  The experimentally observed disappearance below T = 0.5K of the second sound
in liquid He II as a separate wave mode and its subsequent propagation at the
speed of the first sound (Peshkov [3]) may be interpreted as a resonant mode
conversion of the second sound to the first sound. Near the resonant mode
coupling point T = T*, where the frequencies of the two waves become equal, the
anomalous effect of entropy changes on the first sound and density changes on
the second sound, though generally small, become significant. This leads to the
resonant mode coupling of the first sound and the second sound and forces them
to lose their identities and hence pave the way for the resonant mode
conversion of the second sound to the first sound. We give a theoretical
framework for this proposition and an estimate for the fraction of the second
sound that is mode-converted to the first sound.
