LAUSR

Abstract 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 Earthś 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 with respect to the Earth's rotational axis 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 ( ∼ 10 − 4 − 10 − 5 s − 1 ) long wavelength ( ∼ 10 3 − 10 4 km ) 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.