The different orbital modes that appear from the misalignment of the laser
beams in the xy plane of a low-density magneto-optical trap are studied. Us
ing the circular motion approximation, one can obtain a qualitative descrip
tion of the phenomenon and can calculate the ring radii, the atom's velocit
y in the ring, and the trap parameters in which the rings appear. A careful
analysis of some theories previously employed in researching this topic sh
ows that they have extrapolation problems that led to unreasonable conclusi
ons. In particular, we compare the results of the circular motion approxima
tion itself with those obtained from a direct numerical integration of the
atomic trajectory. Within this approximation the influence of the increase
in the number of trapped atoms over the ring is also investigated, and we o
bserve an instability in a ring-shaped structure when the interaction betwe
en the atoms is included. (C) 2000 Optical Society of America [S0740-3224(0
0)01004-3]. OCIS codes: 020.0020, 140.0140, 020.7010, 140.7010.