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Introduction

Field-aligned currents (FAC, also called the Birkeland currents) are essential to the coupling between the solar wind - magnetosphere system and the ionosphere. The main large scale FAC systems are the Region 1 and 2 currents.

Region 1 currents

  • Inner ring driving the R1/R2 system
  • Directed into the ionosphere in the morning hemisphere
  • Directed out of the ionosphere in the evening hemisphere
  • Expands to lower latitudes with increasing activity (see also the auroral ovals)
  • Gets weaker during weak activity (northward IMF, i.e., NBZ)
  • Maximizes between 0800 and 1000 MLT in the morning side and between 1400 and 1600 MLT in the afternoon side
  • Current increase as the electric field associated with the solar wind/IMF increases (but is non-zero even during zero electric field, just like the polar cap potential difference)

Region 2 currents

  • Outer ring following the R1 system
  • Current directions opposite to the Region 1 system
  • Responds to the activity level as Region 1 current ring

Mantle/NBZ currents

  • Poleward of the R1 near local noon (see cusp)
  • Current directions opposite to the adjacent R1 currents
  • Strong IMF By effect: for By > 0 predominantly upward in the norther hemisphere and downward in the southern, and the other way round for By < 0
  • Strong IMF Bz effect
    • Bz < 0: mantle (previously called the cusp) currents (see Lin and Lee, 1994)
      • Well localized, weak currents
    • Bz > 0: NBZ currents
      • Expand and become as strong as the weakened R1/R2 currents

Special FAC types

References

  • Lin, Y. and L. C. Lee, Generation of Region 1 and mantle field-aligned currents by the secondary rotational discontinuity, in Solar Wind Sources of Magnetospheric Ultra-Low-Frequency Waves, Geophysical Monograph 81, 213-221, 1994.
  • Yamamoto, T., S. Inoue, N. Nishitani, M. Ozaki, and C.-I. Meng, A theory for generation of the paired region 1 and region 2 field-aligned currents, J. Geophys. Res., 101, 27199-27222, 1996.
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