A compressible intensification mechanism for a thin, nearly vertical flux tube in a superadiabatic atmosphere. An unstable downflow drains material and lowers internal gas pressure; approximate lateral balance then permits the tube to contract and its field to strengthen at nearly conserved magnetic flux. It relies on compressibility and thermal evolution, which are absent from a strict Boussinesq approximation. Dynamic photospheric structures need not satisfy thin-tube equilibrium, so this mechanism does not explain every intense field concentration.
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