On the fine structure of sunspot penumbrae

Abstract

Abstract. We investigate the fine structure of the sunspot penumbra by means of a model that allows for a flux tube in horizontal pressure balance with the magnetic background atmosphere in which it is embedded. We apply this model to spectropolarimetric observations of two neutral iron lines at 1.56 µm and invert several radial cuts in the penumbra of the same sunspot at two different heliocentric angles. In the inner part of the penumbra we find hot flux tubes that are somewhat inclined to the horizontal. They become gradually more horizontal and cooler with increasing radial distance. This is accompanied by an increase in the velocity of the plasma and a decrease of the gas pressure difference between flux tube and the background component. At large radial distances the flow speed exceeds the critical speed and evidence is found for the formation of a shock front. These results are in good agreement with simulations of the penumbral fine structure and provide strong support for the siphon flow as the physical mechanism driving the Evershed flow.

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

“…The presence of field-free gaps or dynamically weak magnetic fields in the penumbra had been previously ruled out (Mathew et al 2003;Borrero et al 2004;Bellot Rubio et al 2004;Borrero et al 2005;Cabrera Solana et al 2008) from observations of the same deep-forming Fe i spectral lines around 1565 nm used in this work (see Table 1). However, those older investigations were carried out with data at relatively low spatial resolution (1 arcsec) and without accounting for wide-angle scattered light.…”

“…At larger heliocentric angles (NOAA 12045; Θ = 20.5 • ) this is not the case anymore. Moreover, for the latter sunspot the limb-side penumbra displays highly asymmetric three-lobe Stokes profiles similar to those in Borrero et al (2004Borrero et al ( , 2005. This is undoubtedly a sign of two distinct polarities present in the resolution element and it explains the larger number of coefficients needed to reproduce I obs in NOAA 12045.…”

“…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).…”

Modeling the Subsurface Structure of Sunspots

“…• Bellot Rubio (2003) and Borrero et al (2004Borrero et al ( , 2005Borrero et al ( , 2006: Inverted the polarization profiles from the penumbra and considered the uncombed penumbral model with two magnetic components, where the component with a more horizontal and weaker field harbors the Evershed flow.…”