Collimation of auroral particles by time-varying acceleration

IT is generally accepted that charged particles aligned with the magnetic field, which occur in some forms of auroral precipitation 1–4 , are collimated within a few thousand kilometres of the Earth's surface, by an acceleration parallel to the geomagnetic field. Static electric fields are thought to provide this acceleration, and various modes of injection of charged particles have been invoked to account for the details of observed energy spectra and angular distributions 1–4 . A time-varying acceleration of an incident stream of charged particles, provides an alternative explanation.