VLBI properties of compact interplanetary scintillators detected by the
  Murchison Widefield Array
  Interplanetary scintillation (IPS) provides an approach for identifying the
presence of sub-arcsec structures in radio sources, and very long baseline
interferometry (VLBI) technique can help verify whether the IPS sources have
fine structures on milli-arcsec (mas) scales. We searched the available VLBI
archive for the 244 IPS sources detected by the Murchison Widefield Array at
162~MHz and found 63 cross-matches. We analysed the VLBI data of the 63 sources
and characterised the compactness index in terms of the ratio $R$ of the
VLBI-measured flux density at 4.3~GHz to the flux density estimated using the
Very Large Array Sky Survey (VLASS) at 3~GHz and NRAO VLA Sky Survey (NVSS) at
1.4~GHz ($S_{\rm VLBI}/S_{\rm SA}$). Eleven sources are identified as blazars
according to their flat spectra and strong variability. They show high
compactness indices with $R>0.4$, compact core-jet structure, and a broad
distribution of normalised scintillation index (NSI). Other sources show
diverse morphologies (compact core, core and one-sided jet, core and two-sided
jets), but there is a correlation between $R$ and NSI with a correlation
coefficient $r=0.47$. A similar $R$--NSI correlation is found in sources
showing single steep power-law or convex spectra. After excluding blazars
(which are already known to be compact sources) from the VLBI-detected IPS
sources, a strong correlation is found between the compactness and
scintillation index of the remaining samples, indicating that stronger
scintillating sources are more compact. This pilot study shows that IPS offers
a convenient method to identify compact radio sources without the need to
invoke high-resolution imaging observations, which often require significant
observational time.
