Auroral zone electric fields from DE 1 and 2 at magnetic conjunctions

Nearly simultaneous measurements of auroral zone electric fields are obtained by the Dynamics Explorer spacecraft at altitudes below 900 km and above 4500 km during magnetic conjunctions. The measured electric fields are usually nearly perpendicular to the magnetic field lines. The north‐south meridional electric fields are “projected” to a common altitude by a mapping function which accounts for the convergence of the magnetic field lines. When plotted as a function of invariant latitude, graphs of the projected electric fields measured by both DE 1 and DE 2 show that the large‐scale electric field is the same at both altitudes, as expected. Superimposed on the large‐scale fields, however, are small‐scale features with wavelengths of less than 100 km which are larger in magnitude at the higher altitude. Fourier transforms of the electric fields show that the magnitudes depend on wavelength. Outside of the auroral zone the electric field spectrums are nearly identical. But within the auroral zone the high‐ and low‐altitude electric fields have a ratio which increases with the reciprocal of the wavelength. The small‐scale electric field variations are associated with field‐aligned currents. These currents are measured with both a plasma instrument and magnetometer on DE 1. A Fourier transform of the east‐west magnetic field component measured on the high‐altitude satellite is found to be nearly identical to the Fourier transform of the north‐south electric field measured on the low‐altitude satellite, with a constant ratio. This ratio is proportional to the ionospheric conductivity. The experimental measurements are found to agree with a steady state theory which postulates that there are parallel potential drops associated with the variations in the perpendicular electric fields. It is assumed that there is a linear relationship between the field‐aligned current and the total parallel potential drop and that the field‐aligned currents close through Pedersen currents in the ionosphere. The theory predicts that the ratio between the low‐ and high‐ altitude electric fields varies with the wavelength. Below a “critical” wavelength the electric field is not effectively transmitted to low altitudes. Owing to the good agreement between the theory and observations, it is concluded that the linear relationship between the current density and potential drop is a valid approximation.