Abstract
AbstractField‐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 (>450 km) are well correlated and have a one‐to‐one fit up to 50% of the time, whereas small‐scale (<50 km) 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.
Highlights
Electric currents are ubiquitous in magnetized plasmas; currents flowing perpendicular to a background magnetic field introduce shears and separate regions of differing magnetic field connectivity or direction, whereas currents flowing along the direction of a background field can result in momentum and energy transfer between different plasma regimes
By assuming that Field-aligned currents (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
We present a new technique that combines cross correlation and linear fitting of multiple spacecraft measurements to determine the reliability of the FAC estimates
Summary
Electric currents are ubiquitous in magnetized plasmas; currents flowing perpendicular to a background magnetic field introduce shears and separate regions of differing magnetic field connectivity or direction, whereas currents flowing along the direction of a background field can result in momentum and energy transfer between different plasma regimes. A system of field-aligned currents (FACs) in the magnetosphere was first proposed by Birkeland, [1908] and later confirmed by spacecraft observations of the deflection of the magnetic field above the auroral zone [Zmuda et al, 1966]. These currents, on average, form a two-region current system, with Region 1 currents flowing into the polar region on the dawnside and out of the polar region on the duskside, and the adjacent Region 2 currents flowing into and out of the ionosphere in the opposite directions at lower latitudes [Iijima and Potemra, 1976]. Under typical large-scale plasma conditions in the magnetosphere, the displacement current
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