Whistlers recorded at Siple, Antarctica (76¿S, 84¿W; L?4), were used to monitor the east-west component of electric fields in the plasmasphere during moderate substorm activity. The results presented here cover L?3.5--5 and 1400--2100 MLT. During substorms, eastward electric fields of ~0.2 mV/m in the equatorial plane are observed in the 1600--2000 MLT sector. Inside this MLT sector the electric field appears within the experimental uncertainty of ~¿10 min of substorm onset, determined from auroral latitude magnetograms. The field amplitude does not depend strongly on the intensity of the magnetic disturbance observed on the ground. Simultaneous ionospheric observations show that these substorm E fields can explain large enhancements in f0F2 frequently observed in middle latitudes during summer evening hours. These results are consistent with theoretical predictions based on a model ionosphere-magnetosphere system.