Signatures of the high‐altitude polar cusp and dayside auroral regions as seen by the Viking Electric Field Experiment

Abstract

Electric field and satellite potential (which is a rough measure of plasma density) observations along 42 Viking orbits in the high‐altitude (2 RE) polar cusp and dayside auroral region have been examined. A useful, straightforward way to identify the polar cusp from the satellite potential is demonstrated, the results being in excellent agreement with those based on particle data. Within the cusp the plasma density usually reaches a maximum, and it is typically very homogeneous, in contrast to the irregular and lower density in the cleft and dayside auroral regions. The maxima in the plasma density are sometimes anticorrelated with the magnetic field strength, indicating a diamagnetic effect. The entire cusp and dayside auroral regions are characterized by irregular and burstlike electric fields, comprising field reversals on various scales (up to 3 min or 500 km), the larger scales, however, being rare in the cusp. Another common feature in these regions is the high correlation between mutually orthogonal components of the electric and magnetic fields, both for large‐scale variations across spatial structures (associated with the magnetosphere‐ionosphere coupling through field‐aligned currents) and for waves and pulsations in the ULF frequency range. The electric field signatures in the cusp (in the 1100–1300 MLT sector) are, however, characteristically different from the cleft and oval field signatures in that the electric field is usually less intense and less structured and not correlated with the substorm activity level.