Equatorial variation of the 27-day oscillation in the horizontal intensity

Abstract

Mean daily values of the horizontal intensity for four observatories in the equatorial region of South India have been subjected to spectral analysis. The power densities near the 27-day period have been computed separately for periods of high, moderate and low solar activity. The spectrums indicate that, with decreasing dip, the amplitude of the 27-day oscillation increases during high activity period but it decreases during the low and inclining phase. During the declining phase of moderate activity the amplitude is about the same at all the four stations. The spectrums also show that with solar activity the change in the amplitude of the oscillation is largest near the dip equator, the power density decreasing from the high to the low activity period by a factor of nine.