Abstract

The birth of a new spot on the solar disk indicates the emergence of magnetic flux through the photosphere, a process which is key to the solar cycle. The presence of a sunspot leads to a solar region being assigned an active region number by the NOAA Space Weather Prediction Center so that its evolution and activity can be tracked. We analysed data from the Mount Wilson solar observatory, by means of Virtual Observatory tools provided by Astrogrid, to obtain statistics of emergences of 6862 sunspot regions. The strong East-West asymmetry in the location of new emergences that we obtain, first reported by A.S.D. Maunder, allows us to derive the visibility function of sunspots, the rate of magnetic flux emergence and the ratio between the average durations of growth and decay phases of solar active regions. We find that the visibility of small sunspots has a strong center-to-limb variation, far larger than would be expected from geometrical (projection) effects. This results in a large number of young spots being invisible, with as many as 44% of new regions emerging in the West of the Sun going undetected.