Asymptotic g modes: Evidence for a rapid rotation of the solar core

¹ Université Côte d’Azur, Observatoire Côte d’Azur, CNRS, Laboratoire Lagrange, CS 34229, Nice Cedex 4, France
e-mail: eric.fossat@oca.eu
² Institut d’Astrophysique Spatiale, Université Paris-Sud and CNRS (UMR 8617), Bâtiment 121, 91405 Orsay Cedex, France
³ Laboratoire AIM Paris-Saclay, CEA/DRF-CNRS-Univ. Paris Diderot – IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette, France
⁴ LAB, 2 rue de l’Observatoire, BP89, 33271 Floirac Cedex, France
⁵ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Spain
⁶ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁷ DAP/IRFU/CEA, UMR AIM, University Paris-Saclay, CE Saclay, 91191 Gif-sur-Yvette, France
⁸ Department of Physics and Astronomy Department, University of California at Los Angeles, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA
⁹ LPHEA Laboratory, Oukaimeden Observatory, Cadi Ayyad University FSSM, BP 2390 Marrakech, Morocco

Received: 18 January 2017
Accepted: 24 May 2017

Abstract

Context. Over the past 40 years, helioseismology has been enormously successful in the study of the solar interior. A shortcoming has been the lack of a convincing detection of the solar g modes, which are oscillations driven by gravity and are hidden in the deepest part of the solar body – its hydrogen-burning core. The detection of g modes is expected to dramatically improve our ability to model this core, the rotational characteristics of which have, until now, remained unknown.

Aims. We present the identification of very low frequency g modes in the asymptotic regime and two important parameters that have long been waited for: the core rotation rate, and the asymptotic equidistant period spacing of these g modes.

Methods. The GOLF instrument on board the SOHO space observatory has provided two decades of full-disk helioseismic data. The search for g modes in GOLF measurements has been extremely difficult because of solar and instrumental noise. In the present study, the p modes of the GOLF signal are analyzed differently: we search for possible collective frequency modulations that are produced by periodic changes in the deep solar structure. Such modulations provide access to only very low frequency g modes, thus allowing statistical methods to take advantage of their asymptotic properties.

Results. For oscillatory periods in the range between 9 and nearly 48 h, almost 100 g modes of spherical harmonic degree 1 and more than 100 g modes of degree 2 are predicted. They are not observed individually, but when combined, they unambiguously provide their asymptotic period equidistance and rotational splittings, in excellent agreement with the requirements of the asymptotic approximations. When the period equidistance has been measured, all of the individual frequencies of each mode can be determined. Previously, p-mode helioseismology allowed the g-mode period equidistance parameter P₀ to be bracketed inside a narrow range, between approximately 34 and 35 min. Here, P₀ is measured to be 34 min 01 s, with a 1 s uncertainty. The previously unknown g-mode splittings have now been measured from a non-synodic reference with very high accuracy, and they imply a mean weighted rotation of 1277 ± 10 nHz (9-day period) of their kernels, resulting in a rapid rotation frequency of 1644 ± 23 nHz (period of one week) of the solar core itself, which is a factor 3.8 ± 0.1 faster than the rotation of the radiative envelope.

Conclusions. The g modes are known to be the keys to a better understanding of the structure and dynamics of the solar core. Their detection with these precise parameters will certainly stimulate a new era of research in this field.