Magnetic field emergence in mesogranular-sized exploding granules observed with sunrise/IMaX data

Abstract

We report on magnetic field emergences covering significant areas of exploding granules. The balloon-borne mission Sunrise provided high spatial and temporal resolution images of the solar photosphere. Continuum images, longitudinal and transverse magnetic field maps and Dopplergrams obtained by IMaX onboard Sunrise are analyzed by local correlation traking (LCT), divergence calculation and time slices, Stokes inversions and numerical simulations are also employed. We characterize two mesogranular-scale exploding granules where ∼10 18 Mx of magnetic flux emerges. The emergence of weak unipolar longitudinal fields (∼100 G) start with a single visible magnetic polarity, occupying their respective granules' top and following the granular splitting. After a while, mixed polarities start appearing, concentrated in downflow lanes. The events last around 20 min. LCT analyses confirm mesogranular scale expansion, displaying a similar pattern for all the physical properties, and divergence centers match between all of them. We found a similar behaviour with the emergence events in a numerical MHD simulation. Granule expansion velocities are around 1 km s −1 while magnetic patches expand at 0.65 km s −1 . One of the analyzed events evidences the emergence of a loop-like structure. Advection of the emerging magnetic flux features is dominated by convective motion resulting from the exploding granule due to the magnetic field frozen in the granular plasma. Intensification of the magnetic field occurs in the intergranular lanes, probably because of being directed by the downflowing plasma.

Supergranular-scale magnetic flux emergence beneath an unstable filament

“…The rate of flux emergence, which is about 2 × 10 17 Mx min −1 , is at least one order of magnitude larger than those reported by Palacios et al (2012). However, the expansion velocity of the flux emergence is much slower, at 0.16 km s −1 , than in Palacios et al, which is approximately 0.6 km s −1 .…”

“…The second method we used is visually measuring the distance between the two main polarities (as a diameter and obtaining the radius), getting 0.100 ± 0.003 km s −1 . The third procedure is the one described in Palacios et al (2012), measuring the area and assuming it as circular, subsequently calculating the radius from it. This yields a velocity of 0.157 ± 0.004 km s −1 .…”

Flux emergence event underneath a filament

“…Granular-scale simulations have also been performed, e.g., in Tortosa & Moreno-Insertis (2009). At the mesogranular scale, flux emergence cases found by Palacios et al (2012) shows growth rates of 10 18 Mx in about 20 min. Spectral line inversions show field strengths of hectogauss in these mesogranular-sized patches (see Guglielmino et al (2013).…”

“…Most likely, a mechanism of magnetic field intensification in the form of 'convective collapse' (Parker 1978, Spruit 1979) took place in this network patch, since it exhibits signals of small pore growth processes. For some case studies on this physical mechanism, see e.g., Vargas Domínguez et al (2015) and references therein.…”

“…An amount of cases have been described in, e.g., , Gömöry et al (2010), Palacios et al (2012). Loop statistics, lifetimes and energy implications are described in Martínez González et al Martínez González et al (2010).…”