Interplanetary magnetic correlation and low-frequency spectrum over many solar rotations

TL;DR

This study analyzes long-term interplanetary magnetic field data to understand low-frequency fluctuations, revealing a broadband 1/f spectrum linked to solar rotation and possibly originating from the solar dynamo.

Contribution

It uncovers a continuous 1/f spectral signature extending to very low frequencies and relates solar rotation harmonics to this spectrum, offering new insights into solar wind turbulence.

Findings

1/f spectrum extends down to ~5×10^{-7} Hz.

Solar rotation harmonics are consistent with the 1/f spectrum.

Broadband 1/f spectrum is present across different solar wind conditions.

Abstract

Fluctuations and structure across a wide range of spatial and temporal scales are frequently studied in the solar wind. The properties of the low-frequency fluctuations are of relevance to turbulent energy injection into the plasma and the transport of high-energy cosmic rays. Correlation analysis of decade-long intervals of interplanetary data permits study of fluctuations at time scales much longer than suitably defined correlation times, and therefore at frequencies well below those associated with the Kolmogorov inertial range of {\it in situ} turbulence. At the frequencies of interest, we study the familiar occurrence of the $1/f$ spectral signature. We also study point spectral features due to solar rotation and their relation with the $1/f$ signal. We report novel properties at timescales ranging from minutes up to years, using data selected by wind speed, phase of solar cycle, and cartesian components of the magnetic field. A surprising finding is that the power in solar rotation harmonics is consistent with an extension of the $1/f$ spectrum, down to frequencies as low as around $\unit[5 \times 10^{-7}]{Hz}$. The presence of a broadband $1/f$ spectrum across different wind types supports the interpretation that $1/f$ signals may be related to or even originate from the solar dynamo.