Third-sound propagation in thick films of superfluid 4He

Atkin's theory of third sound in thick films of superfluid helium-4 has been extended to include a heat source for excitation. Calculations based on the resulting equations reveal the physical structure of third sound, and it is shown that this structure is not consistent with the standard description, which is based largely on qualitative analysis. Calculations also predict the existence of solitary waves, a new type of third-sound signal excited by very short heat pulses. They have not yet been observed experimentally. Additional calculations show in detail how such waves are generated at a localized heater. Results of various observations of third sound reported in the literature during the past 30 years are studied with the aid of the more complete theory. This study indicates a number of factors that complicated interpretation of the observations and suggests that large discrepancies between theoretical and experimental results that have been reported may be largely due to inappropriate comparisons. Conditions favorable for observing individual normal modes of third sound, where interpretation of the results should be particularly transparent, are investigated and could serve as a useful basis for new experiments. A direct method for accurately measuring the vaporization coefficient utilizing results in this article is described. This coefficient is of central importance in accounting for third-sound attenuation. Failure to account for that attenuation was believed to be a major deficiency of the theory in the past. Also, an energy conservation law is derived and then used to help explain the physical nature of third-sound attenuation.