Viking investigations of auroral electrodynamical processes

Recent results from the Viking electric field experiment and their contribution to a better understanding of the aurora and of associated ionosphere‐magnetosphere processes are briefly reviewed. The high‐resolution electric field data have provided new and important results in a number of different areas, including auroral electrodynamics both on the arc scale size and on the global scale, the auroral acceleration process, the current‐voltage relationship, substorms, and the dynamics of the polar cusp. After a short introduction presenting some of the characteristic features of the high‐altitude electric field data the remainder of this paper focuses on the role of the electric field in auroral electrodynamics and in the auroral acceleration process. The relationships between the auroral emissions and the associated electric field, current, particle, and conductivity distributions are discussed for both small‐scale and large‐scale auroral distributions on the basis of results from Viking event studies and from numerical model studies. Particular attention is paid to ionospheric convection and field‐aligned current signatures associated with northward interplanetary magnetic field (IMF) auroral distributions, such as the theta aurora or those characterized by extended auroral activity poleward of the classical auroral oval. The evolution of the convection pattern as the current system changes from a typical configuration for southward IMF to a configuration that may be representative of northward IMF conditions is illustrated by numerical model results and discussed in the light of Viking observations. The role of “dc” electric fields for the auroral acceleration process has been further investigated and clarified. From a statistical study of the directly measured parallel electric field and its relation to other relevant parameters the data suggest the possible existence of both upward and downward parallel fields. Intense low‐frequency electric field fluctuations (<1 Hz) have been shown to play an important role in the auroral acceleration process. In this frequency range the electric field appears static for the electrons but not for the ions, giving rise to a selective acceleration. The fluctuations are likely to contribute to the thermal energy of the closely associated escaping ionospheric ions. Estimates of the acceleration potential based on a number of different methods generally show good agreement, providing convincing evidence of the role of “dc” electric fields in the auroral acceleration process.