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Abstract
In this study we performed a detailed analysis on the scale-size of field-aligned currents (FACs) at auroral latitudes, using the well-calibrated magnetic data from the non-dedicated magnetic field mission, Gravity Recovery and Climate Experiment Follow-On (GRACE-FO). With two spacecraft following each other, the GRACE-FO provides a good opportunity to identify the variation of FACs with different scale lengths. The results show that the auroral FACs can be classified into two groups: the small-scale ones, shorter than some tens of kilometers, dominated by kinetic Alfvén waves, are quite dynamic; and the large-scale ones, typically larger than 150 km, can be considered as quasi-static and persist longer than 1 min. The GRACE-FO observations also reveal that the small-scale FACs at the same location sometimes can persist over 25 s, e.g., around dusk and dawn hours, which is longer than the typical persistent period (10 s) of kinetic Alfvén waves as earlier reported. The FAC structures show clear magnetic local time dependence, with higher correlations between the spacecraft around dusk and dawn hours; lower correlations are found around midnight and lowest correlations around noon, implying that the small-scale FACs most frequently appear at the noon cusp region. Slightly better correlations of FACs between two spacecraft are found during local summer, and such seasonal dependence is dominated by the correlations of small-scale FACs at noon. However, further analysis shows that the small-scale FACs at noon have largest occurrence and intensity during local summer, which reveals that when interpreting the cross-correlation analysis the intensity of FACs needs to be taken into account.Graphical
Highlights
Auroral field-aligned currents (FACs) are one of the main energy transfer mechanisms between the magnetosphere and ionosphere
Under southward interplanetary magnetic field (IMF) condition, the region 1 (R1)/region 2 (R2) currents appear much clearer as one pair, while under northward IMF condition, the R1 FACs are sometimes accompanied at the poleward side by an additional pair of currents referred as the Northward IMF Bz (NBZ) (Northward IMF BZ) currents, flowing mainly near the dayside cusp region (e.g., Iijima et al 1984)
The LT coverage of both GRACE FollowOn 1 (GF1) and GRACE Follow-On 2 (GF2) orbits slowly progresses, and considering both the ascending and descending orbits it needs about 161 days to cover the 24 local time hours, which is comparable with the LT precession of the GRACE mission (e.g., Xiong et al 2010)
Summary
Auroral field-aligned currents (FACs) are one of the main energy transfer mechanisms between the magnetosphere and ionosphere. The concept of auroral FACs was first proposed by the Norwegian scientist Kristian Birkeland in the early twentieth century (Birkeland 1908, 1913), but only 60 years later the existence of such currents was first confirmed from magnetic measurements of the 1963 38C satellite (Zmuda 1966). Persistent perturbations along the perpendicular direction of the magnetic field at auroral latitudes were observed, and such magnetic perturbations are believed to be caused by the auroral FACs. Later, with several-year of magnetic measurements from the TRIAD satellite, Iijima and Potemra (1976a, b) revealed the typical magnetic local time (MLT) and magnetic latitude (MLAT) distribution of FACs, that exhibited as a pair of current sheets with opposite polarity (upward/downward). The average strength of R1 FACs is larger than the average strength of R2 FACs (e.g., Benkevich et al 2000; Zhou and Lühr 2017; Laundal et al 2017; Shi et al 2019)
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