Theoretical aspects of weak double layers and their relation to the electrodynamic coupling between the magnetosphere and ionosphere

Weak double layers are small-scale plasma structures with a magnetic-field-aligned electric potential jump on the order of, or less than, the thermal energy of background plasma. On auroral magnetic field lines they have been observed within the altitude range from about 4000 km up to 13500 km. The weak double layers are typically found in association with upward flowing ionospheric ions with energies typically below 1 keV. Certain theoretical aspects of auroral weak double layers are discussed. The physical mechanisms behind the phenomena are still poorly understood. The importance of considering these microscale phenomena in the framework of the global electrodynamic coupling between the magnetosphere and ionosphere is emphasized. The weak double layers may contribute to the parallel potential drop between the magnetosphere and ionosphere