= Effective collision strength
The <effective collision strength> is the dimensionless thermal average of an electron-impact <collision strength>. For a <Maxwellian> electron population, with collision energy $E$ measured above the upper-level threshold,
$$
\Upsilon_{ij}(T)=\int_0^\infty\Omega_{ij}(E)e^{-E/(k_BT)}\,d(E/(k_BT)).
$$
It is a function of temperature, rather than a cross section evaluated at one representative energy.
If $\Omega$ is constant, the normalized exponential weight gives $\Upsilon=\Omega$. Near-threshold resonances can instead strongly change the average, particularly at low temperature. Atomic excitation and de-excitation rate coefficients use this average together with statistical weights and, for excitation, the appropriate Boltzmann threshold factor.<Fine-structure collision strengths and line ratios for Ne V in infrared and optical sources by Michael Dance and others>[Section 2.3, equations (3)–(4)]{ref}
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