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We study the response of the low-degree, solar p-mode frequencies to theunusually extended minimum of solar surface activity since 2007. A total of4768 days of observations collected by the space-based, Sun-as-a-starhelioseismic GOLF instrument are analyzed. A multi-step iterativemaximum-likelihood fitting method is applied to subseries of 365 days and 91.25days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2p-mode frequencies are then obtained from April 1996 to May 2009. While thep-mode frequency shifts are closely correlated with solar surface activityproxies during the past solar cycles, the frequency shifts of the l=0 and l=2modes show an increase from the second half of 2007, when no significantsurface activity is observable. On the other hand, the l=1 modes follow thegeneral decreasing trend of the solar surface activity. The differentbehaviours between the l=0 and l=2 modes and the l=1 modes can be interpretedas different geometrical responses to the spatial distribution of the solarmagnetic field beneath the surface of the Sun. The analysis of the low-degree,solar p-mode frequency shifts indicates that the solar activity cycle 24started late 2007, despite the absence of activity on the solar surface.

The onset of solar cycle 24: What global acoustic modes are telling us