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

Abstract: 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. The GOLF instrument on board the SOHO space observatory has provided two decades of full-disk helioseismic data. In the present study, 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. For oscillatory periods in the range between 9 and nearly 48 hours, 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 unambiguouslyprovide their asymptotic period equidistance and rotational splittings, in excellent agreement with the requirements of the asymptotic approximations. Previously, p-mode helioseismology allowed the g-mode period equidistance parameter $P_0$ to be bracketed inside a narrow range, between approximately 34 and 35 minutes. Here, $P_0$ 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 $\pm$ 10 nHz (9-day period) of their kernels, resulting in a rapid rotation frequency of 1644 $\pm$ 23 nHz (period of one week) of the solar core itself, which is a factor 3.8 $\pm$ 0.1 faster than the rotation of the radiative envelope. 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.

Evidence for a Rapid Rotation of the Solar Core from Asymptotic g Modes

The paper "Asymptotic g modes: Evidence for a rapid rotation of the solar core," authored by E. Fossat et al., explores the long-standing endeavor of detecting the gravity (g) modes in the solar interior, specifically within the solar core. Through an innovative analysis using data from the Global Oscillations at Low Frequency (GOLF) instrument on the SOHO space observatory, the research discusses the successful identification of asymptotic g modes and determines critical parameters such as the core rotation rate and the asymptotic equidistant period spacing of these g modes.

Research Summary

The study capitalized on 16.5 years of data collected by the GOLF instrument. The challenges of detecting g modes are substantial due to their confinement within the deep solar interior and significant attenuation by the convective layers. This challenge impacts the potential to capture their amplitudes at the solar surface. The authors circumvented this by employing a novel method involving the modulation of the sound wave travel time through the solar diameter. This approach allowed for the detection of very low-frequency g modes through their collective impact on the solar p-mode frequency spectrum.

Key Results

• Detection of g Modes: The research identifies a significant number of g modes across periods ranging from approximately 9 to 48 hours. The analysis confirms the asymptotic period equidistance parameter $P_0$ to be precisely 34 minutes and 1 second, with an uncertainty of 1 second.
• Estimation of Solar Core Rotation: One of the study's most notable findings is the quantification of the solar core's rotation rate, derived from g-mode splittings. The core is found to rotate at $1644 \pm 23 \text{ nHz}$, which translates to a period of approximately one week. This rotation rate is approximately 3.8 times faster than the radiative envelope's rotation.
• Autocorrelation and Splitting Analysis: The research applied autocorrelation techniques to identify rotational splittings. A significant peak at 210 nHz was identified, indicating the presence of g-mode triplets.

Implications and Future Work

The implications of detecting a rapid solar core rotation are profound for both solar physics and the broader understanding of stellar processes. This discovery challenges existing models of angular momentum distribution and transport within stars, suggesting the need for mechanisms like internal magnetic braking to account for such rapid rotation rates in the core.

Future research can extend this work by applying the same methodology to other stars or through enhanced model sophistication to improve our understanding of angular momentum evolution in stellar interiors. As g modes are further explored and characterized, they have the potential to significantly refine models of stellar dynamics and evolution.

Conclusion

The findings detailed in the study represent a significant stride in helioseismology, offering a more nuanced characterization of the solar core's rotational dynamics. The results underscore the utility of asymptotic g modes as a diagnostic tool for investigating the internal structure and rotation of solar-type stars. The precision of the measurement sets a benchmark for future research in the field, providing a clear direction for the development of theoretical models that incorporate these new insights into solar and stellar interior dynamics.