  The Solar Dynamics Observatory (SDO) was launched in February 2010 and is now
providing an unprecedented view of the solar activity at high spatial
resolution and high cadence covering a broad range of temperature layers of the
atmosphere. We aim at defining the structure of a polar crown cavity and
describing its evolution during the erupting process. We use high cadence time
series of SDO/AIA observations at 304\AA (50000 K) and 171\AA (0.6 MK) to
determine the structure of the polar crown cavity and its associated plasma as
well as the evolution of the cavity during the different phases of the
eruption. We report on the observations recorded on 13 June 2010 located on the
North-West limb. We observe coronal plasma shaped by magnetic field lines with
a negative curvature (U-shape) sitting at the bottom of a cavity. The cavity is
located just above the polar crown filament material. We thus observe the inner
part of the cavity above the filament as depicted in the classical three part
Coronal Mass Ejection (CME) model composed of a filament, a cavity and a CME
front. The filament (in this case a polar crown filament) is part of the cavity
and makes a continuous structuring from the filament to the CME front depicted
by concentric ellipses (in a 2D cartoon). We propose to define a polar crown
cavity as a density depletion sitting above denser polar crown filament plasma
drained down the cavity due to gravity. As part of the polar crown filament,
plasma at different temperatures (ranging from 50000K to 0.6 MK) is observed at
the same location on the cavity dips and sustained by a competition between the
gravity and the curvature of magnetic field lines. The eruption of the polar
crown cavity as a solid body can be decomposed into two phases: a slow rise at
a speed of 0.6 km/s, and an acceleration phase at a mean speed of 25 km/s.
