Planetary Radio Astronomy Research Articles

James Walter (Jim) Warwick (1924–2013)

Warwick helped establish the field of low frequency radio astronomy. He was principal investigator for NASA’s Voyager I and II Planetary Radio Astronomy instruments, that measured the radio emission from Jupiter, Saturn, Uranus and Neptune.

Planetary radio astronomy: Earth, giant planets, and beyond

Abstract. The magnetospheric phenomenon of non-thermal radio emission is known since the serendipitous discovery of Jupiter as radio planet in 1955, opening the new field of "Planetary Radio Astronomy". Continuous ground-based observations and, in particular, space-borne measurements have meanwhile produced a comprehensive picture of a fascinating research area. Space missions as the Voyagers to the Giant Planets, specifically Voyager 2 further to Uranus and Neptune, Galileo orbiting Jupiter, and now Cassini in orbit around Saturn since July 2004, provide a huge amount of radio data, well embedded in other experiments monitoring space plasmas and magnetic fields. The present paper as a condensation of a presentation at the Kleinheubacher Tagung 2013 in honour of the 100th anniversary of Prof. Karl Rawer, provides an introduction into the generation mechanism of non-thermal planetary radio waves and highlights some new features of planetary radio emission detected in the recent past. As one of the most sophisticated spacecraft, Cassini, now in space for more than 16 years and still in excellent health, enabled for the first time a seasonal overview of the magnetospheric variations and their implications for the generation of radio emission. Presently most puzzling is the seasonally variable rotational modulation of Saturn kilometric radio emission (SKR) as seen by Cassini, compared with early Voyager observations. The cyclotron maser instability is the fundamental mechanism under which generation and sufficient amplification of non-thermal radio emission is most likely. Considering these physical processes, further theoretical investigations have been started to investigate the conditions and possibilities of non-thermal radio emission from exoplanets, from potential radio planets in extrasolar systems.

Observation of planetary radio emissions using large arrays

Planetary radio astronomy mostly concerns plasma phenomena at low frequencies (i.e., below a few hundred MHz). The low frequency limit for ground‐based observations of these phenomena is given by the Earth's ionosphere, which limits ground based radio observations to frequencies ≥10 MHz. We give an overview and update on the status of a few representative ground‐based radio arrays that have been used for planetary studies within the frequency range 10–200 MHz, and we discuss their potential for the four types of planetary radio emissions that can be observed within this frequency range: (1) synchrotron emission from Jupiter's radiation belts, (2) radio bursts caused by solar system planetary lightning, (3) Jupiter's magnetospheric emission, and (4) magnetospheric radio emission from extrasolar planets, for which we also give an update to previous predictive studies. Comparing the four emission modes with the characteristics of existing ground‐based radio arrays, we show that the Low Frequency Array (LOFAR) has the potential to bring considerable advances to those four fields of planetary radio science.

Low Frequency Planetary Radio Astronomy

This paper reviews planetary radio astronomy at low frequencies. At least at frequencies observable from the Earth's surface, this field has been almost solely the study of Jovian magnetospheric emissions. With the discovery of extra-solar planets, and the potential for new telescopes with large collecting areas, we may well see more objects and emission mechanisms becoming detectable.

The density of the Io plasma torus ribbon

We investigate two contradictory determinations of the density of the “ribbon” feature within the Io plasma torus. A value of ∼2000 cm−3 was measured by the Voyager 1 Plasma Science and Planetary Radio Astronomy instruments. Volwerk et al. [1997] recently suggested a value at least 5 times higher, and possibly much greater. Their upward revision is based on rapid EUV intensity variations observed by the Voyager Ultraviolet Spectrometers: Short radiative cooling times are only possible with higher plasma densities. We have modeled emissions from both the Voyager‐based torus model and the high‐density case suggested by Volwerk, and show that the latter is inconsistent with the intensity and morphology of numerous observations. We suggest that the observations by Volwerk et al. [1997] can be explained by a closer examination of the radiative cooling process and possibly by short‐term temperature variations caused by variability of the dawn‐dusk electric field in the Jovian magnetosphere.

Constraints on Saturn's G Ring from the Voyager 2 Radio Astronomy Instrument

We have reanalyzed the data acquired by the planetary radio astronomy (PRA) experiment during the passage of Voyager 2 through the outer part of Saturn's G ring, originally published by Aubieret al.(1983.Geophys. Res. Lett.10, 5–8). This study closely parallels the reanalysis of the Voyager 1 PRA data during the E-ring passage (Meyer-Vernetet al.1996.Icarus123, 113–128). The instrument detected dust grain impacts on the spacecraft in a region of ≈1000 km vertical extent around the ring plane with a maximum at ring plane crossing. The signal is mainly produced by grains of radius of a few micrometers. We find a size distribution less steep than ther−6law inferred for submicrometer grains by Showalter and Cuzzi (1993.Icarus103, 124–143) from photometric data. These results can be reconciled if the slope of the size distribution flattens above 0.5 μ. Assuming a rough continuity between the distributions deduced from the two data sets and anr−qlaw for the grains detected by PRA, we infer that the differential power law indexq< 3.5 for grain radii between about a half micrometer and a few micrometers. From the observed vertical profile, we deduce an effective ring vertical thicknessH≈ 1200/(q− 1) km. Whenqvaries in the range 3.5–2,Hvaries in the range 500–1200 km and the geometric cross section per unit area is a few times 10−6.

Constraints on Saturn's E Ring from the Voyager 1 Radio Astronomy Instrument

We have reanalyzed the data acquired by the planetary radioastronomy (PRA) experiment during the passage of Voyager 1 through Saturn's E ring. Depending on the distance from the ring plane, the instrument detected (i) dust grain impacts on the spacecraft and/or (ii) plasma waves or noise. The signal produced by the dust can be recognized by its power spectrum. It is dominant in a region of ≈12,000 km vertical extent around the ring plane, and has a maximum at roughly 5000 km southward of equator (at 6.1RSfrom Saturn). Assuming that the grain concentration is given by the model of Showalteret al.(Showalter, M. R., J. N. Cuzzi, and S. M. Larson 1991.Icarus94, 451–473) derived from optical observations, we infer from the mean PRA voltage and from the histogram of the data that the particles have a mean radiusr≈ 1 μm and a narrow size distribution of fractional dispersion between 10 and 30%. These values agree with the above model. We have also investigated the ring thickness. The PRA signal has a full vertical width at half-maximum of ≈8000 km, which is 2.3 times less than that given by the optical model. Since the signal produced by the dust varies strongly with the grain size (asr6), our measurements can be made compatible with the optical observations if the particle mean size decreases slightly with vertical distance, by about 10% over 4000 km.

Voyager electron density measurements on Saturn: Analysis with a time dependent ionospheric model

We have used a one‐dimensional chemical diffusive model of the ionosphere, in conjunction with the measured Voyager ultraviolet spectrometer (UVS) upper atmospheric temperature and composition structure, to analyze the Voyager measurements of Saturn's upper ionospheric electron densities. Electron density measurements are available from the analysis of the radio science (RSS) experiments. In addition, if interpreted as an atmospheric phenomenon the Saturn electrostatic discharges (SED), detected by the Voyager planetary radio astronomy instrument, the measurements also yield important information on electron densities at the peak. These latter data suggest a strong day‐to‐night variation in the peak electron densities with a maximum of ∼5 × 105 cm−3 near noon and a minimum of ∼103 cm−3 near midnight. The analysis of RSS data yields a peak of ∼104 cm−3 in the electron density profiles near the local terminators. As with simpler ionospheric models, we find serious difficulties in interpreting these electron density measurements because time constants in the model for the chemical sinks in the ionosphere, of mostly H+ ions, are &gt;106 s much longer than a Saturnian day, suggesting little diurnal variation. We have investigated the effects of H2 vibrational excitation (characterized by a single vibrational temperature) and a gaseous H2O influx from Saturn's rings, as enhancers of plasma recombination, since they result in the conversion of slowly recombining H+ ions to more rapidly recombining molecular ions. This does introduce a larger diurnal variation in the peak electron densities, but not as strong as that deduced from the analysis of SED measurements. Furthermore, it also reduces the magnitude of the electron densities at the peak well below that inferred from the SED data. Based on the model results and extrapolation of calculations by Kim and Fox [1991] from Jupiter, we suggest that SEDs may not be associated with atmospheric storm systems. In our attempt to interpret the RSS measurements of electron density profile at sunrise, we have invoked an upward field‐aligned drift, H2 vibrational excitation and an influx of H2O to explain the results, and, in particular, maintain plasma peak at the measured altitude.

Evidence for an Io plasma torus influence on high‐latitude Jovian radio emission

We report the discovery with the Ulysses unified radio and plasma wave (URAP) instrument of features in the Jovian hectometer (HOM) wavelength radio emission spectrum which recur with a period about 2–4% longer than the Jovian System III rotation period. We conclude that the auroral HOM emissions are periodically blocked from “view” by regions in the torus of higher than average density and that these regions rotate more slowly than System III and persist for considerable intervals of time. We have reexamined the Voyager planetary radio astronomy (PRA) data taken during the flybys in 1979 and have found similar features in the HOM spectrum. Contemporaneous observations by Brown (1994) show an [SII] emission line enhancement in the Io plasma torus that rotates more slowly than System III by the same amount as the HOM feature.

Structure within Jovian hectometric radiation

Observations of Jovian hectometric radio emission (HOM) by the Voyager planetary radio astronomy (PRA) experiment at frequencies from 300 kHz to 1.3 MHz indicate persistent dynamic spectral features that had not been previously studied. The features of interest appear as “lanes” of decreased emission intensity within the otherwise persistent HOM. The lanes are apparent in intensity and occurrence probability spectrograms of frequency versus Jovian System III (1965) longitude. In the investigation of the morphology of these features, we use inbound and outbound Voyager 2 data at Jupiter to show that the lane occurrence and characteristics do not depend on local time over the range sampled. Occurrence probability spectrograms of frequency versus magnetic latitude are created from the portion of the data when the spacecraft was between 0° and +10° magnetic latitude. These spectrograms represent both the inbound and outbound passes and are quite similar despite the different longitude ranges. A simple extension of decametric (DAM) arc features into the HOM wavelength does not account for all the lane features, giving further evidence that HOM is an independent emission component. Polarization signatures for the data show that the polarization is predominantly right‐hand circular and that it does not reverse across the lanes, suggesting the emission is from the same hemisphere. In addition, we investigate possible effects due to solar wind variations and find that the occurrence of the lanes appears to be independent of times of low and high solar wind densities. The intensity of the HOM emission on either side of the lanes is comparable, implying that the lane is probably not a result of a gap between fundamental and second harmonic emission regions. We present these data and analyses as a morphological study to establish that the lane features are an important part of the HOM emission and should be considered in HOM emission models. At this time, no theory of the source of the lanes explains all the observed features.

Neptune radio emission in dipole and multipole magnetic fields

We study Neptune's smooth radio emission in two ways: we simulate the observations and we then consider the radio effects of Neptune's magnetic multipoles. A procedure to deduce the characteristics of radio sources observed by the Planetary Radio Astronomy experiment minimizes limiting assumptions and maximizes use of the data, including quantitative measurement of circular polarization. Study of specific sources simulates time variation of intensity and apparent polarization of their integrated emission over an extended time period. The method is applied to Neptune smooth recurrent emission (SRE). Time series are modeled with both broad and beamed emission patterns, and at two frequencies which exhibit different time variation of polarization. These dipole‐based results are overturned by consideration of more complex models of Neptune's magnetic field. Any smooth emission from the anticipated auroral radio source is weak and briefly observed. Dominant SRE originates in complex fields at midlatitude. Possible SRE source locations overlap that of “high‐latitude” emission (HLE) between +(out) and −(in) quadrupoles. This is the first identification of multipolar magnetic structure with a major source of planetary radio emission.

Absence of magnetic field constraints on the source region of Jovian decametric radiation

We have re‐examined the Voyager 1 and 2 planetary radio astronomy radio emission data in light of new magnetic field models. A previous study based on earlier models [Genova and Aubier, 1985] indicated that an apparent offset (∼70° measured at the orbit of Io) exists between the instantaneous Io flux tube and the source region of Jovian decametric radiation (DAM). Our new survey indicates that no such offset is necessary based on more recent magnetic field models. In addition, examples of DAM that are probably due to 2nd harmonic gyroemission were observed, as well as emission that may be due to mechanisms other than the cyclotron maser instability.

The sidereal rotation period of Neptune

The two main, low frequency radio components discovered at Neptune by the Planetary Radio Astronomy experiment carried aboard Voyager 2 have a well defined periodicity at about 16.1 hour. By analyzing all the available data, i.e. about 60 days around the closest approach for the “Burst” component and 15 days for the “Smooth” component, we determine, for each component, the best estimate of the radio period. We conclude that the two estimates are not statistically different. While the two kinds of radio emissions have very different characteristics (in particular their frequency ranges and beaming properties), and probably correspond to different emission processes, their modulation is very likely due to the rotation of the planetary magnetic field tied to the core of the planet, as it has already been assumed for the other giant planets. The deduced estimate of the sidereal rotation period of Neptune is 16.108 ± 0.006 (or 16h06.5m ± 00.4m).

The uncertainty of the Uranian radio source location due to the nonuniqueness of the planetary magnetic field model

Since the Voyager 2 encounter in early 1986, several investigators have attempted to localize the source regions of the smooth high‐frequency radio emission which was observed by the planetary radio astronomy experiment at the nightside of Uranus. The various studies (most of them are based on the offset tilted dipole (OTD) model of the Uranian magnetic field) yielded significantly different source locations around the southern magnetic pole of Uranus. This may be a consequence of the individual a priori assumptions of the source model. However, the simplicity of the OTD model (Ness et al., 1986) also cannot adequately represent the complexity of the magnetic field at the radio source locations near the planet. The aim of this study is twofold. (1) We reanalyze the various source locations given in the literature (most of them are based on the OTD model) in the frame of the Q3 magnetic field model (Connerney et al., 1987). Our analysis moves some of the previously determined source locations from open toward closed field lines; however, the uncertainty due to the nonuniqueness of the Q3 model remains too large to exclude the possibility that open field lines are the source of smooth Uranian kilometric radiation. (2) We calculate the uncertainty of the radio source locations imposed by the nonuniqueness of the Q3 and OTD magnetic field models. We construct solutions by using generalized inversion techniques (Connerney, 1981) to obtain estimates of those magnetic field parameters (spherical harmonic coefficients up to degree and order 6) that are constrained by the magnetometer observations. The nonuniqueness of the resulting magnetic field models translates into an uncertainty about the radio source locations of some 20° in Uranocentric coordinates at altitudes of about 1.5 Uranian radii (RU). The present results are important for radio source locations at all the outer planets whose magnetic field geometries are represented by nonunique magnetic field models.

Phenomenology of Neptune's radio emissions observed by the Voyager Planetary Radio Astronomy Experiment

The Neptune flyby in 1989 added a new planet to the known number of magnetized planets generating nonthermal radio emissions. We review the Neptunian radio emission morphology as observed by the planetary radio astronomy experiment on board Voyager 2 during a few weeks before and after closest approach. We present the characteristics of the two observed recurrent main components of the Neptunian kilometric radiation, i.e., the “smooth” and the “bursty” emissions, and we describe the many specific features of the radio spectrum during closest approach.

Neptune's radio emissions

Neptune's radio emission was discovered by the Planetary Radio Astronomy experiment (PRA) a few days before Voyager 2 closest approach. However, its detection was possible as early as 30 days before encounter and 22 days after encounter. At least two types of emissions, a smooth and a bursty emission were recorded in the frequency range of 20 kHz to 1300 kHz. They differ by their spectral, temporal and polarization characteristics. The smooth emission exhibits both polarizations depending on the frequency of occurrence and on the longitude of Neptune (NLS); the pattern which is repeated before and after encounter is located in the same NLS range. The bursty emission consists of very short duration bursts, narrow-banded, and also strongly polarized. The bursts, as well as the smooth emission, can be described in terms of rotation in a period of 16.10 hours. The fore shock of Neptune's magnetosphere, the impacts of dust at ring-crossing and many different low frequency waves were also detected by the PRA experiment during the period of the encounter. We describe here the polarization response of the instrument in order to determine the true polarization of the incoming waves. At the closest approach the smooth radio source was occulted by the planet. By modeling the intensity profile at the encounter we have been able to conclude that there is at least one radio source in the auroral zone of the northern magnetic hemisphere where a good fit to the observations can be obtained when assuming an emission cone model. The implications on the magnetic field strength will be discussed.

Scintillations of the Uranian kilometric radiation: Implications for the downstream magnetopause

From January 27 to 30, 1986, while Voyager 2 was outbound from Uranus, the planetary radio astronomy experiment recorded strong (∼3–6 dB) modulations of the broadband smooth Uranian radio emission (Warwick et al., 1987a) at frequencies below 500 kHz. These fluctuations exhibited simultaneously two different time scales (∼100 s and ∼10 s) and were exclusively observed outside the Uranian magnetopause during six planetary rotations. The modulations were only present during a part of the total duration of each broadband smooth episode. This phenomenon is interpreted as being due to propagation effects through the magnetopause. The long‐period modulation is due to the influence of ∼8000‐km wavelength surface waves on Uranus' downstream magnetopause. Such waves have only been studied in detail for the inbound passage (Lepping et al., 1987). The short‐period oscillation might be the signature of a second magnetopause ripple period, similar to that measured by Aubry et al. (1971) for the Earth's magnetopause. There is strong evidence that the surface wave fluctuations causing the scintillations are induced by the solar wind interaction with the magnetosphere. We discuss why these modulations are confined to certain periods of time and to a certain frequency range.

A theory for narrow‐banded radio bursts at Uranus: MHD surface waves as an energy driver

We describe a possible scenario for the generation of the narrow‐banded radio bursts (n bursts) detected at Uranus by the Voyager 2 planetary radio astronomy experiment. This particular radio emission is suspected to originate within the Uranian northern polar cusp region. In order to account for the emission burstiness which occurs on time scales of hundreds of milliseconds, we propose that ULF magnetic surface turbulence, generated at the frontside magnetopause, propagates down the open/closed field line boundary and mode converts to kinetic Alfvén waves (KAW) deep within the polar cusp. The oscillating KAW potentials then drive a transient electron stream (beam or loss cone) that creates the bursty radio emission. To substantiate these ideas, we show Voyager 2 magnetometer measurements of enhanced ULF magnetic activity at the frontside magnetopause. We then demonstrate analytically that such magnetic turbulence should mode convert deep in the cusp at a radial distance of 3 RU. A condition for mode conversion from magnetic surface waves to KAW is ƒpe/ƒce &lt; 0.2, which is also a condition favorable for the generation of the R‐X mode radio bursts. The fact that a similar plasma condition is required for both processes lends strong support that the active regions for mode conversion and R‐X mode wave generation are one and the same.

Dust distribution around Neptune: Grain impacts near the ring plane measured by the Voyager Planetary Radio Astronomy Experiment

During the Voyager 2 flyby of Neptune the planetary radio astronomy (PRA) experiment recorded an intense noise near the equatorial plane around 3.4 and 4.2 RN, as already observed during previous Voyager ring plane crossings at Saturn and Uranus. This noise is interpreted as being due to impact ionization of dust grains striking the spacecraft. We deduce a power law index of the grain mass distribution of about 2. The PRA system is sensitive to particles with radii larger than ∼1.6 μm, and the largest particles, detected near the ring plane, are evaluated to have a radius of ∼10 μm. The spatial dust distribution along the spacecraft trajectory around the two equatorial crossings is found not to be symmetrical with respect to the ring plane and spread over wide regions: over ∼2 RN perpendicularly to the equatorial plane with the densest part concentrated within ∼700 km. The vertical optical depth τ of this dense region is found to be 10−6 – 10−8.

Emission characteristics and source location of the smooth Neptunian kilometric radiation

The observations of the planetary radio astronomy (PRA) experiment aboard Voyager 2 reveal the existence of smooth and bursty radio emission. Both recur in a rather regular pattern with a 16.1‐hour period (the Neptunian spin period). We describe the phenomenology of the smooth component in terms of frequency, polarization, and occurrence in magnetic longitude and latitude. The existence of both right‐handed and left‐handed polarized emissions is consistent with two sources (one in each hemisphere) which radiate independently in the TUX mode. Because Voyager passed Neptune at less than 5000 km from the surface at high northern magnetic latitudes, the radio sources were occulted by the planet near the encounter. We have taken advantage of this occultation to locate the northern hemisphere sources by calculating the radio horizon (based on the offset tilted dipole (OTD2) model) for two spacecraft positions close to the encounter. We find that the northern source is located at high magnetic latitudes δm &gt; 40°. By using a geometrical beaming model which assumes emission in a hollow cone pattern we fit the observed PRA intensity profile. The best fit is obtained for a radio source at L=6, thus confirming δm &gt; 40°. The longitudinal extent of the source is at least 180°, from −90° to +90° magnetic longitude. To locate the southern sources we use the large excursions of Voyager 2 to high southern magnetic latitudes. The source is found to be located also at high magnetic latitudes but is possibly more limited in longitude than the northern source. We estimate the uncertainty in source location due to the very limited knowledge of the magnetic field near the Neptunian surface, and we show that the current OTD2 model is satisfactory for the source locations for the lowest observed frequencies; however, an angular uncertainty of about 20° remains for sources in the northern hemisphere. The observed pattern of the smooth emission is strongly frequency dependent, which is in agreement with the azimuthal asymmetries of the magnetic field as predicted by the OTD2 model.