During a stratospheric balloon flight on April 9, 1980, large scale electric fields have been measured, for the first time, at the Brazilian Magnetic Anomaly with a double-probe detector. We report here the results of this flight performed during a time interval with fair weather at the observing region and enhanced geomagnetic activity. These results suggest that a BAND of precipitating energetic particles, moving equatorwards with respect to the balloon, could have produced a conductivity enhancement down to the middle atmosphere, distorting the vertical atmospheric electric field and giving rise to the type of horizontal fields observed.