Letter to the Editor

Invisible sunspots and rate of solar magnetic flux emergence^*

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.