Volume 504, Number 1, September II 2009
|Page(s)||L1 - L4|
|Published online||18 August 2009|
Letter to the Editor
The onset of solar cycle 24
What global acoustic modes are telling us
Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain e-mail: firstname.lastname@example.org
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
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 , 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 and modes increase from the second half of 2007, when no significant surface activity is observable. On the other hand, the modes follow the general decreasing trend of solar surface activity. The different behaviors between the and modes and the 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
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