Intensity contrast of the average supergranule

Abstract

While the velocity fluctuations of supergranulation dominate the spectrum of solar convection at the solar surface, very little is known about the fluctuations in other physical quantities like temperature or density at supergranulation scale. Using observations from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO), we characterize the intensity contrast of solar supergranulation at the solar surface. We identify the positions of ∼ 104 outflow and inflow regions at supergranulation scales, from which we construct average flow maps and co-aligned intensity and magnetic field maps. In the average outflow center, the maximum intensity contrast is (7.8 ± 0.6) × 10-4 (there is no corresponding feature in the line-of-sight magnetic field). This corresponds to a temperature perturbation of about 1.1 ± 0.1 K, in agreement with previous studies. We discover an east-west anisotropy, with a slightly deeper intensity minimum to the east of the outflow center. The evolution is asymmetric in time: the intensity excess is larger eight hours before the reference time (the time of maximum outflow), while it has almost disappeared eight hours after the reference time. In the average inflow region, the intensity contrast mostly follows the magnetic field distribution, except for an east-west anisotropic component that dominates eight hours before the reference time. We suggest that the east-west anisotropy in the intensity is related to the wave-like properties of supergranulation.