The magnetic field vector in solar chromospheric structures: the diagnostic potential of the near infrared He I 1083nm triplet

The solar chromosphere is permeated by solar structures such as sunspots, surges, flare ribbons, prominences (filaments) or spicules, where non-local thermodynamic equilibrium effects dominate the state of the plasma and where the magnetic fields are expected to be much lower in intensity than in the photosphere underneath. These solar chromospheric structures are typically being observed mainly in quasi-monocromatic wavelengths or with spectroscopy. However, imaging and spectroscopic observations do not contain the required information to infer the vector magnetic field, except perhaps through the analysis of waves. Measuring magnetic fields in these chromospheric structures requires the observation of spectral lines in polarization. In particular, spectral lines that are sensitive to the joint action of atomic level polarization and the Hanle and Zeeman effect such as the He 1083nm triplet are of great interest. Here, a brief overview of the diagnostic potential of the He 10830 nm triplet for measuring chromospheric plasmas and its application to solar chromospheric structures is given, with particular emphasis on the magnetic structure of solar prominences and spicules.