The top panel of Figure 1 shows the interplanetary magnetic field (IMF), in GSM coordinate system, observed with the INTERBALL-1 spacecraft during an interval 0000-0800 UT on March 28, 1996. The Russian INTERBALL-1 satellite was launched on August 3, 1995. The magnetic field experiment onboard INTERBALL-1 is described by, e.g., Klimov et al. . The position of the satellite at 0300 UT on March 28, 1996 was =(14.4,21.6,2.3) () in GSE coordinates and the satellite was moving toward the Earth. We roughly estimate the propagation time lag from INTERBALL-1 to the Earth to be ~3 min, by simply dividing the spacecraft position by 440 km/s, which is the assumed nominal solar wind speed (we have no solar wind plasma data). The figure shows a southward turning of the IMF around 0242 UT. IMF Bz reached a minimum value nT at 0301 UT, remained that level (Bz < -4 nT) until 0413 UT, and then started to recover toward zero.
A southward IMF of the strength and duration shown in the top panel of Figure 1 could lead to substorm activity, and the top panel of Figure 2 supports that expectation. The panel shows X components of the ground magnetometer data from the CANOPUS network [e.g., Rostoker et al., 1995], from the Geological Survey of Canada (GSC), and from the STEP Polar Network run by the University of Tokyo, for the same interval as that of Figure 1. Table 1 lists the stations and their locations in geomagnetic coordinates, assuming the geomagnetic north pole at 79.34 in geographic latitude and 288.51 in geographic longitude (based on IGRF 95). Among the listed stations, Poste-de-la-Baleine, Ottawa, and Iqualuit are operated by the GSC, Schefferville is part of the STEP Polar Network, and the others are part of the CANOPUS network. The figure shows substorm activity during the interval 0300-0700 UT. More detailed discussion, including the explanation of the lines A-D in the figure, is given in the next section.
The bottom panel of Figure 1 shows data from the magnetic field experiment (MFE) on board the POLAR spacecraft [ Russell et al., 1995], in the GSM coordinate system (solid lines). Dotted lines show the model field, calculated as the summation of the IGRF95 and the Tsyganenko 1995 model [ Tsyganenko, 1995]. The satellite was located at (-0.8,-0.6,8.5) () in GSM coordinate system at 0300 UT, and was outbound. The subsequent apogee passage took place around 0511 UT and the apogee position was (-2.7,-0.4,8.5). The figure shows that the observed total magnetic field strength BT deviated from the model value , during the interval from 0300 UT to 0700 UT. This interval is about the same as the substorm interval shown in Figure 2.