Kilometric Radio Emissions Research Articles

Feasibility of Passive Sounding of Uranian Moons Using Uranian Kilometric Radiation

AbstractWe present a feasibility study for passive sounding of Uranian icy moons using Uranian Kilometric Radio (UKR) emissions in the 100–900 kHz band. We provide a summary description of the observation geometry, the UKR characteristics, and estimate the sensitivity for an instrument analogous to the Cassini Radio Plasma Wave Science (RPWS) but with a modified receiver digitizer and signal processing chain. We show that the concept has the potential to directly and unambiguously detect cold oceans within Uranian satellites and provide strong constraints on the interior structure in the presence of warm or no oceans. As part of a geophysical payload, the concept could therefore have a key role in the detection of oceans within the Uranian satellites. The main limitation of the concept is coherence losses attributed to the extended source size of the UKR and dependence on the illumination geometry. These factors represent constraints on the tour design of a future Uranus mission in terms of flyby altitudes and encounter timing.

Simultaneous UV Images and High‐Latitude Particle and Field Measurements During an Auroral Dawn Storm at Jupiter

AbstractWe present multi‐instrument Juno observations on day‐of‐year 86, 2017 that link particles and fields in Jupiter's polar magnetosphere to transient UV emissions in Jupiter's northern auroral region known as dawn storms. Juno ranged from 42°N to 51°N in magnetic latitude and 5.8–7.8 Jovian radii (1 RJ = 71,492 km) during this period. These dawn storm emissions consisted of two separate, elongated structures which extended into the nightside, rotated with the planet, had enhanced brightness (up to at least 1.4 megaRayleigh) and high color ratios. The color ratio is a proxy for the atmospheric penetration depth and therefore the energy of the electrons that produce the UV emissions. Juno observed electrons and ions on magnetic field lines mapping to these emissions. The electrons were primarily field‐aligned, bidirectional, and, at times, exhibited sudden intensity decreases below ∼10 keV coincident with intensity enhancements up to energies of ∼1,000 keV, consistent with the high color ratio observations. The more energetic electron distributions had characteristic energies of ∼160–280 keV and downward energy fluxes (∼70–135 mW m−2) that were a significant fraction needed to produce the UV emissions for this event. Magnetic field perturbations up to ∼0.7% of the local magnetic field showing evidence of upward and downward field‐aligned currents, whistler mode waves, and broadband kilometric radio emissions were also observed along Juno's trajectory during this time frame. These high‐latitude observations show similarities to those in the equatorial magnetosphere associated with dynamics processes such as interchange events, plasma injections, and/or tail reconnection.

Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events

AbstractSaturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large‐scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures—separated by ~4 hr UT—form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout ~9 hr observation. Mapping of the ionospheric equatorward motion post‐dawn indicates a factor ~5 reduction of the magnetospheric source region's radial speed at a distance of ~14‐20 RS, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field‐aligned currents linked with the southern planetary period oscillation system, or the re‐energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning.

A new view of Jupiter's auroral radio spectrum

AbstractJuno's first perijove science observations were carried out on 27 August 2016. The 90° orbit inclination and 4163 km periapsis altitude provide the first opportunity to explore Jupiter's polar magnetosphere. A radio and plasma wave instrument on Juno called Waves provided a new view of Jupiter's auroral radio emissions from near 10 kHz to ~30 MHz. This frequency range covers the classically named decametric, hectometric, and broadband kilometric radio emissions, and Juno observations showed much of this entire spectrum to consist of V‐shaped emissions in frequency‐time space with intensified vertices located very close to the electron cyclotron frequency. The proximity of the radio emissions to the cyclotron frequency along with loss cone features in the energetic electron distribution strongly suggests that Juno passed very close to, if not through, one or more of the cyclotron maser instability sources thought to be responsible for Jupiter's auroral radio emissions.

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.

Generation of periodic signatures at Saturn through Titan's interaction with the centrifugal interchange instability

AbstractThe origin of the periodicities in the radio, plasma, and magnetic fields of Saturn has long been debated. Given the high degree of alignment of Saturn's dipole with its rotation axis, no strong rotational periodicities are expected. However, Cassini data demonstrated the existence of such periodicities not only in Saturn's kilometric radio emissions (SKR) but in the plasma and magnetic field signatures. It is shown that the development of the centrifugal interchange instability that originates from mass loading from the Enceladus torus contains information about the planetary rotation period. However, the planetary period is masked by high‐frequency components of the instability. The presence of Titan is shown to damp the high‐frequency components and enables the fundamental frequency near the planetary rotation frequency to grow at the expense of the high‐frequency components. As a result, the interchange instability is seen to change from one where five to seven large interchange fingers dominate to one where there are about three which cause the modulation of magnetospheric parameters near the planetary period. This modulation includes the movement of the magnetopause, the injection of energetic particles into the inner magnetosphere and the plasma density at high latitudes, both of which control SKR. Controlling factors on the frequency include Titan's drag on the plasma sheet which produces asymmetries between the Northern and Southern Hemispheres, solar wind conditions, and the density of the Enceladus plasma torus. The resultant magnetic perturbations are shown to have similar size and frequency as that seen in Cassini data.

Longitude dependences of Saturn's ultraviolet aurora

AbstractBased on periodicities in the kilometric radio emissions, the Saturn Longitude System 4 (SLS4) was used to organize the far ultraviolet (120–150 nm) aurora observed by the Ultraviolet Imaging Spectrograph on the Cassini spacecraft. Individual Ultraviolet Imaging Spectrograph pixels were projected onto the ionosphere of Saturn, transformed into the SLS4 north and SLS4 south longitude systems, accumulated over all over auroral observations from 2007 to early 2009, and binned into 1°×1° bins of colatitude. The intensity of the northern aurora showed little variation in its SLS4 north system, but the intensity of the southern aurora exhibited an enhancement of over ~10 kR between ~140–280° SLS4 south longitude. This enhancement may represent the auroral signature of a southern ionospheric vortex proposed in MHD models of Saturn's magnetosphere to explain its periodicities. The loci of the northern intensity peaks and the 3 kR boundaries varied little over 360° of longitude, while the equatorward boundary of the southern aurora varied by ~5° in SLS4 south longitude, reaching its most equatorward location of ~23° colatitude between 100° and 180° longitude. The polygonal centroids of the aurora in both north and south were consistent with offsets of no more than ~1° in both hemispheres.

Plasma injection events in Saturn's magnetosphere

Periodic radio emissions, including the strong radio emissions known as the Saturn kilometric radio (SKR) emissions, originate from the northern and southern polar regions and are observed throughout Saturn's magnetosphere. Scientists have been studying these phenomena but have not been able to pin down the physical mechanism linking periodicities between the inner and outer magnetospheres.

Ordering of injection events within Saturnian SLS longitude and local time

AbstractPeriodicities similar to the Saturn kilometric radio (SKR) emissions have been observed throughout the magnetosphere in both the magnetic field and the plasma. An outstanding question is what mechanism links these periodicities between the inner and outer magnetospheres. It had been postulated that the interchange instability, where narrow injections return the magnetic flux carried by the bulk plasma outflow, could play a role in determining the periodicities, but early analysis found no ordering of the injection events in the SKR‐derived Saturnian longitude system (SLS). In this study, we reexamine this possibility by limiting our data set to the young injection events observed by the Cassini radio and plasma wave science instrument. We find that the young injection events observed near midnight local time are strongly ordered by SLS. Further, this ordering varies with the Saturnian season. Pre‐equinox, the northern hemisphere's longitude system controls the event occurrence. Post‐equinox, the events are ordered by the southern hemisphere‐derived longitude system. We suggest that this may be an effect in the variations in ionospheric conductivity or due to change in the magnetosphere's orientation relative to the solar wind.

Observations of auroral kilometric radio emission on the Interball-1 satellite in the active Sun period 1999–2000

Results of almost two years (January 1999–October 2000) of continuous observations of auroral kilometric radio emission with the instrument AKR-X onboard the high-apogee satellite of the Earth Interball-1 are presented. The observations were conducted at the growth stage (in 1999) and in the maximum (2000) of solar activity within the 100–1500 kHz frequency band. The results of AKR detection in the vicinity of the maximum of its spectrum at a frequency of 252 kHz are presented. Both similarity (for example, the character of global directivity) and important differences from the AKR emission observed during the solar activity minimum [5] are found. Together with very high sporadicity, strong seasonal changes in the intensity are typical for the emission. It is completely absent in the spring-summer period in the Northern Hemisphere and is strongly suppressed in this period in the Southern Hemisphere. Probable nature of these features of AKR is discussed.

Post-equinox periodicities in Saturn's energetic electrons

[1] Since Saturn's vernal equinox in August 2009 (day 223), energetic electrons (110–365 keV) have exhibited a variety of periodic and aperiodic behavior within a spectral window of 5–15 hours. From late 2009 through the end of 2010, when the observed at dusk, a single period near 10.7 hours dominated the Lomb spectra of these particles. Near the end of 2010, however, the energetic electrons displayed multiple periods, with the strongest at 10.65 hours. The periodicity observed after equinox has a mean value of 10.69 ± 0.06 hours and agreed closely with that of Saturn kilometric radio (south) emissions. By early 2011, when the observer had moved to the dayside, the periodicities abruptly disappeared and the Lomb spectra show no periodicity. This behavior may suggest changes in Saturn's ionosphere as a result of seasonal change, or may alternately imply a local time dependence of periodicity caused by magnetodisk thickness asymmetry.

Auroral kilometric radio emission in the period of quiet sun in 1996

The auroral kilometric radio emission (AKR) is the most powerful sporadic radio emission of the terrestrial magnetosphere. It was discovered in 1965 by Soviet scientists in the experiment onboard the Electron-2 satellite [1]. The AKR still continues to stay an object of a large interest and detailed study (see, for example, a review by Gurnett [2]). The mechanism of cyclotron maser instability proposed by Wu and Lee [3] is a commonly accepted mechanism of AKR generation. We have demonstrated the presence of powerful AKR simultaneously in both hemispheres of the Earth in the period from August 1995 to August 1997, including summer-winter periods, on particular examples of registration of this emission in [4] where the directivity and mechanism of the emission were studied. Since in that period AKR was observed in the vicinity of perigees of the satellite orbit in both hemispheres almost at every orbit (3.8 days), we have a possibility to trace in more detail the changes in the emission power from one orbit to another in 1996 during a deep minimum of solar activity.

Direct evidence of differences in magnetic rotation rate between Saturn's northern and southern polar regions

[1] The discovery of a difference in apparent rotation rate between Saturn kilometric radio emission from northern and southern hemispheres leads immediately to the question of whether there is a counterpart detectable in the Saturnian magnetic field. We report direct detection of a difference in the rotating magnetic signal measured by the Cassini spacecraft magnetometer consistent with that seen in the radio signals. Theoretical consequences of the direct detection are discussed.

In situ observations of the effect of a solar wind compression on Saturn's magnetotail

In this paper we explore the dynamic response of Saturn's magnetotail to an episode of solar wind compression that took place while Cassini was sampling Saturn's nightside equatorial magnetosphere in 2006. Following an initial increase in solar wind dynamic pressure the magnetosphere was compressed, but over several subsequent days the flaring of the tail increased as open flux built up in the tail lobes. Several days later the current sheet was displaced southward from its previously hinged position, and magnetic signatures consistent with the passage of a plasmoid were observed. Concurrently, Saturn's kilometric radio emissions were enhanced and the spectrum displayed a continuous extension to lower frequency, corresponding to radio sources detected at higher altitudes. We suggest that all of the above features are a common consequence of the impact of a solar wind compression on Saturn's magnetosphere.

Phase relations between energetic neutral atom intensities and kilometric radio emissions at Saturn

The intensities of energetic neutral atoms (ENAs) from Saturn's ring current were cross‐correlated with Saturn kilometric radiation (SKR, 100–400 kHz) to determine their phase relations. The cross correlations were obtained for three intervals in early 2007 when Cassini was at high northern latitudes and the Ion and Neutral Camera (INCA) could optimally view the entire ring current. During these intervals, the hydrogen, oxygen, and SKR signals all exhibited ∼10.8 h periodicities. A consistent phase relation was not observed between the ENA and the SKR. Cross‐correlation amplitudes in these intervals varied from 0.25 to 0.79, and the phase offsets varied from ∼1 h to several hours and differed according to ENA species. Not only did the SKR and ENA signal show phase offsets, but the H and O signals also displayed offsets. That is, all periodicities at ∼10.8 h had no consistent synchronization.

A north‐south difference in the rotation rate of auroral hiss at Saturn: Comparison to Saturn's kilometric radio emission

Broadband whistler‐mode emissions, commonly observed by the Cassini spacecraft at high latitudes in Saturn's magnetosphere at frequencies below about 100 Hz, have characteristics very similar to auroral hiss observed at high latitudes in Earth's magnetosphere. In contrast to terrestrial auroral hiss, which shows no obvious rotational modulation, Saturnian auroral hiss shows a very pronounced rotational modulation. We show that the rotation period of the auroral hiss is different in the northern and southern hemispheres, with a period of about 10.6 hours in the northern hemisphere and about 10.8 hours in the southern hemisphere. To within experimental error the rotation periods in the two hemispheres match the rotation periods of Saturn Kilometric Radiation, an intense radio emission generated along the auroral field lines at frequencies from about 20 to 500 kHz. These north–south asymmetries have potentially important implications on how angular momentum is transferred from the planet to the magnetospheric plasma.

Thermal electron periodicities at 20RS in Saturn's magnetosphere

Cassini fields and particles observations show clear evidence of periodic phenomena in Saturn's magnetosphere. Periodicities have been observed in kilometric radio emissions, total electron density (in the inner magnetosphere), magnetic fields, and energetic particles (in the outer magnetosphere). In this letter the first analysis of periodicities in thermal electron densities in Saturn's outer magnetosphere are presented. Plasma sheet electron densities and temperatures at 20 ± 2 RS in Saturn's magnetosphere are studied and examined as a function of SLS3 longitude. Evidence for a density minimum at 170° is presented which is in excellent agreement with total electron density results in the 3–5 RS range. The density asymmetry is interpreted as the result of a periodic plasma sheet motion where the northward offset of the plasma sheet varies with longitude hence producing a density modulation in the equatorial plane. The effect of magnetospheric compressions on the dayside density asymmetry are discussed.

Short‐burst auroral radiations in Alfvénic acceleration regions: FAST observations

We examined particle and field data during the first 3 years (1997–1999) of the FAST mission searching for short‐burst auroral kilometric radio (AKR) emissions in Alfvénic auroral acceleration regions. Eight such events were found at altitudes between 2500 and 3600 km in the midnight local time sector during winter months when the AE index was observed to be greater than 150. The emission source regions are estimated to be ∼300–900 km below the satellite. The frequency of observed short‐bursts is in the range between 400 and 600 kHz. The average bandwidth of the burst, Δf/f, is approximately 2 × 10−2. The reoccurrence frequency of discrete short bursts is estimated to be between 7 and 18 Hz, which is similar to that of Jovian S‐bursts. The negative drift of each discrete burst may be associated with upward moving electrons. As in the case for ordinary AKR, short‐bursts can be explained by the electron cyclotron maser instability due to the positive gradient in the phase space of an unstable electron distribution. Shell, conic, and ring distributions, and a combination of these three, were observed in each of the eight events. Snapshots of observed multiple shells and rings are suggested to be the results of the temporal effect, where low energy electrons are accelerated by the Alfvénic perturbation at an earlier time, while high energy electrons are newly accelerated by later Alfvénic pulses. Results presented within this paper suggest multiple Alfvénic disturbances in the magnetotail as the source of the multiple shell (and ring) distributions. Multiple Alfvénic disturbances may also explain the observed reoccurrence pattern of short‐bursts.

An update to a Saturnian longitude system based on kilometric radio emissions

The period of Saturn kilometric radiation modulation as determined by Voyager forms the basis for a longitude system (SLS) recognized by the International Astronomical Union. However, Ulysses and Cassini observations have shown that this modulation period varies by the order of one percent on timescales of a few years and, hence, does not represent the internal rotation period of the planet. A new longitude system was proposed based on ∼2 years of Cassini observations of the kilometric radio emissions and accounts for the variable radio period (SLS2) valid over the time interval from day 001, 2004 through day 240, 2006. Early uses of this longitude system have revealed a number of magnetospheric phenomena which appear to be locked to the radio period, such as variations in the external magnetic field, the plasma density in the inner magnetosphere, and enhanced intensities of energetic ions. Analysis of the radio emissions since the new system was proposed revealed that the radio period continued to evolve, even showing a second, shorter period at times. The subsolar longitude of the peak of Saturn kilometric radio emissions begins to deviate from that given by the SLS2 system almost immediately after the previous analysis interval. Here, we provide a definition for SLS3, an extension to the longitude system valid over the interval from day 001, 2004 through day 222, 2007 based on variable period radio emissions.

Saturnian magnetospheric dynamics: Elucidation of a camshaft model

Periodic modulation of magnetospheric phenomena at Earth and Jupiter results principally from the tilt of the dipole axis relative to the rotation axis. Saturn's nearly aligned dipole moment is tilted by less than 0.5° from the spin axis, yet the power of radio‐frequency emissions, the orientation of the magnetic field, and many properties of the magnetospheric plasma vary periodically at the approximate rate of Saturn's rotation. Here we examine properties of the periodic magnetic signal detected in the magnetospheric regions inside ∼12–15 RS. We show that it is associated with a rotating nonaxisymmetric system of field‐aligned currents flowing on magnetic shells bounding the region where the signals are seen. Magnetohydrodynamic ideas suggest that these currents would drive rotating plasma flow patterns in the northern and southern ionospheres, with the flows oppositely directed in the two hemispheres. On magnetic shells beyond the sheets of field‐aligned current, the magnetic perturbations generate an effective rotating equatorial dipole moment that when added to the planetary dipole moment, produces a dipole moment tilted relative to the spin axis at an angle of order 12–15°. The overt source of the north–south asymmetric ionospheric circulation could link to different ionospheric conductances that result from nonuniform solar illumination. Other possible sources are discussed but, although we can elucidate much, the origin of the cam signal (as well as other phenomena such as the Saturn kilometric radio emission with close to the same period) remains enigmatic.