Figure. 1 Comparison of the tropical mean daytime averaged and 20-35 day filtered MLS OH at ~78 km altitude (upper panel) and filtered (central panel) and unfiltered (lower panel) H₂O at ~80 km altitude with SOLSTICE/SORCE Ly-_ line irradiance. The comparison is performed for the period from August 2004 to December 2006.

The AURA Microwave Limb Sounder (MLS) data reveal a strong OH and H₂O response to the 27-day solar irradiance variability.

We show that the mesospheric Hydroxide (OH) and Water (H₂O) correlate well with the solar irradiance. The OH variation correlates positively and the H₂O variation correlates negatively with solar irradiance. Both species demonstrate a pronounced 27-day periodicity, which resembles the solar rotational cycle (Fig. 1). The 27-day harmonic in OH and H₂O power spectra is strong during the period of high solar activity and negligible for the solar minimum conditions (Fig.2.) This proves that the 27-day variability in the OH and H₂O data is caused by the solar rotational cycle.

Figure. 2 The normalized power spectra of the tropical mean unfiltered daytime OH (top panel) and H₂O (bottom panel).