Auroral arc electrodymanic parameters are studied using coordinated measurements between the AE-C satellite and the Chatanika radar. On January 4, 1978, the spacecraft twice crossed an east--west aligned arc at widely spaced longitudes, spanning more than 3 hours in local time. The Chatanika radar was scanning in elevation at a magnetic longitude equidistant from the two AE-C crossings. The electric field pattern around this arc was remarkably similar at the three longitudes. Equatorward of the arc the north--south field was very intense (>60 mV/m), whereas poleward of the arc it was small (~10 mV/m). The east--west field was small and almost constant across the arc. The same arc was observed by the radar for about 1 hour, and this electric field pattern did not change, even though the arc location, intensity, and width changed substantially. The ion and electron temperature variations around this arc were also studied. For a given electric field the AE-C measured ion temperature was substantially different during each of the arc crossings. This dissimilarity is attributed to differences in the F region neutral wind at the two longitudes. We show that the electric field variations are consistent with the existence of polarization charges within the arc, even though the electric field remained small poleward of the arc.