Intense fluctuations in the electric field at high altitudes in the auroral zone are frequently measured by the Viking satellite. We have made an analysis of the origin of electric and magnetic fluctuations in the frequency range of 0.1--1 Hz by assuming four different sources for the signals: (1) spatial structures, (2) spatial structures with a parallel potential drop below the satellite, (3) traveling shear Alfv¿n waves, and (4) interfering shear Alfv¿n waves. We will show that these different sources of the signals may produce similar amplitude ratios and phase differences between the perpendicular electric and magnetic fields. Since the different sources have different frequency dependencies, this can be used as an additional test if the signals are broadband. In other cases, additional information is needed, for example, satellite particle measurements or ground magnetic measurements. The ideas presented in the theory were tested for one Viking eveningside pass over Scandinavia, where ground-based magnetometer and EISCAT radar measurements were available. The magnetic conditions were active during this pass and several interfering shear Alfv¿n waves were found. Also, a spatial structure with a parallel potential drop below the satellite was identified. The magnitude of the 10-km-wide potential drop was at least 2 kV and the upward field-aligned current 26 &mgr;A m-2 (value mapped to the ionospheric level). The field-aligned conductance was estimated as 1.3-2.2¿10-8 S m-2. ¿ American Geophysical Union 1996