A heuristic model of an energetic plasma expanding into an initially uniform magnetic field is presented. In this model the plasma is cylindrically shaped, perfectly diamagnetic, and allowed to expand freely in the direction of the magnetic field. It results in a time evolution of the cylindrical radius described by Airy functions and parameterized by the plasma &bgr;. For instance, the maximum extent of the radius scales as &bgr;1/3 while the maximum radial deceleration scales as &bgr;-1/3. Time evolution and scaling are compared both with other models and with two-dimensional electromagnetic particle simulations. A spherically symmetric model is also found to agree well with the simulations and results in simpler expressions for the maximum radius and maximum radial deceleration. The larger deceleration obtained in these models, as compared with models that ignore the axial expansion, would shift toward longer wavelengths the peak growth of the instability thought to be responsible for the field-aligned structures seen in the Active Magnetospheric Particle Tracer Explorers (AMPTE) magnetotail releases. ¿ American Geophysical Union 1989