Rossby wave effect on the organization of photospheric magnetic fields

Photospheric magnetic fields present organization in the form of stable, quasi-starionary, preferred longitudes. These preferred longitudes consist of locations associated with clustering of active regions and recurrent emergences of several active regions at the same position. In this study, we will present an analysis of the longitudinal organization of photospheric magnetic fields based on MDI/HMI magnetic field synoptic maps. The drift speeds of these coherent structures is inferred from analysing time by longitude (Hovmoller) diagrams. The drift speeds are shown to significantly vary as a function the latitudes and are compatible with the phase speeds of magnetized Rossby waves at the solar tachocline embedded in a differential rotation profile.

We use information theory to quantify the degree of organization of these slowly drifting magnetic field structures. The analysis of the Shannon entropy demonstrates that the degree of organization of the longitudinal magnetic field structures at the photospheric level is proportional to the level of activity at a given latitude and time.

Our results suggest that Rossby waves contribute to the origin of the preferred longitudinal structures, acting as an organizing principle of the photospheric magnetic fields.