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.
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Acknowledgements
We thank the reviewer for useful comments. This work utilizes data obtained by the Global Oscillation Network Group (GONG) program, managed by the National Solar Observatory, which is operated by AURA, Inc. under a cooperative agreement with the National Science Foundation. The data were acquired by instruments operated by the Big Bear Solar Observatory, High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de Astrofísico de Canarias, and Cerro Tololo Interamerican Observatory. This work is partially supported by NASA grants NNG05HL41I and NNG08EI54I to the National Solar Observatory.
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Tripathy, S.C., Jain, K. & Hill, F. Acoustic Mode Frequencies of the Sun During the Minimum Phase Between Solar Cycles 23 and 24. Sol Phys 282, 1–13 (2013). https://doi.org/10.1007/s11207-012-0130-6
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DOI: https://doi.org/10.1007/s11207-012-0130-6