Sub-Auroral Polarization Streams (SAPS) Event
This figure illustrates a SAPS event. Plotted from top to bottom are:
|panel 1||EDI drift velocity (black=radial, cyan=east, orange=background magnetic field aligned)|
EDI electric field
|panel 3||Magnetic field at Cluster after subtracting an empirically-based model magnetic field of the inner-magnetosphere (the IGRF)|
The magnitude of the magnetic field
|panel 5||A measure of 1keV electron background counts|
|panel 6||A proxy for the electron density derived from the spacecraft potential|
The SAPS electric field is identified at 06:44 UT (see panels 1 and 2). Mapping of this field to the ionosphere yields a westward drift velocity of 2500 ms-1, consistent with DMSP measurements of SAPS at this time.
The storm-time nature of this event is evident from the overall depression and oscillations in the magnetic field (panels 3 and 4).
Panels 5 and 6 allow identification of the plasmasphere entry at 06:23 and exit at 06:48 UT.
Notice that the SAPS electric field structure is only found near the outbound plasmapause crossing, not the inbound. It is possible to explain this in terms of the timing of geomagnetic storm events during this period. It is known from Los Alamos National Laboratory (LANL) geosynchronous data that a storm-related energetic particle injection occurred at approximately 06:37 UT. Therefore, SAPS-type electric fields were observed only after this injection.