Acoustic Mode Frequencies of the Sun During the Minimum Phase Between Solar Cycles 23 and 24
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Acoustic Mode Frequencies of the Sun During the Minimum Phase Between Solar Cycles 23 and 24 S.C. Tripathy · K. Jain · F. Hill
Received: 6 June 2012 / Accepted: 10 September 2012 © Springer Science+Business Media Dordrecht 2012
Abstract We investigate the spatial and temporal variations of the high-degree mode frequencies calculated over localized regions of the Sun during the extended minimum phase between solar cycles 23 and 24. The frequency shifts measured relative to the spatial average over the solar disk indicate that the correlation between the frequency shift and magnetic field strength during the low-activity phase is weak. The disk-averaged frequency shifts computed relative to a minimal activity period also reveal a moderate correlation with different activity indices, with a maximum linear correlation of about 72 %. From the investigation of the frequency shifts at different latitudinal bands, we do not find a consensus period for the onset of solar cycle 24. The frequency shifts corresponding to most of the latitudes in the northern hemisphere and 30° south of the equator indicate the minimum epoch to be February 2008, which is earlier than inferred from solar activity indices. Keywords Data analysis · Helioseismology · Solar cycle 1. Introduction The analysis of continuous helioseismic data over solar cycle 23 has produced many interesting results related to the cyclic variation of the oscillation frequencies and other mode parameters. Most of the studies are confined to the low- and intermediate-degree modes (Jain, Tripathy, and Hill, 2011; and references therein) which reinforces the idea that the mode frequencies change with solar activity level. However, the period of extended minimum between cycles 23 and 24 produced some surprises, most notably a disagreement between the low- and intermediate-degree modes regarding the onset of cycle 24. Based on S.C. Tripathy () · K. Jain · F. Hill National Solar Observatory, Tucson, AZ 85719, USA e-mail: [email protected] K. Jain e-mail: [email protected] F. Hill e-mail: [email protected]
S.C. Tripathy et al.
data from the Global Oscillation at Low Frequency (GOLF) instrument on-board the Solar and Heliospheric Observatory (SOHO), Salabert et al. (2009) pointed out that cycle 24 started in late 2007. This was later substantiated by the analysis of Birmingham Solar Oscillation Network (BiSON) data (Fletcher et al., 2010). But the analysis of intermediate-degree mode frequencies from Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) did not confirm the early onset (Jain et al., 2010). Instead the occurrence of the minimum was shown to be around the middle of 2008 for MDI data and the end of 2008 for GONG data (Tripathy et al., 2010b). The latter period approximately coincided with the minimum period observed in solar activity indices. A more detailed investigation of lowand intermediate-degree modes from GONG data, to eliminate the instrument dependence, demonstrated that the extended minimum period as seen in oscillation frequencies cons
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