Observations of large‐scale Birkeland currents with Viking

The Viking spacecraft carries a high‐resolution Magnetic Field Experiment that acquires approximately 53 vector samples per second. It has four automatically switchable ranges from ±1024 to ±65,536 nT (full scale) and resolutions commensurate with a 13 bit A/D converter in each range (±0.125 to ±8 nT). Data acquired on March 25, 1986 (just a month after Viking's launch) at ∼ 13,000 km altitude over the morning sector near 08:30 MLT indicate that the large‐scale (>0.5° invariant latitude wide) region 1 and region 2 Birkeland current systems can be readily identified. A narrow (∼0.2° wide) upward flowing Birkeland current exists poleward of the region 1 (downward) flowing current. The density of the large‐scale currents is about 0.1 µA/m², which is the value expected from a mapping of the densities of 2 to 4 µA/m² usually observed at ∼ 800 km altitudes. Small‐scale currents embedded in the region 1 system include an intense 8 µA/m² earthward‐flowing current that is only 8 km wide. That current can be associated with an upward flux of electrons with energies ≤ 1 keV that can account for nearly all of the current. Comparison of the magnetic field observations with particle measurements from the Hot Plasma Experiment confirms that the earthward‐flowing large‐scale region 1 system is carried primarily by upward‐flowing low‐energy (< 10 eV) electrons. Embedded in this low‐energy flux are smaller regions of more energetic electrons that produce the small‐scale currents. The correlation of magnetic and particle observations supports previous suggestions that the region 1 currents in the morning sector have their source in the boundary layer and that the region 2 currents are associated with the plasma sheet.