  We have resolved the scatter-broadened image of PSR B0329+54 and detected
substructure within it. These results are not influenced by any extended
structure of a source but instead are directly attributed to the interstellar
medium. We obtained these results at 324 MHz with the ground-space
interferometer RadioAstron which included the space radio telescope (SRT),
ground-based Westerbork Synthesis Radio Telescope and 64-m Kalyazin Radio
Telescope on baseline projections up to 330,000 km in 2013 November 22 and 2014
January 1 to 2. At short 15,000 to 35,000 km ground-space baseline projections
the visibility amplitude decreases with baseline length providing a direct
measurement of the size of the scattering disk of 4.8$\pm$0.8 mas. At longer
baselines no visibility detections from the scattering disk would be expected.
However, significant detections were obtained with visibility amplitudes of 3
to 5% of the maximum scattered around a mean and approximately constant up to
330,000 km. These visibilities reflect substructure from scattering in the
interstellar medium and offer a new probe of ionized interstellar material. The
size of the diffraction spot near Earth is 17,000$\pm$3,000 km. With the
assumption of turbulent irregularities in the plasma of the interstellar
medium, we estimate that the effective scattering screen is located 0.6$\pm$0.1
of the distance from Earth toward the pulsar.
