A boundary layer model for magnetospheric substorms

Magnetospheric substorm activity is presently understood as the combined effect of energy transport and dissipation which is directly driven through the solar‐terrestrial interaction and of the unloading of energy stored in the magnetotail. Many studies of particle and field signatures of substorms in the magnetotail have led to the proposal that a neutral line forms close to the Earth at the time of expansive phase onset with the plasma sheet population on the anti‐Earthward side of the neutral line ultimately being lost to the solar wind through the ejection of the plasmoid. While the near‐Earth neutral line model has provided a useful working model for substorm phenomenology, it is not a unique description for understanding the observations. The purpose of this paper is to present an alternative framework in which substorm effects in the magnetosphere can be understood. We contend that observations of magnetic field and plasma flow variations in the magnetotail can be explained in terms of the passage of the plasma sheet boundary layer over the satellite detecting the tail signatures. We shall show, by providing a plausible mapping scheme from the high‐latitude ionosphere to the magnetotail, that field‐aligned currents and particle acceleration processes on magnetic field lines threading the ionospheric Harang discontinuity lead to the distinctive particle and field signatures observed in the magnetotail during substorms. In particular, we shall demonstrate that edge effects of field‐aligned currents associated with the westward traveling surge can lead to the negative B z perturbations observed in the tail that are presently attributed to observations made on the anti‐Earthward side of a near‐Earth neutral line. Finally, we shall demonstrate that our model is capable of providing a physical explanation of both the driven system and the loading‐unloading system whose combined effects provide the observed substorm perturbation pattern in the magnetosphere and ionosphere. In this context, a substorm expansive phase can be interpreted as the development of a Kelvin‐Helmholtz instability on magnetic field lines which thread the velocity shear zone separating the central plasma sheet and the plasma sheet boundary layer/low‐latitude boundary layer regions.