Abstract
AbstractPrincipal component analysis is performed on Birkeland or field‐aligned current (FAC) measurements from the Active Magnetosphere and Planetary Electrodynamics Response Experiment. Principal component analysis (PCA) identifies the patterns in the FACs that respond coherently to different aspects of geomagnetic activity. The regions 1 and 2 current system is shown to be the most reproducible feature of the currents, followed by cusp currents associated with magnetic tension forces on newly reconnected field lines. The cusp currents are strongly modulated by season, indicating that their strength is regulated by the ionospheric conductance at the foot of the field lines. PCA does not identify a pattern that is clearly characteristic of a substorm current wedge. Rather, a superposed epoch analysis of the currents associated with substorms demonstrates that there is not a single mode of response, but a complicated and subtle mixture of different patterns.
Highlights
The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) [Anderson et al, 2000; Waters et al, 2001; Anderson et al, 2002, 2008] has provided measurements of the Birkeland currents or field-aligned currents (FACs) in the Northern and Southern Hemispheres at 10 min cadence from 2010 to 2013, using magnetometer observations from the Iridium constellation of close to 70 satellites
Principal component analysis is performed on Birkeland or field-aligned current (FAC)
Murphy et al [2013] and Sergeev et al [2014] have used AMPERE observations to explore the structure of nightside FACs during substorms, attempting to elucidate the structure of the substorm current wedge [McPherron et al, 1973], while Wilder et al [2013] studied cusp currents associated with magnetic tension forces during periods of nonzero interplanetary magnetic field (IMF) BY
Summary
The Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) [Anderson et al, 2000; Waters et al, 2001; Anderson et al, 2002, 2008] has provided measurements of the Birkeland currents or field-aligned currents (FACs) in the Northern and Southern Hemispheres at 10 min cadence from 2010 to 2013, using magnetometer observations from the Iridium constellation of close to 70 satellites. Clausen et al [2012, 2013a, 2013b] and Coxon et al [2014a, 2014b] have demonstrated that the region 1 and 2 current systems [Iijima and Potemra, 1976] observed by AMPERE undergo cycles of expansion to lower latitudes and contractions to higher latitudes in tune with the substorm cycle, and Anderson et al [2014] showed dayside followed by nightside activations of the current systems in response to southward turnings of the interplanetary magnetic field (IMF) Both sets of observations are consistent with the expanding/contracting polar cap (ECPC) model of the Dungey cycle of magnetospheric and ionospheric convection [Dungey, 1961; Siscoe and Huang, 1985; Cowley and Lockwood, 1992; Lockwood and Cowley, 1992; Milan et al, 2007; Milan, 2015]. He et al [2012] used an empirical orthogonal function decomposition (closely related to PCA) to investigate FAC patterns observed by the CHAMP satellite. Kim et al [2012] applied PCA to polar cap ionospheric convection observations and found two dominant modes of response: uniform
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