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We study the interplay among cooling, heating, conduction and magnetic fields in gravitationally stratified plasmas using simplified, plane‐parallel numerical simulations. Since the physical heating mechanism remains uncertain in massive haloes such as groups or clusters, we adopt a simple, phenomenological prescription which enforces global thermal equilibrium and prevents a cooling flow. The plasma remains susceptible to  local  thermal instability, however, and cooling drives an inward flow of material. For physically plausible heating mechanisms in clusters, the thermal stability of the plasma is independent of its convective stability. We find that the ratio of the cooling time‐scale to the dynamical time‐scale  t  cool / t  ff  controls the non‐linear evolution and saturation of the thermal instability: when  t  cool / t  ff ≲ 1, the plasma develops extended multiphase structure, whereas when  t  cool / t  ff ≳ 1 it does not. (In a companion paper, we show that the criterion for thermal instability in a more realistic, spherical potential is somewhat less stringent,  t  cool / t  ff ≲ 10.) When thermal conduction is anisotropic with respect to the magnetic field, the criterion for multiphase gas is essentially independent of the thermal conductivity of the plasma. Our criterion for local thermal instability to produce multiphase structure is an extension of the cold versus hot accretion modes in galaxy formation that applies at all radii in hot haloes, not just to the virial shock. We show that this criterion is consistent with data on multiphase gas in galaxy groups and clusters; in addition, when  t  cool / t  ff ≳ 1, the net cooling rate to low temperatures and the mass flux to small radii are suppressed enough relative to models without heating to be qualitatively consistent with star formation rates and X‐ray line emission in groups and clusters.

Thermal instability in gravitationally stratified plasmas: implications for multiphase structure in clusters and galaxy haloes