  We analyze the observations from Solar TErrestrial RElations Observatory
(STEREO)-A\&B/COR-1 of an eruptive prominence in the intermediate corona on 7
June 2011 at 08:45 UT, which consists of magnetic Rayleigh-Taylor (MRT)
unstable plasma segments. Its upper northward segment shows spatio-temporal
evolution of MRT instability in form of finger structures upto the outer corona
and low inter-planetary space. Using method of Dolei et al.(2014), It is
estimated that the density in each bright finger is greater than corresponding
dark region lying below of it in the surrounding intermediate corona. The
instability is evolved due to wave perturbations that are parallel to the
magnetic field at the density interface. We conjecture that the prominence
plasma is supported by tension component of the magnetic field against gravity.
Using linear stability theory, magnetic field is estimated as 21-40 mG to
suppress growth of MRT in the observed finger structures. In the southward
plasma segment, a horn-like structure is observed at 11:55 UT in the
intermediate corona that also indicates MRT instability. Falling blobs are also
observed in both the plasma segments. In the outer corona upto 6-13 solar
radii, the mushroom-like plasma structures have been identified in the upper
northward MRT unstable plasma segment using STEREO-A/COR-2. These structures
most likely grew due to the breaking and twisting of fingers at large spatial
scales in weaker magnetic fields. In the lower inter-planetary space upto 20
solar radii, these structures are fragmented into various small-scale localized
plasma spikes most likely due to turbulent mixing.
