  Thermal non-equilibrium (TNE) is a fascinating situation that occurs in
coronal magnetic flux tubes (loops) for which no solution to the steady-state
fluid equations exists. The plasma is constantly evolving even though the
heating that produces the hot temperatures does not. This is a promising
explanation for isolated phenomena such as prominences, coronal rain, and
long-period pulsating loops, but it may also have much broader relevance. As
known for some time, TNE requires that the heating be both (quasi) steady and
concentrated at low coronal altitudes. Recent studies indicate that asymmetries
are also important, with large enough asymmetries in the heating and/or
cross-sectional area resulting in steady flow rather than TNE. Using reasonable
approximations, we have derived two formulae for quantifying the conditions
necessary for TNE. As a rough rule of thumb, the ratio of apex to footpoint
heating rates must be less than about 0.1, and asymmetries must be less than
about a factor of 3. The precise values are case dependent. We have tested our
formulae with 1D hydrodynamic loop simulations and find a very acceptable
agreement. These results are important for developing physical insight about
TNE and assessing how widespread it may be on the Sun.
