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Abstract
A comprehensive description of electromagnetic processes related to equatorial plasma depletions (EPDs) is essential for understanding their evolution and day-to-day variability. Recently, field-aligned currents (FACs) flowing at both western and eastern edges of EPDs were observed to be interhemispheric rather than anti-parallel about the dip equator, as suggested by previous theoretical studies. In this paper, we investigate the spatial and temporal variability of the FACs orientation using simultaneous measurements of electron density and magnetic field gathered by ESA’s Swarm constellation mission. By using empirical models, we assess the role of the Pedersen conductance in the preference of the FACs to close either in the northern or southern magnetic hemisphere. Here we show that the closure of the FACs agrees with an electrostatic regime determined by a hemispherical asymmetry of the Pedersen conductance. That is, the EPD-related FACs close at lower altitudes in the hemisphere with the highest conductivity. The evidence of this conclusion stands on the general agreement between the longitudinal and seasonal variability of both the conductivity and the FACs orientation.
Highlights
The occurrence of EPDs with FACs closing to the southern magnetic hemisphere maximizes about 40°W and decreases toward the African sector
By comparing with the orientation of the FACs, we found good agreement between the hemispherical asymmetry of ΣP and the direction to which the FACs close
The FACs close mostly northward though ΣP is not more significant than in the southern hemisphere. This observation might be explained by an effect of zonal thermospheric winds that cannot be adequately detected by empirical models
Summary
The Swarm constellation mission[16,17] was successfully launched into a near-polar, circular orbit on 22 November 2013. Only depletions are identified as EPDs if the percent change between the background and residual (δNe/N0) is higher than 20%, over a minimum distance of about 23 km (3 s of satellite flight) In this way, we account for decreasing N0 along the study period due to the currently declining solar cycle. For each EPD detected, both δNe and δBrad across the depletion must present a correlation coefficient (cc) with absolute values larger or equal than 0.6 (|cc| ≥ 0.6) This approach guarantees that the considered EPDs have comparable sheets of field-aligned currents flowing in opposite directions at their western and eastern edges.
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