Solar wind fluctuations at large scale - A comparison between low and high solar activity conditions

Details Solar wind fluctuations at large scale - A comparison between low and high solar activity conditions Solar wind fluctuations at large scale - A comparison between low and high solar activity conditions

Feb 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991jgr....96.1737b&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 96, Feb. 1, 1991, p. 1737-1744. Research supported by CNR.

Other

17

Helios Project, Magnetohydrodynamic Waves, Solar Activity Effects, Solar Magnetic Field, Solar Wind, Energy Spectra, Frequency Distribution, Solar Cycles, Solar Terrestrial Interactions, Solar Wind Velocity

Scientific paper

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 to 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. The Alfvenic contribution to the solar wind energy spectrum comes out to be dominant at all solar activity conditions. These findings support the conclusion that the solar cycle evolution of the large-scale velocity pattern is the factor governing the observed variations.

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