Abstract
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.
Highlights
IntroductionOcker et al (2021) (see Burlaga et al 2021) have recently discovered a weak continuous line close to the local plasma frequency on the Voyager 1 Plasma Wave System (PWS) instrument (Scarf & Gurnett 1977) in the interstellar medium
Ocker et al (2021) have recently discovered a weak continuous line close to the local plasma frequency on the Voyager 1 Plasma Wave System (PWS) instrument (Scarf & Gurnett 1977) in the interstellar medium. The origin of this line was not understood since a plasma quasi-thermal noise (QTN) origin was deemed problematic
We have shown that superimposing a minute quantity of electrons with a hard power-law velocity distribution above about 100 eV on the local interstellar medium (LISM) Maxwellian at 7000 K produces a QTN peak that explains the observed line
Summary
Ocker et al (2021) (see Burlaga et al 2021) have recently discovered a weak continuous line close to the local plasma frequency on the Voyager 1 Plasma Wave System (PWS) instrument (Scarf & Gurnett 1977) in the interstellar medium. Since the discovery of this noise in the interplanetary medium (Meyer-Vernet 1979; Couturier et al 1981), calculations of it have been extended to flat-top (Chateau & Meyer-Vernet 1989) and Kappa velocity distributions (Chateau & Meyer-Vernet 1991; Zouganelis 2008; Le Chat et al 2009), to magnetised plasmas (Sentman 1982; Meyer-Vernet et al 1993), and to include ions (Issautier et al 1999) Spectroscopy of this noise has been used to measure in situ the electron density and temperature and the properties of supra-thermal electrons in the solar wind as well as in planetary and cometary environments on the spacecraft ISEE3-ICE, Ulysses, WIND, Cassini (Meyer-Vernet et al 2017, and references therein), and more recently, Parker Solar Probe (PSP; Moncuquet et al 2020). To explain the observed Voyager thin line, it is not necessary to assume 50% supra-thermals, which increases the pressure by a large factor, as Gurnett et al (2021) did
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