PARTICLE MOTION IN X-DEPENDENT HARRIS-LIKE MAGNETOTAIL MODELS

The dynamics of charged particles in x-dependent magnetotail models is examined, where x is along the Sun-Earth direction. An earlier paper showed that particle motion in a class of x-dependent Harris-like equilibrium models can be significantly different from the motion in x-independent magnetotail field models. In the present paper, it is shown that these Harris-like equilibrium models have ''bulb-shaped'' field lines and that this property leads to the reported differences. Furthermore, it is shown that the scale length of variation of the Harris-like models in the x direction, L(x), is comparable to the typical excursion distance, DELTA, of particles in the x direction and to rho(n) which is the gyroradius based on the magnetic field component B(n) normal to the plane of the current sheet. It is argued that neither bulb-shaped field lines nor scale lengths L(x) comparable to DELTA or rho(n) apply to the Earth's magnetotail. It is suggested that a key criterion for applicability of x-dependent models to the terrestrial magnetotail is DELTA, rho(n) much less than L(x).