  Thermal quench of a nearly collisionless plasma against a cooling boundary or
region is an undesirable off-normal event in magnetic fusion experiments, but
an ubiquitous process of cosmological importance in astrophysical plasmas.
There is a well-known mismatch that what experimentally diagnosed is the drop
in perpendicular electron temperature $T_{e\perp},$ but the parallel transport
theory of ambipolar-constrained tail electron loss produces parallel electron
temperature $T_{e\parallel}$ cooling. Here two collisionless mechanisms,
dilutional cooling by infalling cold electrons and wave-particle interaction by
two families of whistler instabilities, are shown to enable fast $T_{e\perp}$
cooling that closely tracks the mostly collisionless crash of $T_{e\parallel}.$
