Two-dimensional spectroscopy of a sunspot

Abstract

We investigate the thermal and kinematic configuration of a sunspot penumbra using high spectral and spatial resolution intensity profiles of the non-magnetic Fe i 557.6 nm line. The data set was acquired with the 2D solar spectrometer TESOS. The profiles are inverted using a one-component model atmosphere with gradients of the physical quantities. From this inversion we obtain the stratification with depth of temperature, line-of-sight velocity, and microturbulence across the penumbra. Our results suggest that the physical mechanism(s) responsible for the penumbral filaments operate preferentially in the lower photosphere. The spot, located at an heliocentric angle of 23• , exhibits larger continuum intensities in the center-side penumbra as compared with the limb side, which translates into an average temperature difference of 100−150 K at log τ 500 = 0. We investigate the nature of the bright ring that appears in the inner penumbra when sunspots are observed in the wing of spectral lines. It is suggested that the bright ring does not reflect a temperature enhancement in the mid photospheric layers. The line-of-sight velocities retrieved from the inversion are used to determine the flow geometry at different heights in the photosphere. Both the flow speed and flow angle increase with optical depth and radial distance. Downflows are detected in the mid and outer penumbra, but only in deep layers (log τ 500 ≥ −1.4). We demonstrate that the velocity stratifications retrieved from the inversion are consistent with the idea of penumbral flux tubes channeling the Evershed flow. Finally, we show that larger Evershed flows are associated with brighter continuum intensities in the inner center-side penumbra. Dark structures, however, are also associated with significant Evershed flows. This leads us to suggest that the bright and dark filaments seen at 0. 5 resolution are not individual flow channels, but a collection of them. Our analysis highlights the importance of very high spatial resolution spectroscopic and spectropolarimetric measurements for a better understanding of sunspot penumbrae.

Supersonic Downflows at the Umbra-Penumbra Boundary of Sunspots

“…The EF is observed as a shift of the spectral lines, due to a nearly horizontal, radial outflow of plasma that starts as upflows in the deep layers of the inner penumbra (Bellot Rubio et al 2006;Rimmele & Marino 2006;Franz & Schlichenmaier 2009) and ends in a ring of downflow channels in the mid and outer penumbra (Westendorp Plaza et al 1997;Schlichenmaier & Schmidt 1999;Mathew et al 2003;Bellot Rubio et al 2004;Borrero et al 2005). The weak Evershed upflows in the inner penumbra are associated with bright penumbral grains that migrate toward the umbra-penumbra boundary with speeds of ≈1 km s −1 (Rimmele & Marino 2006) while individual velocity packets are observed to propagate toward the periphery of the sunspot with velocities of 2-5.5 km s −1 (Rimmele 1994) and vary on a timescale of 10-20 minutes (Rimmele 1994;Shine et al 1994;Rouppe van der Voort 2003;Cabrera Solana et al 2007).…”

Recent Evidence for Convection in Sunspot Penumbrae

“…Note the similarity of the magnetic field configuration above the flux tube in this model and that of the convective gap models, shown in Fig.4 flux tubes. Forward calculations of Stokes spectra (Martínez Pillet 2000) and unpolarized spectra (Rimmele 1995;Bellot Rubio et al 2006) based on flux tube models also demonstrate consistency. In some cases, it was demonstrated also that the observed data could equally well be reproduced with flux tube representations and models with smooth gradients (Martínez Pillet 2000; Rouppe van der Voort 2002; Borrero et al 2004;Bellot Rubio et al 2006).…”

The Evershed Flow and the Brightness of the Penumbra

“…The results of this analysis indicate that, on average, the flow is slightly inclined upward in the inner penumbra, then becomes horizontal at about 0.8 penumbral radii, and finally returns to the solar surface in the middle and outer penumbra with inclinations of 10 • -30 • to the horizontal (Schlichenmaier and Schmidt 2000;Bellot Rubio et al 2003;Tritschler et al 2004;Bellot Rubio et al 2006, and references therein). This must be understood as an average behavior; of course, upflows also exist in the outer penumbra, but the downflows dominate the azimuthal average.…”

“…An intriguing feature revealed by high-resolution Dopplergrams (Hirzberger & Kneer 2001;Bellot Rubio et al 2006) and spectropolarimetric measurements Ichimoto et al 2008) is that there is no place in the penumbra where the velocities drop to zero, except where they should vanish because of projection effects. For example, in the inner penumbra the stronger flows occur in the bright filaments, but smaller velocities are also seen in between them.…”