Whistlers recorded in the Antarctic were used to infer the east-west component of electric field in the nightside plasmasphere between L~3 and 6 during moderate substorms. The results show eastward electric fields of ~0.2 mV/m in the equatorial plane in the premidnight local time sector. Near midnight there is a sharp transition from these eastward fields to westward fields of ~0.2-0.6 mV/m. The existence of westward substorm fields in the postmidnight sector has been reported in several previous papers as a prominent feature in the whistler data. These fields are distinct from quiet time fields that are directed eastward over the entire nightside and have an average magnitude of ~0.05 /mV/m. The substorm electric field pattern described here is consistent with previous ionosonde observations of large-scale distortions of the nightside F layer showing an uplifting in the premidnight sector and a lowering in the postmidnight sector.