Ionospheric effects in VLBI measured with space-ground interferometer
  RadioAstron
  We report on slow phase variations of the response of the space-ground radio
interferometer RadioAstron during observations of pulsar B0329+54. The phase
variations are due to the ionosphere and clearly distinguishable from effects
of interstellar scintillation. Observations were made in a frequency range of
316-332~MHz with the 110-m Green Bank Telescope and the 10-m RadioAstron
telescope in 1-hour sessions on 2012 November 26, 27, 28, and 29 with
progressively increasing baseline projections of about 60, 90, 180, and 240
thousand kilometres. Quasi-periodic phase variations of interferometric
scintles were detected in two observing sessions with characteristic
time-scales of 12 and 10 minutes and amplitudes of up to 6.9~radians. We
attribute the variations to the influence of medium-scale Travelling
Ionospheric Disturbances. The measured amplitude corresponds to variations in
vertical total electron content in ionosphere of about $0.1\times10^{16}\,
\mathrm{m}^{-2}$. Such variations would noticeably constrain the coherent
integration time in VLBI studies of compact radio sources at low frequencies.
