Activity and interest in these areas increased during the past four years
in part because of the growing interest in future imaging of
both the plasmasphere(using
304 Å
solar emission resonantly-scattered from plasmaspheric He
) and
the ring current(using detection of charge-exchange-produced
energetic neutral atoms),
and also because appropriate
models were finally developed for treating such
long-outstanding problems as early-stage plasmasphere
refilling following storms, the transport and decay of the ring current
and the various types of interactions between the plasmasphere and ring
current.
Carpenter and colleagues used both ground-based radio wave detection and
spacecraft measurements to investigate the plasmasphere's response
to storms and inner-middle magnetosphere density distributions.
Carpenter et al. [[1993]] concluded that during
storm periods, the plasmasphere
may be thought of as composed of a main plasmasphere together
with a dusk bulge
region which consists of outward extending plasma regions resulted
from erosion of the main plasmasphere.
Carpenter and Anderson [[1992]] developed
obtained relatively simple empirical formulae
describing the equatorial density distributions for each of
the ``saturated'' inner plasmasphere, the steep density gradient
plasmapause regions, and the outer plasma trough.
In related theoretical modeling,
Khazanov et al. [[1994]] considered analytically the effects
of convection on the inner magnetosphere(such as plasmasphere) distributions
of density and other parameters, and suggested that the relation of
the density n
with the magnetic field B might go as 
, where the
parameter
lies between 4/3 and 2. The former value would correpond to
the case where parallel transport dominates convective motions, whereas
the 
case corresponds to convective-dominated transport.
How the depleted outer part of the plasmasphere evolves following
magnetic storms received greatly increased attention during this
period, and was extensively reviewed by Singh and
Horwitz [[1992]] in a special issue of
the Journal of Geophysical Research.
This volume contained the first semikinetic modeling
efforts on early-stage refilling, in which
Wilson et al. [[1992]]
emphasized the effects of Coulomb collisions on the plasma evolution,
whereas Lin et al. [[1992a]] examined the effects of perpendicular
ion heating around the equator on the large-scale refilling process.
More recently, Lin et al. [[1994]]
have allowed for unequal inflows and demonstated the effect of
hemispheric decoupling of the northern and southern hemispheres
of the flux tube as caused by equatorial perpendicular ion heating creating
a positive potential peak about the magnetic equator. The predicted
hemispherical assymmetries
in latitudinal density distributions may have been observed by
Olsen [[1992]]. Miller et al. [[1993]] also developed
a semikinetic model for plasmasphere refilling and included the effects
of inward flux tube convection and magnetospheric particle injection,
and noted the significant
parallel acceleration of the outflowing
ionospheric H
onto these closed field lines during inward convection.
Singh [{[1991]] examined the
role of temperature anisotropy in plasmasphere refilling
using a two-stream
model. The inclusion of temperature anisotropy led to significantly longer
refilling times because
the resulting anistropies involve a downward force
which inhibits the ionospheric
inflow.
Singh [[1993]] also performed a particle-in-cell simulation of
field-aligned plasma flow onto an artificially short flux tube
with equatorially-trapped hot plasmas, finding
that the equatorially-trapped
ions set up an electrostatic potential barrier for the incoming
cold ion beams and cause them to be reflected back toward their
injection location, which would be the ionosphere in the realistic
case. Gentle ionosphere-plasmasphere coupling and flows were examined by
Guiter et al. [[1991]], who treated diurnal variations
on a plasmaspheric flux tube with a multispecies hydrodynamic model.
The unique ``L-shell-skimming'' properties of the Dynamics Explorer-1
orbit(when apogee was near the magnetic equator, the spacecraft
orbit was often nearly aligned with the L=4.6 shell) were again used by
Olsen et al. [[1994]] to examine the distribution
functions of trapped H
ions along the L=4.6 shell from Dynamics
Explorer-1 data.
The changing biMaxwellian parameters measured along the magnetic field
line showed reasonable agreement with a simple mapping procedure
based on Liouville's theorem.
Miller and Khazanov [[1993]] also obtained the
self-consistent electrostatic potential distribution along
magnetic field lines laden with trapped plasmas.
Though most plasmasphere-ionosphere modeling and measurements continued to focus on ion behavior, new models for the treatment of superthermal electron(e.g., ionospheric photo-electrons) transport were developed by Khazanov and colleagues. Khazanov et al. [[1992]] developed analytic solutions for describing the kinetic aspects of superthermal electron transport through the plasmasphere, while Khazanov et al. [[1993]] developed a new time-dependent transport description for superthermal electrons, such as photo-electrons, traversing the plasmasphere. They noted that when the inner plasmasphere is highly depleted in density level, the time scales for superthermal electrons of energies less than 30 eV to reach equilibrium conditions can be several hours long.
Progress on ring current-plasmasphere interaction effects was also seen
in the areas of ring current decay
and Sub-Auroral-Red(SAR)-arc production.
Representative of the Michigan group's efforts were the
model calculations by Kozyra et al. [[1993]]
and Fok et al. [[1993]],
who provided further support that SAR arcs can be powered by Coulomb
energy transfer from ring current O
to plasmaspheric electrons.
On the other hand, Erlandson et al. [[1993]] report
a close relationship
between the intensity of wave magnetic field fluctuations associated
with electromagnetic ion cyclotron waves and electron
temperature enhancements along SAR arc field lines, suggesting that
wave-particle processes
are significant at times creating SAR arcs.
Fok et al. [[1993]]
also found that ring current-plasmasphere collisions create an energy-degraded
population of low-energy(
500 eV) ions within the plasmasphere.
Interesting findings bearing on middle-magnetosphere ion energization
and transport were presented by Liu et al. [[1994]] and
Li et al. [[1993]]. Liu et al. [[1994]] used Dynamics
Explorer-1 ion measurements to show how magnetic field line
dipolarization events lead
to centrifugal acceleration of ionospheric ions out toward the middle
magnetosphere.
Li et al. [[1993]] modeled the loss of ring current O
ions in
their interaction with micropulsation waves.
These authors showed that such waves can bounce-drift resonate
with energetic O
ions, so as to allow
these ions to be on open convection trajectories which intersect the dayside
magnetopause. This mechanism may help explain the rapid
decay of O
ring current flux after magnetic storms.