  The binary system AR Sco contains an M star and the only known radio-pulsing
white dwarf. The system shows emission from radio to X-rays, likely dominated
by synchrotron radiation. The mechanism that produces most of this emission
remains unclear. Two competing scenarios have been proposed: Collimated
outflows, and direct interaction between the magnetospheres of the white dwarf
and the M star. The two proposed scenarios can be tested via very long baseline
interferometric radio observations. We conducted a radio observation with the
Australian Long Baseline Array (LBA) on 20 Oct 2016 at 8.5 GHz to study the
compactness of the radio emission. Simultaneous data with the Australian
Telescope Compact Array (ATCA) were also recorded for a direct comparison of
the obtained flux densities. AR Sco shows radio emission compact on
milliarcsecond angular scales ($\lesssim 0.02\ \mathrm{AU}$, or $4\
\mathrm{R_{\odot}}$). The emission is orbitally modulated, with an average flux
density of $\approx 6.5\ \mathrm{mJy}$. A comparison with the simultaneous ATCA
data shows that no flux is resolved out on mas scales, implying that the radio
emission is produced in this compact region. Additionally, the obtained radio
light curves on hour timescales are consistent with the optical light curve.
The radio emission in AR Sco is likely produced in the magnetosphere of the M
star or the white dwarf, and we see no evidence for a radio outflow or
collimated jets significantly contributing to the radio emission.
