D. Píša

Unexpected Very Low Frequency (VLF) Radio Events Recorded by the Ionospheric Satellite DEMETER

DEMETER was a low Earth orbiting microsatellite in operation between July 2004 and December 2010. Its scientific objective was the study of ionospheric perturbations in relation to seismic activity and man-made activities. Its payload was designed to measure electromagnetic waves over a large frequency range as well as ionospheric plasma parameters (electron and ion densities, fluxes of energetic charged particles). This paper will show both expected and unusual events recorded by the satellite when it was in operation. These latter events have been selected from the DEMETER database because they are rare or even have never been observed before, because they have a very high intensity, or because they are related to abnormalities of the experiments under particular plasma conditions. Some events are related to man-made radio waves emitted by VLF ground-based transmitters or power line harmonic radiation. Natural waves, such as atypical quasi-periodic emissions or uncommon whistlers, are also shown.

Electrostatic solitary waves observed at Saturn by Cassini inside 10 Rs and near Enceladus

We have analyzed the Cassini Radio and Plasma Wave Science Wideband Receiver (WBR) data specifically looking for the presence of bipolar electrostatic solitary waves (ESWs). Typical examples of these ESWs are provided to show that when they are present, several of them may be detected over a few to several millisecond time span. We carried out an event study of an Enceladus encounter which took place on 9 October 2008. Approximately 30u2009min prior to and during the crossing of the Enceladus dust plume, several ESWs are observed with amplitudes of about 100u2009μV/m up to about 140u2009mV/m, and time durations of several tens of microseconds up to 250u2009µs. The highest amplitudes (over 10u2009mV/m) were observed only during the closest approach to Enceladus. We also carried out an ESW survey using the WBR for all years from 2004 to 2008 for distances less than 10 Rs. The survey clearly shows that most of the ESWs are found on the nightside, with a high percentage of them in the range of 4–6 Rs. This location is consistent with the densest part of Saturns E ring and Enceladus orbit. These are the first extended survey results of ESWs near Saturn and the first reported ESWs in connection with Enceladus. We discuss possibilities for the generation of these nonlinear ESWs, which involve current, beam, and acoustic, including dust, instabilities.

VESPA: A community-driven Virtual Observatory in Planetary Science

The VESPA data access system focuses on applying Virtual Observatory (VO) standards and tools to Planetary Science. Building on a previous EC-funded Europlanet program, it has reached maturity during the first year of a new Europlanet 2020 program (started in 2015 for 4 years). The infrastructure has been upgraded to handle many fields of Solar System studies, with a focus both on users and data providers. This paper describes the broad lines of the current VESPA infrastructure as seen by a potential user, and provides examples of real use cases in several thematic areas. These use cases are also intended to identify hints for future developments and adaptations of VO tools to Planetary Science.

Statistics of Langmuir wave amplitudes observed inside Saturn's foreshock by the Cassini spacecraft

We present the first systematic study of Langmuir wave amplitudes in Saturns foreshock using the Cassini Radio and Plasma Wave Science/Wideband Receiver measurements. We analyzed all foreshock crossings from June 2004 to December 2009 using an automatic method to identify Langmuir waves. Using this method, almost 3 × 105 waveform intervals of typical duration of about a minute were selected. For each selected waveform interval the position of the satellite inside the foreshock was calculated using an adaptive bow shock model, which was parametrized by the observed magnetic field and plasma data. We determined the wave amplitudes for all waveform intervals, and we found that the probability density function amplitudes follow a lognormal distribution with a power law tail. A nonlinear fit for this tail gives a power law exponent of −1.37 ± 0.01. The distribution of amplitudes as a function of the depth in the foreshock shows the onset of the waves near the upstream boundary with its maximum slightly shifted inside the foreshock (∼1u2009RS). The amplitudes then fall off with increasing depth in the downstream region. Our results are in agreement with previous observations and roughly follow the generally accepted stochastic growth theory mechanism for the foreshock region, with an exception at the highest observed amplitudes. The estimated energy density ratio W for largest amplitudes does not exceed 10−2, suggesting that modulational instability is not relevant for a large majority of waves. The decay instability can be important for the stronger electrostatic waves in Saturns foreshock, as was previously reported for multiple solar system planets.

EMIC waves observed by the low‐altitude satellite DEMETER during the November 2004 magnetic storm

This paper presents an analysis of ULF (0–20 Hz) waves observed by the low-altitude satellite Detection of ElectroMagnetic Emissions Transmitted from Earthquake Regions (DEMETER) during the magnetic storm of November 2004. Since these ULF waves are measured by both electric and magnetic antennas, they may be identified as electromagnetic ion cyclotron (EMIC) waves. While EMIC waves have been previously observed in the low-altitude ionosphere, this is the first time that they are observed for such extensive time periods and at such high frequencies. A common feature of these emissions is that their observation region in the low-altitude ionosphere extends continuously from the high-latitude southern trough in one side up to the high-latitude northern trough. The analysis of wave propagation points to the possible source region placed in the inner magnetosphere (L∼2–3). Observed wave frequencies indicate that waves must be generated much farther from the Earth compared to the satellite orbit. Exceptionally high frequencies of about 10 Hz can be explained by the source region placed in the deep inner magnetosphere at L ∼2.5. We hypothesize that these waves are generated below the local helium gyrofrequency and propagate over a large range of wave normal angles to reach low altitudes at L ∼1.11. In order to investigate this scenario, a future study based on ray tracing simulations will be necessary.

A case study of a density structure over a vertical magnetic field region in the Martian ionosphere

One of the discoveries made by the radar sounder on the Mars Express spacecraft is the existence of magnetically controlled structures in the ionosphere of Mars, which result in bulges in the ionospheric electron density contours. These bulges lead in turn to oblique echoes, which show up as hyperbola-shaped features in the echograms. A hyperbola-shaped feature observed over an isolated region of strong crustal magnetic field is associated with a plasma cavity in the upper ionosphere and a corresponding density enhancement in the lower levels of the ionosphere. We suggest that along open magnetic field lines, the solar wind electrons are accelerated downward and the ionospheric ions are accelerated upward in a manner similar to the field line-driven auroral acceleration at Earth. This heating due to precipitating electrons may cause an increase in the scale height and may drive a loss of ionospheric plasma at high altitudes.

First Observation of Lion Roar Emission in Saturn's Magnetosheath

We present an observation of intense emissions in Saturn’s magnetosheath as detected by the Cassini spacecraft. The emissions are observed in the dawn sector (magnetic local time ∼06:45) of the magnetosheath over a time period of 11 h before the spacecraft crossed the bow shock and entered the unshocked solar wind. They are found to be narrow-banded with a peak frequency of about 0.16 fce, where fce is the local electron gyrofrequency. Using plane wave propagation analysis, we show that the waves are right hand circularly polarized in the spacecraft frame and propagate at small wave normal angles (<10∘) with respect to the ambient magnetic field. Electromagnetic waves with the same properties known as “lion roars” have been reported by numerous missions in the terrestrial magnetosheath. Here we show the first evidence such emission outside the terrestrial environment. Our observations suggest that lion roars are a solar-system-wide phenomenon and capable of existing in a broad range of parameter space. This also includes 1 order of magnitude difference in frequencies. We anticipate our result to provide new insight into such emissions in a new parameter regime characterized by a higher plasma beta (owing to the substantially higher Mach number bow shock) compared to Earth.

Spatial distribution of Langmuir waves observed upstream of Saturn's bow shock by Cassini: LANGMUIR WAVES AT SATURN

We present the spatial distribution and spectral properties of Langmuir waves observed upstream of Saturn’s bow shock by the Cassini spacecraft. The entire 10 kHz wideband data set obtained between June 2004 and December 2014 has been analyzed using an automated procedure. Almost 10 waveform snapshots with intense narrowband emissions in the frequency range of 1–10 kHz were detected. A typical wave spectrum exhibits a single intense peak (62% of all selected waveforms). However, spectra with a superposition of two (25%) or more (13%) intense peaks are also observed. Using magnetic field observations and a model of the bow shock, plasma wave activity across Saturn’s foreshock has been mapped. The plasma wave occurrence increases steeply behind the tangent magnetic field line, i.e., the sunward foreshock boundary, and rises with increasing distance from the tangential line into the downstream region. The single peak spectra are observed across the entire foreshock, while more complicated spectra are more likely measured deeper inside the foreshock and closer to the bow shock. We confirm that the most intense waves occur close to the tangent point and decrease both deeper in the foreshock and along the tangential line.

Auroral Hiss Emissions During Cassini's Grand Finale: Diverse Electrodynamic Interactions Between Saturn and Its Rings

The Cassini Grand Finale orbits offered a new view of Saturn and its environment owing to multiple highly inclined orbits with unprecedented proximity to the planet during closest approach. The Radio and Plasma Wave Science instrument detected striking signatures of plasma waves in the southern hemisphere. These all propagate in the whistler mode and are classified as (1) a filled funnel-shaped emission, commonly known as auroral hiss. Here however, our analysis indicates that they are likely associated with currents connected to the rings. (2) First observations of very low frequency saucers directly linked to the planet on field lines also connected to the rings. The latter observations are unique to low altitude orbits, and their presence at the Earth and Saturn alike shows that they are fundamental plasma waves in planetary ionospheres. Our results give an insight, from a unique perspective, into the dynamic and diverse nature of Saturn’s environment. Plain Language Summary Cassini’s high-inclination Grand Finale orbits brought the spacecraft closer to Saturn than ever before, with the closest approach between the cloud tops and the inner edge of the D ring. This unprecedented set of orbits introduced a new view of Saturn’s system by enabling direct measurements of the topside ionosphere as well as the rings’ surrounding environment. Here we present evidence of communication between Saturn’s ionosphere and rings via plasma waves. These results underline Saturn’s system as one of the most dynamic and diverse in the solar system.

Source region and growth analysis of narrowband Z‐mode emission at Saturn

Intense Z-mode emission is observed in the lower density region near the inner edge of the Enceladus torus at Saturn, where these waves may resonate with MeV electrons. The source mechanism of this emission, which is narrow banded and most intense near 5u2009kHz, is not well understood. We survey the Cassini Radio and Plasma Wave Science (RPWS) data to isolate several probable source regions near the inner edge of the Enceladus density torus. Electron phase space distributions are obtained from the Cassini Electron Spectrometer (ELS), part of the Cassini Plasma Spectrometer (CAPS) investigation. We perform a plasma wave growth analysis to conclude that an electron temperature anisotropy and possibly a weak loss cone can drive the Z-mode as observed. Electrostatic electron acoustic waves and perhaps weak beam modes are also found to be unstable coincident with the Z-mode. Quasi-steady conditions near the Enceladus density torus may result in the observations of narrow band Z-mode emission at Saturn.