Spectra of solar shallow-water waves from bright point observations

Astronomy & Astrophysics:  Rossby waves, large-scale meandering patterns drifting in longitude, detected in the Sun, were recently shown to a play a crucial role in understanding "seasons” of space weather. Unlike purely classical Earth's atmospheric Rossby waves, the solar counterparts are strongly magnetized and most likely originate in the tachocline. Because of their deeper origin, detecting these magnetized Rossby waves is a challenging task, relying on careful observations of long-lived longitudinally-drifting magnetic patterns at the surface and above.
Here, we utilize 3 years of global, synchronous observations of coronal bright point densities to obtain empirical signatures of dispersion relations that can be attributed to the simulated waves in tachocline. By tracking the bright point densities at selected latitudes, we compute their wave-number X frequency spectra. Wave-number X frequency spectra is computed utilizing the Wheeler-Kiladis method. This method has been extensively used in the identification of equatorial waves in Earth's atmosphere by highlighting spectral peaks in wave-number X frequency space.
Our results are compatible with the predictions of magneto-Rossby waves with typical periods of several months and inertio-gravity waves with typical periods of a few weeks, depending on the background magnetic field's strength and stratification at convection-zone base. Our analysis suggests that magnetized Rossby waves originate from the tachocline toroidal field of up to 15 kG. Global observations of bright points over extended periods will allow us better constraint the stratification and magnetic field strength in the tachocline.