Polar Mesospheric Clouds

The summer polar region at altitudes of about 80 to 85 km just below the mesopause is the terrestrial environment most similar to those near the water snow lines in protoplantary discs. Consequently clouds, called Polar Mesospheric Clouds when they are optically detectable from the ground or from space, of particles composed mostly of water ice populate this upper polar mesospheric region. Such clouds were first observed in 1884 through the visual detection of light emitted by the Sun when it was below the horizon and reflected by the clouds. Polar Mesospheric Clouds that can be observed in this fashion are called Noctilucent Clouds. Some researchers believe that the existence of these clouds in the world's coldest environment may have been triggered by industrial activity, but meteoric smoke certainly plays a role in the nucleation of the icy particles.

The existence and nature of Polar Mesospheric Clouds are in themselves of considerable interest. In addition, an understanding of processes occurring in them may illuminate how the ionisation structure changes near the water snow line in a protoplanetary disc. Water snow lines in protoplanetary discs may be sites of planetary birth. The fractional ionisation and, thus, the conductivity of a disc change near a water snow line. These changes affect the level of hydromagnetic turbulence and, as a result, the angular momentum transport in a disc in a way that may cause the gas and dust densities to build up at the water snow line. Coordinated campaigns involving simultaneous observations with radars and lidars, as well as measurements with rocket-borne instruments, provide the best means for investigating Polar Mesospheric Clouds. During a campaign in which the Astrophysics Group participated, a rocket launched on 30 June 2016 at the Andøya Space Center in Norway carried a new type of instrument into the upper polar summer mesosphere (See image below). The aim was to understand better the composition of the icy particles, which should provide more insight into how they are nucleated.