Planetary Radio Emissions VIII

The Juno spacecraft successfully entered Jupiter orbit on 5 July 2016. One of Juno’s primary objectives is to explore Jupiter’s polar magnetosphere for the first time. An obvious major aspect of this exploration includes remote and in-situ observations of Jupiter’s auroras and the processes responsible for them. To this end, Juno carries a suite of particle, field, and remote sensing instruments. One of these instruments is a radio and plasma wave instrument called Waves, designed to detect one electric field component of waves in the frequency range of 50 Hz to 41 MHz and one magnetic field component of waves in the range of 50 Hz to 20 kHz. Juno’s first perijove pass with science observations occurred on 27 August 2016. This paper presents some of the first observations of the Juno Waves instrument made during that first perijove. Among radio emissions, kilometric, hectometric, and decametric emissions were observed. From a vantage point at high latitudes, many of Jupiter’s auroral radio emissions appear as V-shaped emissions in frequency–time space with vertices near the electron cyclotron frequency where the emissions intensify. In fact, we present observations suggesting Juno flew through or close to as many as five or six sources of auroral radio emissions during its first perijove. Waves made in-situ observations of plasma waves on auroral field lines such as whistler–mode hiss, a common feature of terrestrial auroral regions. We also discuss observations of dust at the equator and lightning whistlers observed over mid-latitudes.

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Observations of Jupiter’s low-frequency radio emissions were made using the radio and plasma wave instrument (Waves) onboard the Juno spacecraft in polar orbit around Jupiter since July 5, 2016. Waves is designed to monitor the electric fields of waves from 50 Hz to 41 MHz with an electric dipole antenna and the magnetic fields of waves from 50 Hz to 20 kHz with a magnetic search coil sensor. The Juno spacecraft rotates with a period of 30 s, which modulates the spectral intensity sensed with the dipole antenna. We report early results of the Juno Waves investigation on (1) the direction–finding measurements of broadband kilometric (bKOM) radiation just before Juno’s first perijove, (2) one concurrent decametric (DAM) radiation from the Juno spacecraft near Jupiter and the Nan¸cay Decameter Array in France during Juno’s interplanetary cruise, and (3) the statistical properties of Jovian DAM radio occurrence probability at 16 MHz obtained from Juno data of June 21 to December 10, 2016, then compared with the previous statistical results of the Cassini and Voyager observations during their Jupiter flybys.

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The Long Wavelength Array Station 1 (LWA1) is located in central New Mexico, USA. It consists of 256 pairs of ‘droopy–dipole’ antennas operating between 10 and 88 MHz. The antennas are distributed in a pseudo–random layout across a 110 m × 100 m region. Observations with the LWA1 are based on peer reviewed proposals for open–skies observing time. LWA1 is an excellent planetary radio emission instrument due to its sensitivity and the low radio frequency interference environment where it is located. We have undertaken several Jovian observing campaigns using the LWA1. We show that LWA1 data provide excellent spectral detail in Jovian decametric emission such as simultaneous left hand circular (LHC) and right hand circular (RHC) polarized Io-related arcs and source envelopes, modulation lane features, S-bursts structures, narrow band N-events, and apparent interactions between S-bursts and N-events. The start of the LHC Io-C source region was traced to earlier longitudes than typically found in the literature. Early LWA1 observations revealed a wealth of Io-D emission, including detection of rare S-bursts during an Io-D event. These initial results have led to new programs to explore the spectral characteristics of Io-D events, investigate modulation lanes of Io-B/Io-C events and examine the beaming structure of S-bursts combining LWA1, NDA, and URAN2. In addition, LWA1 is one of the ground-based support facilities for the NASA JUNO1 mission.

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Analyzing a database of 26 years of observations of Jupiter from the Nan¸cay Decameter Array, we study the occurrence of Io–independent emissions as a function of the orbital phase of the other Galilean satellites and Amalthea. We identify unambiguously the emissions induced by Ganymede and characterize their intervals of occurrence in CML and Ganymede phase and longitude. We also find hints of emissions induced by Europa and, surprisingly, by Amalthea. The signature of Callisto–induced emissions is more tenuous.

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The high latitude radio emissions produced by the Cyclotron Maser Instability (CMI) in Jupiter’s magnetosphere extend from a few kHz to 40 MHz. Part of the decametric emission is of auroral origin, and part is driven by the moons Io, Europa and Ganymede. After summarizing the method used to identify Jupiter– satellite radio emissions, which consists in comparing space- and ground-based radio observations to ExPRES simulations of CMI–driven emissions in the time–frequency plane, we present a parametric study of the free parameters required by the ExPRES code (electron distribution function and resonant energy, magnetic field model, lead angle, and altitude of the ionospheric cut-off) in order to assess the accuracy of our simulations in the Io–Jupiter case. We find that Io–DAM arcs are fairly modeled by loss–cone driven CMI with electrons of 1–10 keV energy, using the ISaAC, VIPAL, or VIP4 magnetic field model and a simple sinusoidal lead angle model. The altitude of the ionospheric cut-off has a marginal impact on the simulations. We discuss the impact of our results on the identification of Europa–DAM and Ganymede–DAM emissions.

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Observations of Jupiter’s Io-D decametric radio emissions have been made with the Long Wavelength Array Station 1 (LWA1) from 2012–2015. The LWA1 data show new characteristics in dynamic spectra of Jupiter Io-D emission, including a double–envelope structure filled with spectral arc features, persistent narrowband events, and ubiquitous modulation lanes and S-bursts. We observe an Io-D peak frequency of 26.5 MHz implying a lower limit of 9.3 Gauss for the southern hemisphere magnetic field magnitude. We estimate a Jovian longitude of 235° for the center location of the Io-D peak frequency source. Many S-burst drift rates were measured as a function of frequency and were found to be consistent with other Iorelated sources. The Io-D source occurrence boundaries require new limits: CML = 0° − 287° and ΦIo = 50° − 135°.

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The information about Jupiter’s decametric radio source locations is a very important key to understand the emission mechanism. By using the modulation lanes in the dynamic spectra of Jupiter’s decametric emissions we developed a remote sensing tool to investigate Jupiter’s radio source locations. This modulation lane method provides a very unique opportunity to know the source locations. Recently we have used it with data taken by the Long Wavelength Array Station 1 (LWA1). The high sensitivity of the LWA1 allows us to measure the slope of the modulation lanes more precisely for many Io–related sources in comparison to previous observations. The source locations and beam parameters can be calculated by these slope measurements. In this analysis we found the existence of two independent radio sources in the case of Io-C and Io-B events. We named the new components Io-C’ (Io-C–prime) and Io-B’ (Io-B–prime).

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We report the first systematic analysis of zebra–like fine spectral structures in the decametric frequency range of Jovian radio emission (DAM). These zebra patterns are observed in the frequency range from 12 to 30 MHz as a quasi-harmonically related bands (from 3 to 9) of enhanced brightness. The features have been observed by the ground-based radio telescope URAN-2 (Poltava, Ukraine). In total, 55 zebra pattern events have been detected during the period from September 2012 to March 2016. The minimum duration of one single zebra pattern was 20 s, and the maximum one was 4 min 50 s. The intensity of the zebra stripes is 1–2 magnitudes lower than the intensity of Io–controlled DAM. The numbers of stripes in one event may vary in time. The frequency interval between neighboring stripes is from 0.26 to 1.5 MHz. The zebra patterns are strongly polarized and have been observed as right-handed and left-handed polarized radio emission. The zebra patterns are mainly detected in two active sectors of Jovian CMLs: 100° to 160° for northern sources (righthanded polarized) and between 275° and 60° (via 360°) for the southern sources (left-handed). No correlation with the position of Io has been detected. We conclude that the observed zebra patterns are a new type of narrow band spectral structures in the Jovian decametric radio emission.

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A new passive subsurface radar technique using interference patterns in the spectrum of the Jovian hectometric and decametric radiation (HOM/DAM) has been proposed, and investigated for implementation on JUICE (Jupiter Icy Moons Explorer)/ RPWI (Radio and Plasma Wave Instrument). When there occurs interference among Jovian radio waves directly from Jupiter (W1), those reflected at the ice crust surface (W2), and those reflected at the subsurface reflectors in the ice crust (W3), fine and wide interference patters can be found in the spectrum. Fine patterns are caused by interference between W1 and W2, and between W1 and W3. Wide patterns are caused by interference between W2 and W3. In order to observe these interference patterns, the receiver of JUICE/RPWI is required to resolve 100 Hz, and possess a downlink spectra with a frequency range of 2 MHz and resolution of 1 kHz. Based on the calculation of the attenuation rate of the radio waves in the ice from 80 K (surface) to 250 K (just above the subsurface ocean), the intensity of the subsurface echo was estimated. The radar waves are expected to reach just above the ice crust/liquid ocean boundary. However, due to extremely high attenuation, it is difficult to detect the echoes from ice crust/liquid ocean boundary. In order to apply the new passive subsurface radar methods, the duration of the coherence of the Jovian radio wave should be long enough (>3.3 ms if spacecraft’s altitude is 500 km).

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The measurement of the H/H2 brightness ratio of giant planets’ far-ultraviolet (FUV) aurora is a proxy for precipitating soft (a few 10s keV) electrons. We investigate the relevance of this H/H2 indicator with the Jupiter auroral observations obtained by the Hubble Space Telescope. The H/H2 ratio does not show any clear relationship with the FUV color ratio which is sensitive to more energetic electrons. Compared to the same analysis applied for Saturn’s aurora, the relationship for Jupiter mainly shows decreasing flux with increasing energy without acceleration features.

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The internal magnetic field of Jupiter is known to be highly multi-polar, not only from the direct measurements performed by the Voyager and Pioneer probes but also from the unusually complex shape of the northern auroral oval. The limited amount of data obtained from the Voyager and Pioneer flybys do not permit accurate determination of the topology of the magnetic field, as they barely constrain, even the octupole contribution to the field. This does not allow one to reproduce the position of the auroras nor satisfactorily explain the shape and frequency range of the Jovian radio arcs. Successive attempts have been made to constrain the higher– degree field using the position of the Io auroral footprint where the auroras are due to currents generated close to Io and carried along the magnetic field lines. Thus, the auroral spots should map to Io’s orbit. VIPAL, the latest model of this kind is a 5th degree model. However, the VIPAL model was limited by three factors: the main constraints come from a unique L-shell, the difficulty of mixing Jovigraphic and magnetic data, and the non-linearity of the problem. These issues lead to numerically demanding computations, with the scale of computation increasing as the square of the model degree. We have developed a new method for computing the magnetic field using in-situ and auroral constraints (ISaAC) which we have applied to the computation of the Jovian magnetic field, based on Voyager, Pioneer, Galileo magnetic measurements and constrained by Io’s, Europa’s and Ganymede’s auroral footprint locations.

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The Saturnian Kilometric Radiation (SKR) is radiated from the auroral regions surrounding the Kronian magnetic poles, above the ionosphere up to a few planetary radii. It directly compares to the auroral radio emissions emanating from other planetary magnetospheres such as Earth and the giant planets. Our knowledge on SKR relied on remote observations of the Voyagers (flybys in 1980 and 1981) and Ulysses (distant observations in the 1990s) until Cassini started to orbit Saturn in 2004. Since then, it has been routinely observed from a large set of remote locations, but also in-situ for the first time at a planet other than Earth. This article reviews the state of the art of SKR average remote properties, the first insights brought by in-situ passes within its source region, together with some remaining questions before the Cassini Grand Finale and its close-in polar orbits.

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Despite the axisymmetry of Saturn’s internal magnetic field, a variety of magnetospheric properties and radio emissions exhibit periodic modulations due to Saturn’s rotation. Previous studies have shown that Saturn kilometric radiation (SKR) and auroral hiss have two different modulation rates, one associated with each hemisphere, which also vary over the time scale of a Saturn year. The narrowband emissions exhibit dual periodicities in each hemisphere. We update the modulation analysis of Saturn’s radio emissions to the end of year 2016. It is shown that the northern SKR rotation slowed to around 800°/day in 2016, while the south remained around 809°/day as of late 2015 (no clear rotation signal for southern SKR in 2016). When Cassini shifted to high inclination orbits at the end of 2016, a single modulation signal of the narrowband emission showed up around 800°/day. A rotational modulation signal for northern auroral hiss also showed up around 800°/day, but there was no signal for the southern auroral hiss.

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The energy flux of the northern (N–) and southern (S–) Saturn kilometric radiation (SKR) bursts are statistically compared. We investigated the N– and S–SKR bursts from 2005 DOY 250 to 2006 DOY 200, when Cassini was close to the equatorial plane and RPWS could simultaneously observe both N– and S–SKR. We identified 38 burst events, and compared their flux from southern (summer–side) and northern (winter–side) hemispheres. In the main band (100–400 kHz), S–SKR bursts from the summer–side hemisphere were 5–6 times stronger than the N–SKR bursts from the winter–side. This is not far from the flux ratio in the non-burst status. In the low-frequency extension (10–50 kHz) of SKR bursts, this ratio is smaller, about 2–3 times.

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The present research is devoted to the study of parameters of Saturn Electrostatic Discharges (SED) according to the data obtained during the observations of the initial period of storm J (December 2010) or the so-called Great White Spot (GWS). The ground-based detection was provided by the Ukrainian radio telescope UTR-2 at frequencies from 8 to 33 MHz in a wide range of time scales: from the day–to–day SED investigations to the temporal fine structure study up to microseconds. In this paper we describe our methods of data cleaning and the search for Saturn lightning in detail. The sensitivity of the observations allowed us to resolve the temporal micro–structure of lightning discharges. We determined the average signal’s dispersion delay for a session equal to (4.4±0.8) · 10−5 pc cm−3. It is close to the predicted value along the ray path from the storm to the radio telescope.

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Observations of whistler–mode chorus made by the THEMIS spacecraft inside the equatorial chorus source region often exhibit the following peculiar features. Two groups of chorus elements are visible simultaneously which are approaching the spacecraft from two different directions: either along or against the direction of the ambient magnetic field. Furthermore, both groups are slightly shifted in frequency with respect to each other and elements are of different intensities. We interpret these features in the frame of the backward–wave oscillator theory by means of two exemplary events, yielding insight into the nonlinear generation mechanism and the specific source–observer geometry during the time of observation.

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The problem of resolving spatial and temporal properties of waves, so-called “space–time ambiguity”, is a longstanding issue of single–spacecraft measurements. The general case can be insoluble, but in special cases in which certain assumptions hold, such as when each frequency corresponds to a single wave vector, the ambiguity can be resolved. Recently a method has been proposed to obtain wave vectors from single–spacecraft measurements of Alfvén wave–modulated magnetic fields and currents [Bellan, 2016], through application of the Wiener–Khinchin theorem to cross-correlation of the current density J and magnetic field B, and to the autocorrelation of B. We apply this method to spacecraft data, obtained by culling, from a large database of inertial Alfvén waves observed by the FAST satellite, two case study intervals during which extraordinarily large modulated currents were measured by the FAST particle detectors in burst mode. Results of this analysis for at least one of the two case studies appear consistent with known properties of ionospheric inertial Alfvén waves and pass error and consistency checks within the analysis method.

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We have investigated the propagation and amplification of Auroral Kilometric Radiation (AKR) in a narrow three-dimensional plasma cavity, in which a weakly relativistic electron beam propagates along the magnetic field. Both electron beam velocity components, parallel and perpendicular to the magnetic field are taken into account. Although the energy of transverse electron motion serves as a source of free energy for the development of electron cyclotron maser instability, it is found that for correct description of the AKR spectrum formation it is necessary to take the velocity component of electron motion directed along the magnetic field into account, because it gives a possibility for wave generation in a much wider frequency range. The results of calculations performed for thousands of waves have strengthened the assumption made in our previous paper [Burinskaya, 2013] on a basis of calculations for several waves, that the main factor, determining the wave energy at the time of wave escape from a source, is the duration of wave lifetime inside an amplification region. Thus, the global magnetic field inhomogeneity plays a key role in the formation of the AKR spectrum, because it defines for each wave its lifetime inside an amplification region and, by this means, the wave spectral intensity.

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Data gathered by the POLRAD swept frequency radiospectrograph (Interball–2 mission) have been used for a preliminary analysis of a number of short bursts of the Auroral Kilometric Radiation (AKR) as a function of their intensity. The AKR intensity samples consisted of data snapshots integrated over 6 ms time periods. The histograms based on 53 data sets, containing up to several thousand samples, exhibit a power–law fall for higher intensities, characteristic for Self–Organized Criticality (SOC). The scaling parameter α varies for most cases between 2 and 3 with the dominant value 2.5, and an error of the order of 0.1. The SOC approach has already been used for an interpretation of some magnetospheric processes, but never for AKR.

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The intensity of Jupiter’s auroral radio emission quickly gave rise to the question whether a comparable coherent emission from the magnetosphere of an extrasolar planet could be detectable. A simple estimation shows that exoplanetary auroral radio emission would have to be at least 1000 times more intense than Jupiter’s emission to be detectable with current radio telescopes. Theoretical models suggest that, at least in certain cases, the radio emission of giant exoplanets may indeed reach such an intensity. At the same time, in order to generate such an emission, an exoplanet would need to have a sufficiently strong intrinsic planetary magnetic field. Extrasolar planets are indeed expected to have a magnetic field, but to date, their magnetic field has never been detected. As discussed elsewhere [Grießmeier et al., 2015], the most promising technique to unambiguously observe exoplanetary magnetic fields is to search for the planetary auroral radio emission. The detection of such an emission would thus constitute the first unambiguous detection of an exoplanetary magnetic field. We review recent theoretical studies and discuss their results for the two main parameters, namely the maximum emission frequency and the intensity of the radio emission. The predicted values indicate that detection should be possible using modern low-frequency radio telescopes. We also review past observation attempts, and compare their sensitivity to the predicted emission.

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Detection of radio emission from exoplanets can provide information on the star– planet system that is difficult to study otherwise, such as the planetary magnetic field, magnetosphere, rotation period, interior structure, atmospheric dynamics and escape, and any star–planet interactions. Such a detection in the radio domain would open up a whole new field in the study of exoplanets. However, currently there are no confirmed detections of an exoplanet at radio frequencies. In this study, we search for non-thermal radio emission from the 55 Cnc system which has 5 known exoplanets. According to theoretical predictions 55 Cnc e, the innermost planet, is among the best targets for this search. We observed for 18 hours with the Low-Frequency Array (LOFAR) Low Band Antenna in the frequency range 26– 73 MHz with full polarization and covered 85% of the orbital phase of 55 Cnc e. During the observations four digital beams within the station beam were recorded simultaneously on 55 Cnc, nearby “empty” sky, a bright radio source, and a pulsar. A pipeline was created to automatically find and mask radio frequency interference, calibrate the time–frequency response of the telescope, and to search for bursty planetary radio signals in our data. Extensive tests and verifications were carried out on the pipeline. Analysis of the first 4 hours of these observations do not contain any exoplanet signal from 55 Cnc but we can confirm that our setup is adequate to detect faint astrophysical signals. We find a 3-sigma upper limit for 55 Cnc of 230 mJy using the pulsar to estimate the sensitivity of the observations and 2.6 Jy using the time series difference between the target and sky beam. The full data set is still under-going analysis. In the near future we will apply our observational technique and pipeline to the most promising exoplanet candidates for which LOFAR observations have already been obtained.

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A study of the plasma conditions in the atmosphere and ionosphere of the Hot Jupiter HD 209458b and for an HD 209458b-like planet at orbit locations of 0.2– 1 AU around a Sun-like star is presented. It is discussed how these conditions influence the radio emission expected from the planet’s magnetosphere. We find that the cyclotron maser instability (CMI) most likely will not operate at Hot Jupiters. It is found that close–in gas giants possess hydrodynamically expanding atmospheres and extended ionospheres with too high plasma densities within their magnetospheres, i.e. the plasma frequency is much higher than the cyclotron frequency, which is a contradiction to the necessary condition for the production of radio emission and also prevents the escape of radio waves for close–in extrasolar planets at distances <0.05 AU from a Sun-like host star. The structure of the upper atmosphere of Hot Jupiters around stars similar to the Sun changes for orbital distances between 0.2 and 0.5 AU from the hydrodynamic to a hydrostatic regime. This results in conditions where the plasma frequency can be lower than the cyclotron frequency, because a region of depleted plasma between the exobase and magnetopause can form. Like for e.g. Earth, in such an environment a beam of highly energetic electrons can propagate and be accelerated along the field lines towards the planet to produce radio emission. We also investigate the possible radio emission of the Hot Jupiter Tau Bootis b by placing it at different orbital distances from the host star, i.e. 0.046, 0.1 and 0.2 AU. It is checked if the atmosphere of Tau Bootis b at 0.046 AU is in the hydrodynamic or hydrostatic regime. In the hydrodynamic regime its ionosphere is extended and constitutes an obstacle for possibly generated radio waves; also, the generation via the Cyclotron Maser Instability (CMI) might be fully prevented.

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Extrasolar planets appear in a chemical diversity unseen in our own solar system. Despite their atmospheres being cold, continuous and transient plasma processes do affect these atmosphere where clouds form with great efficiency. Clouds can be very dynamic due to winds for example in highly irradiated planets like HD189733b, and lightning may emerge. Lightning, and discharge events in general, leave spectral fingerprints, for example due to the formation of HCN. During the interaction, lightning or other flash–ionization events also change the electromagnetic field of a coherent, high energy emission, which results in a characteristic damping of the initial, unperturbed (e.g. cyclotron emission) radiation beam. We summarize this as ’recipe for observers’. External ionization by X-ray or UV e.g. from within the interstellar medium or from a white dwarf companion will introduce additional ionization leading to the formation of a chromosphere. Signatures of plasma processes therefore allow for an alternative way to study atmospheres of extrasolar planets and brown dwarfs.

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Lightning induced radio emission has been observed on solar system planets. Lecavelier des Etangs et al. [2013] carried out radio transit observations of the exoplanet HAT-P-11b, and suggested a tentative detection of a radio signal. Here, we explore the possibility of the radio emission having been produced by lightning activity on the exoplanet, following and expanding the work of Hodos´an et al. [2016a]. After a summary of our previous work [Hodos´an et al. 2016a], we extend it with a parameter study. The lightning activity of a hypothetical storm is largely dependent on the radio spectral roll–off, n, and the flash duration, τfl. The best–case scenario would require a flash density of the same order of magnitude as can be found during volcanic eruptions on Earth. On average, 3.8×106 times larger flash densities than Earth–storms with the largest lightning activity are needed to produce the observed signal from HAT-P-11b. Combined with the results of Hodos´an et al. [2016a] regarding the chemical effects of planet–wide thunderstorms, we conclude that future radio and infrared observations may lead to lightning detection on planets outside the solar system.

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The Sun is an active source of radio emission which is often associated with the acceleration of electrons arising from processes such as solar flares and coronal mass ejections (CMEs). At low radio frequencies (<100 MHz), numerous solar S bursts (where S stands for short) and storms of type III radio bursts have been observed, that are not directly relates to flares and CMEs. Here, we expand our understanding on the spectral characteristic of these two different types of radio bursts based on observations from the Low Frequency Array (LOFAR). On 9 July 2013, over 3000 solar S bursts accompanied by over 800 type III radio bursts were observed over a time period of ∼8 hours. The characteristics of type III radio bursts presented here are consistent with previous studies. S bursts are shown to be different compared to type III bursts: they show narrow bandwidths, short durations and drift rates of about 1/2 the drift rate of type III bursts. Both type III bursts and solar S bursts occur in a region in the corona where plasma emission is the dominant emission mechanism as determined by data constrained density and magnetic field models.

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On 9 July 2013 from 5:30 UT till 13:28 UT more than 1000 S-bursts were recorded by the Ukrainian radio telescope UTR-2 operated in the frequency band 9–32 MHz. All S-bursts were low intensity events with an average flux of about 10 s.f.u. and a minimum flux as low as 0.2 s.f.u. which made their detection with small instruments practically impossible. New methods of observations allowed to retrieve the weakest S-bursts with fluxes comparable to the background level. The durations and frequency drift rates of these bursts as well as the dependencies of these parameters on frequency were found. The obtained results complement the analysis by Morosan et al. [2015] with data at lowest frequencies accessible for ground-based observations.

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On April 7, 2011 a CME was observed originating above the AR NOAA 1176, located behind the solar limb. This CME was associated with type IV burst, type II bursts and groups of J-bursts and type III bursts. Groups of J-bursts and type III bursts were observed from 10:50 to 11:20 UT. There was a group of J-bursts from 10:52 to 10:57 UT, originating from accelerated electrons propagating along high magnetic loops connected with the active region NOAA 1176. The group of type III bursts continued from 11:00 to 11:20 UT. There were a lot of spikes and type IIIb bursts with polarizations up to 80% during the group of type III bursts. Type IV burst began at 11:20 UT at 32 MHz and continued for more than 3 hours. Its maximum flux was about 200 s.f.u., and the polarization achieved 40%. There were 3 type II bursts superimposed by type IV burst. Their drift rates were approximately 0, 10 kHz/s, 25 kHz/s, and it appears that they were radio emissions from different regions of the spherical shock produced by the CME. All type II bursts consisted of tadpoles with ”heads” and ”tails”. Their durations were 4 s and 2 s, and their polarizations were about 10% and 40%, correspondingly. The frequency bands of the ”tails” was up to 10 MHz. Their frequency drift rates could be positive and negative ones, and their absolute values changed from 0.4 MHz/s to 4 MHz/s. Previously we found that decameter type IV bursts oscillated with periods of tens of minutes. The type IV burst observed on April 7, 2011 showed oscillations also. Fourier and wavelet analyses showed periods of about 40 minutes for both fluxes and polarizations. Moreover, it turned out that these periods decreased with time with rates of 0.03–0.07. Interpretations of all decameter phenomena are discussed.

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In this paper we consider solar bursts with a high-frequency cut-off, simultaneously observed by several ground-based radio observatories located at significant distances from each other. The events were correlated with the emergence of a new group of solar spots on the far side of the Sun. The cut-off effect of the solar bursts was caused by the occultation of their radiating sources by the solar corona for observers on Earth. Based on the radio occultation of the low-frequency burst sources in the solar corona, we have estimated their angular sizes. This makes it possible to obtain the dynamic spectra of the solar bursts in terms of brightness temperature.

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HeRO (Heliophysics Radio Observer) is a hybrid ground and space mission concept for radio interferometry of solar radio bursts. The space segment (HeRO-S) covers low frequencies, 100 kHz–20 MHz, and is composed of 6 free–flying Cube- Sats equipped with vector sensors. The ground segment (HeRO-G), covers higher frequencies, 15 MHz–300 MHz. HeRO will explore conditions and disturbances in a key region of the heliosphere, from two to tens of solar radii, using interferometric observations of solar radio bursts over three decades in frequency. Spot mapping across the full frequency range will provide precise positions and basic structural information about type II and III radio bursts. The morphology of CME shock fronts will be traced via type II burst emissions, and heliospheric magnetic field geometries will be probed by measuring precise trajectories of type III bursts. Refraction in the heliospheric plasma on large and intermediate scales will be investigated throughout large volumes via the frequency dependence of accurate interferometric positional data on bursts. The HeRO data will be information rich with high resolution in time, frequency and spatial position, and high SNR, creating fertile ground for discovery of new phenomena.

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Low frequency imaging radio arrays such as MWA, LWA and LOFAR have been recently commissioned, and significantly more advanced and flexible arrays are planned for the near term. These powerful instruments offer new opportunities for direct solar imaging at high time and frequency resolution. They can also probe large volumes of the heliosphere simultaneously, by virtue of very large fields of view. They allow highly detailed, spatially resolved study of solar and heliospheric radio bursts, which are complemented by heliospheric propagation studies using both background astronomical radio emissions as well as the bursts themselves. In this paper, the state of the art in such wide field solar and heliospheric radio studies is summarized, including recent results from the Murchison Widefield Array (MWA). The prospects for major advances in observational capabilities in the near future are reviewed, with particular emphasis on the RAPID system developed at Haystack Observatory.

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The LOw–Frequency ARray (LOFAR) is a radio interferometer operating in the frequency range 10 – 240 MHz (corresponding to wavelengths of 30 – 1.2 m). Important issues of its broad scientific program are the solar and space weather investigations. We are expecting that the LOFAR telescope will bring interesting results in these fields. Three new LOFAR stations were built in Poland in 2015 and have been operating since the beginning of 2016. By including these stations to the ILT (International LOFAR Telescope), the resolution and sensitivity of the whole interferometer were improved and they are, for 240 MHz, 0.1 arcsec and 9.17 mJy/beam, respectively. Using a single LOFAR station, spectroscopic observations of the Sun can be performed; more stations allow us to obtain solar radio images.

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Observations of solar radio bursts is a useful tool to study non-thermal electron acceleration and the plasma environment in the solar corona. The radio bursts in a frequency range from 150 to 500 MHz with fine temporal and spectral resolutions (10 ms and 61 kHz) have been observed with the AMATERAS radio spectro– polarimeter installed at the Iitate Planetary Radio Telescope since 2010. Here we review results obtained from the AMATERAS observation and introduce the database which is open to the public. The AMATERAS receiver consists of a wideband and low-noise front–end receiver and a digital spectrometer. Both right and left-hand polarized components are simultaneously observed. The combination of a large aperture area of the telescope and the digital receiver enables us to observe the radio burst with high dynamic range and fine spectral resolution. After a daily observation of the Sun, a data processing pipeline generates low and high resolution data sets. The low resolution data with reduced resolutions of 1 s, 1 MHz, and 8 bits is converted to the FITS format and distributed through the AMATERAS Data Center. Quick look (PNG format) and meta-data of the FITS–format file are registered to the Virtual European Solar and Planetary Access (VESPA) and Inter– university Upper atmosphere Global Observation NETwork (IUGONET) database. The high resolution data set has fine resolutions of 10 ms and 61 kHz, but the dynamic range is reduced to be 8 or 16 bits depending on the intensity of the radio burst observed. It is currently provided on request basis. ∗

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The Nançay Decameter Array (NDA) routinely observes low frequency (10– 100 MHz) radio emissions of Jupiter and the Sun since 4 decades. The NDA observations, acquired with a variety of receivers with increasing performances, were the basis for numerous studies of Jovian and solar radio emissions and now form a unique long-term database spanning ≥ 3 solar cycles and Jovian revolutions. In addition, the NDA historically brought a fruitful support to space-based radio observatories of the heliosphere, to multi-wavelength analyses of solar activity and contributes to the development of space weather services. After having summarized the NDA characteristics, this article presents latest instrumental and database developments, some recent scientific results and perspectives for the next decade.

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Rapid progress currently takes place in the field of low-frequency radio astronomy in the meter–decameter–hectometer range of wavelengths. It is caused by a radical modernization of the existing radio telescopes, creation of a new generation of instruments, space-borne observations, and by the development of research on all classes of astrophysical objects, including the Solar System. On the other hand, a range of difficulties specific to low-frequency radio astronomy is known, which are caused by technical, methodological, and physical limitations. An effective strategy for overcoming these difficulties is based on synchronous observations using several radio telescopes separated by distances from a few to several thousand kilometers. This provides an opportunity to reduce and identify radio interference and the influence of the propagation media, to increase the sensitivity and resolution, and to solve many problems with higher efficiency. In recent years such simultaneous observations were carried out for the Sun, Jupiter, Saturn, interplanetary medium, pulsars, exoplanets, and transients using the radio telescopes UTR-2, URAN, GURT, NDA, NenuFAR, LOFAR and other. Parallel observations with the space missions WIND, STEREO, Cassini and Juno also facilitate improvement of the quality and reliability of low-frequency radio astronomical experiments.

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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.

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The ExoMars 2020 Surface Platform will conduct environmental and geophysical measurements with the aim of studying the Martian surface and subsurface environment at the landing location. The Surface Platform instrumentation will include the Wave Analyzer Module (WAM) as a European contribution to the Russian–led Martian ground electromagnetic tool (MAIGRET) instrument. The wave analyzer module will be dedicated to measurements of magnetic field fluctuations in the frequency band from 100 Hz to 20 kHz. The scientific questions which we plan to address by measurements of the WAM have never been answered by direct measurements of the fluctuating magnetic fields in the appropriate range of frequencies directly on the surface of the planet. The immediate questions related to these targets are: 1. Can we observe electromagnetic radiation from electric discharges in the Martian dust storms? 2. Can we observe electromagnetic radiation propagating from the interplanetary space down to the surface of the planet?

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We present science objectives of the Software–type Wave–Particle Interaction Analyzer (S–WPIA), which will be realized as a software function of the Low- Frequency receiver (LF) running on the DPU of RPWI (Radio and Plasma Waves Investigation) for the ESA JUICE mission. S–WPIA conducts onboard computations of physical quantities indicating the energy exchange between plasma waves and energetic ions. Onboard inter–instruments communications are necessary to realize S–WPIA, which will be implemented by efforts of RPWI, PEP (Particle Environment Package) and J–MAG (JUICE Magnetometer). By providing the direct evidence of ion energization processes by plasma waves around Jovian satellites, S–WPIA increases the scientific output of JUICE while keeping its impact on the telemetry data size to a minimum; S–WPIA outputs 0.2 kB at the smallest from 440 kB waveform and particle raw data.

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Voyager/PRA (Planetary Radio Astronomy) data from digitized tapes archived at CNES have been reprocessed and recalibrated. The data cover the Jupiter and Saturn flybys of both Voyager probes. We have also reconstructed goniopolarimetric datasets (flux and polarization) at full resolution. These datasets are currently not available to the scientific community, but they are of primary interest for the analysis of the Cassini data at Saturn, and the Juno data at Jupiter, as well as for the preparation of the JUICE mission. We present the first results derived from the re-analysis of this dataset.

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Short dipole or monopole radio antennas are defined as being small in length relative to the wavelength of the frequency of operation. The reception properties of short linear antennas can be described by the so-called effective length vector which is pointing along the direction of minimum gain in the toroidal radiation pattern. We deal here with such antennas, and additionally the word ”short” also means a small antenna with respect to a large spacecraft body. Using numerical computer simulations we calculate the reception properties of an antenna system consisting of three short monopoles positioned on a large spacecraft body in the frequency range of several hundred kHz. It turns out that such a configuration has the major disadvantage that the angular separation between its three effective length vectors is quite small, which would lead to large errors in polarization and direction–finding measurements. We will show ways how to overcome this problem by changing the configuration to an antenna triad consisting of three short dipoles mounted on a boom. The calculations were employed to find a good configuration of the radio antennas for ESA’s future JUICE (Jupiter Icy Moons Explorer) mission.

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