  Relative transverse drift (with respect to the ambient magnetic field)
between the weakly magnetized electrons and the unmagnetized ions at the lower
altitude (80 km) and between the weakly magnetized ions and unmagnetized dust
at the higher altitude (90 km) gives rise to the finite Hall conductivity in
the Earth's mesosphere. If, on the other hand, the number of free electrons is
sparse in the mesosphere and most of the negative charge resides on the weakly
magnetized, fine, nanometre sized dust powder and positive charge on the more
massive, micron sized, unmagnetized dust, the sign of the Hall conductivity due
to their relative transverse drift will be opposite to the previous case. Thus
the sign of the Hall effect not only depends on the direction of the local
magnetic field but also on the nature of the charge carrier in the partially
ionized dusty medium.
  As the Hall and the Ohm diffusion are comparable below 80 km, the low
frequency long wavelength waves will be damped at this altitude with the
damping rate typically of the order of few minutes. Therefore, the ultra--low
frequency magnetohydrodynamic waves can not originate below 80 km in the
mesosphere. However, above 80 km since Hall effect dominates Ohm diffusion the
mesosphere can host the ultra--low frequency waves which can propagate across
the ionosphere with little or, no damping.
