The Long-standing Closure Crisis in Coronal Plasmas

Coronal and solar wind physics have long used plasma fluid models to motivate physical explanations of observations; the hypothesized model is introduced into a fluid simulation to see if observations are reproduced. This procedure is called Verification of Mechanism (VoM) modeling; it is contingent on the self consistency of the closure that made the simulation possible. Inner corona VoMs typically assume weak gradient Spitzer?Braginskii closures. Four prominent coronal VoMs in place for decades are shown to contradict their closure hypotheses, demonstrably shaping coronal and solar wind research. These findings have been possible since 1953. This unchallenged evolution is worth understanding, so that similarly flawed VoMs do not continue to mislead new research. As a first step in this direction, this paper organizes four a posteriori quantitative tests for the purpose of easily screening the physical integrity of a proposed VoM. A fifth screen involving the thermal force, the tandem of the heat flux, has been shown to be mandatory when VoMs involve species-specific energy equations. VoM modeling will soon be required to advance Parker Solar Probe and Solar Orbiter science. Such modeling cannot advance the physical understanding sought by these missions unless the closures adopted (i) are demonstrated to be self consistent for the VoM plasma Knudsen numbers, (ii) are verified a posteriori as possessing nonnegative VDFs throughout the simulated volume, and (iii) include the physical completeness of thermal force physics when the VoM requires species-specific energy equations.