Storm time field-aligned currents and magnetosphere-ionosphere coupling process

Field aligned currents (FACs), also known as Birkland current, are flowing along the magnetic field line between the ionosphere and magnetosphere. They play an essential role in transferring mass, energy, and momentum from the magnetosphere to the ionosphere at high latitudes. FACs arouse much interest in magnetospheric and ionospheric physics even about 60 years after their first experimental discovery. Many critical physical processes in the geospace (e.g., aurora, geomagnetic storm, substorm et al.) are believed to be related to FACs. Scientists have performed a lot of researches on FACs' characteristics and generation mechanisms using ground and space observations. This paper reviews different types of large-scale ionospheric FACs and their temporal and spatial distribution during active geomagnetic conditions (magnetic storm and substorm). Firstly, the paper briefly reviews how the term of FACs were proposed and the related historical disputes. With the launch of the manufactured satellites and the advent of the space age, the in situ observation of magnetic fields from low Earth orbit satellite provides direct evidence for FACs. The paper summarizes the characteristics of traditional region 1 and region 2 FACs (R1, R2), northward interplanetary magnetic field (IMF, NBZ) and IMF By (DPY) FACs, newly discovered IMF Bx FACs, and ionospheric terminal FACs. FACs are related to the divergence of the magnetospheric currents and magnetospheric electric field. The ionosphere acts as a load in the current circuit. We review the research process on the characteristics of the magnetospheric source region of R1 and R2 FACs. The magnetospheric source of FACs exhibits noticeable local time differences. The source of the dayside FACs behaves as a voltage source, while that of the nighttime FACs is like a current source. The paper summarizes the research results on the relative intensity of R1 and R2 FACs under different external conditions. Typically R1 FACs are larger than R2 FACs, but R2 FACs can be larger than R1 FACs under certain external IMF conditions. The dipole tilt angle is revealed to be vital in the transformation process from NBZ to DPY FACs and vice versa. A new type of FACs in the cusp region related to IMF Bx and related effects on the ionosphere and thermosphere is disclosed. In addition to the above currents generated in the magnetosphere, a new type of FACs is revealed related to the ionospheric conductivity gradient in the daytime-nighttime terminal. This paper summarizes the latitudes, polarities and strengths variation of large scale FACs with geomagnetic storm phases. The controlling roles of solar wind and magnetospheric coupling parameters are also discussed. The local time asymmetry of FACs (like the dawn versus dusk sector, daytime versus nighttime) during storm periods and its specific physical mechanisms are summarized. Several different physical modes of substorm FACs are reviewed. The observation and related arguments of single substorm current wedge (SCW) and double SCW are detailed. The shift of the SCW center from midnight to the dawn sector is discussed in the framework of IMF By and ionospheric conductivity. Two SCW systems are coexisting in the pre-midnight and postmidnight sectors. In addition, the relative importance of ionospheric remote and local closed channels of SCW is discussed. Finally, some open scientific questions, such as the small scale FACs, hemispheric and local time asymmetry of FACs during storm and substorm periods, are put forward for future work.