Response of Middle Atmospheric Hydroxyl Radical to the 27-day Solar Forcing King-Fai Li 1, Qiong Zhang 2, Shuhui Wang 3, Yuk L. Yung 2, and Stanley P. Sander 3 2 Department of Applied Mathematics, University of Washington, Seattle, WA, USA 2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA. 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA Abstract In this study, we use the Aura Microwave Limb Sound (MLS) data to examine the response of middle atmospheric hydroxyl radicals (OH) to the 27-day solar rotational variability during MLS’s first measurement year (2004–2005) when solar activities are moderately strong. During this period, OH shows a positive response to the 27-day solar forcing without significant phase lag, while H 2 O shows a negative response to the solar forcing with a time lag of 7 days. OH response vertical profiles show similar features at different latitudes. The observation results are compared to the simulations of a 1-D photochemical model KINETICS using different UV variabilities as input. The comparison shows a significant discrepancy between satellite observation and model simulation. In the MLS data, OH 27-day solar response is ~1% per 1% change in Lyman-α peaked at 78 km. However, the peak response in the photochemical model simulation is ~0.3% per 1% change in Lyman-α. Since the model underestimates OH solar cycle response and the results are insensitive to the selection of solar spectral irradiance (SSI) forcing, further sensitivity tests on model parameters, such as reaction rate coefficients are eddy diffusion coefficients are needed to resolve the discrepancy between model and observation. Data and method Conclusions Model simulation  OH in the mesosphere has a good correlation with Lyman-α index without phase- lag. Its largest solar cycle response is ~1% per 1% change in Lyman-α peaked at 78 km. OH response shows similar vertical profiles in different latitudes.  H 2 O has a weaker correlation with Lyman-α index than OH with a time lag of 7 days. At 80 km, H 2 O solar cycle response is % per 1% change in Lyman-α.  Comparison between MLS data and model simulation shows that photochemical model significantly underestimates the OH solar cycle response in the mesosphere. The underestimation is not caused by the selection of SSI input. Further sensitivity tests are needed.  OH solar cycle response profiles show similar features at different latitudes with a peak ~1% per 1% change in Lyman-α. This result is similar to Shapiro et al. [2012]. The largest response appears in the tropics.  H 2 O in the stratosphere is not mainly destructed by photolysis and its abundance shows a weak correlation to the 27-day solar cycle [Brasseur, 1993]. Solar cycle response of H 2 O is only reliable above 75 km where its abundance is small [Gruzdev et al., 2009].  Model simulation shows a much smaller OH response peak ~ 0.3 % per 1% change in Lyman-α. The profile is insensitive to the selection of input SSI.  At hPa (78 km) in the tropics, OH has a peak solar cycle response as 1% per 1% change in Lyman-α index. Since its life time is short, there is no phase lag between OH and Lyman-α.  We also perform the same regression analysis to test the residual data to show the robustness of the band-pass filter. Residual data of OH and H 2 O have no significant correlations with Lyman-α index.  The correlation between H 2 O and Lyman-α index is weak. At 0.01 hPa (80 km), H 2 O has a negative solar cycle response as % per 1% change in Lyman-α. The 7- day time lag is calculated to yield that largest negative correlation between H 2 O and Lyman-α index. Figure 2. (a) Variation of the filtered data relative to the mean value. (b) Linear fit of the filtered OH (H 2 O) variation and Lyman-α variation, the regression slope is the OH (H 2 O) response to the 27-day solar forcing. (c) Same as a, but for the residual data (original data – filtered data). (d) Same as b, but for the residual data. OH and H 2 O solar cycle response OH 27-day solar cycle response  Both OH and H 2 O show the 27-day period variation modulated by the solar spectral irradiance change. Since OH in the mesosphere is primarily produced by the photolysis of H 2 O and the reaction of H 2 O with O( 1 D) [Canty and Minschwaner, 2002], the 27-day variation amplitude in OH is much larger than in H 2 O, whose abundance is not dominated by the photochemical reactions.  The correlation of OH or H 2 O with Lyman-α is only significant after the band- pass filter is applied. The results are insensitive to the filter window size. H 2 O 27-day solar cycle response Figure 3. MLS OH solar cycle response in the (a) tropics averaged between 25°S - 25°N, (b) northern hemisphere mid-latitude region averaged between 25°N - 50°N, and (c) southern hemisphere mid-latitude region averaged between 25°S - 50°S. (d) Simulation by the Caltech-JPl 1-D KINETICS photochemical model with different input solar spectral irradiance. References: Brasseur, JGR, 1993; Canty and Minschwaner, JGR, 2002; Gruzdev et al., ACP, 2009; Shapiro et al. ACP, Figure 1. Filtered and unfiltered time series of SORCE Lyman-α, MLS OH and H 2 O abundances from Dec to Dec A day Fast Fourier Transform (FFT) band- pass filter is applied. OH data at hPa and H 2 O data at 0.01 hPa are shown. Tropics (d) Northern mid-latitudeSouthern mid-latitude