OBSERVATIONAL SUPPORT FOR THE CURRENT SHEET CATASTROPHE MODEL OF SUBSTORM CURRENT DISRUPTION

It has recently been found that a one-dimensional current sheet equilibrium with a non-zero convection electric field, E(y), and a non-zero normal magnetic field component, B(n), can reach a point of catastrophe through either the reduction of the drift velocity, v(D) = cE(y)/B(n) or the increase of B(n). This point of catastrophe coincides with a value of kappa(A) congruent-to 0.7, where kappa(A) is the self-consistent value of kappa = (R(min)/rho(A max))1/2 corresponding to ions of average energy. (Here R(min) is the minimum field-line radius of curvature and rho(A max) is the maximum gyroradius for ions of average energy.) The point of catastrophe was found to be preceded by a twisting of the current sheet field-lines into the dawnward direction, i.e. by the development of a y-component of B, with odd symmetry in z, and a sign opposite B(x), where positive x is earthward, positive y is in the dawn-to-dusk direction and positive z is northward. Since the loss of the current sheet would cause the local configuration to become more dipolar, it was suggested that the catastrophic loss of the local current sheet equilibrium could correspond to local current disruption and dipolarization. In this paper, observations of some of the signatures predicted by theory are presented.