Pulsed particle injection in a reconnection-driven dynamic trap model in solar flares

The time structure of hard X-ray emission during solar flares shows subsecond pulses, which have an energy-dependent timing that is consistent with electron time-of-flight delays. The inferred time-of-flight distances imply an injection height about 50% above the soft X-ray-bright flare loops, where electrons are injected in a synchronized way. No physical injection mechanism is known that can account for the energy synchronization and duration of subsecond pulses. Here we propose a model in terms of dynamic loss cone angle evolution of newly reconnected magnetic field lines that relax from the cusp at the reconnection point into a force-free configuration, which can explain the subsecond pulse structure of injected particles as well as the observed correlation between the pulse duration and flare loop size. This quantitative model predicts that the pulse duration of hard X-ray or radio pulses scales with t(w) approximate to 2L(B)/v(A) (s), which is the Alfvenic transit time through the magnetic outflow region of a Petschek-type X-point with magnetic length scale L-B, and thus provides a direct diagnostic of the magnetic reconnection geometry. It also demonstrates that the observed pulse durations are primarily controlled by the injection time rather than by the acceleration timescale.