Thermal quench of a nearly collisionless plasma against an isolated cooling boundary or region is an undesirable off-normal event in magnetic fusion experiments, but an ubiquitous process of cosmological importance in astrophysical plasmas. Parallel transport theory of ambipolar-constrained tail electron loss is known to predict rapid cooling of the parallel electron temperature \(T_{e\parallel },\) although \(T_{e\parallel }\) is difficult to diagnose in actual experiments. Instead direct experimental measurements can readily track the perpendicular electron temperature \(T_{e\perp }\) via electron cyclotron emission. The physics underlying the observed fast drop in \(T_{e\perp }\) requires a resolution. 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 \(T_{e\parallel }.\) These findings motivate both experimental validation and reexamination of a broad class of plasma cooling problems in laboratory, space, and astrophysical settings.
