Supersonic downflows in sunspots

Discoveries of supersonic downflows both in the middle penumbra and on a light bridge are reported through the analysis of spectropolarimetric data.

Sea-serpent structures detected on the penumbra of a sunspot

Our observations show small-scale, elongated, bipolar magnetic structures that appear in the mid penumbra and move radially outward. They occur in between the more vertical fields of the penumbra, and can be associated with the horizontal fields that harbor the Evershed flow. Many of them cross the outer penumbral boundary, becoming moving magnetic features in the sunspot moat. The bipolar patches can be interpreted as being produced by sea-serpent field lines that originate in the mid penumbra and eventually leave the spot in the form moving magnetic features.

Temporal evolution of the Evershed effect in sunspots: observational and physical properties of Evershed clouds

Evershed clouds are observationally characterized and their physical properties retrieved from Stokes inversion of simultaneous visible and infrared observations. The vector magnetic field deduced from the data is consistent with penumbral flux tubes continuing outward the external penumbral border in the form of a sea-serpent that gives rise to the apparent moving magnetic features.

The origin of dark-cored penumbral filaments

The 2D stationary heat transfer equation is solved in a stratified atmosphere consisting of nearly horizontal magnetic flux tubes embedded in a stronger and more vertical field. The tubes carry an Evershed flow of hot plasma. This model produces bright filaments with dark cores as a consequence of the higher density of the plasma inside the tubes, which shifts the surface of optical depth unity toward higher (cooler) layers. Our calculations suggest that the surplus brightness of the penumbra is a natural consequence of the Evershed flow, and that magnetic flux tubes about 250 km in diameter can explain the morphology of sunspot penumbrae.

Flux tubes as the origin of the net circular polarization in sunspots

A three-dimensional magnetohydrostatic model of a horizontal flux tube, embedded in a magnetic surrounding atmosphere, is used to successfully reproduce the azimuthal and center-to-limb variations of the net circular polarization observed in sunspot penumbrae.

Properties of G-band bright points on the moat of a sunspot

We find that (a) 94% of the BPs are associated with magnetic fields; (b) their field strengths range between 500 and 1400 G, with a rather flat distribution; (c) the contrast of BPs in the G-band depends on the angle between the vector magnetic field and the line of sight; (d) the BPs harbor downflows of magnetized plasma and exhibit Stokes V profiles with large area and amplitude asymmetries; (e) the magnetic interior of BPs is hotter than the immediate field-free surroundings by about 1000 K at equal optical depth; and (f) the mean effective diameter of BPs in our data set is 150 km, with very few BPs larger than 300 km.

Evershed clouds as precursors of moving magnetic features

An analysis of simultaneous visible and infrared spectropolarimetric data provides evidence for a clear link between the Evershed effect and the so-called moving magnetic features that propagate from the outer skirts of sunspots outwards.

Doppler shifts along the dark cores of penumbral filaments

Significantly larger shifts have been found in the dark cores of penumbral filaments than in their brighter surroundings. Such shifts increase with depth in the photosphere and with the heliocentric distance. The shifts correspond to upflows and the cores display weaker magnetic field strengths than their surroundings.

Magnetic flux cancellation at the moat of a sunspot

Multi-wavelength observations of the cancellation of a moving magnetic feature and a plage element at the outer edge of the moat of an isolated, round sunspot are reported. The decrease in magnetic flux is compatible with cancellation due to magnetic reconnection.

Optical tomography of a sunspot

Inversion of the radiative transfer equation is applied to a whole Advanced Stokes Polarimeter map in order to derive the 3D structure of the thermodynamic, magnetic (vector), and dynamic quantities characterizing the state of the sunspot atmosphere. Among the prominent results are the natural detection of a magnetic canopy in the outer (and external) upper parts of the sunspot, the decrease of the field strength upwards and outward from the sunspot center, and the discovery of magnetic downflows on the lower layers of the outer periphery of the sunspot that provide an explanation to the fate of the long-time known Evershed mass flux.