High-Q spectral peaks and non-stationarity in the solar wind and geomagnetic field over the 200–4000 μHz band

Abstract This paper examines 60 day sections of magnetic field variations from the Autonomous Compositional Explorer satellite located at the L1 Lagrange point within the solar wind and commensurate data from Honolulu Observatory. Averaged spectra for the data sections at ACE show nearly identical results for all three components with a spectral slope of about −5/3, consistent with Kolmogorov turbulence. By contrast, averaged spectra for HON have different spectral slopes, and the north and vertical components display excess variability over 2000–4000 μHz. A mixture central/noncentral chi square fit to the spectra over 1000 μHz wide bands ranging over 1000–4000 μHz show a noncentral fraction that varies systematically with frequency, peaking over 2000–3000 μHz with a value of ~ 0.35. Standardized spectra for ACE show the presence of narrowband (a few μHz), high Q (100–1000), moderate p -quantile (≥ 0.99) peaks superimposed on a turbulence signature. Offset coherences for ACE suggest only weak non-stationarity. By contrast, standardized spectra for HON show numerous narrowband, high Q , high p -quantile (≥ 0.999) peaks, with a concentration that substantially exceeds that for ACE. Offset coherences for HON display pervasive non-stationarity in the form of high coherences between frequencies with probabilities of occurrence as high as 0.9999. Canonical coherences between all three magnetic components at ACE and HON have high values at some of the coincident spectral peaks in standardized spectra, suggesting that there is coupling of narrowband processes contained within the solar wind into Earth’s local geomagnetic environment. However, the pervasive non-stationarity at HON is not coherent with ACE, suggesting a local origin within the magnetosphere and ionosphere. A solar normal mode origin for the narrowband peaks in the spectra was evaluated, with indeterminate results due to masking by the high non-stationarity at HON. The implications of narrowband spectral variability over 200–4000 μHz for electromagnetic induction studies are also considered. Graphical abstract