The Response of the Magnetosphere to the Sudden Impulse of September 24, 1998


C. T. RUSSELL, X-W. Zhou, P. Chi, J. Raeder and R. J. Strangeway (All at Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA, email:

H. J. Singer (Space Environment Center, NOAA, Boulder, CO, USA)

H. Kawano (Kyushu University, Fukuoka, Japan)

T. E. Moore (Goddard Space Flight Center, Greenbelt, MD, USA)

W. K. Peterson (Lockheed Martin, Palo Alto, CA, USA)

J. B. Cladis (Lockheed Martin, Palo Alto, CA, USA)

E. Zesta (Atmosphere Sciences, University of California Los Angeles, CA, USA)


At approximately 2344 UT on September 24, 1998 the nose of the magnetosphere was suddenly compressed by a strong interplanetary shock wave. As evidenced by successive compressions of the magnetic field at GOES 10, POLAR and GOES 8, the compressional wave moved rapidly through the magnetosphere. At POLAR the compression caused the magnetopause to move closer to the spacecraft, causing rapid cross field flows and altering the location of the conjugate point in the ionosphere. The pre-existing flows were heated locally and only slightly by betatron acceleration because the flows were mainly field aligned. Waves also did not lead to much heating because they were small in amplitude. Centrifugal acceleration may have been a more important local process as evidenced by an increase in the parallel velocity components of the ions during the local rotation of the magnetic field. Some of the observed changes in the plasma observed at POLAR were associated with the motion of the non-uniform polar cap outflow across POLAR. Snapshots of the polar cap by FAST confirm that the outward polar cap flows are non-uniform at this time but also show that strong heating is present and subsequently is very important in determining the outflow of plasma. Ground station recordings, synchronized with GPS signals, enable the disturbance to be followed through the magnetosphere from high to low latitudes. This event teaches us much about how the ionosphere is coupled to the magnetosphere and demonstrates the symbiosis of ground based, ionospheric and magnetospheric observations with modeling in magnetospheric studies.