Precipitating radiation belt electrons and enhancements of mesospheric hydroxyl during 2004-2009

Energetic particle precipitation leads to enhancement of odd hydrogen (HOx) below 80 km altitude through water cluster ion chemistry. Using measurements from the Microwave Limb Sounder (MLS/Aura) and Medium Energy Proton and Electron Detector (MEPED/POES) between 2004-2009, we study variations of nighttime OH caused by radiation belt electrons at geomagnetic latitudes 55-65 degrees. For those months with daily mean 100-300 keV electron count rate exceeding 150 counts/s in the outer radiation belt, we find a strong correlation (r >= 0.6) between OH mixing ratios at 70-78 km (0.046-0.015 hPa) and precipitating electrons. Correlations r >= 0.35, corresponding to random chance probability p <= 5%, are observed down to52 km (0.681 hPa), while no clear correlation is observed at altitudes below. This suggests that the fluxes of >= 3 MeV electrons were not high enough to cause observable changes in OH mixing ratios. At 75 km, in about 34% of the 65 months analyzed we find a correlation r >= 0.35. Although similar results are obtained for both hemispheres in general, in some cases the differences in atmospheric conditions make the OH response more difficult to detect in the South. Considering the latitude extent of electron forcing, we find clear effects on OH at magnetic latitudes 55-72 degrees, while the lower latitudes are influenced much less. Because the time period 2004-2009 analyzed here coincided with an extended solar minimum, and the year 2009 was anomalously quiet, it is reasonable to assume that our results provide a lower-limit estimation of the importance of energetic electron precipitation at the latitudes considered.