Search

We study 10 years (1995–2004 inclusive) of auroral kilometric radiation (AKR) radio emission data from the Wind spacecraft to examine the link between AKR and terrestrial substorms. We use substorm lists based on parameters including ground magnetometer signatures and geosynchronous particle injections as a basis for superposed epoch analyses of the AKR data. The results for each list show a similar, clear response of the AKR power around substorm onset. For nearly all event lists, the average response shows that the AKR power begins to increase around 20 min prior to expansion phase onset, as defined by the respective lists. The analysis of the spectral parameters of AKR bursts show that this increase in power is due to an extension of the source region to higher altitudes, which also precedes expansion phase onset by 20 min. Our observations show that the minimum frequency channel that observes AKR at this time, on average, is 60 kHz. AKR visibility is highly sensitive to observing spacecraft location, and the biggest radio response to substorm onset is seen in the 21:00–03:00 hr local time sector. Plain Language Summary: Substorms are an energetic disturbance to the magnetic environment of the Earth. They represent the driving of the terrestrial magnetosphere by particles from the Sun and the subsequent response in various parts of this environment, in both its inner and outer boundaries. These effects are mostly constrained to the nightside of Earth, and can be observed by both ground‐based and remote sensing instruments. In this work, we select auroral kilometric radiation (AKR) observations from 10 years (from 1995 to 2004 inclusive) of radio data from the Wind/WAVES instrument, and compare this with lists of substorm expansion phase onsets that are derived from various observational signatures. After accounting for visibility of the radio sources, we show that the AKR response correlates with the size/strength of the substorm, based on the sensitivity of the list. Our results show that the AKR source region tends to increase in size along magnetic field lines while the emission intensifies, using a longer data set to corroborate previous results. Key Points: Auroral kilometric radiation (AKR) observations made over 10 years are compared with four event lists of substorm onsets using superposed epoch analysesOn average, AKR power increases and source region extends to higher altitudes in the 20 min prior to onsetThe occurrence of AKR power at higher altitudes is sensitive to the substorm size [ABSTRACT FROM AUTHOR]

A new radio component namely Saturn Anomalous Myriametric Radiation (SAM) is reported. A total of 193 SAM events have been identified by using all the Cassini Saturn orbital data. SAM emissions are L‐O mode radio emission and occasionally accompanied by a first harmonic in R‐X mode. SAM's intensities decrease with increasing distance from Saturn, suggesting a source near Saturn. SAM has a typical central frequency near 13 kHz, a bandwidth greater than 8 kHz and usually drifts in frequency over time. SAM's duration can extend to near 11 hr and even longer. These features distinguish SAM from the regular narrowband emissions observed in the nearby frequency range, hence the name anomalous. The high occurrence rate of SAM after low frequency extensions of Saturn Kilometric Radiation and the SAM cases observed during compressions of Saturn's magnetosphere suggest a special connection to solar wind dynamics and magnetospheric conditions at Saturn. Plain Language Summary: This paper reports a new type of radio emission at Saturn. We name this emission "Saturn Anomalous Myriametric Radiation" (SAM). Due to the different morphological behavior of SAM emissions in the spectrogram, compared to the regularly observed narrowband emissions, SAM emissions are identified as a new radio component. We search for SAM events by using all the available data acquired by the Cassini spacecraft. A total of 193 SAM events are found and we summarized its characteristics. The spatial distribution of SAM shows a high latitude preference in occurrence. The observed intensities of SAM increase as the spacecraft approaches Saturn, suggesting a source near Saturn. The typical central frequency of SAM is near 13 kHz and the duration of SAM can extend to near 11 hr and even longer. The majority of the 193 SAM emissions are observed after the enhancement of the dominant Saturn radio emission Saturn Kilometric Radiation (SKR), which is deemed to be closely connected to the solar wind dynamics at Saturn. Therefore, SAM emissions are also possibly related to the conditions that cause the enhancement of SKR. This newly identified SAM emission may be used to study the magnetospheric dynamics of Saturn in the future. Key Points: A new radio component namely the Saturn Anomalous Myriametric Radiation (SAM) is reportedThe characteristics of SAM are given including its spatial distribution, typical frequency, bandwidth and time durationSAM shows a possible connection to the compression of the magnetosphere [ABSTRACT FROM AUTHOR]

Auroral Kilometric Radiation (AKR) is the strongest terrestrial radio emission, and emanates from the same electron acceleration regions from which particles precipitate into the ionosphere, exciting the aurorae and other phenomena. As such, AKR is a barometer for the state of solar wind ‐ magnetosphere ‐ ionosphere coupling. AKR is anisotropically beamed in a hollow cone from a source region generally found at nightside local times, meaning that a single source region cannot be viewed from all local times in the magnetosphere. In radio data such as dynamic spectra, AKR is frequently observed simultaneously to other radio emissions which can have a similar intensity and frequency range, making it difficult to automatically detect. Building on a previously published pipeline to extract AKR emissions from Wind/WAVES data, in this paper a novel automated AKR burst detection technique is presented and applied to Wind/WAVES data. Over a five year interval, about 5000 AKR bursts are detected with median burst length ranging from about 30 to 60 min. During detected burst windows, higher solar wind velocity is observed, and the interplanetary magnetic field clock angle is observed to tend toward BZ < 0, BY < 0, when compared with the entire statistical interval. Additionally, higher geomagnetic activity is observed during burst windows at polar, high and equatorial latitudes. Plain Language Summary: Auroral Kilometric Radiation (AKR) is a terrestrial radio emission which is excited by the same electrons which enhance the aurorae. Due to a combination of complex beaming, and the statistical position of the source region, an AKR event cannot be observed at all positions in the Earth's magnetosphere. A combination of different radio emissions are simultaneously observed in the radio data, including both AKR and non‐AKR sources. Building on previous work, in this paper individual AKR burst events are automatically detected from Wind/WAVES data over a five year interval. About 5000 events are detected over the interval, during which the observed geomagnetic activity was higher. Higher solar wind velocity and differences in the morphology of the interplanetary magnetic field are also observed during burst windows, both of which are known to excite magnetospheric dynamics. Key Points: A novel technique has been developed to detect individual Auroral Kilometric Radiation bursts in Wind/WAVES dataWhen the technique is applied to 2000–2004 data, about 5000 bursts are detected with median duration 30–60 minDuring burst windows, higher solar wind velocity, more negative IMF BZ and greater geomagnetic activity is observed [ABSTRACT FROM AUTHOR]

We investigate the beaming of 11 Io‐Jupiter decametric (Io‐DAM) emissions observed by Juno/Waves, the Nançay Decameter Array, and NenuFAR. Using an up‐to‐date magnetic field model and three methods to position the active Io Flux Tube (IFT), we accurately locate the radiosources and determine their emission angle θ from the local magnetic field vector. These methods use (a) updated models of the IFT equatorial lead angle, (b) ultraviolet (UV) images of Jupiter's aurorae, and (c) multi‐point radio measurements. The kinetic energy Ee− of source electrons is then inferred from θ in the framework of the Cyclotron Maser Instability. The precise position of the active IFT achieved from methods (b and c) can be used to test the effective plasma density of the Io torus. Simultaneous radio/UV observations reveal that multiple Io‐DAM arcs are associated with multiple UV spots and provide the first direct evidence of an Io‐DAM arc associated with a trans‐hemispheric beam UV spot. Multi‐point radio observations probe the Io‐DAM sources at various altitudes, times and hemispheres. Overall, θ varies a function of frequency (altitude), by decreasing from 75°−80° to 70°−75° over 10−40 MHz with slightly larger values in the northern hemisphere, and independently varies as a function of time (or longitude of Io). Its uncertainty of a few degrees is dominated by the error on the longitude of the active IFT. The inferred values of Ee− also vary as a function of altitude and time. For the 11 investigated cases, they range from 3 to 16 keV, with a 6.6 ± 2.7 keV average. Plain Language Summary: The auroral decametric emissions of Jupiter induced by Io (Io‐DAM) are radiated along high latitude magnetic field lines at large aperture angles from the local magnetic field vector, forming a thin hollow cone. In this study, we determine the emission angle θ of 11 cases of Io‐DAM emissions observed by Juno/Waves, the Nançay Decameter Array and the NenuFAR radiotelescope with an up‐to‐date magnetic field model and three different methods aimed at minimizing the uncertainty on θ. These methods accurately position the active Io magnetic Flux Tube (IFT) which hosts the decametric radiosources by using (a) models of the active IFT, (b) ultraviolet images of Jupiter's aurorae, and (c) multi‐point radio measurements. most notably, we found that θ varies within 70°–80° as a function of the source altitude along the field line and independently as a function of time. Assuming that the Io‐DAM emissions are driven by the Cyclotron Maser Instability from energetic electrons, we infer from the measured θ the kinetic energy Ee− of the source electrons accelerated by the Io‐Jupiter interaction. The obtained values of Ee− also depend on altitude and time and vary between 3 and 16 keV, with a ∼6.5 keV average, in agreement with Juno in situ measurements. Key Points: We derive the Io‐decametric emission angle θ from Juno, Nançay Decameter Array, and NenuFAR data using 3 methods to locate the radio sourcesθ(f) decreases from 75°−80° to 70°−75° over 10–40 MHz and varies both as a function of frequency (or altitude) and time (or longitude of Io)The inferred electron energies amplifying Io‐decametric waves range from 3 to 16 keV also vary as a function of altitude and time [ABSTRACT FROM AUTHOR]

The reflection‐by‐sheath mechanism of 5 kHz narrowband emissions (NB) at Saturn is confirmed by Cassini observations during several crossings of the magnetopause, which show that the 5 kHz NB can be prevented from escaping Saturn's magnetosphere. The L‐O mode 5 kHz NB remained visible in areas of low plasma density but disappeared in regions of high plasma density. In three cases, NB disappeared immediately after the crossings of Saturn's magnetopause. A possible reflected NB event observed near the magnetosheath is discussed. This mechanism can help explain the 5 kHz NB observed at low latitudes outside the Enceladus plasma torus and their upper frequency limit variations. This mechanism significantly improves the current understanding of the 5 kHz NB. Plain Language Summary: Very low frequency narrowband radio emissions have been observed at about 5 and 20 kHz by the Voyager and Cassini spacecraft at Saturn. Recently, a statistical survey of Saturn narrowband emissions indicated that the 5 kHz narrowband emissions could be reflected by Saturn's magnetosheath due to the high plasma density within. The work presents evidence to confirm the reflection and refraction of 5 kHz narrowband emissions by Saturn's magnetosheath. The observations of narrowband emissions near Saturn's magnetosheath show that sometimes the 5 kHz narrowband emissions cannot pass the magnetosheath when the plasma density is high enough, at other times the polarization reverses near the magnetosheath, which indicates a reflected signal. This survey confirms the reflection‐by‐sheath mechanism, and it is of special importance because this mechanism significantly improves the current understanding of the Saturn narrowband emissions and explains several features of them. Key Points: The 5‐kHz narrowband emissions (NB) from Saturn can be reflected by density structures in the magnetosheathReflection leads to depolarization and trapping of NB inside Saturn's magnetosphereThe upper frequency limit of trapped NB depends on variations in the magnetosheath and the solar wind [ABSTRACT FROM AUTHOR]

The reception properties of the Radio Wave Instrument (RWI) onboard JUICE (Jupiter Icy Moons Explorer) have been determined using numerical methods applied to a mesh-grid model of the spacecraft. The RWI is part of the RPWI (Radio and Plasma Wave Investigation) and consists of three perpendicular dipoles mounted on a long boom. We determined their effective lengths vectors and capacitive impedances of 8.9 pF. We also investigated the change in effective antenna angles as a function of solar panel rotation and calculated the directivity of the antennas at higher frequencies up to the maximum frequency of 45 MHz of the receiver. We found that the RWI dipoles can be used for directionfinding with an accura....cy of 2. up to a frequency of 1.5 MHz. Additionally we calculated the influence of strong pulses from the JUICE active radar on RPWI and found that they should do no harm to its sensors and receivers. [ABSTRACT FROM AUTHOR]

Reanalyzing Cassini radio observations performed during Jupiter's flyby of 2000–2001, we study the internal (rotational) versus external (solar wind) control of Jupiter's radio emissions, from kilometer to decameter wavelengths, and the relations between the different auroral radio components. For that purpose, we build a database of the occurrence of Jovian auroral radio components bKOM, HOM, and DAM observed by Cassini, and then frequency‐longitude stacked plots of the polarized intensity of these radio components. Comparing the results obtained inbound and outbound, as a function of the Observer's or Sun's longitude, we find that HOM & DAM are dominantly rotation‐modulated (i.e., emitted from searchlight‐like sources fixed in Jovian longitude), whereas bKOM is modulated more strongly by the solar wind than by the rotation (i.e., emitted from sources more active within a given Local Time sector). We propose a simple analytical description of these internal and external modulations and evaluate its main parameters (the amplitude of each control) for HOM + DAM and bKOM. Comparing Cassini and Nançay Decameter Array data, we find that HOM is primarily connected to the decameter emissions originating from the dusk sector of the Jovian magnetosphere. HOM and DAM components form a complex but stable pattern in the frequency‐longitude plane. HOM also seems to be related to the "lesser arcs" identified by Voyager. bKOM consists of a main part above ∼40 kHz in antiphase with HOM occurrence, and detached patches below ∼80 kHz in phase with HOM. The frequency‐longitude patterns formed by DAM, HOM and bKOM remain to be modeled. Key Points: We build synthetic frequency‐longitude maps of polarized Jovian auroral radio emissions, DAM, HOM, and bKOMJovian HOM and DAM are dominantly rotation‐modulated, bKOM dominantly solar wind‐modulatedHOM appears primarily connected to decameter sources from the dusk side of the Jovian magnetosphere [ABSTRACT FROM AUTHOR]

Auroral Kilometric Radiation (AKR) is terrestrial radio emission that originates in particle acceleration regions along magnetic field lines, coinciding with discrete auroral arcs. AKR viewing geometry is complex due to the confinement of the source regions to nightside local times (LTs) and the anisotropy of the beaming pattern, so observations are highly dependent on spacecraft viewing position. We present a novel, empirical technique that selects AKR emission from observations made with the spin‐axis aligned antenna of the Wind/WAVES instrument, based on the rapidly varying amplitude of AKR across spacecraft spin timescales. We apply the technique to Wind/WAVES data during 1999 day of year 227–257, when the Cassini spacecraft flew past Earth and provided an opportunity to observe AKR from two remote locations. We examine the AKR flux and power, with observations made from LTs of 1700–0300 hr having an average power up to 104 Wsr‐1 larger than those on the dayside and an increasing AKR power observed at higher magnetic latitudes. We perform a linear cross‐correlation between the Wind AKR power and the spacecraft magnetic latitude, showing positive then negative correlation as Wind travels from the Northern to Southern magnetic hemisphere. Statistically significant diurnal modulations are found in the whole 30‐day period and in subsets of the data covering different local time sectors, indicative of a predominantly geometrical effect for remote AKR viewing. The reproduction of well‐known features of the AKR verifies the empirical selection and shows the promise of its application to Wind/WAVES observations. Plain Language Summary: Auroral Kilometric Radiation (AKR) is naturally occurring radio emission from the Earth's Northern and Southern polar regions, which becomes more intense as the aurora brightens. In this work, we examine data from the Wind spacecraft WAVES instrument from a 30‐ day interval in 1999 when a second spacecraft, Cassini, was also flying near Earth and measuring the AKR from a different viewpoint. In this work, we select the AKR using an empirical measure of the variability observed by the WAVES instrument, and compare the distribution and time profile of AKR intensity. Comparing measurements of this radio emission from different spacecraft positions help us to understand how the AKR is best viewed and illustrate the constrained beaming of the emission. This information is important for anyone wanting to attempt to interpret measurements of the AKR. Key Points: Novel, empirically based method to extract Auroral Kilometric Radiation (AKR) from Wind/WAVES is presented and applied to observations made during the Cassini flybySelected data show a distribution of AKR power with expected longitudinal and latitudinal visibility constraintsDiurnal temporal modulation observed, suggesting the dominance of a geometric viewing effect and agreeing with previous AKR observations [ABSTRACT FROM AUTHOR]

The observations from the Juno spacecraft in polar orbit of Jupiter provide for the first time a complete view of Jupiter's radio emissions from all latitudes. Characterizing the latitudinal distribution of radio emissions' occurrence and intensity is a useful step for elucidating their origin. Here, we analyze for that purpose the first 3 years of observations from the Waves experiment on the Juno spacecraft (mid‐2016 to mid‐2019). Two prerequisites for the construction of the latitudinal distribution of intensities for each Jovian radio component are (a) to work with absolute flux densities and (b) to be able to associate each radio measurement with a specific radio component. Accordingly, we develop a method to convert the Juno/Waves data in flux densities and then we build a catalog of all Jovian radio components over the first 3 years of Juno's orbital mission. From these, we derive occurrence and intensity distributions versus observer's latitude and frequency for each component; these will be the basis for future detailed studies and interpretations of each component's characteristics and origin. Key Points: We build a processing pipeline of Juno/Waves data that include conversion to absolute flux densitiesWe build a catalog of all Jovian radio components over the first 3 years of Juno's orbital missionWe derive occurrence and intensity distributions versus observer's latitude and frequency for each component [ABSTRACT FROM AUTHOR]

Virtual Observatory tools are specifically designed for astronomical data, but they can be adapted to work with geospatial data by providing existing Geographical Information System tools with Simple Application Messaging Protocol interface. Open source QGIS package was chosen as a platform for this. The Simple Application Messaging Protocol interface was made with Python plug‐ins. Geospatial data were exposed to Virtual European Solar and Planetary Access via a dedicated German Astronomical Virtual Observatory Data Center Helper Suite server and several tables exposing existing Open Geospatial Consortium—compliant planetary services were published to Virtual European Solar and Planetary Access providing a variety of geospatial data types: vector data and spectral cube rasters, as well as Open Geospatial Consortium Web Map Service of planetary maps. Plain Language Summary: Geospatial Science and Astronomy are closely related fields which nevertheless use different tools, standards, interfaces, formats, and protocols. Heterogeneous Astronomical data are accessed routinely via standard system largely known as Virtual Observatory, whereas in Planetary Science, capabilities of cross‐mission and cross‐archive data discovery are lacking. The present work tries to bridge these two communities by leveraging on commonalities in data types and tools while exposing Geospatial data sets to the Virtual Observatory and creating necessary interfaces in Geographical Information Systems. Key Points: Geospatial data sets were exposed to Virtual ObservatoryQGIS plug‐ins interface to Virtual Observatory was developed [ABSTRACT FROM AUTHOR]

Planetary science encompasses a broad number of research fields and brings together several research communities (geologists, astronomers, physicists, geochemists, etc.). Planetary missions produce an impressively growing amount of diverse data requiring an evolution from a mostly manual‐based visual analysis to a more automated and quantitative analysis. Interoperability, openness of data formats, and shared processing techniques are a necessity to efficiently extract scientific information from the data and to guarantee the reproducibility of the scientific results. Unfortunately, the technologies and data formats used by researchers for planetary surface studies and the field of astronomy diverged as these related, but almost completely isolated, domains evolved. In this paper we will describe how a small addition to the Flexible Image Transport System (FITS) standard, widely used in the astronomy investigations, will allow FITS to be more easily used in planetary surface investigations. We will also show how FITS metadata can easily be transformed in the Planetary Data System version 4 metadata archival model and lastly provide example implementations. More than imposing a formal data model, a FITS description for planetary data aims to simplify sharing data across the planetary and astronomy domains and promoting interoperability from raw data formatting to final visualization. Key Points: We present an extension of FITS metadata for planetary surface investigationsA FITS description for planetary data aims to simplify sharing data across the astronomy and planetary domainsAn open FITS portrayal will promote interoperability from raw data formatting to final visualization [ABSTRACT FROM AUTHOR]

Throughout 2017, the Hubble Space Telescope (HST) observed the northern far‐ultraviolet aurorae of Saturn at northern solstice, during the Cassini Grand Finale. These conditions provided a complete viewing of the northern auroral region from Earth and a maximal solar illumination, expected to maximize the ionosphere‐magnetosphere coupling. We analyze 24 HST images concurrently with Cassini measurements of Saturn's kilometric radiation and solar wind parameters predicted by two magnetohydrodynamic models. The aurorae reveal highly variable components, down to timescales of minutes, radiating 7 to 124 GW. They include a nightside‐shifted main oval, unexpectedly frequent and bright cusp emissions, and a dayside low‐latitude component. On average, these emissions display a strong local time dependence with two maxima at dawn and premidnight, the latter being newly observed and attributed to nightside injections possibly associated with solstice conditions. These results provide a reference frame to analyze Cassini in situ measurements, whether simultaneous or not. Plain Language Summary: In 2017, the Hubble Space Telescope regularly observed the northern ultraviolet aurorae of Saturn in coordination with Cassini in situ measurements obtained during the Grand Finale, when the spacecraft flew across magnetic field lines connected to the aurorae. Hubble imaged Saturn's aurorae at 24 occasions spread over 7 months during northern solstice, when the northern auroral region was both fully visible from Earth and permanently illuminated by the Sun. The observed aurorae display a variety of components observed poleward of 68° latitude with different properties, some of which were unreported before. These emissions strongly vary with time, down to a few minutes, and radiate from 7 to 124 GW. On average, the auroral intensity also strongly varies with local time (a Sun‐referenced frame) and peaks at dawn, as previously observed, and also premidnight, pointing to a recurrent nightside activity of the magnetosphere. These results provide a reference basis to analyze Cassini in situ measurements. Key Points: Saturn's northern UV aurorae at solstice were sampled from HST observations coordinated with Cassini's Grand FinaleThe observed aurorae are highly variable with powerful events, radiating up to 124 GW, controlled by solar wind and planetary rotationThe average auroral brightness strongly varies with LT with two maxima at dawn (previously known) and premidnight (newly identified) [ABSTRACT FROM AUTHOR]

We compare observations from the Juno/Waves radio experiment with simulations of radio «arcs» in the time-frequency plane resulting from the Io-Jupiter interaction, performed with the ExPRES code. We identify the hemisphere of origin of the observed arcs directly from simulations and confirm this identification through comparison with Juno, Nançay, and Wind observations. The occurrence and shape of observed arcs are well modeled, at low latitudes with their usual shapes as seen from Earth, as well as at high latitudes with longer, bowl-shaped, arcs observed for the first time. Predicted emission is actually observed only when the radio beaming angle θ = ( k, B) ≥ 70° ± 5°, providing new constraints on the generation of the decameter emission by the Cyclotron Maser Instability. Further improvements of ExPRES are outlined, which will then be applied to Juno and Earth-based observations of radio emissions induced by other Galilean satellites or associated to the main auroral oval. [ABSTRACT FROM AUTHOR]

The Jovian high-latitude radio emissions produced by Jupiter's magnetosphere extend from a few kilohertz to 40 MHz. Part of the decametric (DAM) emissions are driven by the Galilean moon Io (Io-DAM). As UV aurorae have been detected at the footprint of Europa and Ganymede, we expect that these moons drive Jovian radio emissions as well. To check this assumption, we used the ExPRES simulation code (Exoplanetary and Planetary Radio Emissions Simulator) to predict dynamic spectrum (time-frequency spectograms) of the radio emissions controlled by the four Galilean moons. Then we compared the simulations to the Voyager/PRA and Cassini/RPWS radio observations of Jupiter (1979, and between 2000 and 2003, respectively). We present the first clear evidence for the existence of decametric emissions controlled by Europa and Ganymede. Their statistical analysis allows us to describe the average properties of the Europa-DAM and Ganymede-DAM emissions such as their spectrum, temporal variability, and occurrence as a function of moon phase and subobserver's longitude. [ABSTRACT FROM AUTHOR]

We conduct a survey of electrostatic electron cyclotron harmonic (ECH) emissions observed at Saturn by the radio and plasma wave science investigation on board the Cassini spacecraft. These emissions are known to be effective at interacting with electrons in the terrestrial inner magnetosphere, producing electron scattering into the loss cone and acceleration (cf. Horne and Thorne, 2000; Thorne et al., 2010). At Saturn ECH emission occurs with high probability and at strong intensity near the magnetic equator, outside the Enceladus torus in the range ~5 < L < ~10. Inside the inner boundary of the torus, ECH emissions are also observed near the equator and at higher latitude. Intensity levels of ECH emission are comparable to those observed at Earth, higher than Saturn chorus and Z-mode emission, and are likely to scatter electrons into the loss cone as at Earth. ECH waves are particularly intense and extend to higher harmonics within some plasma injection regions. We present results for a survey of over 8 years of Saturn data for fundamental and up to three harmonics of f ce, the electron cyclotron frequency. [ABSTRACT FROM AUTHOR]