  A robust method to handle vacuum and near vacuum regions in hybrid
simulations for space and astrophysical plasmas is presented. The conventional
hybrid simulation model dealing with kinetic ions and a massless
charge-neutralizing electron fluid is known to be susceptible to numerical
instability due to divergence of the whistler-mode wave dispersion, as well as
division-by-density operation in regions of low density. Consequently, a pure
vacuum region is not allowed to exist in the simulation domain unless some ad
hoc technique is used. To resolve this difficulty, an alternative way to
introduce finite electron inertia effect is proposed. Contrary to the
conventional method, the proposed one introduces a correction to the electric
field rather than the magnetic field. It is shown that the generalized Ohm's
law correctly reduces to Laplace's equation in a vacuum which therefore does
not involve any numerical problems. In addition, a variable ion-to-electron
mass ratio is introduced to reduce the phase velocity of high frequency
whistler waves at low density regions so that the stability condition is always
satisfied. It is demonstrated that the proposed model is able to handle near
vacuum regions generated as a result of nonlinear self-consistent development
of the system, as well as pure vacuum regions set up at the initial condition,
without losing the advantages of the standard hybrid code.
