Alain Lecacheux

The recent paper by Li et al. on electron quasi-thermal noise in the outer heliosphere is flawed. It assumes the plasma drift speed to be much smaller than the electron thermal speed, even though both quantities are of the same order of magnitude in the outer heliosphere inward of the termination shock, because of the low plasma temperature. In this case, the Langmuir wave dispersion equation and the quasi-thermal noise in the antenna frame are completely changed. Furthermore, these calculations neglect the shot noise, which should produce a large contribution below the plasma frequency with the Voyager antennas in the outer heliosphere.

The recent paper by Li et al. on electron quasi-thermal noise (QTN) in the outer heliosphere is flawed. It assumes the plasma drift speed to be much smaller than the electron thermal speed, even though both quantities are of the same order of magnitude in the outer heliosphere inward of the termination shock, because of the low plasma temperature. In this case, the Langmuir wave dispersion equation and the QTN in the antenna frame are completely changed. Furthermore, these calculations neglect the shot noise, which should produce a large contribution below the plasma frequency with the Voyager antennas in the outer heliosphere.

The quasi-thermal motion of plasma particles produces electrostatic fluctuations, whose voltage power spectrum induced on electric antennas reveals plasma properties. In weakly magnetised plasmas, the main feature of the spectrum is a line at the plasma frequency – proportional to the square root of the electron density – whose global shape can reveal the electron temperature, while the fine structure reveals the suprathermal electrons. Since it is based on electrostatic waves, quasi-thermal noise spectroscopy (QTN) provides in situ measurements. This method has been successfully used for more than four decades in a large variety of heliosphere environments. Very recently, it has been tentatively applied in the very local interstellar medium (VLISM) to interpret the weak line discovered on board Voyager 1 and in the context of the proposed interstellar probe mission. The present paper shows that the line is still observed in the Voyager Plasma Wave Science data, and concentrates on the main features that distinguish the plasma QTN in the VLISM from that in the heliosphere. We give several tools to interpret it in this medium and highlight the errors arising when it is interpreted without caution, as has recently been done in several publications. We show recent solar wind data, which confirm that the electric field of the QTN line in a weakly magnetised stable plasma is not aligned with the local magnetic field. We explain why the amplitude of the line does not depend on the concentration of suprathermal electrons, and why its observation with a short antenna does not require a kappa electron velocity distribution. Finally, we suggest an origin for the suprathermal electrons producing the QTN and we summarise the properties of the VLISM that could be deduced from an appropriate implementation of QTN spectroscopy on a suitably designed instrument.

The quasi-thermal motion of plasma particles produces electrostatic fluctuations, whose voltage power spectrum induced on electric antennas reveals plasma properties. In weakly magnetised plasmas, the main feature of the spectrum is a line at the plasma frequency -- proportional to the square root of the electron density -- whose global shape can reveal the electron temperature, while the fine structure reveals the suprathermal electrons. Since it is based on electrostatic waves, quasi-thermal noise spectroscopy (QTN) provides in situ measurements. This method has been successfully used for more than four decades in a large variety of heliosphere environments. Very recently, it has been tentatively applied in the very local interstellar medium (VLISM) to interpret the weak line discovered on board Voyager 1 and in the context of the proposed interstellar probe mission. The present paper shows that the line is still observed in the Voyager Plasma Wave Science data, and concentrates on the main features that distinguish the plasma QTN in the VLISM from that in the heliosphere. We give several tools to interpret it in this medium and highlight the errors arising when it is interpreted without caution, as has recently been done in several publications. We show recent solar wind data, which confirm that the electric field of the QTN line in a weakly magnetised stable plasma is not aligned with the local magnetic field. We explain why the amplitude of the line does not depend on the concentration of suprathermal electrons, and why its observation with a short antenna does not require a kappa electron velocity distribution. Finally, we suggest an origin for the suprathermal electrons producing the QTN and we summarise the properties of the VLISM that could be deduced from an appropriate implementation of QTN spectroscopy on a suitably designed instrument.

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.

Since 2003, significant efforts have been devoted to the understanding of the radio emission of extensive air shower in the range [20-200] MHz. Despite some studies led until the early nineties, the [1-10] MHz band has remained unused for 20 years. However, it has been measured by some pioneering experiments that extensive air shower emit a strong electric field in this band and that there is evidence of a large increase in the amplitude of the radio pulse at lower frequencies. The EXTASIS experiment, located within the Nançay Radioastronomy Observatory and supported by the CODALEMA experiment, aims to reinvestigate the [1-10] MHz band, and especially to study the so-called “Sudden Death” contribution, the expected electric field emitted by shower front when hitting the ground level. Currently, EXTASIS has confirmed some results obtained by the pioneering experiments, and tends to bring explanations to the other ones, for instance the role of the underlying atmospheric electric field. Moreover, CODALEMA has demonstrated that in the most commonly used frequency band ([20-80] MHz) the electric field profile of EAS can be well sampled, and contains all the information needed for the reconstruction of EAS: an automatic comparison between the SELFAS3 simulations and data has been developed, allowing us to reconstruct in an almost real time the primary cosmic ray characteristics.

The present state of solar wind investigations by IPS (interplanetary scintillations) observations at decameter wavelengths is discussed. Radio telescopes, equipment and methods which are used in these experiments are shown. We also describe some interesting results devoted to the long term monitoring of the solar wind beyond Earth’s orbit, the detection of the large scale disturbances associated with active processes at the Sun and the reconstructions of the solar wind stream structure. Emphasis is placed on perspectives of low frequency IPS investigations which are particularly connected with the creation of the Giant Ukrainian Radio Telescopes (GURT) and UTR-2 – URAN – GURT – LOFAR collaboration.

The “8th International Workshop on Planetary, Solar and Heliospheric Radio Emissions” was held in October 2016 in the historical castle Schloss Seggau, about 35 km south of Graz, Austria. The proceedings of this workshop are now available as the book Planetary Radio Emissions VIII (PRE VIII), which is a continuation of the “PRE silver series” issued by the Austrian Academy of Sciences Press. All contributions were peer-reviewed under the guidance of the four editors. For more than three decades the developments in the field of planetary and solar radio emissions have been documented in the PRE proceedings. The new volume PRE VIII contains articles about the first observations from the Juno spacecraft, which started to orbit Jupiter in mid-2016, and articles analysing the data of Cassini, whose mission ended in September 2017. A large number of contributions deals with the topic of Jovian radio emissions, where the ground-based support of space missions has become increasingly important, and where the statistical analysis of years of data have led to new conclusions about the influence of the Galilean moons. The large ground-based radio telescopes (LOFAR, UTR-2, GURT, LWA, NDA and others) have other important scientific targets besides Jupiter, namely the Sun and exoplanets. The articles about exoplanets in PRE VIII give the impression that a first detection of radio emission signatures could happen in a not too distant future. Other contributions deal with terrestrial radio emissions and theory. Finally, the technical developments in instrumentation have led to the discovery of new fine structures in radio emissions or to improved solar radio imaging, and newly developed databases should make radio data available to all interested scientists around the globe.