The Ring Current
If the cloud advances at 1000 km/s, the cavity will be fully formed in a few minutes. A typical magnetic storm, however, lasts much longer. Its main features are a "main phase" in which the north-south component of the Earth's field (at low and middle latitudes) gradually weakens over 6-12 hours, followed by a slow recovery of the field lasting 1-3 days. This part of the storm disturbance can be far more intense than the "sudden commencement," yet the Chapman-Ferraro cavity provided no good explanation.
In the early 1900s the idea arose that a "ring current" of trapped particles might exist around the Earth's equatorial plane. Electrons and positive ions of sufficiently high energy could circle the Earth's equatorial plane in opposite directions, each contributing an electric current in the same sense, which always weakens the Earth's main field as observed.
Carl Stoermer proposed such a ring current [Stoermer, 1910, 1911, 1912] to overcome a discrepancy in his theory, which predicted the aurora far closer to the magnetic pole than where it was observed [Smith, 1963; Chapman and Bartels, 1940, section 24.13]. Soon afterward, however, Adolf Schmidt suggested that a ring current was also the cause of the main phase of magnetic storms [Schmidt, 1924].
The main problem was that the energy required for motions like those suggested by Stoermer was rather high: such orbits, when close to the Earth (distant orbits have other problems) are now recognized as appropriate for cosmic ray particles. As part of their theory, Chapman and Ferraro also proposed their own version of the ring current concept, set up (somehow) inside the Chapman-Ferraro cavity [Chapman and Ferraro, 1933; Smith, 1963]; the curved arrows in Figure 3 are related to their theory. This was later expanded by Martyn  and Stoermer, [1955, section 60]. But as Chapman remarked [cited by Hulburt, 1937],
The whole theory is necessarily both speculative and difficult; probably the most doubtful feature is that relating to the ring current, the existence and formation of which are still very uncertain.
Shortly before the discovery of the radiation belt, Singer  pointed out that trapped particles of low energy could also carry a ring current, even though their motion was more complex. An ion confined to the equatorial plane, for instance, tends to circle locally around field lines, but its circle will be slightly tighter where it comes closest to Earth, because the field there is slightly stronger. This causes the mean position of the ion to drift slowly in longitude, gradually carrying it around the Earth (Figure 4); ions and electrons drift in opposite directions, and therefore a neutral plasma yields a net circulating current.