Stretched magnetic fields and the formation of inertial currents in the outer magnetosphere

Abstract

Summary form only given, as follows. When magnetic field lines are sufficiently stretched during the substorm growth phase the earthward E/spl times/B ion drift velocity can become comparable to the gyration velocity. Under these conditions inertial currents can become quite important. Using the Rostoker-Skone magnetic field model and a cross tail electric field the authors find that cold (<=200 eV) O/sup +/ ions injected from the ionosphere into the equatorial plane at high latitudes will drift eastward at radial distances less than -10 R/sub E/, due to inertial effects and westward at distanced closer to the Earth due to magnetic drift. The inertial eastward drift gives rise to a current which in terms of J/spl times/B is consistent with the convective deceleration of the earthward drift velocity due to higher values of B. Similarly momentum balance also requires that the convective acceleration of the westward drift velocity consistent with a tailward inertial current. Therefore, an equatorial current wedge system with an eastward and tailward current naturally arises from the ion dynamics and a simple, yet realistic magnetic field model. More complicated current systems can be modelled by adding a radial electric field structure. For example, a weak electric field gradient leads to an earthward inertial current that may dominate the tailward current just discussed. These current structures will be examined for different cross tail electric field values and magnetic field configurations in order to hopefully shed some light on the substorm onset process. Small scale structures related to the phase effects of the oxygen ions will also be discussed.