Magnetospheric observation of large sub‐auroral electric fields

An example of large sub‐auroral poleward electric fields, similar to those observed on OGO‐6, S3‐2 and AE‐C (recently referred to as SAID) has been found in the magnetosphere near L = 4 and 2300 MLT using ISEE‐1 electric field data. The event is located adjacent to and outside the plasmapause and occurs 1½ hours into a substorm. The event is accompanied by a significant penetration of the convection electric field inside the plasmasphere. Data from similar regions on the next orbit occurring near the beginning of a substorm did not exhibit these effects. Recent theoretical models predict SAID to occur in the trough regions, where substorm dynamics force currents to flow in regions of low conductivity. These models provide a first‐order interpretation of this phenomena; however, the overall picture is more complex.