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Keywords:

  • mesospheric clouds;
  • global change;
  • radiation

[1] Earlier studies have indicated that there is a secular increase in the occurrence frequency of polar mesospheric clouds (PMC), along with an anti-correlation with the solar activity. The combined data records from the Solar Backscatter Ultraviolet (SBUV and SBUV/2) instruments provide the longest satellite record (28 years) of the PMC frequency of occurrence. This record has been analyzed to determine the long-term variation in the PMC occurrence frequency in each of three latitude bands (54°–64°N, 64°–74°N, 74°–82°N). This analysis includes an adjustment for changes in the local time of measurement due to the satellite orbital drift, to take into account diurnal variations in the PMC frequency. Multiple linear regression fits using solar activity and time show that the occurrence frequency nearly doubles from solar maximum to solar minimum in all latitude bands. There is a long-term increase in the occurrence frequency ranging from 7% per decade at 64°–74°N to 20% per decade at 74°–82°N. These secular increases are significant at the 95% level for the 74°–82°N and all latitudes combined. We find a time lag of half a year (with an uncertainty of one year) between the minimum solar activity and the maximum PMC activity, consistent with our previous findings for PMC albedo.