Striation and convection in penumbral filaments

Abstract

Observations with the 1-m Swedish Solar Telescope of the flows seen in penumbral filaments are presented. Time sequences of bright filaments show overturning motions strikingly similar to those seen along the walls of small isolated structures in the active regions. The filaments show outward propagating striations with inclination angles suggesting that they are aligned with the local magnetic field. We interpret it as the equivalent of the striations seen in the walls of small isolated magnetic structures. Their origin is then a corrugation of the boundary between an overturning convective flow inside the filament and the magnetic field wrapping around it. The outward propagation is a combination of a pattern motion due to the downflow observed along the sides of bright filaments, and the Evershed flow. The observed short wavelength of the striation argues against the existence of a dynamically significant horizontal field inside the bright filaments. Its intensity contrast is explained by the same physical effect that causes the dark cores of filaments, light bridges and "canals". In this way striation represents an important clue to the physics of penumbral structure and its relation with other magnetic structures on the solar surface. We put this in perspective with results from the recent 3-D radiative hydrodynamic simulations.

Deep probing of the photospheric sunspot penumbra: no evidence of field-free gaps

“…wide-angle scattered light) within the instrument, we have applied, prior to the inversion, a PCA deconvolution method using an empirical point spread function. Our results show no evidence for the presence of weak-field regions (B < 500), let alone of dynamically weak fields (Spruit et al 2010) or field-free regions in the deepest regions of the photosphere (log τ 5 = 0). This agrees with previous observational results, in particular with , 2010; Tiwari et al (2013) and with three-dimensional MHD simulations of sunspot fine-structure (Rempel et al 2009;Nordlund & Scharmer 2010;Rempel 2011Rempel , 2012.…”

“…The regions where the magnetic field strength is smaller than 500 Gauss represent only 0.5 % and 0.2 % of the total area in Figures 7 (left) and 8 (left), respectively. These areas would be even smaller if we had employed a 300 Gauss limit found by Spruit et al (2010). At close inspection we notice that these tiny regions where the magnetic field is below 500 Gauss correspond to pixels in the outer penumbra where granules enter into the sunspot.…”

“…These results rule out the existence, at an optical depth of log τ 5 = 0, of dynamically weak magnetic fields (cf. Scharmer 2008;Spruit et al 2010), and even more strongly so, the presence of field-free gaps in the deep photospheric layers of the penumbra . This is in agreement with previous results obtained from Hinode/SP data Puschmann et al 2010;Ruiz Cobo & Asensio Ramos 2013;Tiwari et al 2013Tiwari et al , 2015 and SST/CRISP data (Scharmer et al 2008b;Scharmer & Henriques 2012;Scharmer et al 2013).…”

“…Three-dimensional magnetohydrodynamic simulations of penumbral fine structure also yield magnetic field values of the order to 1-1.5 kG inside penumbral intraspines (Rempel 2012) irrespective of the boundary conditions and grid resolution. Spruit et al (2010) interpreted the striations seen perpendicular to the penumbral filaments in high-resolution continuum images as a consequence of fluting instability, and established an upper limit of B ≤ 300 Gauss for the magnetic field inside intraspines. This redefines field-free to mean instead dynamically weak magnetic fields, where the magnetic pressure is smaller than the kinematic pressure.…”

“…2.2 in Scharmer et al 2013); and (c) the impossibility to probe layers located deep enough to detect them (see Sect. 5.4 in Spruit et al 2010). In this work we will address these issues by employing spectropolarimetric observations of the Fe i spectral lines at 1565 nm recorded with the GRIS instrument at the GREGOR Telescope.…”