A mechanism for prompt trapping of solar energetic particles (SEPs) in the inner magnetosphere during severe storms is numerically demonstrated. The mechanism can be understood in terms of a generalization of St¿rmer theory to a time dependently perturbed dipole magnetic field. Test particle Lorentz trajectories are computed in fields from a Lyon-Feder-Mobarry (LFM) global MHD magnetospheric model simulation of the 24 Nov 2001 storm to show energetic particles may be impulsively injected into the magnetosphere and trapped on a time scale of minutes by a sudden compression and de-compression of the magnetosphere. It is also shown that SEPs with access to the innermost L-shells enter through the magnetopause on the day side.