THE COSMIC-RAY 1.68-YEAR VARIATION - A CLUE TO UNDERSTAND THE NATURE OF THE SOLAR-CYCLE

Using the maximum entropy method (MEM), the cosmic-ray power spectral density in the frequency range 3 x 10(-9)-2 x 10(-7) Hz has been estim ated for the period 1947-1990. Cosmic-ray intensity data were integrat ed from the ion chamber at Huancayo and the neutron monitor at Deep Ri ver, following the method of Nagashima and Morishita (1980). The estim ated spectrum shows power-law dependence (f(-1.62)), With several peak s superimposed. Periodicities of the different peaks are identified an d related to solar activity phenomena; most of them were reported in t he past. Once the Il-yr variation is eliminated, the most prominent fe ature in the spectrum is a variation, not reported before, with a peri od of 1.68 yr (604.8 d). This peak is correlated with fluctuations of similar periodicities found in the southern coronal hole area and in l arge active regions. The importance that this variation may have to el ucidate the solar magnetic flux emergence and the activity cycle is di scussed.