  A two-dimensional dust crystal levitated in the sheath of a modified Gaseous
Electronics Conference (GEC) reference cell is manipulated by heating or
cooling the lower electrode. The dust charge is obtained by measuring global
characteristics of the levitated crystal obtained from top-view pictures. From
the force balance, the electric field in the sheath is reconstructed. From the
Bohm criterion, we conclude that the dust crystal is levitated mainly above and
just below the classical Bohm point.
