Towards inertial-mode helioseismology: Direct sensing of solar rotation at 75 deg latitude and 0.8 Rsun

TL;DR

This paper demonstrates that high-latitude inertial modes can be used to directly measure the solar rotation rate at 75 degrees latitude and 0.8 solar radii, providing new insights into the Sun's internal dynamics.

Contribution

It introduces a method to constrain solar rotation using inertial mode frequencies, offering the first spatially resolved inertial-mode helioseismology at high latitudes.

Findings

Inferred solar rotation rate near 75° latitude and 0.8 R_sun is 365.3 nHz.

Sensitivity kernel peaks at 75° latitude and 0.8 R_sun with 7° and 0.13 R_sun widths.

Observed mode frequency exceeds the p-mode estimate by 8.1 nHz.

Abstract

Solar internal rotation at high latitudes is poorly constrained by acoustic-mode helioseismology. Global inertial modes observed on the Sun are highly sensitive to solar differential rotation and may provide new diagnostics of rotation in these regions. We aim to constrain solar rotation with the measured frequency of the $m=1$ high-latitude inertial mode, starting from the HMI/SDO reference rotation profile given by p-mode helioseismology for 2010-2024. Using a validated and accurate eigenvalue solver, we compute the perturbation to the mode frequency resulting from localised changes in the differential rotation rate throughout the solar interior. We find that the linear sensitivity kernel of the $m=1$ high-latitude mode peaks at latitude 75 deg and radius $0.8 R_\odot$, with full widths of 7 deg and $0.13 R_\odot$. From the observed mode frequency in the Carrington frame, $-87.9 \pm 1.9$ nHz (retrograde, averaged over 2010-2024), we infer that the solar rotation rate near this location is $365.3\pm 2.0$ nHz, which exceeds the reference p-mode estimate by $8.1$ nHz. Additionally, we propose a latitudinally smooth, radially independent modification to the rotation rate at high latitudes beyond the linear (small-perturbation) regime. This work demonstrates that individual inertial modes can provide direct constraints on rotation in the bulk of the solar convection zone, well below the surface, representing the first example of spatially resolved inertial-mode helioseismology.