The Variation of Solar Wind Correlation Lengths Over Three Solar Cycles

TL;DR

This study analyzes 35 years of solar wind data, revealing that magnetic field correlation lengths increase during solar maxima, indicating changes in solar wind turbulence and energy structures over solar cycles.

Contribution

It provides the first long-term analysis of solar wind correlation lengths over multiple solar cycles, linking magnetic energy and structure size to solar activity phases.

Findings

Magnetic field correlation length doubles at solar maxima.

Turbulent correlation length remains around 1.4 million km.

Correlation length distribution shifts with solar activity.

Abstract

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(|B|) 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(|B|), VB^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 geo-effective structures which is in turn proportional to VB^2. By looking at the distribution of daily correlation lengths we show that the solar minimum values of lambda(|B|) correspond to the turbulent outer scale. A tail of larger lambda(|B|) values is present at solar maximum, causing the increase in mean value.