By John Coxon (University of Southampton)
Birkeland currents are the mechanism by which information is communicated from Earth’s magnetopause to the ionosphere. Understanding the timescales of these currents is very useful for understanding the ionosphere’s reaction to magnetopause phenomena. We use the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) dataset, which uses magnetometers on 66 spacecraft in low Earth orbit to derive Birkeland current density on a grid of colatitude and magnetic local time. The current densities are derived in a ten minute sliding window, evaluated every two minutes.
We use the SPatial Information from Distributed Exogenous Regression (SPIDER) technique (), which treats each coordinate of a global dataset (e.g. AMPERE or SuperMAG) independently, regressing the time series in each coordinate against some external driver to find the time lag that maximises the correlation of the two.
The figure below shows the correlation (left) and lag (centre) of the current densities with Interplanetary Magnetic Field (IMF) Bz. We focus on the R1 and R2 regions (right) here. Southward (negative) Bz drives Birkeland current as a result of magnetic reconnection, as shown by the correlations. Looking at the lags on the dayside, the poleward lags are 10–20 minutes, reflecting the time taken for the Birkeland currents to start to react to magnetic reconnection. At all MLT, the equatorward lags are 60–90 minutes, reflecting the time at which the polar cap is largest. On the nightside, the poleward lags are 90–150 minutes, reflecting how long it takes the polar cap to contract during nightside reconnection. More details on the R1/R2 correlations, and other correlations between Birkeland current and IMF Bz and By, are available in the full study.
For more information, please see the paper:
2019). Timescales of Birkeland currents driven by the IMF. Geophysical Research Letters, 46, 7893– 7901. https://doi.org/10.1029/2018GL081658, , , , , , et al. (
Figure: Correlation (left) and lag (centre) of AMPERE current density with IMF Bz in March 2010. A key to the regions visible is presented in the right-hand panel, to allow easy references in the text above.