A bevy of satellites buzzing around in the Earths magnetosphere has found at least part of the answer to a long-standing puzzle about the source of the charged particles that feed the aurora.
Three-dimensional computer simulation of the space waves or vortices that inject the solar wind plasma into the Earths magnetic field. Green-blue areas represent the solar wind plasma, and red-orange areas represent plasma trapped in Earths magnetic field. Earths magnetosphere develops ripples and folds like a flag in the wind as the solar wind blows past. This turbulence creates rolling waves on the edges of the magnetosphere that engulf the solar wind into the magnetosphere (see orange wavelike structure in the cut-away portion of the image). The blue, green and orange swirl behind the wave is the vortex that mixes the solar wind into the magnetosphere. The vortices are huge structures, measuring more than 20,000 miles across. (Kentaro Tanaka of Tokyo Institute of Technology)
The charged particles come from explosions on the sun and smash into the Earths magnetic field, which repels the bulk of them. But many slip through, often via a physical process called magnetic reconnection, where the magnetic field traveling with the particles breaks and reconnects with the Earths field, opening a window for the particles to surge through. Once inside, these excited particles can spiral down toward the poles and create brilliant auroras when they hit the atmosphere.
But magnetic reconnection happens only when the solar winds magnetic field direction is 180 degrees opposite from that of the magnetic field of the Earth. When the two fields are aligned, there is no obvious physical process allowing entry of charged particles, at least at the leading edge of the Earths magnetosphere.
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