It has been suggested that much of the drop and subsequent recovery of
storm time relativistic electron fluxes at geosynchronous orbit can b
e explained in terms of a fully adiabatic response (all three adiabati
c invariants conserved) to magnetic field changes. To calculate this e
ffect, we assume a prestorm electron flux distribution constructed fro
m CRRES satellite data, we use modular magnetospheric magnetic field m
odels to represent the magnetic field configuration before and during
the storm, and we use Liouville's theorem to evolve the prestorm elect
ron flux. In this work we focus on the important special case of equat
orially mirroring electrons. During the main phase of a storm with a D
st minimum of -100 nT we find that the fully adiabatic effect can caus
e a flux decrease of up to 2 orders of magnitude, consistent with obse
rved flux decreases. We also find that the magnitude of the fully adia
batic flux decrease is larger for lower energies, again in agreement w
ith observations. The contribution of prestorm electron fluxes to the
recovery phase flux increase at synchronous orbit is expected to be sm
all because of losses to the dawnside magnetopause. A comparison of fu
lly adiabatic fluxes with measured electron fluxes for the November 2-
5, 1993, storm indicates that for this event the fully adiabatic effec
t may be contributing to the observed decrease but that nonadiabatic e
ffects are clearly important. Overall we conclude that the fully adiab
atic effect can account for a significant fraction of observed flux de
creases and that differences between the observed and the fully adiaba
tic fluxes help to clarify when and where additional loss and source m
echanisms exist.