Evidence of Effective Shielding in the Inner Magnetosphere?

Within the inner magnetosphere, the large scale, solar wind imposed electric field can be significantly modified by internal magnetospheric coupling processes. One of these processes is shielding which acts to reduce the strength of the convection field at low latitudes. The Region-2 field aligned currents, driven by pressure gradients in the ring current/plasma sheet region, can close via a horizontal current across the ionosphere. This closure current in turn generates an electric field that opposes the main convection field. Observations and theoretical models have shown that shielding takes a finite time to be established (on the order of a half hour) and changes in the imposed electric field or other magnetospheric parameters can cause it be ineffective for extended periods. We present a seemingly rare observation from 10 Jul 2001 when it appears that the inner magnetosphere was effectively shielded from the solar wind imposed convection field for an extended period. IMAGE EUV observations show that despite a prolonged period (>12 hours) of southward IMF, a plasmaspheric plume did not form. DMSP field-aligned current observations support the shielding mechanism and suggest that a substorm at ~19UT disrupted the shielding layer and allowed the convection field to penetrate the inner magnetosphere.