Abstract
AbstractWe obtain current densities from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), alongside By and Bz from the Interplanetary Magnetic Field (IMF) for March 2010. For each AMPERE spatial coordinate, we cross‐correlate current density with By and Bz, finding the maximum correlation for lags up to 360 min. The patterns of maximum correlation contain large‐scale structures consistent with the literature. For the correlation with By, the lags on the dayside are 10 min at high latitudes but up to 240 min at lower latitudes. Lags on the nightside are 90–150 min. For Bz, the shortest lags on the dayside are 10–20 min; on the equatorward edge of the current oval, 60–90 min; and on the nightside, predominantly 90–150 min. This novel approach enables us to see statistically the timescales on which information is electrodynamically communicated to the ionosphere after magnetic field lines reconnect on the dayside and nightside.
Highlights
Birkeland currents were first observed by Zmuda et al (1966) as magnetic perturbations, which Cummings and Dessler (1967) later recognized as the current systems of Birkeland (1908, 1913), their name
We lag the Interplanetary Magnetic Field (IMF) By and Bz compared with the current density j reported by AMPERE in order to find the maximum correlation within a lag of 2 hr, and the time lag which yields that maximum correlation
The time lags associated with the Bz correlations show that statistically, on the dayside, the poleward side of the Region 1 (R1) correlation is associated with a timescale of 10–20 min, before the equatorward edge of the R1 correlation at ∼60 min
Summary
Birkeland currents ( known as field-aligned currents) were first observed by Zmuda et al (1966) as magnetic perturbations, which Cummings and Dessler (1967) later recognized as the current systems of Birkeland (1908, 1913), their name. It has long been thought that these By asymmetries propagate into the magnetotail via convecting field lines (Cowley, 1981), and so it can be reasonably supposed that the subsequent reconnection of these asymmetric field lines in the tail will create flow and current asymmetries with timescales similar to the nightside Bz case. It has been suggested that these waves can cause the Birkeland currents to react almost instantaneously to increases in dayside reconnection (Snekvik et al, 2017) and induce a By asymmetry in the magnetotail in the same sense as the IMF on timescales of 15–45 min (Khurana et al, 1996; Tenfjord et al, 2015), and it has been shown that this mechanism can induce asymmetries on closed field lines (Tenfjord et al, 2017, 2018)
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