Browsing by Subject "Interplanetary magnetic fields"

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  • Good, S. W.; Kilpua, E. K. J.; Ala-Lahti, M.; Osmane, A.; Bale, S. D.; Zhao, L. -L. (2020)
    Magnetic clouds are large-scale transient structures in the solar wind with low plasma-beta, low-amplitude magnetic field fluctuations, and twisted field lines with both ends often connected to the Sun. Their inertial-range turbulent properties have not been examined in detail. In this Letter, we analyze the normalized cross helicity, sigma(c), and residual energy, sigma(r), of plasma fluctuations in the 2018 November magnetic cloud observed at 0.25.au by the Parker Solar Probe. A low value of |sigma(c)| was present in the cloud core, indicating that wave power parallel and antiparallel to the mean field was approximately balanced, while the cloud's outer layers displayed larger amplitude Alfvenic fluctuations with high |sigma(c)| values and sigma(r) similar to 0. These properties are discussed in terms of the cloud's solar connectivity and local interaction with the solar wind. We suggest that low |sigma(c)| is likely a common feature of magnetic clouds given their typically closed field structure. Antisunward fluctuations propagating immediately upstream of the cloud had strongly negative sigma(r) values.
  • Good, S. W.; Ala-Lahti, M.; Palmerio, E.; Kilpua, E. K. J.; Osmane, A. (2020)
    The sheaths of compressed solar wind that precede interplanetary coronal mass ejections (ICMEs) commonly display large-amplitude magnetic field fluctuations. As ICMEs propagate radially from the Sun, the properties of these fluctuations may evolve significantly. We have analyzed magnetic field fluctuations in an ICME sheath observed by MESSENGER at 0.47 au and subsequently by STEREO-B at 1.08 au while the spacecraft were close to radial alignment. Radial changes in fluctuation amplitude, compressibility, inertial-range spectral slope, permutation entropy, Jensen-Shannon complexity, and planar structuring are characterized. These changes are discussed in relation to the evolving turbulent properties of the upstream solar wind, the shock bounding the front of the sheath changing from a quasi-parallel to quasi-perpendicular geometry, and the development of complex structures in the sheath plasma.