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Astron. Astrophys. 318, 293-307 (1997)

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Is the longitudinal distribution of solar flares nonuniform?

L. Jetsu 1, 2, S. Pohjolainen 3, J. Pelt 4 and I. Tuominen 1, 5

1 Observatory, P.O. Box 14, University of Helsinki, FIN-00014 Helsinki, Finland
2 NORDITA, Blegdamsvej 17, DK-2100 Copenhagen, Denmark
3 Helsinki University of Technology, Metsähovi Radio Research Station, Metsähovintie, FIN-02540 Kylmälä, Finland
4 Tartu Astrophysical Observatory, Tõravere 1-6, Tartu 202444, Estonia
5 Department of Geosciences and Astronomy, University of Oulu, P.O. Box 333, FIN-90571 Oulu, Finland

Received 17 July 1995 / Accepted 22 June 1996


Two methods of searching for periodicity in weighted time point series are applied to earlier major solar flare data during three decades (1956-1985). The results of this analysis are tested against new solar flare data during the four years 1989-1993. Active longitudes rotating with a constant synodic period of [FORMULA] could explain the complete data set over both hemispheres. The nonuniform distribution of the flare activity with this period displays two active longitudes separated by [FORMULA], but also shows other detailed features. No significant periodicity is detected on the southern solar hemisphere, whereas active zones rotating with a constant synodic period of [FORMULA] are present in the earlier and new data on the northern hemisphere. The main peculiarity is that two highly significant periodicities ([FORMULA] / [FORMULA]) are detected in the same data. The phenomena detected are interpreted as a signature of a long-lived nonaxisymmetric component of the solar magnetic field below the visible surface.

Key words: Sun: activity – flares – magnetic fields

Send offprint requests to: L. Jetsu, e-mail: jetsu@nordita.dk

© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998