Acoustic Mode Frequencies of the Sun during the Minimum Phase between   Solar Cycles 23 and 24
  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 degree south of the equator indicate the minimum epoch to be February 2008, which is earlier than inferred from solar activity indices. 
S. C. Tripathy, K. Jain, and Frank Hill
astro-ph.SR