Structure of sunspot penumbral filaments: a remarkable uniformity of properties

Abstract

Context. The sunspot penumbra comprises numerous thin, radially elongated filaments that are central for heat transport within the penumbra, but whose structure is still not clear. Aims. We aim to investigate the fine-scale structure of these penumbral filaments. Methods. We perform a depth-dependent inversion of spectropolarimetric data of a sunspot very close to solar disk center obtained by Solar Optical Telescope/Spectropolarimeter onboard the Hinode spacecraft. We have used a recently developed, spatially coupled 2D inversion scheme, which allows us to analyze the fine structure of individual penumbral filaments up to the diffraction limit of the telescope.Results. Filaments of different sizes in all parts of the penumbra display very similar magnetic field strengths, inclinations, and velocity patterns. The temperature structure is also similar, although the filaments in the inner penumbra have cooler tails than those in the outer penumbra. The similarities allowed us to average all these filaments and to subsequently extract the physical properties common to all of them. This average filament shows upflows associated with an upward-pointing field at its inner, umbral end (head) and along its axis, as well as downflows along the lateral edge and strong downflows in the outer end (tail) associated with a nearly vertical, strong, and downward-pointing field. The upflowing plasma is significantly, i.e., up to 800 K, hotter than the downflowing plasma. The hot, tear-shaped head of the averaged filament can be associated with a penumbral grain. The central part of the filament shows nearly horizontal fields with strengths in the range of 1 kG. The field above the filament converges, whereas a diverging trend is seen in the deepest layers near the head of the filament. The fluctuations in the physical parameters along and across the filament increase rapidly with depth. Conclusions. We put forward a unified observational picture of a sunspot penumbral filament. It is consistent with such a filament being a magneto-convective cell, in line with recent magnetohydrodynamic simulations. The uniformity of its properties over the penumbra sets constraints on penumbral models and simulations. The complex and inhomogeneous structure of the filament provides a natural explanation for a number of long-running controversies in the literature.

Peripheral downflows in sunspot penumbrae

“…1, is the granular flow pattern that is clearly visible around both the sunspots. This contrasts with the penumbra, that shows many compact regions containing predominantly upflows in the inner and downflows in the outer penumbra, actually at the inner and outer ends of penumbral filaments (Tiwari et al 2013). The downflows completely dominate over the upflows on the border between the penumbra and the granulation surrounding it, resulting in a clearly defined ring of downflows surrounding the entire sunspot.…”

“…Such downflows are found at the ends of all penumbral filaments, although in the case of simple filaments, with a single head and a single tail, they only reach speeds of 6 km s −1 on average, so that many of them remain subsonic (Tiwari et al 2013). The supersonic downflows of this type are found at the ends of more complex filaments, particularly those with multiple heads that merge to form a single tail (or alternatively multiple filaments merge).…”

The magnetic field in the solar atmosphere

“…The flows also move the magnetic features around, causing each to carry out a random walk, although the exact nature of the motion can differ, depending on the location of the magnetic feature (Abramenko et al 2011;Jafarzadeh et al 2014a). Tiwari et al (2013) and van Noort et al (2013) The random walk of the magnetic patches, imposed by the convective motions, necessarily leads to the encounter of opposite polarity fields. These, in the case of smaller flux tubes, often do not correspond to the other footpoint of the original -loop, so that in larger ARs, a fair amount of cancelation takes place .…”

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

“…At the same time, Solanki & Montavon (1993) established that these two distinct components also interlace vertically, thereby explaining the asymmetries in the observed circular polarization profiles (Stokes V ). It was later found that the vertical and horizontal interlacing of these two components implies that the magnetic field in the spines wraps around the intraspines , with the latter remaining unchanged at all radial distances from the sunspot's center (Borrero et al 2005(Borrero et al , 2006Tiwari et al 2013) and the former being nothing but the extension of the umbral field into the penumbra (Tiwari et al 2015). It has also been confirmed that the Evershed flow can reach supersonic and super-Alvénic values, not only on the outer penumbra (Borrero et al 2005;van Noort et al 2013), but also close to the umbra (del Toro Iniesta et al 2001;Bellot Rubio et al 2004) and has a strong upflowing component at the inner penumbra that turns into a downflowing component at larger radial distances (Franz & Schlichenmaier 2009Tiwari et al 2013).…”

“…It was later found that the vertical and horizontal interlacing of these two components implies that the magnetic field in the spines wraps around the intraspines , with the latter remaining unchanged at all radial distances from the sunspot's center (Borrero et al 2005(Borrero et al , 2006Tiwari et al 2013) and the former being nothing but the extension of the umbral field into the penumbra (Tiwari et al 2015). It has also been confirmed that the Evershed flow can reach supersonic and super-Alvénic values, not only on the outer penumbra (Borrero et al 2005;van Noort et al 2013), but also close to the umbra (del Toro Iniesta et al 2001;Bellot Rubio et al 2004) and has a strong upflowing component at the inner penumbra that turns into a downflowing component at larger radial distances (Franz & Schlichenmaier 2009Tiwari et al 2013). Finally, there is strong evidence for an additional component of the velocity field in intraspines that appears as convective upflows along the center of the intraspines that turns into downflows at the filaments' edges (Zakharov et al 2008;Joshi et al 2011;Scharmer et al 2011;Tiwari et al 2013).…”