Quantifying the Anisotropy and Solar Cycle Dependence of the "1/f" Energy Range of Solar Wind Fluctuations Observed by ACE

Details Quantifying the Anisotropy and Solar Cycle Dependence of the "1/f" Energy Range of Solar Wind Fluctuations Observed by ACE Quantifying the Anisotropy and Solar Cycle Dependence of the "1/f" Energy Range of Solar Wind Fluctuations Observed by ACE

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh43a1630n&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SH43A-1630

Other

2159 Plasma Waves And Turbulence, 2162 Solar Cycle Variations (7536), 2169 Solar Wind Sources, 3270 Time Series Analysis (1872, 4277, 4475), 4440 Fractals And Multifractals

Scientific paper

In-situ observations of the solar wind from satellites such as ACE provide measurements of bulk plasma parameters in the solar wind on timescales spanning seconds to years, enabling exploration of the full solar cycle. At frequencies lower than the inertial range of turbulence, solar wind power spectra typically show a region of "1/f" inverse power law dependence. This "1/f" energy range is believed to be of more direct coronal origin than is the inertial range, so that the "1/f" fluctuations in the solar wind may directly embody aspects of the complex magnetic field structure of the solar corona, and of footpoint stirring in the solar photosphere. Analysis of these fluctuations may thus shed light on intriguing physical questions, such as the extent to which the possibly fractal nature of the coronal magnetic carpet is projected into the solar wind. Here we present statistical analyses that quantify the scaling properties of solar wind fluctuations in the "1/f" energy range, focusing on solar cycle dependence and on anisotropy with respect to the background magnetic field. In particular, we present structure function analysis of magnetic and velocity field fluctuations, in the directions parallel and perpendicular to the mean background magnetic field. It is necessary to go beyond power spectral analysis, which does not uniquely qualify the statistical scaling of the fluctuations, in order to address the question of fractal and multifractal scaling. We find that the magnetic field fluctuations, unlike the velocity fluctuations, show behaviour close to the expected "1/f" scaling exponents. Parallel and perpendicular fluctuations differ from each other in their scaling, which also varies with the solar cycle. These results point to distinct physical processes in the corona, and to their mapping out into the solar wind. The scaling exponents obtained constrain the models for these processes. RN acknowledges the STFC and UKAEA Culham for financial support and R. P. Lepping and the ACE team for data provision.

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