Periodicities in sunspot activity during solar cycle 23

¹ Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital 263 129, Uttaranchal, India e-mail: bhuwan,ppant@aries.ernet.in
² Radio Astronomy Centre, TIFR, Ooty 643 001, Tamilnadu, India e-mail: mano@ncra.tifr.res.in

Received: 7 February 2006
Accepted: 27 February 2006

Abstract

Aims.The data of sunspot numbers, sunspot areas and solar flare index during cycle 23 are analyzed to investigate the intermediate-term periodicities.

Methods.Power spectral analysis has been performed separately for the data of the whole disk, northern and southern hemispheres of the Sun.

Results.Several significant midrange periodicities (~175, 133, 113, 104, 84, 63 days) are detected in sunspot activity. Most of the periodicities in sunspot numbers generally agree with those of sunspot areas during the solar cycle 23. The study reveals that the periodic variations in the northern and southern hemispheres of the Sun show a kind of asymmetrical behavior. Periodicities of ~175 days and ~133 days are highly significant in the sunspot data of northern hemisphere showing consistency with the findings of Lean (1990) during solar cycles 12-21. On the other hand, southern hemisphere shows a strong periodicity of about 85 days in terms of sunspot activity. The analysis of solar flare index data of the same time interval does not show any significant peak. The different periodic behavior of sunspot and flare activity can be understood in the light of hypothesis proposed by Ballester et al. (2002), which suggests that during cycle 23, the periodic emergence of magnetic flux partly takes place away from developed sunspot groups and hence may not necessarily increase the magnetic complexity of sunspot groups that leads to the generation of flares.