Solar-Wind High-Speed Stream (HSS) Alfvén Wave Fluctuations at High Heliospheric Latitudes: Ulysses Observations During Two Solar-Cycle Minima

Abstract

We study Alfvén wave fluctuations in solar-wind high-speed streams (HSSs) at high heliolatitudes during the last two solar-cycle minima (SCM). Solar-wind plasma and interplanetary magnetic field (IMF) measured by Ulysses during four 50-day intervals in 1994, 1995, 2007, and 2008 were analyzed using wavelet and Fourier analyses, cross-correlation and kurtosis techniques. Intervals during 1994 and 1995 (2007 and 2008) correspond to the Ulysses polar passes through the southern and northern solar hemispheres, respectively, during the minimum between Cycles 22 and 23 or SCM22−23 (the minimum between Cycles 23 and 24 or SCM23−24). The solar-wind plasma density [Np], IMF magnitude [B], and IMF-component variances are found to be lower during SCM23−24 than during SCM22−23 by ≈ 20 – 30%. The cross-correlation between the plasma velocity and IMF vector components, an indicative of Alfvénicity, is smaller during SCM23−24 than during SCM22−23. The Alfvén wave periodicity exhibits a large range, from ≈ 8 hours to 10 days, with peak occurrences near 1 – 5 days during both minima. The statistical kurtosis analysis shows that the IMF distributions are mostly sub-Gaussian. Further, the Fourier power law analysis reveals a higher spectral power of transverse IMF components Bt and Bn than the radial field-aligned component Br. The power spectrum shows a spectral break near 10−4 Hz, with its high-frequency portion following a −1.7 power law dependence (Kolmogorov spectrum), while the low-frequency portion shows an ≈ − 1.0 power law index dependence. This low-frequency index is slightly higher during SCM22−23 (−0.65 to −0.87) than during SCM23−24 (−0.49 to −0.78). We conclude that while the Alfvénicity of the high-latitude HSSs does not vary substantially between the two minima, the amplitude of the Alfvén wave fluctuations is reduced during SCM23−24 compared to SCM22−23. © 2022, The Author(s), under exclusive licence to Springer Nature B.V.