Orbits of charged particles with an azimuthal initial velocity in a dipole magnetic field.

Nonintegrable dynamical systems have complex structures in their phase space. Due to the axisymmetry of the system, motion of a test charged particle in a dipole magnetic field can be reduced to a two-degree-of-freedom (2-DoF) nonintegrable Hamiltonian system (Störmer, Radium (Paris) 4(1):2–5, 1907; Dragt, Rev Geophys, https://doi.org/10.1029/RG003i002p00255, 1965). We carried out a systematic study of orbits of charged particles with an azimuthal initial velocity in a dipole field via calculation of their Lyapunov characteristic exponents (LCEs) and escape times for a dimensionless energy, respectively, less and greater than 1/32, above which most particles will escape from the magnetic dipole to infinity. Meridian plane periodic orbits symmetric with respect to the equatorial plane of the magnetic dipole are then identified. We found that (1) symmetric periodic orbits can be classified into several classes based on their number of crossing points on the equatorial plane; (2) the initial conditions of these classes locate on closed loops or closed curves going through the origin; (3) most isolated regions of stable quasi-periodic orbits are associated asymmetric stable periodic orbits; (4) classes of asymmetric periodic orbits either go through the origin or terminate at flat equatorial plane orbits with the other end approaching centers of spiral structures; (5) there are apparent self-similarities in the above features with the decrease of energy. These results can be used to guide the search for stable orbits that may have applications in broad physical contexts. [ABSTRACT FROM AUTHOR]