Solar-cycle variation of near-sun sky brightness observed with coronagraphs

When the intensity of emission lines in the solar corona is measured with a coronagraph, the background sky brightness caused by aerosol in the earth atmosphere is also measured. Observational records of sky brightness at 50″ above solar limb have been accumulated for many years at coronagraph stations. The data obtained at Norikura, Japan, during the period of 1951-1997 show an 11-year periodicity of solar cycle. This suggests that the density and/or size distributions of aerosol particles might be modulated by solar activity (possibly by UV radiation). Similar data obtained at Lomnický Štít, Slovakia, in the period of 1980-2001 do not show this 11-year periodicity. This may be because the latter data set is not long enough, or could be because of different meteorological conditions in Europe and in Japan.