An analysis of couplings between solar activity and atmospheric opacity at the South Pole

A spectral radiometer located at the South Pole has obtained a long-term record of ground-level solar irradiance at wavelengths from 315 to 600 nm. Data acquired during the sunlit periods from 1992 to 2016 provide information on the scattering properties of the atmosphere and their variation covering more than two solar cycles. Prior to the late 1990s the time-integrated solar energy received over an entire observing season, September to March, displayed no organized trend. However, between 1996 and 2000 the irradiances began a decline which persisted through the end of the record. The observations imply a small systematic increase in attenuation at the South Pole, presumably associated with cloudiness, during the current century. The dataset allows a search for links between atmospheric opacity and solar activity. There is no significant correlation between seasonally-integrated irradiances and the 11-year solar cycle as measured by the 10.7 cm radio flux or the ground-level neutron count. On a much shorter timescale, a statistically significant positive relationship exists between the geomagnetic activity index Ap on one day and ground-level irradiance on the following day. Two days that differ in Ap by 10 units are followed by days whose irradiances differ, on average, by about 0.25 ± 0.19% in the wavelength band 400-600 nm. The wavelength dependence of this effect from the near-ultraviolet into the visible is consistent with a small decrease in the optical thickness of tropospheric clouds or of scattering layers at higher altitudes following magnetic disturbances.