The interaction between two electrically coupled collectors in an infinite-Debye-length magnetoplasma is studied by the use of a simple semianalytical model of particle collection by spherical probes. Because the ambient magnetic field confines the orbits of the particles, the perturbations due to photoelectron emission or screening of the ambient electron flux are ''guided'' along the magnetic field direction. When one collector is used as a density probe, depending on the size of the other collector with respect to the mean Larmor radius of the photoelectrons, the model predicts either an electron temperature threshold or a satellite potential threshold in order that the photoelectron current between the collectors overcomes the ambient electron current to the probe. This explains the variation of the modulation patterns of biased Langmuir probe measurements observed in the vicinity of auroral arcs. ¿ American Geophysical Union 1995