The Response of the Magnetosphere to an Interplanetary Shock: Ground-based Observations of the Sudden Impulse on September 24, 1998

E Zesta1, P J Chi1, C T Russell1, J Raeder1, G Le1, K Yumoto2, H Kawano2, K Kitamura2, V Angelopoulos3, and M Moldwin4


1 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095

2 Department of Earth and Planetary Sciences, Kyushu University 33, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan

3 Space Sciences Laboratory, University of California, Berkeley, CA 94720

4 Florida Institute of Technology, Physics and Space Sciences, 150 W. University Blvd, Melbourne, Florida 32901


The strong interplanetary shock wave that intersected the magnetosphere at about 2344 UT on September 24, 1998, launched a compression of the magnetosphere that was followed around the globe by a new generation of magnetometers with rapid sampling and precise GPS-synchronized timing. Magnetometers of these chains included those in the Circum-pan Pacific Magnetometer Network (CPMN), those in the IGPP/LANL array, those in the MEASURE array and those in the MACCS array. The event exhibited all the classical signatures of an SI including the preliminary decrease before the main increase in the H-component. A preceding depression of the magnetic field with an amplitude of 1--2 nT also occurred approximately 40 seconds prior to the preliminary impulse at some low-latitude stations near local noon. Properties of the SI signatures on the ground, including the propagation of the preliminary impulse, will be presented. The ability to model this event and its induced ionospheric currents using an MHD code allows us to compare the compression as it is seen in the magnetosphere with the response of the ionospheric currents.