EDP Sciences Journals List
Advanced Search
Subscriber Authentication Point
Home All issues Volume 479/1 Article
Free access article

Issue A&A
Volume 479, Number 1, February III 2008
Page(s) L1 - L4
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20078800



A&A 479, L1-L4 (2008)
DOI: 10.1051/0004-6361:20078800

Letter

Invisible sunspots and rate of solar magnetic flux emergence

S. Dalla1, L. Fletcher2, and N. A. Walton3

1  Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK
    e-mail: sdalla@uclan.ac.uk
2  Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK
3  Institute of Astronomy, University of Cambridge, Cambridge CB3 OHA, UK

(Received 5 October 2007 / Accepted 30 November 2007)

Abstract
Aims.We study the visibility of sunspots and its influence on observed values of sunspot region parameters.
Methods.We use Virtual Observatory tools provided by AstroGrid to analyse a sample of 6862 sunspot regions. By studying the distributions of locations where sunspots were first and last observed on the solar disk, we derive the visibility function of sunspots, the rate of magnetic flux emergence and the ratio between the durations of growth and decay phases of solar active regions.
Results.We demonstrate that the visibility of small sunspots has a strong centre-to-limb variation, far larger than would be expected from geometrical (projection) effects. This results in a large number of young spots being invisible: 44% of new regions emerging in the west of the Sun go undetected. For sunspot regions that are detected, large differences exist between actual locations and times of flux emergence, and the apparent ones derived from sunspot data. The duration of the growth phase of solar regions has been, up to now, underestimated.


Key words: sunspots -- Sun: photosphere -- Sun: magnetic fields -- Sun: activity



© ESO 2008


What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.
  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.