The effect of magnetic substorms on near-ground atmospheric current

Ionosphere-magnetosphere disturbances at high latitudes, e.g. magnetic subs torms, are accompanied by energetic particle precipitation and strong varia tions of the ionospheric electric fields and currents. These might reasonab ly be expected to modify the local atmospheric electric circuit. We have an alysed air-earth vertical currents (AECs) measured by a long wire antenna a t Esrange, northern Sweden during 35 geomagnetic substorms. Using superpose d epoch analysis we compare the air-earth current variations during the 3 h before and after the time of the magnetic X-component minimum with those f or corresponding local times on 35 days without substorms. After eliminatio n of the average daily variation we can conclude that the effect of substor ms on AEC is small but distinguishable. It is speculated that the AEC incre ases observed during about 2 h prior to the geomagnetic X-component minimum , are due to enhancement of the ionospheric electric field. During the subs equent 2 h of the substorm recovery phase, the difference between "substorm " and "quiet" atmospheric currents decreases. The amplitude of this "substo rm" variation of AEC is estimated to be less than 50% of the amplitude of t he diurnal variation in AEC during the same time interval. The statistical significance of this result was confirmed using the Van der Waerden X-test. This method was further used to show that the average air-earth current an d its fluctuations increase during late expansion and early recovery phases of substorms.