Antarctic polar plateau vertical electric field variations across heliocentric current sheet crossings

Abstract

A superposed epoch analysis of variations of the vertical electric field measured at Vostok (78.5°S, 107°E; magnetic latitude 83.6°S) during 1998–2002 heliocentric current sheet (HCS) crossings yields no significant variation other than an association imposed by polar-cap potential differences above the site. This result contradicts published reports of a reduction ∼15% in electric field 1–3 days after HCS crossings, an observation initially made ∼30 years ago. If such a reduction had been caused by reductions in stratospheric ionising radiation, the presence of polar stratospheric clouds (PSC) would seem necessary for the occurrence of this effect. PSCs would increase the resistance of the stratosphere thus making ionisation in that region significant in the context of the ionosphere–ground current flow, in a manner analogous to the role of volcanic aerosols in the stratosphere in the explanation of the weakening of northern hemisphere winter cyclones associated with HCS crossings, the so-called ‘Wilcox effect’. However, separating the present data to correspond to the likely presence of PSC above Vostok also does not yield the reported reduction. Significant increases or decreases of the vertical electric field emerge from the observations when the HCS crossings are separated into sets depending on whether the solar wind magnetic field changes from ‘toward-to-away’ (increase of ∼11%) and ‘away-to-toward’ (decrease of ∼8%). Polar-cap potential changes above the site, inferred from solar wind parameters using the Weimer model, also show such step functions that reverse with the sign of HCS transition and are broadly consistent with the measured electric field increases or decreases. Remaining differences between the measurements and the model are consistent with a somewhat stronger solar wind speed and/or magnetic activity influence on polar-cap convection above Vostok than is predicted by the model. Variations in ground-level neutron counts, a measure of cosmic ray influence on the atmospheric conductivity, demonstrate average variations of less than 1% around HCS crossings and at this level would not contribute significantly to the electric field variations. Magnetic activity (measured using the Kp index) and solar wind speed decrease around HCS crossings independent of whether the crossings are ‘toward-to-away’ or ‘away-to-toward’. These decreases are broadly similar in shape to the previously reported Vostok electric field responses. However, their effects on ionospheric potential are included in the Weimer model, and are small. In contrast to sub-auroral latitudes, where the precipitation of MeV electrons from the radiation belts has a characteristic decrease for both types of HCS crossing, there are no known sources of stratospheric ionising radiation in the polar caps that could explain the previously reported effect. Thus, observation and theory are now in agreement for the absence of this effect.