Umbral Light Bridges Research Articles

Properties of sunspot light bridges on a geometric height scale

Context. Investigating light bridges (LBs) helps us comprehend key aspects of sunspots. However, few studies have analyzed the properties of LBs in terms of the geometric height, which is a more realistic perspective given the corrugation of the solar atmosphere. Aims. We aim to shed light on LBs by studying the variation in their physical properties with geometric height. Methods. We used the SICON code to infer the physical quantities in terms of the optical depth and the Wilson depression values of three LBs hosted by a sunspot observed with Hinode/SP in the Fe I 630 nm pair lines. We also used SIR inversions to cross-check the height variation of the field inclination in the LBs. In both output sets, we performed linear interpolation to convert the physical parameters from optical depth into a geometric height scale in each pixel. Results. Depending on their general appearance, we classified each LB as filamentary, grainy, or umbral. They appear as ridges that reach different maximum heights, with the umbral LB being the deepest. While the filamentary LB hosts a plasma inflow from the penumbra, the results for the grainy LB are compatible with an injection of hot plasma through convective cells of reduced field strength. Only a few positions reveal hints suggesting a cusp-like magnetic canopy. Moreover, strong gradients in the magnetic field strength and inclination usually exhibit enhanced electric currents, with the filamentary LB having remarkably strong currents that appear to be related to chromospheric events. Conclusions. The height stratification in filamentary and grainy LBs differ, indicating diverse mechanisms at work. Our results are in general incompatible with a magnetic canopy scenario, and further analysis is needed to confirm whether it exists along the entire LB or only at specific locations. Furthermore, this work assesses the usefulness of the SICON code when determining the height stratification of solar structures.

INTERFERENCE OF THE RUNNING WAVES AT LIGHT BRIDGES OF A SUNSPOT

ABSTRACT The observations of chromospheric oscillations of two umbral light bridges (LBs) within a sunspot from NOAA Active Region 12127 are presented. It was found that the running umbral waves with periods of 2.2–2.6 minutes underwent very fast damping before approaching umbral boundaries, while those with higher periods (>2.6 minutes) could propagate outside umbrae. On two sides of each LB adjacent to umbrae, the cross-wavelet spectra displayed that the oscillations on them had a common significant power region with dominant frequencies of 2–6 minutes and phase differences of ∼90°. A counterstream of two running umbral waves in the 2–6 minute frequency range propagated toward the LBs, where they encountered each other and gave rise to constructive or even destructive interference on the LBs. In addition, the velocity and density perturbations on the LBs were found in opposite phases suggesting that the perturbations were caused by the downward propagating waves.

Magnetic tension of sunspot fine structures

The equilibrium structure of sunspots depends critically on its magnetic topology and is dominated by magnetic forces. Tension force is one component of the Lorentz force which balances the gradient of magnetic pressure in force-free configurations. We employ the tension term of the Lorentz force to clarify the structure of sunspot features like penumbral filaments, umbral light bridges and outer penumbral fine structures. We compute vertical component of tension term of Lorentz force over two active regions namely NOAA AR 10933 and NOAA AR 10930 observed on 05 January 2007 and 12 December 2006 respectively. The former is a simple while latter is a complex active region with highly sheared polarity inversion line (PIL). The vector magnetograms used are obtained from Hinode(SOT/SP). We find an inhomogeneous distribution of tension with both positive and negative signs in various features of the sunspots. The existence of positive tension at locations of lower field strength and higher inclination is compatible with the uncombed model of the penumbral structure. Positive tension is also seen in umbral light bridges which could be indication of uncombed structure of the light bridge. Likewise, the upward directed tension associated with bipolar regions in the penumbra could be a direct confirmation of the sea serpent model of penumbral structures. Upward directed tension at the PIL of AR 10930 seems to be related to flux emergence. The magnitude of the tension force is greater than the force of gravity in some places, implying a nearly force-free configuration for these sunspot features. From our study, magnetic tension emerges as a useful diagnostic of the local equilibrium of the sunspot fine structures.

Fine structure, magnetic field and heating of sunspot penumbrae

We interpret penumbral filaments as due to convection in field-free, radially aligned gaps just below the visible surface of the penumbra, intruding into a nearly potential field above. This solves the classical discrepancy between the large heat flux and the low vertical velocities observed in the penumbra. The presence of the gaps causes strong small-scale fluctuations in inclination, azimuth angle and field strength, but without strong forces acting on the gas. The field is nearly horizontal in a region around the cusp-shaped top of the gap, thereby providing an environment for Evershed flows. We identify this region with the recently discovered dark penumbral cores. Its darkness has the same cause as the dark lanes in umbral light-bridges, reproduced in numerical simulations by Nordlund and Stein (2005). We predict that the large vertical and horizontal gradients of the magnetic field inclination and azimuth in the potential field model will produce the net circular polarization seen in observations. The model also explains the significant elevation of bright filaments above their surroundings. It predicts that dark areas in the penumbra are of two different kinds: dark filament cores containing the most inclined (horizontal) fields, and regions between bright filaments, containing the least inclined field lines.

Spectroscopic observations and models of umbral light bridges

Profiles of the Nai D lines in two moderately bright umbral light bridges are analyzed. A simple method of elimination of stray light is presented. The corrected continuum intensities (λ ≈ 588 nm) of the light bridges are 0.51 and 0.43. For each light bridge, using optimization technique, a semi-empirical model is derived. The models exhibit a steep temperature rise towards the subphotospheric layers. Generally, the temperature of the light bridges under investigation is lower than the temperature of the penumbra as well as the temperature of bright umbral dots.