Populations of trapped particles in Earth's magnetic field are intimately l
inked to prominent magnetospheric phenomena, for example, to the radiation
belts and the ring current. Particles are forced to bounce between both hem
ispheres because field lines converge from the magnetic equatorial region t
oward the poles owing to the dominating dipolar component. The scenario may
change essentially if the so-called paleomagnetosphere is considered, that
is, the terrestrial magnetosphere during apolarity reversal period. This r
eport deals with particle trapping in general field configurations and conc
entrates on the quadrupole case as a potential scenario for paleomagnetosph
eric studies. By using the tensor representation of the quadrupole field, w
e show that its topology is controlled by a single "shape" parameter which
is a measure for the relative strength of quadrupole tensor components, or
equivalently, coefficients in the spherical harmonics expansion of the pote
ntial. In order to locate centers of particle bounce motion the concept of
a magnetic equatorial surface is extended to a more general type of "trappi
ng center surface" by means of geometrical criteria such as B . del B = 0.
The quadrupole case yields two such surfaces. As does the field line topolo
gy, those surfaces and the resulting drift orbits vary strongly with the qu
adrupole shape parameter.