In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium¹. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 au of interstellar space, revealing a large-scale density gradient^2,3 and turbulence⁴ outside of the heliopause. Previous studies of the plasma density relied on the detection of discrete plasma oscillation events triggered ahead of shocks propagating outwards from the Sun, which were used to infer the plasma frequency and, hence, density^5,6. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 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 distance of 0.03 au. We find au-scale density fluctuations that trace interstellar turbulence between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and they could be clarified by new findings from Voyager or a future interstellar mission. The emission’s persistence suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.

2012 年，航海者 1 号成为第一个对非常局部的星际介质¹进行原位探测的探测器。航海者 1 号等离子波系统给出了跨越约 30 au 星际空间的等离子密度的点估计，揭示了日球层顶外的大尺度密度梯度^2,3和湍流⁴。以前对等离子体密度的研究依赖于在从太阳向外传播的冲击之前触发的离散等离子体振荡事件的检测，这些事件被用来推断等离子体频率，因此，密度^5,6. 我们在航海者 1 号数据中检测到一类非常微弱的窄带等离子体波发射，该数据从 2017 年开始持续存在，并且能够在平均采样距离为 0.03 au 的情况下对大约 10 au 范围内的星际等离子体密度进行稳定采样测量。我们发现 au 尺度密度波动可以追踪先前检测到的等离子体振荡事件之间的星际湍流。窄带发射的可能机制包括热激发等离子体振荡和准热噪声，它们可以通过航海者号或未来星际任务的新发现来澄清。发射的持久性表明，在没有冲击产生的等离子体振荡事件的情况下，航海者 1 号可能能够继续跟踪星际等离子体密度。