The Variation of Solar Wind Correlation Lengths Over Three Solar Cycles

We present the results of a study of solar wind velocity and magnetic field correlation lengths over the last 35 years. The correlation length of the magnetic field magnitude lambda(vertical bar B vertical bar) increases on average by a factor of two at solar maxima compared to solar minima. The correlation lengths of the components of the magnetic field lambda(BXYZ) and of the velocity lambda(VYZ) do not show this change and have similar values, indicating a continual turbulent correlation length of around 1.4 x 10(6) km. We conclude that a linear relation between lambda(vertical bar B vertical bar), V B-2, and Kp suggests that the former is related to the total magnetic energy in the solar wind and an estimate of the average size of geoeffective structures, which is, in turn, proportional to V B-2. By looking at the distribution of daily correlation lengths we show that the solar minimum values of lambda(vertical bar B vertical bar) correspond to the turbulent outer scale. A tail of larger lambda(vertical bar B vertical bar) values is present at solar maximum causing the increase in mean value.