Distortion of the nightside boundary of the "firmly-closed" region in the 1996 Tsyganenko magnetic field model

This paper proposes that the outer boundary of the "firmly-closed" region should be represented by field lines with the adiabaticity parameter, K, equal to unity at the equator, where K^2 is the ratio between the radius of the field-line curvature and the Larmor radius of an ion with 1 keV of energy. Just outside the boundary where K = 1, plasma particles (primarily ions) can be nonadiabatically accelerated in the presence of the dawn-to-dusk electric field. An inwardly convecting flux tube will attain the maximum content of nonadiabatically accelerated particles when it passes the K - 1 boundary. Thus, the K = 1 boundary outlines the region of the plasma population with a maximum content of nonadiabatically accelerated particles. In addition, the field lines with K = 1 are shown to have a minimum field strength of roughly 1 nT at the equator. From this fact, a field line with k < 1 may not be considered as being "firmly-closed" in the sense that such a field line may easily merge with an interplanetary field line. The outer boundary of the nightside firmly-closed region in the Tsyganenko model has an IMF Bz dependence that is consistent with observations. Moreover, this boundary is found to be "distorted", favoring the generation of region 1 field-aligned currents