{
  "id": "1810.09358",
  "version": "v1",
  "published": "2018-10-22T15:25:00.000Z",
  "updated": "2018-10-22T15:25:00.000Z",
  "title": "Magnetic properties of a long-lived sunspot - Vertical magnetic field at the umbral boundary",
  "authors": [
    "Markus Schmassmann",
    "Rolf Schlichenmaier",
    "Nazaret Bello González"
  ],
  "comment": "accepted for publication in A&A",
  "doi": "10.1051/0004-6361/201833441",
  "categories": [
    "astro-ph.SR"
  ],
  "abstract": "Context. In a recent statistical study of sunspots in 79 active regions, the vertical magnetic field component $B_\\text{ver}$ averaged along the umbral boundary is found to be independent of sunspot size. The authors of that study conclude that the absolute value of $B_\\text{ver}$ at the umbral boundary is the same for all spots. Aims. We investigate the temporal evolution of $B_\\text{ver}$ averaged along the umbral boundary of one long-lived sunspot during its stable phase. Methods. We analysed data from the HMI instrument on-board SDO. Contours of continuum intensity at $I_\\text{c}=0.5I_\\text{qs}$, whereby $I_\\text{qs}$ refers to the average over the quiet sun areas, are used to extract the magnetic field along the umbral boundary. Projection effects due to different formation heights of the Fe I 617.3 nm line and continuum are taken into account. To avoid limb artefacts, the spot is only analysed for heliocentric angles smaller than $60^{\\circ}$. Results. During the first disc passage, NOAA AR 11591, $B_\\text{ver}$ remains constant at 1693 G with a root-mean-square deviation of 15 G, whereas the magnetic field strength varies substantially (mean 2171 G, rms of 48 G) and shows a long term variation. Compensating for formation height has little influence on the mean value along each contour, but reduces the variations along the contour when away from disc centre, yielding a better match between the contours of $B_\\text{ver}=1693$ G and $I_\\text{c}=0.5I_\\text{qs}$. Conclusions. During the disc passage of a stable sunspot, its umbral boundary can equivalently be defined by using the continuum intensity $I_\\text{c}$ or the vertical magnetic field component $B_\\text{ver}$. Contours of fixed magnetic field strength fail to outline the umbral boundary.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2018-10-22T15:25:00.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "umbral boundary",
      "long-lived sunspot",
      "magnetic properties",
      "vertical magnetic field component",
      "field strength varies"
    ],
    "tags": [
      "journal article"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 0,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}