The contribution of the Joule-Thomson effect to solar coronal heating

Abstract

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.