The event to be studied occurred on November 25, 1997. Figure 1 shows the three components of the magnetic field observed by each of the three spacecraft. At GEOTAIL, located near the computed subsolar magnetopause at 12.5 RE, the BZ component of the magnetic field changes abruptly (2 min) from +3 nT to -13 nT at 1651 UT. The BY and BX components also change during this transition from -17 nT to +10 nT and from -1 to +6 nT, respectively. A similar change in the IMF was measured at both WIND (60 min earlier) and IMP8 (20 min later), increasing confidence that the change at GEOTAIL was spatially extended and of IMF origin. The IMF data have been shifted appropriately in Figure 1. During this period the solar wind density and velocity measured at WIND were steady, implying that no pressure pulses were associated with this structure.
We use the Northern Hemisphere component of the SuperDARN radar network [Greenwald et al., 1995] to determine the high-latitude convection pattern. During the period of the event being studied, line-of-sight (LOS) velocity measurements were available from 5 of the 6 northern SuperDARN radars currently in operation.
Figure 2 shows LOS velocities from 2 of the 5 radars. Each panel displays the data from one beam and a series of range gates. This format is the same as that presented by Ruohoniemi and Greenwald, . The particular beams were chosen to illustrate the wide MLT range of the first observed transition in the convection. The invariant latitude of the selected range gates are listed to the right of each panel. The vertical dotted line drawn through each panel at 1702:49 UT indicates the first discernible transition in convection attributable to the change in IMF observed at GEOTAIL, seen most clearly in Figure 2c. This time was determined from careful examination of the radar scans.
Prior to the transition at 1702:49 UT only minor variations in the ionospheric LOS velocities were seen on 2-min time scales. A significant increase in these velocities of 200-500 m s-1 is seen in all the range gates in Figure 2 within 2 min of the identified onset time. To further characterize this transition the increase in velocities exceeded 750 m s-1 within 6 min in some of the range gates. The response occurs nearly simultaneously in the 10 MLT and 15 MLT sectors (Figures 2a and 2c). A response is not seen in the 12 MLT sector (Figure 2b) until the following scan at 1704 UT, because the beam in that sector is sampled 30 s earlier than those in Figures 2a and 2c. Thus, within 30 s, the transition occurred simultaneously over nearly 7 hours of MLT.
To show the extent of the convection onset the SuperDARN data has been combined in a manner described by Ruohoniemi and Baker  to produce 2-min convection maps. Figure 3a shows the scan preceding the transition (1700-1702 UT) and Figure 3b shows the scan following the transition (1704-1706 UT). The fitted velocity vectors in Figure 3b show an enhanced flow over all MLTs where ionospheric scatter is present and a change from northeastward to northwestward flow in the prenoon sector near 80, both typical of a +BZ, -BY to -BZ, +BY IMF change [Greenwald et al., 1990]. The black rectangular boxes show the locations of the stackplot Doppler data shown in Figure 2.
The CANOPUS magnetometer at Taloyoak, NWT ( 79) showed a change consistent with the onset of enhanced northwestward flow across 12 MLT. The transition occurred within 1 min of the time identified by the radar measurements, confirming the near-noon ionospheric response to 1 min. Magnetometer stations located further equatorward showed no discernible change.