Substructure of visibility functions from scattered radio emission of
  pulsars through space VLBI
  We report on the substructure of visibility functions in the delay domain of
PSRs B0329+54, B0823+26, B0834+06, B1933+16 and B0833-45 (Vela) observed with
earth-earth and RadioAstron space-earth two-element interferometers at
frequencies of 324 MHz and 1668 MHz. All visibility functions display
unresolved spikes distributed over a range of delays. They are due to
band-limited scintillation noise and related to the scattering time. The
envelopes for each but the Vela pulsar are well fit by a single Lorentzian
which we interpret as being indicative of isotropic scattering on the plane of
the sky due to a thin scattering screen between the pulsar and us. In contrast,
the envelope for the Vela pulsar needs to be mostly fit by at least two
Lorentzians, a narrow and a broad one at the same zero delay. We interpret this
characteristic as indicative of anisotropic scattering due to more complex
structure of scattering screens in the supernova remnant. The possibility of
describing the delay visibility functions by Lorentzians is likely a general
property of pulsars and offers a new way of describing scattering parameters of
the intervening interstellar medium. Furthermore, for all our pulsars, the
unresolved spikes in visibility functions of similar projected baselines were
well correlated indicating that the telescopes are located in the same
diffraction spot. The correlation vanished for visibilities from largely
different baselines, when some radio telescopes are not in the same spot.
