PSR B0329+54: Scattering Structures on Two Scales Observed with RadioAstron on Baselines of up to 235,000 km

We studied the scattering properties of pulsar PSR B0329+54 with the RadioAstron ground-space radio interferometer. The instrument included the 10-m Space Radio Telescope, the 110-m Green Bank Telescope, the $14\times25$-m Westerbork Synthesis Radio Telescope, and the 64-m Kalyazin Radio Telescope. The observations were performed at 324 MHz on baselines of up to 235,000 km in November 2012 and January 2014. In the delay domain, on long baselines the interferometric visibility consists of many discrete spikes within a limited range of delays. On short baselines it consists of a sharp spike surrounded by lower spikes. The average envelope of the visibility function shows two exponential scales, with characteristic delays of $\tau_1=4.1\pm 0.3\ \mu$s and $\tau_2=22.5\pm 2.9\ \mu$s, indicating the presence of two scales of scattering in the interstellar medium. The longer scale contains 0.38 times the scattered power of the shorter one. We suggest that the longer tail arises from highly-scattered paths, possibly from anisotropic scattering or from substructure at large angles.