Abstract
AbstractDuring the first several months of the three‐spacecraft Swarm mission all three spacecraft came repeatedly into close alignment, providing an ideal opportunity for validating the proposed dual‐spacecraft method for estimating current density from the Swarm magnetic field data. Two of the Swarm spacecraft regularly fly side‐by‐side in closely similar orbits, while the third at times approaches the other two. This provides a data set which under certain assumptions of stationarity of the magnetic field can produce 2, 3, 4, 5 (or more) point measurements, which can be cross compared. We find that at low Earth orbit the use of time‐shifted positions allow stable estimates of current density to be made and can verify temporal effects as well as validating the interpretation of the current components as arising predominantly from field‐aligned currents. In the case of four‐spacecraft configurations we can resolve the full vector current and therefore can check the perpendicular as well as parallel current density components directly, together with the quality factor for the estimates directly (for the first time in situ at low Earth orbit).
Highlights
The curlometer technique [Dunlop et al, 1988, 2002] and its derivatives have been established over the last decade or so as a key method of estimating the electric current density in a number of regions of the magnetosphere
These methods have been predominantly applied to the outer magnetospheric regions, where the influence of the Earth’s internal field is weak and temporal fluctuations are often dominant, recently, there have been a number of studies using multispacecraft estimates of current density in the inner magnetospheric regions and ring current [Vallat et al, 2005; Zhang et al, 2011; Shen et al, 2014] and in regions supporting field-aligned currents [Marchaudon et al, 2009; Shi et al, 2010, 2012]
In low Earth regions of the magnetosphere, studies have until now relied on estimates arising from single-spacecraft data, for which the disentangling of temporal and spatial variations is difficult [Lühr et al, 2015], and have focused on large- or small-scale field-aligned currents (FACs) and the modeling of signals associated with the external influences in the geomagnetic field
Summary
The curlometer technique [Dunlop et al, 1988, 2002] and its derivatives have been established over the last decade or so as a key method of estimating the electric current density in a number of regions of the magnetosphere. More generalized methods estimating the curvature and gradient of the magnetic field have been extensively applied to multispacecraft magnetic field measurements [Shen et al, 2007, 2012a, 2012b; Vogt et al, 2009] from both Cluster and Time History of Events and Macroscale Interactions during Substorms (THEMIS) [e.g., Dunlop et al, 2002; Shen et al, 2012a, 2012b] These methods have been predominantly applied to the outer magnetospheric regions (dayside magnetopause, magnetotail, and lobes), where the influence of the Earth’s internal field is weak and temporal fluctuations are often dominant, recently, there have been a number of studies using multispacecraft estimates of current density in the inner magnetospheric regions and ring current [Vallat et al, 2005; Zhang et al, 2011; Shen et al, 2014] and in regions supporting field-aligned currents [Marchaudon et al, 2009; Shi et al, 2010, 2012]. We apply the methodology of the curlometer technique to the close configurations of Swarm to achieve 3- and 4-point estimates of electric current density, and using combinations of spacecraft positions we test the temporal stability and the validity of inferring the field-aligned component from time-shifted spacecraft positions (extending the work of Ritter and Lühr [2006] and Ritter et al [2013])
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