroxyl radical (OH) at 13.441 GHz. These important chemical species in the polar middle atmosphere respond strongly to high-latitude geomagnetic activity associated with space weather. Atmospheric model calculations predict that energetic electron precipitation (EEP) driven by magnetospheric substorms produces large changes in polar mesospheric O-3 and OH. The EEP typically peaks at geomagnetic latitudes of similar to 65 degrees and evolves rapidly with time longitudinally and over the geomagnetic latitude range 60-80 degrees. Previous atmospheric modelling studies have shown that during substorms OH abundance can increase by more than an order of magnitude at 64-84 km and mesospheric O-3 losses can exceed 50 %. In this work, an atmospheric simulation and retrieval study has been performed to determine the requirements for passive microwave radiometers capable of measuring diurnal variations in O-3 and OH profiles from high-latitude Northern Hemisphere and Antarctic locations to verify model predictions. We show that, for a 11.072 GHz radiometer making 6 h spectral measurements with 10 kHz frequency resolution and root-mean-square baseline noise of 1 mK, O-3 could be profiled over 8 x 10(-4)-0.22 hPa (similar to 98-58 km) with 10-17 km height resolution and similar to 1 ppmv uncertainty. For the equivalent 13.441 GHz measurements with vertical sensor polarisation, OH could be profiled over 3 x 10(-3)-0.29 hPa (similar to 90-56 km) with 10-17 km height resolution and similar to 3 ppbv uncertainty. The proposed observations would be highly applicable to studies of EEP, atmospheric dynamics, planetary-scale circulation, chemical transport, and the representation of these processes in polar and global climate models. Such observations would provide a relatively low-cost alternative to increasingly sparse satellite measurements of the polar middle atmosphere, extending long-term data records and also providing "ground truth" calibration data.