Kuppers, F., J. Untiedt, W. Baumjohann, K. Lange, and A. Jones. “A Two-Dimensional Magnetometer Array for Ground-Based Observations of Auroral Zone Electric Currents During the International Magnetospheric Study (IMS)”. Journal of Geophysics, Vol. 46, no. 1, Apr. 1979, pp. 429-50, https://journal.geophysicsjournal.com/JofG/article/view/160.