Emergence of Small-Scale Magnetic Loops Through the Quiet Solar Atmosphere

Abstract

We investigate the emergence of magnetic flux in the quiet Sun at very small spatial scales, focusing on the magnetic connection between the photosphere and chromosphere. The observational data consist of spectropolarimetric measurements and filtergrams taken with the Hinode satellite and the Dutch Open Telescope. We find that a significant fraction of the magnetic flux present in internetwork regions appears in the form of Ωshaped loops. The emergence rate is 0.02 loops per hour and arcsec −2 , which brings 1.1x10 12 Mx s −1 arcsec −2 of new flux to the solar surface. Initially, the loops are observed as small patches of linear polarization above a granular cell. Shortly afterwards, two footpoints of opposite polarity become visible in circular polarization within or at the edges of the granule and start to move toward the adjacent intergranular space. The orientation of the footpoints does not seem to obey Hale's polarity rules. The loops are continuously buffeted by convective motions, but they always retain a high degree of coherence. Interestingly, 23% of the loops that emerge in the photosphere reach the chromosphere (16 cases out of 69). They are first detected in Fe I 630 nm magnetograms and 5 minutes later in Mg I b 517.3 nm magnetograms. After about 8 minutes, some of them are also observed in Ca II H line-core images, where the footpoints produce small brightness enhancements.

The magnetic field in the solar atmosphere

“…Below that we generally speak of "ephemeral regions" (10 18 to 10 20 Mx). Even smaller are the smallest so far resolved bipolar features, the internetwork magnetic loops (Martínez González et al 2007;Centeno et al 2007;Martínez González and Bellot Rubio 2009;Danilovic et al 2010a) which emerge throughout the QS (although preferring a meso-scale pattern). They have fluxes of roughly 10 16 to 10 17 Mx (Lin and Rimmele 1999) and display in general weak equipartition (that is, the magnetic energy density is similar to the kinetic energy density of the convective flows) intrinsic fields.…”

“…Note that emerging small-scale loops are observed in the form of newly detected horizontal fields well before vertical flux is detected (see Centeno et al 2007, for a corresponding study based on Hinode/SOT-SP data). In subsequent studies, Martínez Bellot Rubio (2009) andMartínez González et al (2010) presented observations of emerging -loops by combining magnetic and imaging data from Hinode/SOT and DOT. With an even higher resolution, based on the balloon-borne Sunrise/IMaX instrument, Danilovic et al (2010a) investigated a large number of transverse photospheric features.…”

“…Obviously, Aly's condition is not fulfilled in the internetwork photosphere where the horizontal field is on average much stronger than the vertical field, which has important implications for the reliability of force-free magnetic field reconstructions in the QS. The fact that these fields are definitely far from being force-free in the photosphere and that most of the weak (nearly) horizontal fields are the tops of very short and low-lying loops (Martínez González et al 2007;Centeno et al 2007;Martínez González and Bellot Rubio 2009;Danilovic et al 2010a) which do not reach up to layers where the field is closer to being force-free, the impact of the plasma on the field cannot be neglected.…”

Magneto-convection

“…This new flux is first seen as horizontal field (linear polarization in Stokes spectra) inside granules followed by the appearance of vertical field at the ends of the horizontal field (circular polarized Stokes spectra) [50][51][52]. These U-loop footpoints get quickly swept into the intergranular lanes and the horizontal field to the edges of the granules.…”

“…Transient horizontal fields also appear briefly where new downflow lanes form [49]. The flux in these emerging bipoles is small, 10 16 − few × 10 17 Mx, but their rate of appearance is large, around a few ×10 −10 km −2 s −1 , hence their dominant contribution to the emerging flux of the Sun [47,51,52]. Most of these small loops are low lying, with only about a quarter reaching up to chromospheric heights.…”