  Analysis of the Hanle effect in solar molecular lines allows us to obtain
empirical information on hidden, mixed-polarity magnetic fields at
subresolution scales in the (granular) upflowing regions of the `quiet' solar
photosphere. Here we report that collisions seem to be very efficient in
depolarizing the rotational levels of MgH lines. This has the interesting
consequence that in the upflowing regions of the quiet solar photosphere the
strength of the hidden magnetic field cannot be sensibly larger than 10 G,
assuming the simplest case of a single valued microturbulent field that fills
the entire upflowing photospheric volume. Alternatively, an equally good
theoretical fit to the observed scattering polarization amplitudes can be
achieved by assuming that the rate of depolarizing collisions is an order of
magnitude smaller than in the previous collisionally dominated case, but then
the required strength of the hidden field in the upflowing regions turns out to
be unrealistically high. These constraints reinforce our previously obtained
conclusion that there is a vast amount of hidden magnetic energy and unsigned
magnetic flux localized in the (intergranular) downflowing regions of the quiet
solar photosphere.
