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H.J. Hagenaar (Lockheed Martin Solar and Astrophysics Labs)
Ephemeral regions are bipolar regions of magnetic flux, emerging all over the solar surface, with a typical lifetime of 4.4 hrs. Ephemeral regions and active regions are distinguished by a rather arbitrary size limit of 2.5 square degrees.
In this paper, a 3.5 day sequence of full disk magnetograms is studied, taken by the MDI-instrument on SOHO. An algorithm is developed to recognize ephemeral regions on these magnetograms.
The chosen data set was taken with a high frequency of, on average, one magnetogram per minute, during a total time span of 3.5 days. In each image, all coherent magnetic concentrations are selected. Subsequently, two opposite-polarity concentrations of comparable total flux content, situated closer to one another than 10 Mm, are interpreted as bipoles. These bipoles can be associated with newly emerged ephemeral regions, but may also be the result of chance encounters of previously existing opposite-polarity concentrations.
Pairs of images are compared, interspaced by 95 - 100 minutes, in order to determine which bipoles do not overlap magnetic concentrations in the previous image, within a radius of 19 Mm. We find a total number of (1-2)104 ephemeral regions, on 1/4 of the total solar surface.
The total amount of flux emerging in ephemeral regions is estimated to be of order 1016-1017 Mx/sec. This flux, emerging in ephemeral regions, provides a significant fraction of the total flux on the solar surface. We discuss the implications for the total flux balance on the entire sun.
This work was supported by the SOI/MDI project at Stanford and LMSAL (grant NAG5-3077).