Migration of Ca II H bright points in the internetwork

Abstract

Context. The migration of magnetic bright point-like features (MBP) in the lower solar atmosphere reflects the dispersal of magnetic flux as well as the horizontal flows of the atmospheric layer they are embedded in.Aims. We analyse trajectories of the proper motion of intrinsically magnetic, isolated internetwork Ca ii H MBPs (mean lifetime 461± 9 s) to obtain their diffusivity behaviour. Methods. We use seeing-free high spatial and temporal resolution image sequences of quiet-Sun, disc-centre observations obtained in the Ca ii H 3968 Å passband of the Sunrise Filter Imager (SuFI) onboard the Sunrise balloon-borne solar observatory. Small MBPs in the internetwork are automatically tracked. The trajectory of each MBP is then calculated and described by a diffusion index (γ) and a diffusion coefficient (D). We also explore the distribution of the diffusion indices with the help of a Monte Carlo simulation. Results. We find γ = 1.69 ± 0.08 and D = 257 ± 32 km 2 s −1 averaged over all MBPs. Trajectories of most MBPs are classified as super-diffusive, i.e. γ > 1, with the determined γ being the largest obtained so far to our knowledge. A direct correlation between D and timescale (τ) determined from trajectories of all MBPs is also obtained. We discuss a simple scenario to explain the diffusivity of the observed, relatively short-lived MBPs while they migrate within a small area in a supergranule (i.e. an internetwork area). We show that the scatter in the γ values obtained for individual MBPs is due to their limited lifetimes. Conclusions. The super-diffusive MBPs can be described as random walkers (due to granular evolution and intergranular turbulence) superposed on a large systematic (background) velocity, caused by granular, mesogranular, and supergranular flows.

Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

“…This result is in good agreement with the observational findings of Jafarzadeh et al (2014a), obtained by comparing the positions of BPs recorded at different heights by Sunrise. Hence this is another point in which Sunrise data and MHD simulations agree.…”

Surface flux transport simulations: Effect of inflows toward active regions and random velocities on the evolution of the Sun’s large-scale magnetic field

“…To compare with the diffusion approximation, we consider random walks corresponding to a fixed diffusion coefficient of η = 250 km 2 /s. This value is similar to the ∼257 km 2 /s value reported by Jafarzadeh et al (2014) from observations. It is also in the range of diffusivities found in radiative MHD simulations by Cameron et al (2011), and compatible with the evolution of the large-scale fields .…”

Kinematics of Magnetic Bright Features in the Solar Photosphere

“…This is, however, not surprising, since D has been shown to be dependent on spatial and temporal scales (e.g., Abramenko et al 2011;Giannattasio et al 2013;Jafarzadeh et al 2014a). (a), represent the best linear fits to the data points corresponding to the various ROIs.…”

“…Magnitudes of diffusion parameters describe various diffusivity regimes (e.g., Abramenko et al 2011;Jafarzadeh et al 2014a) such as (1) 1 g = , the so-called normal diffusion, where magnetic elements are randomly advected around (random walkers) and the diffusion coefficient (D) is independent of temporal and spatial scales; (2) 1 g < , the sub-diffusive process, where features are trapped at so-called stagnation points (sinks of flow field) and D is anti-correlated with both time and length scales; and (3) 1 g > , the superdiffusive case, indicating regions where small structures quickly move away from their first location (D grows with scales both in time and in length). Magnetic elements in the latter regime are transported with a negative acceleration when 2 g < , with a constant average speed for 2 g = (known as "ballistic" diffusion), and with a positive acceleration when 2 g > (the super-ballistic branch).…”