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

Salabert, D.; García, R. A.; Pallé, P. L.; Jiménez-Reyes, S. J.
Bibliographical reference

Astronomy and Astrophysics, Volume 504, Issue 1, 2009, pp.L1-L4

Advertised on:
9
2009
Number of authors
4
IAC number of authors
3
Citations
81
Refereed citations
47
Description
We study the response of the low-degree, solar p-mode frequencies to the unusually extended minimum of solar surface activity since 2007. A total of 4768 days of observations collected by the space-based, Sun-as-a-star helioseismic GOLF instrument are analyzed. A multi-step iterative maximum-likelihood fitting method is applied to subseries of 365 days and 91.25 days to extract the p-mode parameters. Temporal variations in the l=0, 1, and 2 p-mode frequencies are then obtained from April 1996 to May 2009. While the p-mode frequency shifts are closely correlated with solar surface activity proxies during the past solar cycles, the frequency shifts of the l=0 and l=2 modes increase from the second half of 2007, when no significant surface activity is observable. On the other hand, the l=1 modes follow the general decreasing trend of solar surface activity. The different behaviors between the l=0 and l=2 modes and the l=1 modes may be interpreted as different geometrical responses to the spatial distribution of the solar magnetic field beneath the surface of the Sun. The analysis of the low-degree, solar p-mode frequency shifts indicates that the solar activity cycle 24 started in late 2007, despite the absence of activity on the solar surface.
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