EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Field-aligned current ordering in ground and space measurements

Malcolm Dunlop1,2,7, Junying Yang1, Xiangcheng Dong2, Mervyn Freeman3, Neil Rogers4, Jim Wild4, Colin Forsyth5, Jinbin Cao1, Hermann Lühr6, and Chao Xiong6
Malcolm Dunlop et al.
  • 1School of Space and Environment, Beihang University, 100191, Beijing, China
  • 2Rutherford Appleton Laboratory, Space Sciences Division, SSTD, Oxfordshire, United Kingdom of Great Britain and Northern Ireland (
  • 3British Antarctic Survey, Cambridge, CB3 0ET, UK
  • 4Department of Physics, Lancaster University, Lancaster, LA1 4YB, UK
  • 5Mullard Space Science Laboratory, University College London, Dorking, Surrey, RH5 6NT, UK
  • 6GFZ, German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
  • 7The Blackett Laboratory, Imperial College London, London, SW7 2AZ, UK

The orientation of field-aligned current sheets (FACs) can be inferred from dual-spacecraft correlations of the FAC signatures between two Swarm spacecraft (A and C), using the maximum correlations obtained from sliding data segments. Statistical analysis of both the correlations and the inferred orientations shows clear trends in magnetic local time (MLT) which reveal behaviour of both large and small scale currents. The maximum correlation coefficients show distinct behaviour in terms of either the time shift, or the shift in longitude between Swarm A and C for various filtering levels. The lower-latitude FACs show the strongest correlations for a broad range of MLT centred on dawn and dusk, with a higher correlation coefficient on the dusk-side and lower correlations near noon and midnight, and broadly follow the mean shape of the auroral boundary for the lower latitudes corresponding to Region 2 FACs (and are most well-ordered on the dusk side). Individual events sampled by higher altitude spacecraft (e.g. the 4 Cluster spacecraft), in conjunction with Swarm (mapping both to region 1 and 2), also show two different domains of FACs: time variable, small-scale (10s km), and more stationary large-scale (>100 km) FACs. We investigate further how these FAC regimes are dependent on geomagnetic activity, focusing on high activity events. Both the statistical trends, and individual conjugate events, show comparable effects seen in the ground magnetometer signals (dH/dt) during storm/substorm activity and show distributions that are similar.

How to cite: Dunlop, M., Yang, J., Dong, X., Freeman, M., Rogers, N., Wild, J., Forsyth, C., Cao, J., Lühr, H., and Xiong, C.: Field-aligned current ordering in ground and space measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19203,, 2020

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Display material version 1 – uploaded on 04 May 2020
  • AC1: Comment on EGU2020-19203, Malcolm Dunlop, 05 May 2020

    We use an application of a dual-satellite, cross-correlation technique to the field-aligned current (FAC) estimates of Swarm. The technique can produce the values of the maximum correlations (from sliding data segments) and corresponding current sheet orientations; either as statistical maps or as event information pass by pass. Correlation trends in MLT produce broadly expected large scale behaviour, depending on geomagnetic activity, and suggestions of small scale variations. Favourable comparisons can be made to the maximum variance method (MVA on the magnetic field). For individual passes, the evolution along the orbits can be compared to the positions of the auroral ovals (Xiong et al.). FAC behaviour can be compared to ground estimates of the rate of change of the horizontal field (dH/dt), either statistically or for individual events (see Poster Wei et al. ST2.5).