Is Substorm Onset Seeded by Cross-Tail Current Enhancement Resulting from Parallel Energization of Oxygen Ion Polar Cap Outflow?

Abstract:

The parallel energization of polar cap outflow (PCO) has been shown to be highly effective along newly-created lobe lines where the geometry is such that the Coulomb force due to the dawn-to-dusk electric field is roughly parallel to the curvature drift of the ions. This “Coulomb-curvature” interaction can produce a parallel energization rate that raises the parallel energy of the ions to about 5-6 keV before they reach the Neutral Sheet (NSh) at about 11 earth radii downtail. Then, in the outer NSh, they produce a strong westward curvature current. When they enter the inner neutral sheet (INSh) where they become “unmagnetized”, they are accelerated westward by the dawn-dusk electric field. This causes their perpendicular energy and their pitch angle to increase such that the ions essentially become trapped in the inner NSh while continuing to accelerate westward. This additional westward current in the inner NSh accompanies the westward curvature current produced in the two outer NSh layers. This total ion current is supplemented by the westward curvature current caused by the eastward curvature drift of the electrons. The combined total ion and electron current is sufficient to severely decrease the magnetic field near the earthward end of the NSh. The magnetic pressure decrease is compensated by the particle pressure increase due to inflow of oxygen ions from the northern and southern polar caps. The conditions near the earthward edge of the NSh and in the adjoining plasmasheet regions neighbouring the NSh become ideal for reconnection and the dipolarization that follows.