Decametric Radio Emission Research Articles

Effect of the Earth’s declination variation on characteristics of Jovian decametric radio emissions

Context. The variation in the Jovicentric sub-latitude (declination, DE) of a radio observer of Jupiter has long been known to affect the observation of Jupiter’s decametric (DAM) radio emissions due to these emissions’ anisotropic nature (through cyclotron maser instability beaming cones centered on Jovian magnetic field lines). The effect of the DE variation, however, is still not clearly understood. For ground-based observations of Jupiter, the DE variation, from −4° to +4°, occurs concomitantly with the cyclic variation in the distance to Jupiter, R, and Jupiter’s elongation angle, γ, which also affect the emission observation. Those covariant effects must be removed, then, for an analysis of the pure effect of DE. Aims. The aim of this study is to investigate the pure effect of the Earth’s DE variation on the maximum frequency, duration, average Io phase, and average longitude of Io-induced DAM emissions observed with the Nançay Decameter Array (NDA). Methods. For this purpose, we selected from the NDA/Routine digital catalog Jovian DAM emissions with an intensity (distance-corrected) above or equal to 8.8 dB and a maximum frequency above or equal to 20 MHz (25 MHz) for southern (northern) emissions. Distinct maximum frequency thresholds were adopted because of the typical discrepancy in the emissions’ frequency due to the high amplitude anomaly in the Jovian magnetic field at Jupiter’s northern hemisphere. The selected emissions comprise a new “unbiased” catalog. After analyzing the tenuous variation in the characteristics of the unbiased set of Io-DAM emissions with DE, we compared them with those of matching Io-DAM simulations obtained with the Exoplanetary and Planetary Radio Emissions Simulator (ExPRES). Results. From both the NDA data and the ExPRES simulations, it is observed that the pure DE effect on the Io-DAM emissions characteristics is minor, yet a clear proportionality of the maximum frequency and duration of the northern Io-DAM emissions with DE is noticed. Conclusions. The northern Io-DAM emissions seem to be more strongly affected by the DE variation than the southern emissions. Additionally, ExPRES can predict Io-DAM emissions consistently, from which we conclude that the current understanding of emission generation and propagation is reasonable. This study may be extended for broader ranges of DE, such as Juno’s.

Origin of ‘‘Bursts in Absorption’’ in the Jovian Decameter Radio Emission

Origin of ‘‘Bursts in Absorption’’ in the Jovian Decameter Radio Emission

Statistical study of the Jovian decametric radio emissions based on multiple-view observations from remote radio instruments

Statistical study of the Jovian decametric radio emissions based on multiple-view observations from remote radio instruments

Method to observe Jupiter’s radio emissions at high resolution using multiple LOFAR stations: a first case study of the Io-decametric emission using the Irish IE613, French FR606, and German DE604 stations

ABSTRACT The Low Frequency Array (LOFAR) is an international radio telescope array, consisting of 38 stations in the Netherlands and 14 international stations spread over Europe. Here, we present an observation method to study the Jovian decametric radio emissions from several LOFAR stations (here Birr Castle in Ireland, Nançay in France, and Postdam in Germany), at high temporal and spectral resolution. This method is based on prediction tools, such as radio emission simulations and probability maps, and data processing. We report an observation of Io-induced decametric emission from 2021 June, and a first case study of the substructures that compose the macroscopic emissions (called millisecond bursts). The study of these bursts makes it possible to determine the electron populations at the origin of these emissions. We then present several possible future avenues for study based on these observations. The methodology and study perspectives described in this paper can be applied to new observations of Jovian radio emissions induced by Io, but also by Ganymede or Europa, or Jovian auroral radio emissions.

Sizes and Shapes of Sources in Solar Metric Radio Bursts

Abstract Metric and decametric radio emissions from the Sun are the only direct source of information about the dynamics of nonthermal electrons in the upper corona. In addition, the combination of spectral and imaging (sizes, shapes, and positions) observations of low-frequency radio sources can be used as a unique diagnostic tool to probe plasma turbulence in the solar corona and inner heliosphere. The geometry of the low-frequency sources and its variation with frequency are still not understood, primarily due to the relatively low spatial resolution available for solar observations. Here we report the first detailed multifrequency analysis of the sizes of solar radio sources observed by the Low Frequency Array. Furthermore, we investigate the source shapes by approximating the derived intensity distributions using 2D Gaussian profiles with elliptical half-maximum contours. These measurements have been made possible by a novel empirical method for evaluating the instrumental and ionospheric effects on radio maps based on known source observations. The obtained deconvolved sizes of the sources are found to be smaller than previous estimations, and often show higher ellipticity. The sizes and ellipticities of the sources inferred using 2D Gaussian approximation, and their variation with frequency are consistent with models of anisotropic radio-wave scattering in the solar corona.

Reply to Comment by Lamy et al. on “Locating the source field lines of Jovian decametric radio emissions”

Locating the source of decametric (DAM) radio emissions is a key step in the use of remote radio observations to understand the Jovian magnetospheric dynamics and their interaction with the planet’s moons. Wang YM et al. (2020) presented a method by which recorded arc-shaped DAM emissions in the radio dynamic spectra can be used to locate the source of a DAM. An Io-related DAM event on March 14, 2014 was used to demonstrate the method. A key parameter in the method is whether the DAM is emitted in the northern or the southern hemisphere; the hemisphere of origin can be determined definitively from the polarization of the emission. Unfortunately, polarization information for the emission on March 14, 2014 event was not recorded. Our analysis assumed the source to be in the northern hemisphere. Lamy et al. (2022) argue convincingly that the source was probably in the southern hemisphere. We appreciate the helpful contribution of Lamy et al. (2022) to this discussion and have updated our analysis, this time assuming that the DAM source was in the southern hemisphere. We also explore the sensitivity of our method to another parameter — the height at which the value of <italic>f</italic>_ce,max, which is the maximal electron cyclotron frequency reached along the active magnetic flux tube, is adopted. Finally, we introduce our recent statistical study of 68 DAM events, which lays a more solid basis for testing the reliability of our method, which we continue to suggest is a promising tool by which remote radio observations can be used to locate the emission source of Jovian DAMs.

Comment on “Locating the source field lines of Jovian decametric radio emissions” by YuMing Wang et al.

In this comment on the article “Locating the source field lines of Jovian decametric radio emissions” by Wang YM et al., 2020, we discuss the assumptions used by the authors to compute the beaming angle of Jupiter’s decametric emissions induced by the moon Io. Their method, relying on multi-point radio observations, was applied to a single event observed on 14^th March 2014 by Wind and both STEREO A/B spacecraft from ~5 to ~16 MHz. They have erroneously identified the emission as a northern (Io-B type) instead of a southern one (Io-D type). We encourage the authors to update their results with the correct hemisphere of origin and to test their method on a larger sample of Jupiter-Io emissions.

ДОСЛІДЖЕННЯ СОНЯЧНОГО ДЕКАМЕТРОВОГО РАДІОВИПРОМІНЮВАННЯ ЗА ДОПОМОГОЮ РАДІОТЕЛЕСКОПА УТР-2

Purpose: The overview of the scientifi c papers devoted to the study of the solar decameter radio emission with the world’s largest UTR-2 radio telescope (Ukraine) published for the last 50 years. Design/methodology/approach: The study and analysis of the scientifi c papers on both sporadic and quiet (thermal) radiation of the Sun recorded with the UTR-2 radio telescope at the decameter wavelength range. Findings: The most signifi cant observational and theoretical results of the solar radio emission studies obtained at the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine for the last 50 years are given. Conclusions: For the fi rst time, at frequencies below 30 MHz, the Type II bursts, Type IV bursts, S-bursts, drift pairs and spikes have been recorded. The dependences of these bursts parameters on frequency within the frequency band of 9 to 30 MHz were obtained. The models of their generation and propagation were suggested. Moreover, for the fi rst time the fi ne time-frequency structures of the Type III bursts, Type II bursts, Type IV bursts, U- and J-bursts, S-bursts, and drift pairs have been observed due to the high sensitivity and high time-frequency resolutions of the UTR-2 radio telescope. The super-fi ne structure of Type II bursts with a “herringbone” structure was identifi ed, which has never been observed before. New types of bursts were discovered: “caterpillar” bursts, “dog-leg” bursts, Type III bursts with decay, Type III bursts with changing drift rate sign, Type III-like bursts, Jb- and Ub-bursts, etc. An interpretation of the unusually high drift rates and drift rates with alternating signs of the Type III-like bursts was suggested. Based on the dependence of spike durations on frequency, the coronal plasma temperature profi le at the heliocentric heights of 1.5–3RS was determined. Usage of the heliographic and interferometric methods gave the possibility to start studies of the spatial characteristics – sizes and locations of the bursts emission sources. Thus, it was shown that at the decameter band, the Type III burst durations were defi ned by the emission source linear sizes, whereas the spike durations were governed by the collision times in the source plasma. It was experimentally proved that the effective brightness temperatures of the sources of solar sporadic radio emission at the decameter band may reach values of 1014–1015 K. In addition, it was found that the radii of the quiet Sun at frequencies 20 and 25 MHz are close to the distances from the Sun at which the local plasma frequency is equal to the corresponding observed frequency of radio emission in the Baumbach–Allen model. Key words: UTR-2; Sun; decameter radio emission; radio bursts; corona

Origin of the zebra structure in the Jovian decameter radio emission

Context.We discuss the origin of quasi-harmonic emission bands that have been observed in the dynamic spectra of the Jovian decameter emission.Aims.We aim to show that the interpretation of the observed structure can be based on the effect of double plasma resonance (DPR) at ion cyclotron harmonics.Methods.According to the proposed model, in the extended source in the upper ionosphere of Jupiter, where the DPR condition is satisfied for one of the ion cyclotron frequency harmonics, the ion cyclotron waves are effectively excited at the frequency of the lower hybrid resonance. The observed electromagnetic radiation with a quasi-harmonic structure arises due to scattering of ion cyclotron waves by supra-thermal electrons.Results.Based on the VIP4 magnetic field model, we determine the longitudes at which the source of the considered radiation can be located. The obtained estimates of the plasma density and its height distribution in the source, as well as the energies of emitting ions and scattering electrons provide information about the plasma parameters in the upper ionosphere of Jupiter. Furthermore, these estimates are in good agreement with the observational data.

Ganymede‐Induced Decametric Radio Emission: In Situ Observations and Measurements by Juno

AbstractAt Jupiter, part of the auroral radio emissions are induced by the Galilean moons Io, Europa, and Ganymede. Until now, they have been remotely detected, using ground‐based radio telescopes or electric antennas aboard spacecraft. The polar trajectory of the Juno orbiter allows the spacecraft to cross the magnetic flux tubes connected to these moons, or their tail, and gives a direct measure of the characteristics of these decametric moon‐induced radio emissions. In this study, we focus on the detection of a radio emission during the crossing of magnetic field lines connected to Ganymede's tail. Using electromagnetic waves (Juno/Waves) and in situ electron measurements (Juno/JADE‐E), we estimate the radio source size of ∼250 km, a radio emission growth rate &gt;3 × 10−4, a resonant electron population of energy –15 keV and an emission beaming angle of θ = 76–83°, at a frequency ∼1.005–1.021 × fce. We also confirmed that radio emission is associated with Ganymede's downtail far ultraviolet emission.

Juno Reveals New Insights Into Io‐Related Decameter Radio Emissions

AbstractThe Juno mission is providing stunning new information about Jupiter and its environment. A new magnetic field model (JRM09) with much improved accuracy near the planet provides the basis for a better understanding of Io‐related decametric radio emissions (DAM) and implications for auroral processes. Here, we selected Io‐related DAM events observed by the Juno Waves instrument to shed light into the beaming angle, the resonant electron energy, and radio source location by forward modeling. We use the JRM09 model to better constrain the location and observability of DAM and characterize the loss cone‐driven electron cyclotron maser instability. We obtained good agreement between synthetic and observed arcs with calculated beaming angles ranging from 33° to 85° and resonant electron energies up to 23 times higher than previously proposed. In addition, through a quantitative analysis, we provide an explanation regarding the higher likelihood of observing groups of arcs of Io DAM originating in the northern hemisphere relative to those originating in the southern hemisphere. This is primarily a consequence of the asymmetry of the magnetic field geometry, observer location, and pitch angles of the electrons at the equator.

Juno reveals new insights into Io-related decameter radio emissions

Juno reveals new insights into Io-related decameter radio emissions

Juno reveals new insights into Io-related decameter radio emissions

Juno reveals new insights into Io-related decameter radio emissions

Locating the source field lines of Jovian decametric radio emissions

Decametric (DAM) radio emissions are one of the main windows through which one can reveal and understand the Jovian magnetospheric dynamics and its interaction with the moons. DAMs are generated by energetic electrons through cyclotron-maser instability. For Io (the most active moon) related DAMs, the energetic electrons are sourced from Io volcanic activities, and quickly trapped by neighboring Jovian magnetic field. To properly interpret the physical processes behind DAMs, it is important to precisely locate the source field lines from which DAMs are emitted. Following the work by Hess et al. [2008, 2010], we develop a method to locate the source region as well as the associated field lines for any given DAM emission recorded in a radio dynamic spectrum by, e.g., Wind/WAVES or STEREO/WAVES. The field lines are calculated by the state-of-art analytical model, called JRM09 [Connerney et al., 2018]. By using this method, we may also derive the emission cone angle and the energy of associated electrons. If multiple radio instruments at different perspectives saw the same DAM event, the evolution of its source region and associated field lines is able to be revealed. We apply the method to an Io-DAM event, and find that the method is valid and reliable. Some physical processes behind the DAM event are also discussed.

Comparative analysis of solar radio bursts before and during CME propagation

Context. As is well known, coronal mass ejection (CME) propagation often results in the fragmentation of the solar atmosphere on smaller regions of density (magnetic field) enhancement (depletion). It is expected that this type of fragmentation may have radio signatures. Aims. The general aim of the present paper is to perform a comparative analysis of type III solar and narrow-band type-III-like radio burst properties before and during CME events, respectively. The main goal is to analyze radio observational signatures of the dynamical processes in solar corona. In particular, we aim to perform a comparison of local plasma parameters without and with CME propagation, based on the analysis of decameter radio emission data. Methods. In order to examine this intuitive expectation, we performed a comparison of usual type III bursts before the CME with narrow-band type-III-like bursts, which are observationally detectable on top of the background type IV radio bursts associated with CME propagation. We focused on the analysis of in total 429 type III and 129 narrow-band type-III-like bursts. We studied their main characteristic parameters such as frequency drift rate, duration, and instantaneous frequency bandwidth using standard statistical methods. Furthermore, we inferred local plasma parameters (e.g., density scale height, emission source radial sizes) using known definitions of frequency drift, duration, and instantaneous frequency bandwidth. Results. The analysis reveals that the physical parameters of coronal plasma before CMEs considerably differ from those during the propagation of CMEs (the observational periods 2 and 4 with type IV radio bursts associated with CMEs). Local density radial profiles and the characteristic spatial scales of radio emission sources vary with radial distance more drastically during the CME propagation compared to the cases of quasistatic solar atmosphere without CME(s) (observational periods 1 and 3). Conclusions. The results of the work enable us to distinguish different regimes of plasma state in the solar corona. Our results create a solid perspective from which to develop novel tools for coronal plasma studies using radio dynamic spectra.

Zebra pattern in decametric radio emission of Jupiter

We report the systematic analysis of zebra-like fine spectral structures in decametric frequency range of Jovian radio emission. Observations were performed by the large ground-based radio telescope URAN-2 during three observation campaigns between, Sep., 2012, and May, 2015. In total, 51 zebra pattern (ZP) events were detected. These rare fine radio features are observed in frequency range from 12.5 to 29.7 MHz as quasi-harmonically related bands of enhanced brightness. ZPs are strongly polarized radio emission with a duration from 20 s to 290 s and flux densities ~105−106 Jy (normalized to 1 AU), that is, 1–2 orders lower than for Io-decametric radio emission (DAM). Occurrence of the events does not depend on the position of Io satellite but is strongly controlled by the Jovian central meridian longitude (CML). ZPs are mainly detected in two active sectors of Jovian CMLs: 100∘ to 160∘ for Northern sources (right-handed polarized) and 300∘ and 60∘ (via 360∘) for the Southern sources (left-handed). The frequency interval between neighboring stripes is from 0.26 to 1.5 MHz and in most cases this interval increases with frequency. We discussed the double plasma resonance with electrons or ions as a possible source of the ZPs. The performed analysis of the observations allows us to conclude that the observed ZPs are a new type of narrow band spectral structures in the Jovian DAM.

Automated detection of Sun decametre radio bursts by the radio telescope URAN-3

The problem of the automated determination of the parameters of the sporadic decameter radio emission of the Sun with help of the radio-telescope URAN-3 taking into account its functions in the Ukrainian VLBI URAN structure is analyzed. An overview of alternative solutions of the problem is carried out. The technology of searching and detecting the radio bursts of type II and determination of such parameters as speed of drift in frequency sub bands, intensity, and duration and frequency width of bursts for a number of selected frequencies are considered. The effectiveness of the developed technology is confirmed by the results of long-term radioastronomical observations carried out at the URAN-3 radio telescope in 2011–2017. The formed on the basis of obtained and accumulated experimental data multiparameter relative database of solar activity is intended for further studies of the relation between the helio- and the geophysical phenomena for the purpose of developing forecasting models of geophysical manifestations of solar activity and evaluating the “geoefficiency” of active solar processes.

Statistical analysis of 26 yr of observations of decametric radio emissions from Jupiter

Jupiter is a complex and at the same time very powerful radio source in the decameter wavelength range. The emission is anisotropic, intrinsically variable at millisecond to hour timescales, and also modulated by various external processes at much longer periods, ranging from ~10 h to months or years (including Jovian day and year, solar activity and solar wind variations, and for ground-based observations, terrestrial day and year). As a consequence, long-term observations and their statistical study have proved to be necessary for disentangling and understanding the observed phenomena. We have built a database from the available 26 yr of systematic, daily observations conducted at the Nançay Decameter Array and recorded in digital format. This database contains all observed Jovian decametric emissions, classified with respect to the time-frequency morphology, their dominant circular polarization, and maximum frequency. We present the results of the first statistical analysis of this database. We confirm the earlier classification of Jovian decameter emissions in Io-A, -A′, -B, -C, -D and non-Io-A, -B, -C types, but we also introduce new emission types (Io-A′′ and Io-B′) and precise and characterize the non-Io-D type. We determine the contours of all emission types in the CML− ΦIo plane (Central Meridian Longitude in Jupiter’s System III coordinates versus Io Phase), provide representative examples of their typical time-frequency patterns, and the distribution of emission’s maximum frequency as a function of ΛIo (Io’s Longitude). Finally, we present a statistical analysis of the distributions of the occurrence rate, duration, intensity and polarization for each emission type. non-Io-DAM appears to be related to small-scale, possibly bursty auroral structures.

Latitudinal beaming of Jovian decametric radio emissions as viewed from Juno and the Nançay Decameter Array

AbstractTwo well‐defined Jovian decametric radio arcs were observed at latitudinal separations of 11°–16° from the Juno spacecraft near Jupiter and the Nançay Decameter Array (NDA) at Earth on 17 May and 25 August 2016. These discrete arcs are from the so‐called A source covering both Io‐related and non‐Io‐related emissions. By measuring the wave arrival time at two distant observers with propagation time correction, the remaining delay times are 92.8 ± 1.3 min for the first arc and 116.0 ± 1.2 min for the second arc. This implies that both radio sources are not controlled by the orbital motion of Io but Jupiter's rotation itself. The geometrical information for Juno and NDA and the loss cone‐driven electron cyclotron maser instability theory provide these radio sources that are located at about 173° ± 10° in system III longitude projected onto Jupiter's north surface and imply resonant electron energy ranges from 0.5 to 11 keV.

Progress in low-frequency radio astronomy and I.S. Shklovskii’s contribution to its development

Radio astronomy at decameter wavelengths is currently undergoing very active development. Large-scale, new generation low-frequency radio telescopes are being constructed and already used in many countries around the world. As before, the largest, most sensitive, and most versatile telescope at decameter wavelengths is the Ukrainian UTR-2 radio telescope operating at 8–32 MHz, which has an effective area of more than 105 m2 and an angular resolution of about 0.5◦, as well as the URAN interferometric system based on the UTF-2. Many studies that have been carried out on these facilities have been based on important results and far-sighted predictions of Shklovskii. These include, in particular, studies of dynamical spectra and the brightness distributions of the sporadic and quiescent decameter radio emission of the hot solar corona, complex, broadband radio spectroscopy of the interstellar medium, and multi-frequency monitoring of secular decreases in the flux densities of supernova remnants. The coordinated use of highly effective existing and newly constructed radio telescopes joined into ground networks, as well as specialized space missions, are opening new prospects for low-frequency radio astronomy.