The impact of technical constraints on the possibility of resolving the pulsar magnetosphere by observing its scintillations

Variation in the pulsar dynamic spectra with pulse longitude had in the past been interpreted as a result of the spatial motion of the pulsar emission region and led to some conclusions about the altitude of the emission regions or the magnetosphere structure. Motivated by this research, we used the VLBI observations of PSR B1237+25 with the Arecibo and Green Bank radio telescopes at 324 MHz, performed as part of a RadioAstron observing program, and analyzed scintillations at separate longitudes of the pulse profile. We have found that the fringe phase characteristics of the visibility function vary quasi-sinusoidally as a function of longitude. Also, the dynamic spectra from each of the telescopes shift in frequency as a function of longitude. However, we have found that these shifts arise from the low-level digitizing of the pulsar signal. After correcting for these digital effects, the frequency shifts have largely disappeared. The residual effects may be of instrumental origin or indicate the pulse emission altitudes well below the pulsar light cylinder radius.