  Two of the three gases that display isenthalpic Joule-Thomson (J-T) warming
under laboratory conditions are hydrogen and helium, the main constituents of
the solar plasma, but the temperatures that are attained by this route are at
most a few hundred K. Increases in ion temperature by several orders of
magnitude are claimed for hydrogen plasmas subject to expansion into a vacuum;
modest increases are reported for the shortlived tests of this effect that have
been carried out in space in the wakes of artificial satellites and of the
Moon. Attempts to calculate the J-T coefficient at very high temperatures using
equations of state and thermodynamics remain very preliminary. The potential
contribution of plasma expansion to heating of the solar corona must therefore
be assessed empirically, but this is consistent with how the J-T effect was
first identified. The sunspot record, EUV measurements by the EVE instrument on
the SDO satellite, and solar wind fluctuations documented by the ACE satellite
indicate broadly coherent periodicity from the photosphere to the outer corona
consistent with a non-pulsatory heating process. It comprises three successive
stages characterised by induction, the J-T mechanism, and plasma expansion.
Astronomical data may therefore be used to derive rather than to test an
extension of the J-T effect which could help to explain heating in other solar
system bodies and other stellar coronae.
