In a recent article, Shapiro and Ward [1966] discussed the power spectra of the magnetic character figure, Ci, and of the daily sums of the index of magnetic activity, Kp. In each spectrum, the authors found a principal peak at a period of 27.4 days and subsidiary peaks at 29.4 and 25.6 days. Our purpose in this note is to show that the subsidiary peaks are the result of an amplitude modulation of the 27-day components of these indices by a modulating function with a period of one year.For the simplest case of an amplitude modulation of one sinusoidal function with a frequency ƒ0 by a second sinusoidal function with frequency ƒm, the signal is represented byA(t) = [A1 + A2 cos (2πƒmt + φ)] cos 2πƒ0 = A1 cos 2πƒ0t + (A2/2){cos[2π(ƒ0 + ƒm)t + φ] + cos [2π(ƒ0 - ƒm)t - φ]} The latter form exhibits the three frequency components that include the carrier, ƒ0; the upper sideband, ƒ0 + ƒm; and the lower sideband, ƒ0 - ƒm.
No takes yet. Share an insight, caveat, or question.
Coleman et al. (1966) studied this question.