Prompt response of SAPS to stormtime substorms

We report on satellite, incoherent scatter radar, and ground magnetometer observations of the development of SAPS during a stormtime substorm event on 25 September 1998. The substorm onset was determined from the data of the ground magnetometer array and LANL satellites. The radar and Defense Meteorological Satellite Program (DMSP) satellites identified the SAPS region. The pre-existing SAPS in the afternoon sector moved equatorward and intensified within ∼10min after the substorm onset. It is shown that this transition began when the westward traveling surge (WTS) arrived at the adjacent auroral region. The observations indicate that the stormtime ring current responds to the substorm expansion on a rapid timescale, characteristic for the dipolarization pulse and WTS development. A scenario for the generation of the post-onset SAPS-wave structures is proposed.