Relationships between field-aligned currents and convection observed by EISCAT and implications concerning the mechanism that produces region-2 currents: Statistical study

Fluid theories explain the origin of region-2field-aligned currents as the closure of the ring current, driven itself by theazimuthal pressure gradients generated in the magnetospheric ring plasma by thesunward convection. Although the structure of pressure gradients appearsexperimentally complex, observations confirm that a close connection existsbetween the region-2 field-aligned currents and the ring current. The fluidlinear theory of the adiabatic transport by convection of the ring plasma givesa first estimate of this process, and leads ultimately to phase quadrature (interms of magnetic local time) between the region-2 field-aligned currents andthe convection potential. When significant non-adiabatic processes are takeninto account, such as precipitations at auroral latitudes, the theoretical phasedifference rotates toward opposition. We determine experimentally the phaserelationship between the region-2 field-aligned currents and the convectionpotential from recent statistics, depending on the magnetic activity index K_p,and performed from the EISCAT data base. For geometrical reasons of sufficientprobing of region 2, it is only computed in the case of a moderate magneticactivity corresponding to 2 ≤ K_p<4. Region-2 field-alignedcurrents are found to be in phase opposition with the convection electrostaticpotential at auroral latitudes. This confirms the importance of non adiabaticprocesses, especially ion losses, in the generation of region-2 field-alignedcurrents, as theoretically suggested