Whistler mode auroral hiss emissions observed near Jupiter's moon Io

Observations from the Cassini spacecraft have shown that Saturn's small icy moon Enceladus ejects a plume of water vapor and small ice particles into Saturn's rapidly co-rotating magnetosphere. In this paper we show that the interaction of the moon with the magnetospheric plasma produces a number of electrodynamics effects that are remarkably similar to those observed in Earth's auroral regions and near Jupiter's moon Io. These include whistler-mode emissions similar to terrestrial auroral hiss, magnetic-field-aligned electron beams, and currents associated with a standing Alfven wave excited by the moon. Ray path analyses of the auroral hiss show that the electron beams responsible for the emissions are accelerated very close to the moon, most likely by parallel electric fields associated with the Alfven wave. However, other possibilities such as electric fields due to electrostatic charging of the moon's surface or of particles in the water vapor plume should be considered. Citation: Gurnett, D. A., et al. (2011), Auroral hiss, electron beams and standing Alfven wave currents near Saturn's moon Enceladus, Geophys. Res. Lett., 38, L06102, doi:10.1029/2011GL046854.

We present results obtained from the Galileo satellite's Energetic Particles Detector during its final two encounters in 2001 with Jupiter's moon Io. These encounters returned the first data from just above Io's polar caps. They complement previous low‐latitude data and provide a new perspective of Io's interaction with Jupiter's magnetosphere and ionosphere. The evolution of electron and ion distributions was measured from the upstream region throughout the polar cap traversals. From the time of initial field contact with Io and continuing throughout the encounter these distributions evolve in a manner consistent with adiabatic motion along the Io‐Jupiter field line. At encounter all particles develop narrow trapped‐like distributions indicative of the creation of a near‐Io magnetic bottle caused by an enhancement of field at Io's upstream surface. The measured pitch angle distributions indicate a field enhancement of up to 10%–15% higher than the field observed at Galileo's position. Distribution evolution times agree roughly with particle bounce times on the Io‐Jupiter field line. The ion distribution evolution times provide an estimate of ∼3–7 km/s for the field line convection speed across Io's polar caps, a value small (∼10%) compared with the upstream convection speed. Along with these trapped distributions, beams of ions and electrons are observed streaming into Io's polar caps throughout the encounters. The continued observation of ion beams across the polar cap is consistent with their half‐bounce times. The data further indicate that the convection speed may vary as the polar cap is traversed. The one exception to the adiabatic particle behavior discussed above is the observation of intense electron beams streaming into Io's polar caps. The polar cap electron beams are similar to those previously measured in Io's wake [Williams et al., 1996] and apparently originate from the same source. The source has been located at low (∼0.5 RJ) altitudes on the Io‐Jupiter field line [Williams et al., 1999]. The intensities of the electron beams indicate strong acceleration in the source region. Following the suggestion of Mauk et al. [2001], we conclude that the electron beams most likely form the downward (toward Jupiter) current portion of the overall current system established by Io's interaction with Jupiter's magnetosphere and ionosphere. A map of the observed beam locations for all Io encounters provides a rough measure of the spatial extent of this downward current.

A theory for auroral hiss is developed based on the existence of a beam of energetic particles that is also believed responsible for the visual aurora. A dispersion relation for electromagnetic waves in a plasma consisting of an electron beam and a background plasma is derived. The Hermitian part of the dispersion relation is assumed to be governed by the denser cold background plasma, whereas the anti-Hermitian part is governed by the electron beam. It is shown that the electron beam can excite an electron whistler mode instability near the resonance cone by the Landau interaction because near the resonance cone the phase velocity of the wave can be made arbitrarily small. The instability can be excited at all frequencies between the lower hybrid resonance and the electron plasma frequencies. The wave normal angles along the resonance curve vary between 0° and 90° with respect to the magnetic field. Waves whose wave normal angles are small have the largest growth rates and are most likely to grow to observable amplitudes. Only waves generated within a few degrees of the vertical can reach the ground. The results of the calculations are applied to auroral hiss observations. From the fact that auroral hiss is observed on the ground at frequencies less than 10 kHz it is concluded that at least some of the auroral hiss is generated at altitudes where the ambient electron density is of the order of 1 cm−3. This suggests that auroral electrons are energized in regions where the ambient density is of the order of 1 cm−3 or less.

The polar‐orbiting DE 1 spacecraft is now providing the first measurements of high‐latitude auroral phenomena in the radial distance range between about 2 and 5 RE, where several important types of auroral plasma wave emissions are believed to be generated. This paper describes the initial observations from the DE 1 plasma wave instrument in this interesting region. Three principal types of plasma wave emissions are discussed: auroral hiss, Z mode radiation, and auroral kilometric radiation. Whistler mode auroral hiss emissions are observed on essentially every pass over the auroral zone. The auroral hiss usually has a characteristic ‘funnel‐shaped’ frequency‐time spectrum which can be explained by a simple whistler mode propagation effect if the radiation is emitted from a spatially localized source below the spacecraft. Ray path studies show that the auroral hiss is propagating upward from a source, the lower boundary of which is an altitude of about 0.7–0.9 RE. The DE 1 observations have identified broadband Z mode emissions in the low‐density region over the auroral zone and polar cap. This radiation usually has a sharply defined upper cutoff near the electron gyrofrequency and extends downward in frequency to a cutoff called the fL = 0 cutoff. The Z mode radiation is sometimes difficult to distinguish from the auroral hiss, which occurs in the same general frequency range with about the same intensity. The Z mode radiation probably corresponds to the noise previously identified as ‘continuum radiation’ in the Hawkeye polar region data. The auroral hiss can usually be distinguished from the Z mode radiation by the sharp upper cutoff of the whistler mode at the local electron plasma frequency. Broadband emissions identified as auroral kilometric radiation are frequently observed over the evening auroral regions at frequencies between about 100 kHz and 400 kHz. These emissions are very intense, usually 30–50 dB above the intensity of the auroral hiss and Z mode radiation, and are highly variable, sometimes disappearing completely. The auroral kilometric radiation usually occurs at frequencies above the electron gyrofrequency, consistent with earlier measurements which indicate that this radiation is propagating in the free‐space R‐X mode.

Over the last three years, the Cassini spacecraft has been in a series of high inclination orbits, allowing investigation and measurements of Saturnian auroral phenomena. During this time, the Radio and Plasma Wave Science (RPWS) Investigation on Cassini detected low frequency whistler mode emissions propagating upward along the auroral field lines, much like terrestrial auroral hiss. Comparisons of RPWS data with Cassini Plasma Spectrometer (CAPS) plasma measurements during a high‐latitude pass on 17 October 2008, show that intense upward moving electron beams with energies of a few hundred eV were associated with auroral hiss emissions. In this paper we show that these beams produce large growth rates for whistler‐mode waves propagating along the resonance cone, similar to the generation of auroral hiss at Earth.

The left-hand side of the auroral hiss emission observed by Galileo has a frequency time profile shaped very similar to the funnel shape observed in the Earth's auroral region. This close similarity indicates that we can use the theory of whistler-mode propagation near the resonance cone to locate the emission source. The general characteristics of the whistler mode are discussed. Then the position of the emission source is investigated using a geometrical method that takes into account the trajectory of Galileo. Initially a point source is assumed. Then the possibility of a sheet source aligned along the magnetic field lines which are tangent to the surface of Io is investigated. Both types of sources show that the whistler mode radiation originates very close to the surface of Io.

Saturn auroral hiss is intense whistler mode emission similar in morphology to terrestrial auroral hiss, and is observed at high latitude very often in quasiperiodic episodes with a period of approximately 1 hr. Bader et al. (2019) report auroral pulsations that may be due to duskside magnetodisk reconnection. The source of the 1‐hr period is not definitively known but has been purported to be due to second harmonic Alfven waves standing along near planet magnetic field lines (Yates et al., 2016). Observations of auroral hiss at high latitude along Cassini proximal orbits are often excellent, and we have focused on an event for which we have concurrent ultraviolet auroral images as well as electron flux data. A series of repeating auroral hiss episodes is observed to initiate near the magnetic field line that traverses a Saturn kilometric radiation source region in each hemisphere, with periodic episodes of hiss recurring at higher L‐shells. Magnetic field lines centered on individual hiss episodes have auroral footprints that lie near and within a region of intense auroral ultraviolet emissions. These observations have a parallel in terrestrial return current electron beam‐generated auroral hiss seen near magnetic field lines supporting auroral kilometric radiation source regions. Recent findings link periodic plasma injections with Saturn reconnection sites observed preferentially on the duskside. These injections may spawn Saturn kilometric radiation source regions and periodic auroral hiss emission in nearby return current regions.

The results of a VLF (0.3-18 kHz) experiment aboard OGO 4 are compared with simultaneous data obtained by the satellite on precipitating electrons at 0.7, 2.3, and 7.3 keV to determine the source of the auroral hiss band in the night side auroral zone. At these energies the correlation with VLF auroral hiss is best at 0.7 keV and worst at 7.3 keV. Auroral electrons in the keV range may enhance the intensity of VLF auroral hiss on the night side, but the predominant source of night side hiss appears to be electrons of energies below 0.7 keV. Auroral hiss tends to occur simultaneously over a broad range of frequencies. A study based on OGO 6 data has revealed a lack of correlation between keV electrons and LF auroral hiss. These observations suggest that hiss of all frequencies is generated by electrons with energies below about 1 keV. The excellent correlation between auroral hiss and 0.7 keV electrons in the day time cleft is apparently maintained when the region of very soft electron precipitation is in motion.

Satellite observations have revealed electrostatic solitary structures in the Earth's auroral region. These structures have positive electrostatic potentials and move along the ambient magnetic field. In this paper we performed one‐dimensional electrostatic particle simulations of electrostatic solitary waves (ESW) in plasma composed of three electron components: cold, hot, and beam electrons. First, the nearly monochromatic electrostatic acoustic waves are excited. When the amplitude of the electron acoustic (EA) waves is sufficiently large, part of hot and beam electrons are trapped by the electron acoustic waves. These waves coalesce each other during their nonlinear evolution, and at last the solitary structures with travel speed related to the beam velocity are formed at the quasi‐equilibrium stage. These structures have positive potential signatures, and they seem to be stable. Electron density cavities for cold electron component are always accompanied with these structures. In addition, the corresponding electric fields have a bipolar structure, which has also been observed in the Earth's auroral region recently. The conditions for existence of such solitary structures are investigated through our simulations, and the comparisons between our simulated results and satellite observations in the Earth's auroral regions are also discussed.

Observations of auroral hiss obtained from the Voyager 1 encounter with Jupiter have been reanalyzed. The Jovian auroral hiss was observed near the inner boundary of the warm Io torus and has a low‐frequency cutoff caused by propagation near the resonance cone. A simple ray tracing procedure using an offset tilted dipole of the Jovian magnetic field is used to determine possible source locations. The results obtained are consistent with two sources located symmetrically with respect to the centrifugal equator along an L shell (L ≃ 5.59) that is coincident with the boundary between the hot and cold regions of the Io torus and is located just inward of the ribbon feature observed from Earth. The distance of the sources from the centrifugal equator is approximately 0.58 ± 0.01 RJ. Based on the similarity to terrestrial auroral hiss, the Jovian auroral hiss is believed to be generated by beams of low energy (∼tens to thousands of eV) electrons. The low‐frequency cutoff of the auroral hiss suggests that the electrons are accelerated near the inferred source region, possibly by parallel electric fields similar to those existing in the terrestrial auroral regions. A field‐aligned current is inferred to exist at L shells just inward of the plasma ribbon. A possible mechanism for driving this current is discussed.

The Dust ballerina skirt is a set of well defined streams composed of nanometric sized dust particles that escape from the Jovian system and may be accelerated up to ≥200 km/s. The source of this dust is Jupiter's moon Io, the most volcanically active body in the Solar system. The escape of dust grains from Jupiter requires first the escape of these grains from Io. This work is basically devoted to explain this escape given that the driving of dust particles to great heights and later injection into the ionosphere of Io may give the particles an equilibrium potential that allow the magnetic field to accelerate them away from Io. The grain sizes obtained through this study match very well to the values required for the particles to escape from the Jovian system.

It has been known for many years that several types of electromagnetic plasma wave emissions are generated in the Earth’s polar regions in association with auroras. As long ago as 1933, Burton and Boardman (1933) detected bursts of very-low-frequency (VLF) “static” at high latitudes that occurred simultaneously with flashes of auroral light. Later a variety of investigations using ground VLF radio receivers at high latitudes firmly established that broadband bursts of radio noise are produced in the auroral regions during periods of enhanced auroral activity (Ellis, 1957; Martin et al., 1960; Jorgensen and Ungstrup, 1962; Harang and Larsen, 1964). This type of VLF radio emission came to be known as “auroral hiss,” following the classification scheme of Helliwell (1965). Because of the low frequencies involved, it was realized relatively early that the auroral hiss must be propagating in the whistler mode. The first satellite investigation of auroral hiss was reported by Gurnett (1966) who showed that auroral hiss is closely correlated with intense fluxes of precipitating auroral electrons. This relationship was subsequently confirmed and refined by a number of low-altitude satellite studies.

The intensity of incoherent Cerenkov radiation in the dayside polar-cusp region has been estimated by including the very soft (100 ev to 1 kev) energy range, which recent experimental results have shown to be the source of broad-band auroral hiss at least on the dayside of the earth. Our results show that, for an electron flux spectrum of the approximate form dJ/dE ∝ E−2, electrons in the range 100 ev to 1 kev contribute about 2 orders of magnitude more to the intensity of Cerenkov hiss than those in the range 1–10 kev. If the electron flux is assumed to have the value of 5 × 105 el cm−2 sec−1 ster−1 ev−1 at 700 ev, a peak Cerenkov hiss intensity of ≃10−13 w m−2 Hz−1 results at ionospheric heights at ≃70 kHz and decreases in intensity toward both lower and higher frequencies. A limitation of the present theories is pointed out, and the possible role of anomalous cyclotron radiation in the auroral hiss generation is discussed. It is concluded that, even if the generation mechanism is not totally incoherent, as has been assumed in the calculations, it almost certainly involves electrons that are moving faster than the wave. If the calculations are repeated at a lower latitude, appropriate to the nightside auroral hiss zone, essentially identical results are obtained if the dayside model ionosphere and the dayside electron flux spectrum are used. These results indicate that, if incoherent Cerenkov radiation is the source of auroral hiss, the generally weaker hiss on the nightside of the earth must be due to lower electron fluxes, harder spectra, lower magnetospheric electron densities, or a combination of these factors.

Solar wind pressure effects on Jupiter decametric radio emissions independent of Io

Auroral hiss is observed to propagate over distances comparable to an Earth radius from its source in the auroral oval. The role of Landau damping is investigated for upward propagating auroral hiss. By using a ray tracing code and a simplified model of the distribution function, the effect of Landau damping is calculated for auroral hiss propagation through the environment around the auroral oval. Landau damping is found to be the likely mechanism for explaining some of the one‐sided auroral hiss funnels observed by Dynamics Explorer 1. It is also found that Landau damping puts a lower limit on the wavelength of auroral hiss. Poleward of the auroral oval, Landau damping is found in a typical case to limit ω/k∥ to values of 3.4 × 104 km/s or greater, corresponding to resonance energies of 3.2 keV or greater and wavelengths of 2 km or greater. For equatorward propagation, ω/k∥ is limited to values greater than 6.8 × 104 km/s, corresponding to resonance energies greater than 13 keV and wavelengths greater than 3 km. Independent estimates based on measured ratios of the magnetic to electric field intensity also show that ω/k∥ corresponds to resonance energies greater than 1 keV and wavelengths greater than 1 km. These results lead to the difficulty that upgoing electron beams sufficiently energetic to directly generate auroral hiss of the inferred wavelength are not usually observed. A partial transmission mechanism utilizing density discontinuities oblique to the magnetic field is proposed for converting auroral hiss to wavelengths long enough to avoid damping of the wave over long distances. Numerous reflections of the wave in an upwardly flared density cavity could convert waves to significantly increased wavelengths and resonance velocities.