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Aura Science

Hydroxyl radical (OH) response to the 11-year solar cycle


Observed solar UV changes during 2004 - 2007 from the SORCE satellite are much larger than predicted by established models.


OH data from the Microwave Limb Sounder (MLS) on Aura (~5 yr) and a ground-based FTUVS (~13 yr) suggest a response of the OH column to the solar cycle that is significantly larger than model results using established solar forcing. Model runs using a solar forcing based on SORCE UV data agree better with MLS OH observations.


It is essential to have the proper solar forcing in photochemical models and climate models to understand variability in middle atmospheric hydrogen chemistry that affects ozone and natural influences on climate.

MLS figure

The modeled OH change using established solar forcing (top left) is too small. A factor of ~3 is needed to match the OH change extracted from FTUVS and MLS data (top right). The model using SORCE solar forcing (bottom left) agrees much better with observations (bottom right)


  • The FTUVS is a high resolution Fourier Transform UV Spectrometer located at NASA/JPL's Table Mountain Facility in California [1].
  • The FTUVS OH measurement is for total column (1997 - present). The MLS measurements are integrated to get OH columns that cover ~90% of the atmospheric OH, at ~21.5hPa and above (2004 - 2009) [2].
  • The SORCE (Solar Radiation and Climate Experiment) satellite carries four instruments, two of which measure the solar UV spectral irradiance: SOLSTICE (Solar Stellar Irradiance Comparison Experiment) and SIM (Spectral Irradiance Monitor) [3,4].
  • Since the SORCE data publication, there have been debates on whether the solar UV variability from SORCE or the established models is correct. A recent paper looking into the O3 response to solar cycle was published in Nature in 2010 and support SORCE data [5].
  • There were earlier studies on the O3 response that support established models. We look into OH instead of O3 as the first step, because OH has very short chemical lifetime, is not affected by transport, and reacts/recovers rapidly to solar forcing. Its variability is expected to be much "cleaner" than O3. In other words, it's less likely to get OH right for wrong reasons than in the case of O3.

References [Wang et al., submitted to PNAS, 2011] Cageao RP, et al. (2001) High-resolution Fourier-transform ultraviolet-visible spectrometer for the measurement of atmospheric trace species: application to OH. Appl Opt 40(12):2024-2030. Wang S, et al. (2008) Validation of Aura MLS OH measurements with FTUVS total OH column measurements at Table Mountain California. J Geophys Res 113:D22301 10.1029/2008JD009883. Snow M, McClintock WE, Rottman G, Woods TN (2005) Solar-stellar irradiance comparison experiment II (SOLSTICE II): examination of the solar-stellar comparison technique. Sol Phys 230:295-324. Harder JW, et al. (2010) The SORCE SIM solar spectrum. Comparison with recent observations. Sol Phys 263:3-24. Haigh JD, Winning AR, Toumi R, Harder JW (2010) An influence of solar spectral variations on radiative forcing of climate. Nature 467:696-699.