Geomagnetic activity and brightenings of the Earth’s auroral
emissions are controlled primarily by the interaction of the solar
wind with the Earth’s magnetic field. Geomagnetic substorms occur
during intervals of southward interplanetary magnetic field (IMF),
permitting reconnection between the IMF and geomagnetic fields.
Substorms are manifested as bright auroral emissions, which
typically shift the brightened auroral oval to lower latitudes and
also fill in the midnight sector of the oval1. The Earth’s auroral
oval is adjacent to and equatorward of the boundary between open
and closed magnetic field lines, and its statistical location is fixed
with respect to the solar wind direction rather than rotating with
the planet. By contrast2, Jupiter’s main auroral oval occupies field
lines that extend only to the middle magnetosphere, where outward
diffusing plasma lags behind the rotational speed of Jupiter,
driving strong currents in and out of Jupiter’s ionosphere3,4.
Jupiter’s total auroral power is relatively constant, rarely changing
by as much as a factor of two, whereas the localized auroral storms
that are observed consist of rapid flares poleward of the main oval
and more gradual brightenings of the dawn sector of the main
oval.