We present a novel model in which field lines shortening after localized , three-dimensional reconnection heat the plasma as they compress it .
The shortening progresses away from the reconnection site at the Alfvén speed , releasing magnetic energy and generating parallel , compressive flows .
These flows , which are highly supersonic when \beta \ll 1 , collide in a pair of strong gas-dynamic shocks at which both the mass density and temperature are raised .
Reconnecting field lines initially differing by more that 100 ^ { \circ } can produce a concentrated knot of plasma hotter that 20 MK at the loop ’ s apex , consistent with observations .
In spite of these high temperatures , the shocks convert less than 10 \% of the liberated magnetic energy into heat — the rest remains as kinetic energy of bulk motion .
These gas-dynamic shocks arise only when the reconnection is impulsive and localized in all three dimensions ; they are distinct from the slow magnetosonic shocks of the Petschek steady-state reconnection model .