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Ann. Geophys., 25, 1183-1197, 2007
www.ann-geophys.net/25/1183/2007/
© European Geosciences Union 2007
Solar cycle changes in the geo-effectiveness of small-scale solar wind turbulence measured by Wind and ACE at 1 AU
M. L. Parkinson, R. C. Healey, and P. L. Dyson
Department of Physics, La Trobe University, Victoria 3086, Australia
Abstract. Multi-scale structure of the solar wind in the ecliptic
at 1 AU undergoes significant evolution with the phase of the solar cycle.
Wind spacecraft measurements during 1995 to 1998 and ACE spacecraft
measurements during 1997 to 2005 were used to characterise the evolution of
small-scale (~1 min to 2 h) fluctuations in the solar wind speed
vsw, magnetic energy density B2, and solar wind ε parameter, in the
context of large-scale (~1 day to years) variations. The large-scale
variation in ε most resembled large-scale variations in B2. The
probability density of large fluctuations in ε and B2 both had strong
minima during 1995, a familiar signature of solar minimum. Generalized
Structure Function (GSF) analysis was used to estimate inertial range
scaling exponents aGSF and their evolution throughout 1995 to 2005. For
the entire data set, the weighted average scaling exponent for small-scale
fluctuations in vsw was aGSF=0.284±0.001, a value
characteristic of intermittent MHD turbulence (>1/4), whereas the scaling
exponents for corresponding fluctuations in B2 and ε were
aGSF=0.395±0.001 and 0.334±0.001, respectively. These values
are between the range expected for Gaussian fluctuations (1/2) and
Kolmogorov turbulence (1/3). However, the scaling exponent for ε changed from
a Gaussian-Kolmogorov value of 0.373±0.005 during 1997 (end of solar
minimum) to an MHD turbulence value of 0.247±0.004 during 2003
(recurrent fast streams). Changes in the characteristics of solar wind
turbulence may be reproducible from one solar cycle to the next.
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