Correlation and Taylor scale variability in the interplanetary magnetic field fluctuations as a function of solar wind speed

WEYGAND J.M.; MATTHAEUS W.H.; DASSO S.; KIVELSON M.G.

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2011 vol. 116 p. 8102 - 8122

0148-0227

Simultaneous multiple point measurements of the magnetic field from 11 spacecraftare employed to determine the correlation scale and the magnetic Taylor microscaleof the solar wind as functions of the mean magnetic field direction and solar wind speed.We find that the Taylor scale is independent of direction relative to the mean magneticfield in both the slow (<450 km/s) and the fast (>600 km/s) solar wind, but theTaylor scale is longer along the mean magnetic field direction in the intermediate(600 km/s ≥ speed ≥ 450 km/s) solar wind. The correlation scale, on the other hand, varieswith angle from the mean magnetic field direction. In the slow solar wind the ratio ofthe parallel correlation scale to the perpendicular correlation scale is 2.55 ± 0.76, decreasesto 2.15 ± 0.18 in the intermediate solar wind, and becomes 0.71 ± 0.29 in the fast solarwind. Thus, solar wind turbulence is anisotropic, dominated by quasi two‐dimensionalturbulence in both the slow and intermediate solar wind, and by slab type turbulence in thefast solar wind. The correlation and Taylor scales may be used to estimate effectivemagnetic Reynolds numbers separately for each angular channel. To within theuncertainty, no dependence on the solid angle relative to the mean magnetic field couldbe identified for the Reynolds number. These results may be useful inmagnetohydrodynamic modeling of the solar wind and can contribute to ourunderstanding of solar and galactic cosmic ray diffusion in the heliosphere.