Direct influence of ring current on auroral oval diameter

Abstract

The inside diameter of the auroral oval (conceived as marking the boundary between closed and polar cap field lines) is known to vary with storm intensity, as measured by the Dst index. Part of this variability may result from storm-associated changes in solar wind pressure and/or from storm-associated changes in location of the nightside neutral line (inner edge of neutral sheet). The magnetospheric B field is also directly distorted (stretched outward) by the stormtime ring current so as to increase the amount of polar cap magnetic flux. The magnitude of this last effect on auroral oval diameter is estimated from a simple model in the present work for storms of various intensities. The prototype for an empty magnetosphere in this effort is Dungey's model (dipole plus uniform southward Bz ≈ -9.0 cos6 ΛμT, with Λbeing the invariant latitude of the quiettime auroral oval). Addition of a ring current field ΔBz(r0) with Dst < 0 moves the circular neutral line outward (to a larger radial distance r0 in the equatorial plane) but simultaneously increases the amount of polar cap magnetic flux. The consequent increase in auroral oval diameter can be calculated (in closed form for sufficiently simple ΔBz models) by magnetic flux bookkeeping. In the present model, this increase (≡ -2 ΔΛ) is typically about 4° ± 1° for each -100 nT of Dst. Copyright 1997 by the American Geophysical Union.