Amplitudes of Solar Gravity Modes: A Review

TL;DR

This review discusses the theoretical predictions and recent observational efforts to detect solar gravity modes, emphasizing the importance of the low-frequency domain and the need for improved turbulence modeling.

Contribution

It provides a comprehensive overview of theoretical estimates of solar gravity mode amplitudes and compares them with recent observational thresholds, highlighting future research directions.

Findings

Low-frequency domain (10-100 μHz) is promising for detection.

Theoretical estimates are within a factor of two of observational thresholds.

Recent detections of inertial modes and simulations offer new prospects.

Abstract

Solar gravity modes are considered as the {\it Rosetta Stone} for probing and subsequently deciphering the physical properties of the solar inner-most layers. Recent claims of positive detection therefore shed some new light on the long-standing issue of estimating solar gravity mode amplitudes. In this article, our objective is to review the theoretical efforts intended to predict solar gravity mode amplitudes. Because most of these studies assumed analogous driving and damping properties to those for the observed acoustic modes, we also provide a short overview of our current knowledge for these modes in the Sun and solar-type stars (which show solar-like oscillations) before diving into the specific problem of solar gravity modes. Finally, taking recent estimates into account, we conclude and confirm that the low-frequency domain (typically between $10\,\mu$Hz and $100\,\mu$Hz) is certainly more suited to focus on for detecting solar gravity modes. More precisely, around $60\,\mu$Hz, the theoretical estimates are slightly lower than the observational detection threshold as provided by the GOLF (Global Oscillations at Low Frequencies) instrument by about a factor of two only. This is typically within the current uncertainties associated with theoretical estimates and should motivate us for improving our knowledge on turbulence in the whole solar convective region, which is key for improving the accuracy of $g$-mode amplitude estimates. The recent detection of solar inertial modes (Gizon et al. 2021) combined with the continuous development of numerical simulations provide interesting prospects for future studies.