{
  "id": "0904.4594",
  "version": "v1",
  "published": "2009-04-29T12:30:21.000Z",
  "updated": "2009-04-29T12:30:21.000Z",
  "title": "Effect of Polarimetric Noise on the Estimation of Twist and Magnetic Energy of Force-Free Fields",
  "authors": [
    "Sanjiv Kumar Tiwari",
    "P. Venkatakrishnan",
    "Sanjay Gosain",
    "Jayant Joshi"
  ],
  "comment": "33 pages, 6 figures, 1 table; Accepted for publication in the ApJ",
  "journal": "Astrophys.J.700:199-208,2009",
  "doi": "10.1088/0004-637X/700/1/199",
  "categories": [
    "astro-ph.SR"
  ],
  "abstract": "The force-free parameter $\\alpha$, also known as helicity parameter or twist parameter, bears the same sign as the magnetic helicity under some restrictive conditions. The single global value of $\\alpha$ for a whole active region gives the degree of twist per unit axial length. We investigate the effect of polarimetric noise on the calculation of global $\\alpha$ value and magnetic energy of an analytical bipole. The analytical bipole has been generated using the force-free field approximation with a known value of constant $\\alpha$ and magnetic energy. The magnetic parameters obtained from the analytical bipole are used to generate Stokes profiles from the Unno-Rachkovsky solutions for polarized radiative transfer equations. Then we add random noise of the order of 10$^{-3}$ of the continuum intensity (I$_{c}$) in these profiles to simulate the real profiles obtained by modern spectropolarimeters like Hinode (SOT/SP), SVM (USO), ASP, DLSP, POLIS, SOLIS etc. These noisy profiles are then inverted using a Milne-Eddington inversion code to retrieve the magnetic parameters. Hundred realizations of this process of adding random noise and polarimetric inversion is repeated to study the distribution of error in global $\\alpha$ and magnetic energy values. The results show that : (1). the sign of $\\alpha$ is not influenced by polarimetric noise and very accurate values of global twist can be calculated, and (2). accurate estimation of magnetic energy with uncertainty as low as 0.5% is possible under the force-free condition.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2009-04-29T12:30:21.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "magnetic energy",
      "polarimetric noise",
      "force-free field",
      "analytical bipole",
      "estimation"
    ],
    "tags": [
      "journal article"
    ],
    "publication": {
      "journal": "The Astrophysical Journal",
      "year": 2009,
      "month": "Jul",
      "volume": 700,
      "number": 1,
      "pages": 199
    },
    "note": {
      "typesetting": "TeX",
      "pages": 33,
      "language": "en",
      "license": "arXiv",
      "status": "editable",
      "inspire": 819010,
      "adsabs": "2009ApJ...700..199T"
    }
  }
}