“…To better understand the Van Allen radiation belt discovered in the mid-20th century, there have been rea) Also at Phillips Academy Andover, 180 Main Street, Andover, MA, 01810, USA b) Electronic mail: liusm@pmo.ac.cn newed interests in solving the Störmer's problem. In particular, periodic orbits in the equatorial plane have been explored extensively 7 , and there are also a few branches of periodic orbits in the Meridian plane at relatively high energies with distinct orbital shapes [3][4][5] . At low energies, due to the presence of approximate adiabatic invariant, the problem can be further simplified 13,14 , and the results, the so-called guiding center approximation, have been applied to studies of pulsars, radiation belts, and dynamics of particles in magnetic confinement devices 12,15,16 .…”