Response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field

The response of the dayside auroral and electrodynamic processes to variations in the interplanetary magnetic field (IMF) is examined with multi‐instrument data from the Viking and DMSP‐F7 satellites as well as ground magnetometer data. The event selected, which occurred on October 5, 1986, was previously identified as a high‐latitude dayside form by Murphree and Elphinstone [1988]. IMF By was positive during most of the Viking auroral UV observation, whereas IMF Bz made a transient southward excursion, followed by a sharp increase to +8 nT. The results are summarized as follows: (1) There were two auroral belts extending westward from the early afternoon sector. The equatorward belt was persistent throughout the event and was embedded in the CPS/BPS precipitation region, whereas the poleward belt was in the open field line region and was associated with northward IMF Bz. (2) A new auroral spot emerged in the afternoon sector just poleward of the previously active region, delayed 10 min from the sudden increase in IMF Bz. This time lag is ascribed to the travel time of the new IMF orientation from the subsolar point to a solar wind‐magnetosphere interaction site tailward of the dayside cusp, as well as to the response time of the dayside auroral acceleration process. (3) The new auroral activity expanded both eastward and westward during the first few minutes and then expanded primarily westward across the noon meridian, forming the poleward belt. The speed of the westward extension in the second phase was comparable to the speed of the ionospheric convection. (4) The dayside auroral process has a finite decay constant, which is inferred to be at least 10 min. (5) The response time of the midday ionospheric convection to IMF variations is estimated to be a few to several minutes.