The Evershed Flow and the Brightness of the Penumbra

Abstract

The Evershed flow is a systematic motion of gas that occurs in the penumbra of all sunspots. Discovered in 1909, it still lacks a satisfactory explanation. We know that the flow is magnetized, often supersonic, and that it shows conspicuous fine structure on spatial scales of 0.2"-0.3", but its origin remains unclear. The hope is that a good observational understanding of the relation between the flow and the penumbral magnetic field will help us determine its nature. Here I review advances in the characterization of the Evershed flow and sunspot magnetic fields from high-resolution spectroscopic and spectropolarimetric measurements. Using this information as input for 2D heat transfer simulations, it has been demonstrated that hot Evershed upflows along nearly horizontal field lines are capable of explaining one of the most intriguing aspects of sunspots: the surplus brightness of the penumbra relative to the umbra. They also explain the existence of penumbral filaments with dark cores. These results support the idea that the Evershed flow is largely responsible for the transport of energy in the penumbra.

18 pages, to appear in "Magnetic Coupling between the Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and Space Science Proceedings, Springer, Heidelberg, 2009

References in corpus (23)

• CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
• Radiative MHD simulation of sunspot structure
• Fine structure, magnetic field and heating of sunspot penumbrae
• MHD simulations of penumbra fine structure
• Magnetostatic penumbra models with field-free gaps
• On the fine structure of the sunspot penumbrae. II. The nature of the Evershed flow
• Stokes imaging polarimetry using image restoration at the Swedish 1-m Solar Telescope
• Evidence of convective rolls in a sunspot penumbra
• Two-dimensional spectroscopy of a sunspot. III Thermal and kinematic structure of the penumbra at 0.5"
• A 3-D sunspot model derived from an inversion of spectropolarimetric observations and its implications for the penumbral heating
• Detection of sea-serpent field lines in sunspot penumbrae
• Evershed clouds as precursors of moving magnetic features around sunspots
• On the fine structure of the sunspot penumbrae. III The vertical extension of penumbral filaments
• Evidence of magnetic field wrapping around penumbral filaments
• Vector spectropolarimetry of dark-cored penumbral filaments with Hinode
• Physical properties of the solar magnetic photosphere under the MISMA hypothesis III: sunspot at disk center
• Heat transfer in sunspot penumbrae. Origin of dark-cored penumbral filaments
• The Analysis of Penumbral Fine Structure Using an Advanced Inversion Technique
• Are there field-free gaps near tau=1 in sunspot penumbrae?
• The structure of sunspot penumbrae IV. MHS equilibrium for penumbral flux tubes and the origin of dark core penumbral filaments and penumbral grains
• The Fine-Structure of the Net-Circular Polarization in a Sunspot Penumbra
• Penumbral models in the light of Hinode spectropolarimetric observations
• Recent Evidence for Convection in Sunspot Penumbrae

Cited by in corpus (6)

• A geometrical height scale for sunspot penumbrae
• A new type of small-scale downflow patches in sunspot penumbrae
• Explanation of the sea-serpent magnetic structure of sunspot penumbrae
• Searching for overturning convection in penumbral filaments: slit spectroscopy at 0.2 arcsec resolution
• Properties of Supersonic Evershed Downflows
• Twist, Writhe & Helicity in the inner penumbra of a sunspot