SOLAR-WIND FLUCTUATIONS AT LARGE-SCALE - A COMPARISON BETWEEN LOW AND HIGH SOLAR-ACTIVITY CONDITIONS

The influence of the Sun's activity cycle on the solar wind fluctuations at time scales from 1 hour to 3 days in the inner heliosphere (0.3 ti 1 AU) is investigated. Hourly averages of plasma and magnetic field data by Helios spacecraft are used. Since fluctuations behave quite differently with changing scale the analysis is performed separately for two different ranges in time scale. Between 1 and 6 hours Alfvenic fluctuations and pressure-balanced structures are extensively observed. At low solar activity and close to 0.3 AU Alfvenic fluctuations are more frequent than pressure-balanced structures. This predominance, however, weakens for rising solar activity and radial distance, to the point that a role exchange, in terms of occurrence rate, is found at the maximum of the cycle close to 1 AU. On the other hand, in all cases Alfvenic fluctuations have a larger amplitude than pressure-balanced structures. On the whole, the Alfvenic contribution to the solar wind energy spectrum comes out to be dominant at all solar activity conditions. At scales from 0.5 to 3 days the most important feature is the growth, as the solar wind expansion develops, of strong positive correlations between magnetic and thermal pressures. These structures are progressively built up by the interaction between different wind flows. This effect is more pronounced at low than at high activity. Our findings support the conclusion that the solar cycle evolution of the large-scale velocity pattern is the factor governing the observed variations.