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 4 FAST 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 geosynchronous 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) [see Figure 7], the MACCS array, the MEASURE array and the IGPP/LANL array. The overall ground level geomagnetic history for September 23-26 is shown in Figure 8a and b. Finally, this event is an excellent vehicle for testing and comparing with numerical models especially those that include ionospheric effects. A web page has been established for ground-based observations relating to this event.
Two papers have been published in GRL on the events of the period:
Ionospheric Mass Ejection in Response to a Coronal Mass
Ejection by T. E. Moore, et al.
Sudden Compression of the Outer Magnetosphere Associated with an Ionospheric Mass Ejection by C. T. Russell, et al.
Plasmaspheric and Refilling Associated with the September 25, 1998 Magnetic Storm Observed by Ground Magnetometers L=2 (.ps format) by P. J. Chi, et al. (click here for .pdf format)
Papers in draft form:
Observations of Centrifugal Acceleration of Ions during Compression of the Magnetosphere (pdf document) by J. B. Cladis, et al.
The Interplanetary Shock of September 24, 1998; Arrival at Earth, by C. T. Russell, et al.
FAST observations of electromagnetic stresses applied to the polar ionosphere by Strangeway, R. J., R. C. Elphic, W. J. Peria, and C. W. Carlson,
Link to Plots
|C. T. Russell (firstname.lastname@example.org)||Overview|
|W. Peterson (email@example.com)||Joule Heating|
|R. Strangeway (firstname.lastname@example.org)||FAST Plasma/Current Observations|
|B. Giles (email@example.com)||Plasma Outflow Context|
|J. Spann (firstname.lastname@example.org)||Auroral Response|
|J. Sigwarth (email@example.com)||Polar Cap Dynamics|
|S. Petrinec (firstname.lastname@example.org)||Pixie X-ray Observations|
|P. Anderson (email@example.com)||Polar Cap Potential Drop|
|G. Lu (firstname.lastname@example.org)||Amie Inferences of Field-Aligned Currents|
|J. Raeder (email@example.com)||Simulations|
|P. Chi (firstname.lastname@example.org)||SI Propagation|
|S. Musman (email@example.com)||Total Electron Content from Models of GPS Observations|
|P. Chi (firstname.lastname@example.org)||Equatorial Mass Density|
|J. Fennell (Joseph.F.Fennell@aero.org)||Ring Current|
|C. T. Russell, et al.||The Response of the Magnetosphere to the Sudden Impulse of September 24, 1998|
|M. O. Chandler, et al.||Ion Signatures of Magnetospheric Regions and Boundaries: The September 24,1999 CME Event|
|R. J. Strangeway, et al.||Observations of intense ion outflows and cusp-region field-aligned currents during the September 24th and 25th 1998 magnetic storm|
|E. Zesta, et al.||The Response of the Magnetosphere to an Interplanetary Shock: Ground-based Observations of the Sudden Impulse on September 24, 1998|
|W. K. Peterson, et al.||Energization and escape of O+ into the Earth's polar cap and magnetotail|
|P. D. Craven, et al.||Ionospheric response to the CME passage of September 24, 1998|
|S. M. Petrinec, et al.||Auroral X ray emissions observed by PIXIE during the September 24-25, 1998 storm|
|J. Raeder, et al.||Global Simulation of the Response of Earth's Magnetosphere to the September 24, 1998, Interplanetary Shock|
|P. Chi, et al.||The Response of the Magnetosphere to an Interplanetyary Shock Groundbased Observation of the Sudden Impulse on September 24|
|C. T. Russell, et al.||The Response of the Magnetosphere to an Interplanetary Shock: The Magnetospheric Compression on September 24, 1998|
|J. B. Cladis, et al.||Centrifugal Acceleration of Ions Observed on Polar Spacecraft Near 9 RE Over Polar Cap Upon Arrival of CME-Driven Shock|
|J. Raeder, et al.||The Response of the Magnetosphere to an Interplanetary Shock: Global Simulations of the September 24, 1998 Sudden Impulse Event|
|R. J. Strangeway, et al.||FAST Observations During the September 25th and 25th 1998 Magnetic Storm|
|11:00||C. T. Russell||Event overview: Solar wind conditions and spacecraft configuration|
|11:15||T. Moore||Event overview: Tide and Timas observations|
|11:45||C. T. Russell||Magnetic response at Polar and Goes 8,10|
|Noon||W. Peterson/J. Cladis||Timas observations/Centrifugal acceleration|
|13:30||J. Spann||UVI observations|
|13:45||J. Sigwarth||Auroral Dynamics|
|14:00||R. Strangeway||FAST observations|
|14:30||P. Chi||Groundbased observations|
|14:45||S. Petrinec||Auroral X ray emissions|
|15:30||P. Chi||Inner plasmaspheric response to the storm of Sept 25|