New global angles

Detailed ground-based observations of substorms have often relied on measurements from mesoscale networks (e.g. instrumentation in Canada - CANOPUS, Fenno-Scandinavia - MIRACLE, or Greenland - DMI networks). With the appropriate co-location of instrumentation with the substorm associated phenomena this has lead to the excellent diagnosis of local features of the substorm, i.e. substorm current wedge, westward travelling surge etc. However, the complete substorm process is not necessarily confined to just the local time sector where the substorm current wedge is observed. In fact, Baker et al. (1999) describe the magnetospheric substorm as a "global instability". Global networks such as SuperDARN and those created by integrating such networks as CANOPUS and MIRACLE allow large-scale signatures of substorms to be studied. Boralv et al. (2000) have utilised the longitudinal extent of SuperDARN (also with magnetometer, LANL and EISCAT data) to investigate the responses in the dawn and dusk electrojets at substorm onset. The findings of this study are that there are strong increases in the dawn and dusk electrojets at substorm onset. The response can occur simultaneously with substorm onset. The reaction time is too fast for information to be communicated from the local time sector of magnetic midnight by either particle drifts, the expansion of the substorm current wedge or by Alfvén waves.

 

Figure 2:

An example from the study is given in Figure 2. This is described in detail in the SuperDARN 1999 proceedings and now in Boralv et al. (2000).

A global picture of the response to substorm onset is illustrated in Figure 3. Here four consecutive scans of line of sight velocity in the N. hemisphere are presented. A substorm onset (determined from LANL data, not shown here) occurs at one or two minutes after 0300 UT. The first SuperDARN plot at 03:00 UT we can consider as just before onset. The centre of the field of view is centered over Greenland where it is magnetic midnight. Note the dense scatter over Greenland and the small amount of scatter in the dawn and dusk regions. No radars are monitoring the dayside at this time. At 03:02 UT the substorm is under way. There is a reduction in the scatter at magnetic midnight and the appearance of new scatter in the dawn and dusk regions.

 

Figure 3: