Sensitivity of equatorial atomic oxygen in the MLT region to the 11-year and 27-day solar cycles

We report on 27-day and 11-year solar cycle signatures in atomic oxygen (O) concentrations ([O]) in the MLT (Mesosphere/Lower Thermosphere) region of the terrestrial atmosphere. MLT [O] profiles were retrieved on the base of green line (557.7 nm) nightglow data sets provided by the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) onboard Envisat from 2002 to 2012. A statistically significant solar 27-day signature was identified (and then quantified with respect to the sensitivity and phase relationship to solar forcing) in time series of MLT [O] profiles with use of cross-correlation and superposed epoch analysis techniques. It was the first identification of the solar 27-day signature in MLT atomic oxygen on the base of such experimental data sets. The sensitivity of [O] to solar cycle variability at the 11-year time scale was quantified with use of cross-correlation and multiple-linear regression analysis techniques, which yield results consistent with known studies and, particularly, indicate that the sensitivity of [O] to solar forcing increases with increasing altitude. A comparison of obtained values of atomic oxygen sensitivity in response to solar forcing at the 27-day and 11-year time scales reveals the fact that the sensitivities agree well to each other within their uncertainties during the descending phase of the last (23rd) 11-year cycle of solar activity, whereas the [O] sensitivity values at the 27-day time scale during the last solar minimum phase were lower than those ones during the descending phase. It was also determined that atomic oxygen is in-phase with the solar forcing (in agreement with model results) at the 11-year time scale, whereas the time lag of the 27-day signature in response to solar forcing was about 12 - 14 days.