EDP Sciences
Free access
Volume 504, Number 1, September II 2009
Page(s) L1 - L4
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361/200912736
Published online 18 August 2009
A&A 504, L1-L4 (2009)
DOI: 10.1051/0004-6361/200912736


The onset of solar cycle 24

What global acoustic modes are telling us
D. Salabert1, 2, R. A. García3, P. L. Pallé1, 2, and S. J. Jiménez-Reyes1, 2

1  Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
    e-mail: salabert@iac.es
2  Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
3  Laboratoire AIM, CEA/DSM-CNRS, Université Paris 7 Diderot, IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette, France

Received 21 June 2009 / Accepted 23 July 2009

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.

Key words: methods: data analysis -- Sun: helioseismology -- Sun: activity

© ESO 2009