The ring current: a short biography

Abstract

Abstract. The "ring current" grows in the inner magnetosphere during magnetic storms and contributes significantly to characteristic perturbations to the Earth's field observed at low-latitudes. This paper outlines how understanding of the ring current evolved during the half-century intervals before and after humans gained direct access to space. Its existence was first postulated in 1910 by Carl Størmer to explain the locations and equatorward migrations of aurorae under stormtime conditions. In 1917 Adolf Schmidt applied Størmer's ring-current hypothesis to explain the observed negative perturbations in the Earth's magnetic field. More than another decade would pass before Sydney Chapman and Vicenzo Ferraro argued for its necessity to explain magnetic signatures observed during the main phases of storms. Both the Størmer and Chapman-Ferraro models had difficulties explaining how solar particles entered and propagated in the magnetosphere to form the ring current. During the early 1950s Hannes Alfvén correctly argued that the ring current was a collective plasma effect, but failed to explain particle entry. The discovery of a weak but persistent interplanetary magnetic field embedded in a continuous solar wind provided James Dungey with sufficient evidence to devise the magnetic merging-reconnection model now regarded as the basis for understanding magnetospheric and auroral activity. In the mid-1960s Louis Frank showed that ions in the newly discovered plasma sheet had the energy spectral characteristics needed to explain the ring current's origin. The introduction of ion mass spectrometers on space missions during the 1970s revealed that O + ions from the ionosphere contribute large fractions of the ring current's energy content. Precisely how cold O + ions in the ionosphere are accelerated to ring-current energies still challenges scientific understanding.

Determining the boundaries of the auroral oval from CHAMP field-aligned current signatures – Part 1

“…It was Størmer in 1910 who first argued that a current encircling the Earth in the equatorial plane is needed to shift the artificial auroras of Birkeland's terrella experiment to latitudes where they are actually observed on the ground (Egeland and Burke, 2012). Our observation of a linear relation between Dst and the boundary latitude, as well as other studies (e.g.…”

Current Systems in the Earth's Magnetosphere

“…Due to this process, the plasma pressure distribution in the inner magnetosphere becomes highly asymmetric with a gradient in the azimuthal direction which is reflected in the spatial asymmetry of the ring current. The concept of the partial ring current and its closure to the ionosphere was suggested by Alfv n in 1950s (see the review by Egeland & Burke, ). In situ satellite observations showed a fairly symmetric ring current during geomagnetically quiet conditions and an asymmetric current distribution during storm times, symmetrizing again during the storm recovery phase (e.g., De Michelis et al, ; Ebihara et al, ; Korth et al, ; Le et al, ; Lui, ).…”

“…During a magnetic storm, hot charged particles move from the tail, through the inner magnetosphere, and out to the dayside magnetopause (e.g., Daglis, , ; Daglis et al, ; Ebihara & Ejiri, ; Egeland & Burke, ; Ganushkina et al, ; Kozyra & Liemohn, ; Vasyliunas, ; Williams, ; Wolf et al, , ). The current systems in this region change according to the variation in plasma pressure and magnetic field pressure (see section 3.5).…”

Space Weather Effects Produced by the Ring Current Particles

“…The ring current is one of the oldest concepts in magnetospheric physics (see the historical review by Egeland and Burke 2012). A current flowing around the Earth was first introduced by Störmer (1907) and supported by Schmidt (1917).…”