There's a paper in press at Icarus which also deals with the same data (Icarus in press bit doesn't seem to work now, so I can't give a link yet):
A Schumann-like resonance on Titan driven by Saturn’s magnetosphere possibly
revealed by the Huygens Probe
C. Béghin a,*, F. Simões b, V. Krasnoselskikh a, K. Schwingenschuh c, J.J. Berthelier b, B. Besser c,
C. Bettanini d, R. Grard e, M. Hamelin b, J.J. López-Moreno f, G.J. Molina-Cuberos g,
and T. Tokano h
they conclude that (from the abstract):
QUOTE
[...]
Extremely-low frequency waves generated in the ionosphere of Titan,
driven by the corotating Saturn’s frozen plasma flow, are assumed to be the most likely source for
the observation of the second eigenmode of a Schumann-like resonance at around 36 Hz in the
moon-ionosphere cavity. This particular mode is thought to be enhanced with respect to other
harmonics because of the particular location of the landing site with respect to that of the supposed
sources. The power budget of the observed wave amplitude seems to be consistent with a rough
model of the global current of the wake-ionosphere circuit.
[...]
Though I don't have a good idea about what that all means, it seems that it may be consistent with some sort of ionosphere interaction. In the conclusions they state however:
QUOTE
We cannot firmly conclude yet that only one scenario
can explain the ELF and “36 Hz” line seen in the PWA data,
Instead of a subsurface layer they think aerosols might be responsible for the reflection, as far as I can tell,
QUOTE
Within the low conductivity cavity which is thought to be deepened
by a high concentration of electrophylic aerosols, between the bottom of the regular ionosphere
and the “Huygens” conductivity layer, the emerging 36 Hz wavelets are assumed to be reflected
upwards. Then, the horizontal electric component of those waves may be subsequently enlarged, as
seen on the “36 Hz” between 105 and 70 km.
Note that F. Simoes is 2nd author here, so the sub-surface sea might well be an improvement on this work.