Empirical modeling of plasma pressure and magnetic field for the quiet
time nightside magnetosphere is investigated. Two models are construc
ted for this study. One model, referred to here as T89R, is basically
the magnetic field model of Tsyganenko (1989) but is modified by the a
ddition of an inner eastward ring current at a radial distance of simi
lar to 3 R(E) as suggested by observation. The other is a combination
of the T89R model and the long version of the magnetic field model of
Tsyganenko (1987) such that the former dominates the magnetic field in
the inner magnetosphere, whereas the latter prevails in the distant t
ail. The distribution of plasma pressure, which is required to balance
the magnetic force for each of these two field models, is computed al
ong the tail axis in the midnight meridian. The occurrence of pressure
anisotropy in the inner magnetospheric region is also taken into acco
unt by determining an empirical fit to the observed plasma pressure an
isotropy. This effort is the first attempt to obtain the plasma pressu
re distribution in-force equilibrium with magnetic stresses from an em
pirical field model with the inclusion of pressure anisotropy. The inc
lusion of pressure anisotropy alters the plasma pressure by as much as
a factor of similar to 3 in the inner magnetosphere. The deduced plas
ma pressure profile along the tail axis is found to be in good agreeme
nt with the observed quiet time plasma pressure for geocentric distanc
es between similar to 2 and similar to 35 R(E).