The Viking orbit has been well suited for studies of polar cap phenomena in the altitude range 5000 to 13500 km. A variety of particle features have been observed. Signatures of acceleration below the spacecraft altitude by field-aligned potentials such as narrow beams of upflowing ions and an increased electron loss cone angle are typically observed. Polar arc events are caused by accelerated particle distributions in regions with characteristics similar to those in the plasma sheet boundary layer. The shape of the electron angular distributions indicates that the arcs occur on closed magnetic field lines. Many ''polar cap'' arcs are better described as the poleward edge of an expanded oval during magnetic quiet conditions. ¿American Geophysical Union 1987