Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh31a1660w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SH31A-1660
Physics
2104 Cosmic Rays, 2162 Solar Cycle Variations (7536), 2169 Solar Wind Sources, 4430 Complex Systems, 7863 Turbulence (4490)
Scientific paper
The spatial correlation lengthscale is a key observable for quantitative modelling of fluctuations in the solar wind, in the context of either MHD turbulence or of propagating coherent structures. We present direct measurements of the spatial correlation lengthscale of solar wind magnetic field and ion density, obtained from simultaneous in-situ observations by multiple spacecraft. We focus on comparisons between different parameters and different phases of the solar cycle, which taken together clarify the respective roles of in-situ evolving turbulence and structure of more direct coronal origin. We calculate the spatial correlation properties of the solar wind in the ecliptic at 1AU using simultaneous in-situ observations by the ACE and WIND spacecraft. We present the first direct study of the spatial correlation lengthscale λ of solar wind ion density fluctuations, and find it to be smaller than that of the magnetic field. We find that there is the same statistically significant increase in λ, by a factor ≍ 2 from solar minimum to solar maximum for both density and magnetic field magnitude. In contrast, the λ of the individual components of the magnetic field, shows no discernible solar cycle variation. This behaviour is present both in the inertial range of turbulence and on larger scales. Our results suggest that long range correlation in ion density and |B| is of direct coronal origin, in contrast to that found in the B components, which is more strongly dominated by in situ evolving turbulence. The distinct correlation lengths of the density and magnetic field, together with their solar cycle variation, thus provide new quantitative insights into the mapping of coronal processes out into the solar wind. The lack of variation in the correlation length of the components of the magnetic field implies the continual presence of shear Alfvénic turbulence throughout the solar cycle. The difference between the correlation lengths of the magnetic field magnitude and the magnetic field components could indicate the relative scales of compressive versus shear Alfvénic fluctuations or, at larger temporal scales, different aspects of propagating coherent structures of coronal origin. These results contribute to our understanding of the interplay between in-situ turbulence and fluctuations of coronal origin, and also provide quantitative input for models of cosmic ray propagation within the heliosphere. We acknowledge the ACE and WIND magnetometer and SWE teams for data provision and the EPSRC and UKAEA for support.
Chapman Sandra C.
Dendy R. O.
Wicks Robert T.
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