Trujillo-Bueno, Javier

Contact Email: jtb@ll.iac.es
Institute: Instituto de Astrofisica de Canarias
Research URL:http://www.iac.es
Subject Area: The Sun
Waveband: Optical
Technique: Spectroscopy
Other: IR spectropolarimetry
Presentation: Oral Contribution: Tuesday
Title: Chromospheric Magnetism and the Hanle Effect
Abstract: Magnetograms in the Ca II IR triplet and magnetohydrostatic extrapolations of photospheric magnetic-field measurements led in the past to the suggestion that the solar chromosphere is pervaded by a magnetic canopy with predominantly horizontal fields. It is however necessary to point out that the Ca II IR triplet and other chromospheric lines are relatively broad, which implies that the magnetic fields of the ``quiet'' chromospheric regions are difficult to diagnose via the only consideration of the longitudinal Zeeman effect on which magnetograms are based on. In fact, the only way to obtain reliable empirical information on the intensity and topology of the weak magnetic fields of the ``quiet'' solar chromosphere is via the interpretation of spectropolarimetric observations in suitable chromospheric lines within the framework of a rigorous theory for the generation and transfer of polarized radiation. In this contribution we will begin presenting some observations of the full Stokes vector of the Ca II IR triplet in ``quiet'' atmospheric regions close to the solar limb. These spectropolarimetric observations have been obtained recently by us using different state-of-the-art polarimeters attached to the solar telescopes of the Canary Islands observatories. The Stokes V signals are mainly due to the Zeeman effect, while the Stokes Q and U signals are fully due to scattering processes and to the Hanle effect (which can provide information on extremely weak magnetic fields in the solar atmospheric plasma). Our observations of weak polarization signals in chromospheric lines provide useful constraints to end up with a more rigorous empirical picture of the topology and intensity of the magnetic fields of the ``quiet'' solar chromosphere. The main emphasis of our presentation will precisely consist on showing in some detail how a rigorous theoretical interpretation of such spectropolarimetric observations is giving us a new picture of chromospheric magnetism.