  Super-hot looptop sources, detected in some large solar flares, are compact
sources of HXR emission with spectra matching thermal electron populations
exceeding 30 megakelvins. High observed emission measure, as well as inference
of electron thermalization within the small source region, both provide
evidence of high densities at the looptop; typically more than an order of
magnitude above ambient. Where some investigators have suggested such density
enhancement results from a rapid enhancement in the magnetic field strength, we
propose an alternative model, based on Petschek reconnection, whereby looptop
plasma is heated and compressed by slow magnetosonic shocks generated
self-consistently through flux retraction following reconnection. Under steady
conditions such shocks can enhance density by no more than a factor of four.
These steady shock relations (Rankine-Hugoniot relations) turn out to be
inapplicable to Petschek's model owing to transient effects of thermal
conduction. The actual density enhancement can in fact exceed a factor of ten
over the entire reconnection outflow. An ensemble of flux tubes retracting
following reconnection at an ensemble of distinct sites will have a collective
emission measure proportional to the rate of flux tube production. This rate,
distinct from the local reconnection rate within a single tube, can be measured
separately through flare ribbon motion. Typical flux transfer rates and loop
parameters yield emission measures comparable to those observed in super-hot
sources.
