{
  "id": "1904.04118",
  "version": "v1",
  "published": "2019-04-08T15:10:37.000Z",
  "updated": "2019-04-08T15:10:37.000Z",
  "title": "Three-dimensional local anisotropy of velocity fluctuations in the solar wind",
  "authors": [
    "Andrea Verdini",
    "Roland Grappin",
    "Olga Alexandrova",
    "Luca Franci",
    "Simone Landi",
    "Lorenzo Matteini",
    "Emanuele Papini"
  ],
  "comment": "accepted in MNRAS",
  "categories": [
    "astro-ph.SR",
    "physics.plasm-ph",
    "physics.space-ph"
  ],
  "abstract": "We analyse velocity fluctuations in the solar wind at magneto-fluid scales in two datasets, extracted from Wind data in the period 2005-2015, that are characterised by strong or weak expansion. Expansion affects measurements of anisotropy because it breaks axisymmetry around the mean magnetic field. Indeed, the small-scale three-dimensional local anisotropy of magnetic fluctuations ({\\delta}B) as measured by structure functions (SF_B) is consistent with tube-like structures for strong expansion. When passing to weak expansion, structures become ribbon-like because of the flattening of SFB along one of the two perpendicular directions. The power-law index that is consistent with a spectral slope -5/3 for strong expansion now becomes closer to -3/2. This index is also characteristic of velocity fluctuations in the solar wind. We study velocity fluctuations ({\\delta}V) to understand if the anisotropy of their structure functions (SF_V ) also changes with the strength of expansion and if the difference with the magnetic spectral index is washed out once anisotropy is accounted for. We find that SF_V is generally flatter than SF_B. When expansion passes from strong to weak, a further flattening of the perpendicular SF_V occurs and the small-scale anisotropy switches from tube-like to ribbon-like structures. These two types of anisotropy, common to SF_V and SF_B, are associated to distinct large-scale variance anisotropies of {\\delta}B in the strong- and weak-expansion datasets. We conclude that SF_V shows anisotropic three-dimensional scaling similar to SF_B, with however systematic flatter scalings, reflecting the difference between global spectral slopes.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2019-04-08T15:10:37.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "solar wind",
      "spectral slope",
      "weak expansion",
      "distinct large-scale variance anisotropies",
      "strong expansion"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}