Calculation of the gyro‐frequency in theE‐layer at arctic stations from measurements of critical radio‐frequency differences gives a lower magnetic field than that obtained by extrapolation of the terrestrial field measured at ground‐level, and, at one station, a large semi‐diurnal variation with maxima at 06hand 18hlocal time.Ray‐path deflections which can explain similar effects previously reported for theF‐layer cannot be responsible in theE‐layer because of the opposite sense of this deviation and because of the small layer‐thickness. Moreover, no semi‐diurnal variation was found in theF‐layer.It is shown that these newE‐layer phenomena may be due to a variable concentration of heavy ions rising to over 4,000 times the de