The ring current: a short biography

摘要

The "ring current'' grows in the inner magnetosphere during magnetic stormsand contributes significantly to characteristic perturbations to the Earth'sfield observed at low-latitudes. This paper outlines how understanding ofthe ring current evolved during the half-century intervals before and afterhumans gained direct access to space. Its existence was first postulated in1910 by Carl St?rmer to explain the locations and equatorward migrationsof aurorae under stormtime conditions. In 1917 Adolf Schmidt appliedSt?rmer's ring-current hypothesis to explain the observed negativeperturbations in the Earth's magnetic field. More than another decade wouldpass before Sydney Chapman and Vicenzo Ferraro argued for its necessity toexplain magnetic signatures observed during the main phases of storms. Boththe St?rmer and Chapman–Ferraro models had difficulties explaining howsolar particles entered and propagated in the magnetosphere to form the ringcurrent. During the early 1950s Hannes Alfvén correctly argued that thering current was a collective plasma effect, but failed to explain particleentry. The discovery of a weak but persistent interplanetary magnetic fieldembedded in a continuous solar wind provided James Dungey with sufficientevidence to devise the magnetic merging-reconnection model now regarded asthe basis for understanding magnetospheric and auroral activity. In themid-1960s Louis Frank showed that ions in the newly discovered plasma sheethad the energy spectral characteristics needed to explain the ring current'sorigin. The introduction of ion mass spectrometers on space missions duringthe 1970s revealed that O+ ions from the ionosphere contribute largefractions of the ring current's energy content. Precisely how cold O+ ionsin the ionosphere are accelerated to ring-current energies still challengesscientific understanding.