He I 10830 Å line polarimetry:: A new tool to probe the filament magnetic fields

In this paper, we present a new Stokes polarization observation of a solar filament using the neutral helium line at 10830 Angstrom. Similar to the prominence Hanle effect, the polarization of the filament is due to the resonant scattering and magnetic depolarization of the photospheric radiation; it differs only in the scattering geometry. Since this represents one of the first filament polarization observations, we also present a classical derivation of a set of diagnostic formulae that relate the filament polarization signals to the vector magnetic field. We measured the full Stokes profiles (I, Q, U, V) by scanning the slit of the spectrograph parallel to the axis of a small filament. In one section of the filament, the polarization signals showed that the axial component of the magnetic field reverses direction on either side of the filament axis. This axial field reversal is not predicted by any of the existing magnetic field models of the filament, nor was it observed by previous prominence Hanle effect observations. We propose that a tilted magnetic field loop across the filament axis can explain the observed axial field reversal. This observation also serves to demonstrate that measurements of the polarization of the He I 10830 Angstrom radiation from filaments is a useful new tool for the diagnostics of filament magnetic field structures.