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We analyzed the brightness of the sky background observed with a coronagraph at Norikura, Japan, in the period of 1951-1997. We discovered that the power spectrum shows a clear eleven-year periodicity of solar activity cycle, in addition to strong annual variations. The peaks in the eleven-year component are found in the declining phase of activity, 2-4 years after the sunspot number maximum. The brightness of the continuum corona is far fainter than the observed amplitude of the eleven-year component and cannot account for the observed phenomenon. A possible interpretation is that the solar activity modulates the contents of aerosols in the upper atmosphere of the earth, thus producing variations in scattering of sunlight. Why this effect is largest in the declining phase of solar activity is still unknown.

Eleven-year solar cycle periodicity in sky brightness observed at Norikura, Japan