At a time when most polar cap ionospheric studies are focused on elect
ron density enhancements caused by patches or arcs, this paper discuss
es an F region depletion known as the ionospheric polar hole. Statisti
cally, polar holes tend to develop on the nightside from 2100-0600 MLT
between 70-degrees-80-degrees LAMBDA. Electron concentrations as smal
l as 2 x 10(2) cm-3 at altitudes near 300 km have previously been obse
rved in polar holes during solar minimum winter conditions. Under magn
etically quiet conditions, polar holes are thought to form due to slow
convection of plasma across the polar cap. The electron concentration
is depleted by normal recombination processes during many hours spent
in the dark polar cap. In contrast with previous studies, which have
tended to be statistical in nature, this paper describes the temporal
and spatial development of a single polar hole. During the Geospace En
vironment Modelling Pilot Program of January 16,1990, the development
of the polar hole was monitored by the DMSP F8 and F9 satellites, a di
gital ionosonde and a 250-MHz scintillation receiver. The data reveal
that the polar hole is a very dynamic phenomenon, and its location can
change dramatically within several hours. This is the first study of
the polar hole using ground based instruments. The observations presen
ted here are of interest not only because they contribute to a morphol
ogical understanding of the polar hole, but also because they illustra
te the influence of the polar hole on radio signals propagating in the
ionospheric medium. The results are of direct relevance to applicatio
ns and systems involving ionospheric and transionospheric propagation.
The data set is sufficiently complete that it will provide rigorous c
onstraints for future modeling studies and will thus contribute to a b
etter physical understanding of the polar hole.