IS THERE MEMORY IN SOLAR-ACTIVITY

The Hurst effect is a presumed and unexpected behavior of geoastrophys ical time series by which these time series have persistence or ''memo ry.'' The application of Hurst analysis tc, monthly sunspot numbers [B . B. Mandelbrot and J. R. Wallis, Water Resour. Res. 5, 321 (1969)] yi elded a Hurst exponent H = 0.86 +/- 0.05, suggesting that solar activi ty shows persistence and that the underlying responsible mechanism can guarantee a positive correlation of solar activity during long time l apses, raising, at the same time, the possibility of the existence of long-term memory in solar activity. More recently, radiocarbon data ha ve been used for a similar study [A. Ruzmaikin, J. Feynmann, and P. Ro binson, Sol. Phys. 149, 395 (1994)] resulting in a constant value H = 0.84 between 100 and 3000 years, which indicates persistence of solar activity in such time scales. Furthermore, Mount Wilson rotation measu rements have also been analyzed in the same way [R. W. Komm, Sol. Phys . 156, 17 (1996)] and the results indicate that temporal variations of solar rotation on time scales shorter than the Ii-year cycle are caus ed by a stochastic process which is characterized by persistence. Here , we have followed the scale of fluctuation approach to show that ther e is no incontrovertible evidence for the presence of the Hurst effect in sunspot areas and, therefore, that there is no proof of the existe nce of long-ten memory in solar activity. [S1063-651X(98)02311-3].