Abstract When multiple electron temperature diagnostics converge on the same value, the standard inference is that the measurement is robust. We show that this convergence is a structural consequence of the shared ionization bottleneck in any plasma where the electron Knudsen number exceeds ∼ 0. 01 0. 01: all diagnostics downstream of collisional ionization report the effective temperature T eff of the electron velocity distribution, not the core temperature T core. Their agreement is a single measurement reported N times. We introduce a diagnostic taxonomy classifying methods as ionization-gated (Type A, measures T eff), bulk-sampling (Type B, measures T core), or distribution-resolving (Type C). The ratio R = T A / T B yields κ = 3 R /2 (R − 1) directly. We apply the framework to the solar corona (R = 2. 4, κ ≈ 2. 5) and validate it quantitatively in the tokamak scrape-off layer, where single kappa distributions (κ ≈ 2–10) reproduce published bi-Maxwellian EEDF decompositions to 3–8 % RMS with one fewer free parameter and Thomson scattering independently confirms the predicted Type B temperature. We test the framework’s boundary of applicability in planetary nebulae (the 80-year CEL–ORL abundance discrepancy). Knudsen number calculations, including the Shoub v 4 mean-free-path scaling for tail electrons, show that the ionizing population is collisionless in the corona even when the bulk is fluid; in planetary nebulae, both the ionizing electrons (∼55 eV) and the lower-energy excitation electrons (∼5 eV) that drive the CEL diagnostic are collisional over nebular scales, identifying PNe as the falsification boundary of the framework; in the tokamak SOL, non-local parallel transport maintains suprathermal tails even where local collisionality is high. For plasmas with κ ≈ 3–5, the raw Spitzer–Härm formula with spectroscopic T e as input overestimates parallel heat flux by factors of 3–25×; flux-limited transport models inherit the temperature bias through their boundary conditions, with direct relevance to ITER divertor predictions. Every diagnostic campaign on a weakly collisional plasma should include at least one Type B measurement.
Victor Edmonds (2026) studied this question.