Radial Flow Component of Sun’s High-frequency Retrograde Inertial Waves

Abstract Solar inertial modes have the potential to surpass the diagnostic capabilities of acoustic waves in probing the deep interior of the Sun. The fulfillment of this potential requires an accurate identification and characterization of these modes. Among the set of detected inertial modes, the equatorially antisymmetric “high-frequency retrograde” (HFR) modes has attracted special interest because numerical studies have suggested that they are not purely toroidal, as initial observations suggested, and predicted that they would possess a significant radial flow signal at depth. Here, we analyze ∼13 yr of Helioseismic and Magnetic Imager/Solar Dynamics Observatory 5° ring tiles and discover a horizontal-divergence signal, directly connected to radial flows, in the near-surface layers of the Sun. We demonstrate that this signal is indeed part of the HFR modes and not spatial leakage from prograde flows associated with magnetic regions. The amplitudes of the horizontal divergence are approximately half that associated with radial vorticity. We also report the presence of a ridge of enhanced power, although with a signal-to-noise ratio of 0.3, in the retrograde frequencies that coincides with the HFR latitudinal overtones reported by models. Using numerical linear models, we find reasonable agreement with observations, though future work on boundary considerations and the inclusion of the near-surface may improve future inferences. This is the first instance where numerical studies of solar inertial modes have guided observations, giving further confidence to past inferences that rely upon numerical models.