The electrodynamic, thermal, and energetic character of stable Sun-aligned arcs in the polar cap can be meaningfully diagnosed by an incoherent scatter radar, provided a suitable observing scheme is selected. We report here such measurements of two intense Sun-aligned arcs. The two arcs were diagnosed on two different nights (February 26 and March 1, 1987) using the Sondre Stromfjord radar as a stand-alone diagnostic. Repeatable patterns are found in mesoscale area (order 103 km by 103 km) maps of altitude profiles for observed electron and ion gas number densities, temperatures, and line-of-sight velocities, and projected mesoscale area maps of derived electric fields. Pedersen and Hall conductivities (Ne, Te, Ti, V, E, &sgr;p, &sgr;H), horizontal and field-aligned currents, joule heating rate, and Poynting flux. We confirm, for the first time with continuuous mesoscale area maps, that the arcs have the anticipated simple arc electrodynamics. That is, the visual and enhanced ionization signatures of the arc are produced by incoming energetic electrons carrying the outgoing current from the electric field convergence in the arc. Strong electron temperature enhancements (>2000 K) are found as expected within the sheets of ionizing particle precipitation. Dawn to dusk decreases in the antisunward plasma flow of order 1 km s-1, across order 100 km, correspond to peak electron densities of order 105 cm-3 down to altitudes as low as 120 km, and upward currents of order 1 &mgr;A m-2.

These data also lead to important implications for the physics of polar cap arcs. The high-velocity (antisunward flow on the dawnside) edge of the arc marks the location of strong persistent Joule heating driven by downward Poynting flux. There is a channel of strongly enhanced ion temperature (well above the electron temperature) along the high-velocity edge of the arc, quantitatively accounted for by ion frictional heating as the strong electric field drags the ions through the neutral gas. The deposition rate into the atmosphere of the net electromagnetic energy well exceeds the net particle energy deposited by the ionizing energetic electron flux. This heating is a substantial source of heat into the polar thermosphere. As estimated by heat into the ions, heat lost by the ions to the neutrals, or energy available from the Poynting and energetic particle flux, several ergs cm-2 s-1 are deposited in channels of order 100 km for good fractions of an hour in these arcs: this contributes to resolving the problem of the missing polar thermosphere heat source. Finally, a reasonably simple yet self-consistent, accurate, and comprehensive representation of stable intense Sun-aligned arcs is presented, including the electrodynamic, thermal, and energetic character. ¿1991 American Geophysical Union