  For about a decade, spectro-polarimetry of HeI 10830 has been applied to the
magnetic diagnostics of the solar chromosphere. This resonance line is very
versatile, as it is visible both on disk and in off-limb structures, and it has
a good sensitivity to both the weak-field Hanle effect and the strong-field
Zeeman effect. Recent observations of an active-region filament showed that the
linear polarization was dominated by the transverse Zeeman effect, with very
little or no hint of scattering polarization. This is surprising, since the HeI
levels should be significantly polarized in a conventional scattering scenario.
To explain the observed level of atomic depolarization by collisional or
radiative processes, one must invoke plasma densities larger by several orders
of magnitude than currently known values for prominences. We show that such
depolarization can be explained quite naturally by the presence of an
unresolved, highly entangled magnetic field, which averages to give the ordered
field inferred from spectro-polarimetric data, over the typical temporal and
spatial scales of the observations. We present a modeling of the polarized HeI
10830 in this scenario, and discuss its implications for the magnetic
diagnostics of prominences and spicules, and for the general study of
unresolved magnetic field distributions in the solar atmosphere.
