Structure of sunspot light bridges in the chromosphere and transition region

Light bridges (LBs) are elongated structures with enhanced intensity embedded in sunspot umbra and pores. We studied the properties of a sample of 60 LBs observed with the Interface Region Imaging Spectrograph (IRIS). Using IRIS near- and far-ultraviolet spectra, we measured the line intensity, width, and Doppler shift; followed traces of LBs in the chromosphere and transition region (TR); and compared LB parameters with umbra and quiet Sun. There is a systematic emission enhancement in LBs compared to nearby umbra from the photosphere up to the TR. Light bridges are systematically displaced toward the solar limb at higher layers: the amount of the displacement at one solar radius compares well with the typical height of the chromosphere and TR. The intensity of the LB sample compared to the umbra sample peaks at the middle/upper chromosphere where they are almost permanently bright. Spectral lines emerging from the LBs are broader than the nearby umbra. The systematic redshift of the Si IV line in the LB sample is reduced compared to the quiet Sun sample. We found a significant correlation between the line width of ions arising at temperatures from 3x10^4 to 1.5x10^5 K as there is also a strong spatial correlation among the line and continuum intensities. In addition, the intensity-line width relation holds for all spectral lines in this study. The correlations indicate that the cool and hot plasma in LBs are coupled. Light bridges comprise multi-temperature and multi-disciplinary structures extending up to the TR. Diverse heating sources supply the energy and momentum to different layers, resulting in distinct dynamics in the photosphere, chromosphere, and TR.Comment: 12 pages, 9 figures, accepted in A&

Primordial vorticity and gradient expansion

The evolution equations of the vorticities of the electrons, ions and photons in a pre-decoupling plasma are derived, in a fully inhomogeneous geometry, by combining the general relativistic gradient expansion and the drift approximation within the Adler-Misner-Deser decomposition. The vorticity transfer between the different species is discussed in this novel framework and a set of general conservation laws, connecting the vorticities of the three-component plasma with the magnetic field intensity, is derived. After demonstrating that a source of large-scale vorticity resides in the spatial gradients of the geometry and of the electromagnetic sources, the total vorticity is estimated to lowest order in the spatial gradients and by enforcing the validity of the momentum constraint. By acknowledging the current bounds on the tensor to scalar ratio in the (minimal) tensor extension of the $\Lambda$CDM paradigm the maximal comoving magnetic field induced by the total vorticity turns out to be, at most, of the order of $10^{-37}$ G over the typical comoving scales ranging between 1 and 10 Mpc. While the obtained results seem to be irrelevant for seeding a reasonable galactic dynamo action, they demonstrate how the proposed fully inhomogeneous treatment can be used for the systematic scrutiny of pre-decoupling plasmas beyond the conventional perturbative expansions.Comment: 36 page