September 1998 has also been selected as a study event for the Second IACG Campaign on Boundaries in Collisionless Plasma ( see http://www-ssc.igpp.ucla.edu:80/IACG/mag_io_coupling/index.html). The following description and links was copied from that site.
On September 24, 1998 at 2320 UT a strong interplanetary shock crossed the WIND spacecraft about 185 RE upstream of Earth. As shown in Figure 1 ACE and WIND were both well upstream of the Earth near the Earth-sun line with WIND in a little better position to monitor the solar wind plasma hitting the magnetosphere. At about 2344 UT the pressure applied to the nose of the magnetosphere jumped from about 2 nPa to 15 nPa. In about one minute this pressure pulse had reached the terminators and rapidly moved down the tail. As shown in Figure 1 Geotail and IMP 8 are in the solar wind and INTERBALL 1 is in the dawn magnetosheath during this event. Figure 2 shows the view from the sun. Geotail is above the GSM equator; Interball 1 and Imp 8 are below it. The Polar spacecraft is high above the equator in the region of the magnetosphere attached to the polar cap. Figure 3 show the magnetic field and Corall ion spectral observed by Interball. The spacecraft is clearly in the magnetosheath throughout and the shock passes at 2348:15 UT. At 2340 UT Interball was at a location of (-11.46, -22.93, -4.56) RE in GSM. This event is particularly well documented with interplanetary, high altitude, low altitude and ground-based data and is important because it illustrates well the response of the magnetosphere to a rapid pressure increase, a situation that frequently precedes a geomagnetic storm. The interplanetary magnetic field was initially horizontal, its most probable direction. After about 2 hours it turned southward and a strong geomagnetic storm began. At POLAR the compression of the magnetic field was seen as a gradual rise in the field from 2345:20 to 2348 UT. Accompanying this compression were rapid cross-field flows as the magnetospheric boundaries compressed and a rapid change in the ion flows coming from the ionosphere representing changes to convecting inhomogeneous plasma, local heating and ionospheric heating. Over the entire length of the POLAR pass that included the development of the storm main phase extremely large changes occurred in the flux and energy of the ion beams.
As shown in Figure 4FAST was in a noon-midnight orbit with apogee at 4000 km over the northern hemisphere during this time providing snapshots of the polar fields and plasmas but sparse temporal coverage. FAST captured the sudden impulse itself on the dayside at mid-latitudes and documents well the spatial inhomogenity of the polar cap before and after the sudden commencement. FAST also documents well the dramatic changes in the plasma in the polar region. Other spacecraft at low altitudes at this time are DMSP 13 and 14, Sampex and Akebono. The latter two spacecraft are in the southern hemisphere.
At geosynchronoous orbit both GOES 10 and GOES 8 are making measaurements. Their positions are shown projected in the magnetic equator in Figure 5 and in the view from the sun in Figure 6. At synchronous orbit the magnetic signature varies significantly with local time. At GOES 10, in mid-afternoon, the response is sharp almost shock-like; at GOES 8, postdusk, the response is slower and weaker but still more rapid than at POLAR. The magnetospheric compression was also captured across the globe by modern magnetometer arrays with rapid sampling and precision timing. Such arrays include the Circum Pan Pacific Magnetic Network (ex -210 MM chain) the MACCS array, the MEASURE array and the IGPP/LANL array. Finally, this event is an excellent vehicle for testing and comparing with numerical models especially those that include ionospheric effects.
