Modeling the relationship between substorm dipolarization and dispersionless injection

[1] Significant populations of electrons with energies of tens of keV appear in the Earth's inner central plasma sheet during the substorm expansion phase. Increasing observational evidence indicates that these injections begin at a radially narrow but azimuthally extended transition between very stretched and less stretched field lines around 8 RE. In this work we suggest that the tailward retreat of this transition region can be responsible for adiabatic acceleration of electrons which is sufficient to cause ionospheric signatures of the dispersionless injections observed by riometers. To support this mechanism we develop a novel conceptual magnetotail magnetic field model with a few adjustable parameters which can be easily constrained by observations. Our calculations show that a tailward motion of the transition region at the speed of 0.8 RE/min is required to achieve good agreement with riometer observations.