Observations of 1/f Noise at Mercury’s Solar Wind Using MESSENGER Data

Abstract

Low-frequency signals having a band-limited scale-invariant power, or “1/f” noise, have been detected in the interplanetary medium and inferred from coronal and photospheric observations. Their origin remains under debate, with the principal issue being whether the signal originates from local processes or has a solar origin, possibly emerging from a superposition of scale-invariant structures. Detection of such interplanetary signals at frequencies as low as a few times 10−6 Hz at distances well within 1 au is important in developing an understanding of this phenomenon, but this is challenging due to a lack of the requisite long-duration data records. Here, we employ magnetic field data from the MESSENGER spacecraft, processed to remove Mercury’s wake, to analyze spectra from a year of data. The 1/f signal is detected down to the lowest expected frequencies, consistent with the observed lognormal distribution of correlation times and the Machlup–Montroll–Shlesinger superposition principle. This supports the hypothesis that the observed 1/f signals have an origin in solar processes, possibly the dynamo, and disfavors local in situ generation, as described in W. H. Matthaeus & M. L. Goldstein and W. H. Matthaeus et al.