The development of auroral and geomagnetic substorm activity after a southward turning of the interplanetary magnetic field following several hours of magnetic calm


  • Knud Lassen,

  • J. R. Sharber,

  • J. D. Winningham


A comprehensive study of growth phase and substorm activity following a period of magnetic calm has been conducted through a network of all-sky camera stations, auroral zone magnetic observatories, and particle detectors aboard the Isis 1 satellite. We have carefully documented the observations with the following results. The preexpansive phase arc which extended at least from 17 to 05 MLT was responsible for an energy input rate of ≃3 × 1016 ergs/s before breakup. An equatorward drift of this arc of 6 km/min, observed only in the evening sector, remained until after the expansive phase, when its motion stopped abruptly at the time of the maximum poleward displacement of the arcs. Electrons responsible for the prebreakup arc had energies of ≃1–5 keV. Protons of ≃4-keV energy were measured equatorward of the electron arc. During the expansive phase, symmetrically traveling disturbances were observed propagating eastward in the evening sector and westward in the morning sector. The propagation stopped for 1–2 min at the time of maximum expansion and then continued, thus suggesting a momentary variation in the rate of convection. Equivalent currents consistent with observed magnetic perturbations represented approximately the same DPZ (twin vortex) pattern before the expansive phase as during it; however, although the magnitude of the currents was greater during the expansive phase, the dominant feature during this phase was an intense westward auroral electrojet. The camera observations of diffuse cloudlike aurora showed an injection of ≃40-keV electrons during the expansive phase along the auroral oval between midnight and 0400 corrected geomagnetic time. Movement of the cloud indicated an eastward gradient drift of the electron population.