  A weak continuous line has been recently discovered onboard Voyager 1 in the
interstellar medium, whose origin raised two major questions. First, how can
this line be produced by plasma quasi-thermal noise on the Voyager short
antenna? Second, why does this line emerge at some distance from the
heliopause? We provide a simple answer to these questions, which elucidates the
origin of this line. First, a minute quantity of supra-thermal electrons, as
generally present in plasmas, whence the qualifier quasi-thermal, can produce a
small plasma frequency peak on a short antenna, of amplitude independent of the
concentration of these electrons; furthermore, the detection required long
spectral averages, alleviating the smallness of the peak compared to the
background. We therefore attribute the observed line to a minute proportion of
fast electrons that contribute negligibly to the pressure. Second, we suggest
that, up to some distance from the heliopause, the large compressive
fluctuations ubiquitous in this region prevent the line to emerge from the
statistical fluctuations of the receiver noise because it is blurred out by the
averaging required for detection,especially in the presence of short-wavelength
density fluctuations. These results open up novel perspectives for interstellar
missions, by showing that a minute proportion of fast electrons may be
sufficient to measure the density even with a relatively short antenna, because
the quietness of the medium enables a large number of spectra to be averaged.
