  In 2012, Voyager 1 became the first in situ probe of the very local
interstellar medium. The Voyager 1 Plasma Wave System has given point estimates
of the plasma density spanning about 30 astronomical units (au) of interstellar
space, revealing a large-scale density gradient and compressive turbulence
outside the heliopause. Previous studies of the plasma density relied
exclusively on the detection of discrete plasma oscillation events that are
triggered ahead of shocks propagating outwards from the Sun, which were used to
infer the plasma frequency and hence density. We present the detection of a
class of very weak, narrowband plasma wave emission in the Voyager 1 Plasma
Wave System data that persists from 2017 onwards and enables a steadily sampled
measurement of the interstellar plasma density over about 10 au with an average
sampling time of 3 days, or 0.03 au. We find au-scale density fluctuations that
trace turbulence in the interstellar medium between episodes of previously
detected plasma oscillations. Possible mechanisms for the narrowband emission
include thermally excited plasma oscillations and quasi-thermal noise, and
could be clarified by new findings from Voyager or a future interstellar
mission. The persistence of the emission suggests that Voyager 1 may be able to
continue tracking the interstellar plasma density in the absence of
shock-generated plasma oscillation events.
