A realignment of Earth's magnetic field lines rather than a disruption of current across its magnetosphere triggers the geomagnetic substorms that lead to luminous polar auroras. That, at least, is what the first results from NASA's five-satellite THEMIS mission suggest.
The magnetosphere is a volume enveloping the planet that protects it from a stream of charged particles from the Sun. Before a substorm, Earth's magnetic field becomes entwined with this solar wind. When the field snaps back into place, it prompts the chain of events that causes the substorm.
Vassilis Angelopoulos of the University of California, Los Angeles, and his team used THEMIS to observe a nascent substorm in February. They found that the snapping back, or realignment, of the magnetic field preceded a brightening of the aurora. Disruption to the magnetospheric current came after that.
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Planetary physics: Storm warning. Nature 454, 554 (2008). https://doi.org/10.1038/454554a
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DOI: https://doi.org/10.1038/454554a
