Forsyth, C; Rae, IJ; Mann, IR; Pakhotin, IP; (2017) Identifying intervals of temporally invariant field-aligned currents from Swarm: Assessing the validity of single-spacecraft methods. Journal of Geophysical Research: Space Physics , 122 (3) pp. 3411-3419. 10.1002/2016JA023708.

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Abstract

Field-aligned currents (FACs) are a fundamental component of coupled solar wind-magnetosphere-ionosphere. By assuming that FACs can be approximated by stationary infinite current sheets that do not change on the spacecraft crossing time, single-spacecraft magnetic field measurements can be used to estimate the currents flowing in space. By combining data from multiple spacecraft on similar orbits, these stationarity assumptions can be tested. In this technical report, we present a new technique that combines cross correlation and linear fitting of multiple spacecraft measurements to determine the reliability of the FAC estimates. We show that this technique can identify those intervals in which the currents estimated from single-spacecraft techniques are both well correlated and have similar amplitudes, thus meeting the spatial and temporal stationarity requirements. Using data from European Space Agency's Swarm mission from 2014 to 2015, we show that larger-scale currents (>450km) are well correlated and have a one-to-one fit up to 50% of the time, whereas small-scale (<50km) currents show similar amplitudes only ~1% of the time despite there being a good correlation 18% of the time. It is thus imperative to examine both the correlation and amplitude of the calculated FACs in order to assess both the validity of the underlying assumptions and hence ultimately the reliability of such single-spacecraft FAC estimates. PLAIN LANGUAGE SUMMARY: Electric currents flowing along the Earth's magnetic field link the stream of particles coming off the Sun with the Earth's upper atmosphere and allowing the Earth to gain energy from this interaction. These currents have a multitude of widths, with the widest currents being linked to the circulation of charged particles in Earth's upper atmosphere and the narrowest being associated with bright aurora. Detecting the currents directly is very challenging; however, in principle, the currents can be measured by detecting the magnetic field associated with them using spacecraft orbiting the Earth. This type of detection requires a number of assumptions to be made in order to calculate the strengths of the currents from the measured magnetic field. Using multispacecraft observations, these assumptions can be tested. In this paper, we examine a new way of comparing the currents estimated from two coorbiting spacecraft to determine when and where our estimates of these currents is most reliable.

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