Sudden commencement is usually followed by a temporary decrease in geomagnetic field strength know as the main phase decrease. This period of decreased field strength, which may last for several days, is believed to be caused by solar flare cosmic rays trapped in spiral orbits around the Earth's magnetic field lines. These cosmic rays would form a storm-time radiation belt similar in nature to the Van Allen radiation belts which girdle our planet. As a result, the motion of these particles would produce a ring current which in turn would generate a magnetic field opposed to the Earth's dipole field. The inner Van Allen radiation belt.
Arrows indicate the directions of particle drift which collectively constitute the ring current.
Suppose that during the proposed period of solar chromospheric activation a solar flare were to occur that was 100 times more intense than the most energetic solar flare observed during the past few decades, of a size typical of flares observed in T Tauri stars. Thus ring current magnetic fields 100 times stronger than those thus far observed could conceivably occur, the applied opposed field reaching such a high intensity that it would essentially neutralize the Earth's dipole field. It is difficult to say what would happen when the intensity of the ring current field exceeded that of the geomagnetic field. Very likely the ring current field lines would close to form a doughnut-shaped field configuration having a symmetry axis that passed through the Earth's geomagnetic axis. The net field at the Earth's surface would then be reduced in intensity or even reversed in direction by the dominant ring current field.