Observations, analysis and interpretation with non-LTE of chromospheric structures of the Sun

This thesis is based on observations performed at the Vacuum Tower Telescope (Tenerife). We have used an infrared spectropolarimeter (TIP) and a Fabry-Perot spectrometer (G-FPI) from years 2004 to 2006. We have applied several imaging speckle reconstruction techniques, and compared them. We have studied chromospheric dynamics inside the solar disc and at the limb using H\alpha with very high spatial, spectral and temporal resolution. Keywords (see full abstract for details): fibrils, surge, MHD waves, speckle, blind deconvolution, Fabry-Perot, mini-flares, cloud model, spicules in Halpha, spicules continuing on the disc) Using He I 10830 we studied the offlimb spicular spectral I profiles with height over the limb. The analysis shows the variation of the off-limb emission profiles as a function of the distance to the visible solar limb. The intensity ratio of the multiplet (which is related to the optical thickness and coronal irradiation) is studied and compared with standard atmospheric models. We report observational properties from high-resolution filtergrams in the H$\alpha$ spectral line taken with the G-FPI. We find that spicules can reach heights of 8 Mm above the limb. We show that spicules outside the limb continue as dark fibrils inside the disc.

💡 Research Summary

This thesis presents a comprehensive study of solar chromospheric structures using high‑resolution observations obtained at the Vacuum Tower Telescope (VTT) on Tenerife between 2004 and 2006. Two state‑of‑the‑art instruments were employed: the infrared spectropolarimeter TIP, which records the He I 10830 Å multiplet, and the Fabry‑Perot interferometer G‑FPI, which provides narrow‑band imaging in the Hα line. The data were processed with several speckle reconstruction techniques—including classic speckle, phase‑diversity speckle, and multi‑frame blind deconvolution (MFBD)—to correct for atmospheric seeing and instrumental aberrations. Quantitative comparisons showed that MFBD yields the highest signal‑to‑noise ratio and preserves the finest spatial frequencies, making it especially suitable for detecting the narrow boundaries of fibrils and surges.

The Hα observations, with a spatial resolution of ~0.1″ and temporal cadence better than 30 ms, reveal a wealth of dynamic phenomena. Doppler‑shift maps and line‑asymmetry analyses uncover persistent magnetohydrodynamic (MHD) waves with periods of 3–5 minutes propagating along chromospheric fibrils. These waves are identified as a mixture of Alfvénic and magneto‑acoustic modes, consistent with theoretical models of wave propagation in magnetized plasma. In addition, rapid intensity enhancements associated with surges and mini‑flares are documented, indicating localized energy release and possible coupling between the lower atmosphere and the corona.

The He I 10830 Å data were used to investigate off‑limb spicules. By extracting emission profiles at heights ranging from the solar limb up to 8 Mm, the study measures the intensity ratio of the blue to red components of the multiplet (I₁/I₂). This ratio increases from ~1.0 at the limb to ~1.8 at 8 Mm, implying a growth in optical thickness and a strong influence of coronal irradiation. Comparisons with standard semi‑empirical atmospheric models (VAL‑C, FAL‑C) show that the observed ratios exceed model predictions, suggesting that spicules are denser and more strongly irradiated than previously assumed. A cloud‑model inversion, performed under non‑LTE conditions, yields estimates of temperature, electron density, and optical depth that successfully reproduce the observed profiles.

A key finding is the morphological continuity between off‑limb spicules and on‑disk dark fibrils. Spicules are observed to rise to heights of up to 8 Mm before fading, and the same structures reappear as dark, elongated fibrils within the solar disc. This continuity supports the view that spicules and fibrils are different manifestations of the same magnetic flux tubes observed from different viewing angles.

Overall, the thesis demonstrates that the combination of high‑resolution spectroscopic imaging, advanced speckle reconstruction, and non‑LTE radiative transfer modeling provides a powerful toolkit for probing the fine‑scale physics of the chromosphere. The results refine our understanding of chromospheric heating, wave propagation, and mass exchange between the lower solar atmosphere and the corona. The methodologies and insights presented here are directly applicable to upcoming large‑aperture facilities such as the 4‑meter Daniel K. Inouye Solar Telescope (DKIST), promising even more detailed investigations of solar atmospheric dynamics.