SUBSTORM CHANGES OF THE ELECTRODYNAMIC QUANTITIES IN THE POLAR IONOSPHERE - CDAW-9

On the basis of ground magnetometer data from 75 northern hemisphere s tations and the ionospheric conductivity distribution estimated from V iking satellite observations of auroral images, various electrodynamic quantities in the polar ionosphere are calculated for the April 1, 19 86, Coordinated Data Analysis Workshop (CDAW) 9 substorm. Since the Sc andinavia and Russia chains of magnetometers were located in the premi dnight-midnight sector during this interval and the estimated conducti vity distribution is instantaneous, our data set provides us with a un ique opportunity to examine some long-standing problems associated wit h the substorm expansion onset. Several important findings of this stu dy are summarized as follows: (1) Before the expansion onset of the su bstorm, intensifications of ionospheric currents or the cross-polar ca p potential are very weal; in this particular example. Both quantities begin to increase notably only with the initiation of the substorm ex pansion onset. (2) The intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the mi dnight sector during the expansion phase, while its equatorward half i s occupied by a weak eastward electrojet. (3) The Joule heating rate a nd the energy input rate of auroral particles are quite comparable pre ceding the expansion onset. During the expansion phase of the substorm , however, Joule heating shows a marked intensification, but the latte r increases only moderately, indicating that the Joule dissipation is more effective than auroral particle energy input during substorm time s. (4) The Hall currents are not completely divergence-free. The corre sponding field-aligned currents show highly localized structures durin g the maximum epoch of the substorm, with the upward current being loc ated in the region of the steepest conductivity gradient on the polewa rd side of the westward electrojet in the midnight sector. This is ind irect evidence that the so-called imperfect Cowling channel is effecti ve behind the westward traveling surge.