Advances in Space Research 33 (2004) 2045–2060 www.elsevier.com/locate/asr
Radio and plasma waves at the outer planets
P. Zarka *
Observatoire de Paris – CNRS, LESIA, 5 Place J. Janssen, 92195 Meudon Cedex, France
Received 3 March 2003; received in revised form 5 July 2003; accepted 9 July 2003
Abstract
We review our present knowledge of plasma waves and non-thermal radio emissions at the outer planets. The review mainly concerns waves linked to electron dynamics. After a summary of the basics of radio and plasma wave modes as derived from the theory and from observations in the Earth’s vicinity, we discuss the counterpart of these waves as observed in outer planets’ magnetospheres. Plasma wave spectra display a remarkable overall similarity at all magnetized planets in spite of the very diverse magnetospheric configurations, emphasizing the importance of similar microphysics processes at all magnetized planets, in par- ticular the outer giant planets. But the magnetospheres of the outer planets also give birth to a large diversity of radiosources, reflecting their complex structure (plasma reservoirs, gradients, regimes of fpe=fce, etc.) and dynamics (locations and distributions of energetic electrons). We discuss in more details these radio components, including their generation mechanisms. Emphasis is put on the direct generation of the very intense high latitude radio emissions, but conversion processes also occur in low latitude regions. The Earth stands as a reference in our comparative approach, but the study of radio and plasma waves at the outer planets allows to apply the concepts developed at Earth in different plasma environments, broadening the regime of plasma parameters involved in the generation of these waves. One specific Jovian radio component is identified (made of sporadic bursts), whose study could bring significant conceptual advances in the physics of the generation process involved. Finally, radio waves, because they can freely propagate far from their source regions, are of utmost importance as they allow for remote sensing of magnetospheric plasmas. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved.
Keywords: Magnetospheres; Outer planets; Radio emission; Plasma waves; Plasma environment
1. Introduction role in particle acceleration and plasma heating. Radio waves, because they can freely propagate far from their Planetary magnetospheres are a vast and complex source regions, are of utmost importance as they allow zoo of electromagnetic and electrostatic radio and for remote sensing of magnetospheric plasmas. For ex- plasma waves. These waves are generated by out-of- ample, they played recently a major role in uncovering equilibrium populations of charged particles (electrons, the large scale dynamics of the Jovian magnetosphere. ions) whose distribution possesses non-thermal features They also allow for unrecoverable release of free energy o o ð f = mk;? > 0Þ. Plasma waves – especially low frequency from collisionless plasmas. waves – play a fundamental role in the collisionless The aim of this paper is to provide a synthetic and magnetospheric plasma: they transfer momentum and up-to-date view of plasma waves and radio emissions energy between the various populations of particles, and how they compare among the various outer planets. playing the role of collisions through which the plasma The review mainly concerns waves linked to electron relaxes towards equilibrium. They also play a dominant dynamics. In the next section, we briefly summarize the basics of radio and plasma wave modes as derived from the theory and from observations in the Earth’s vicinity. Then, we discuss their observation in outer planets’ * Tel.: +33-01-45-07-76-63; fax: +33-01-45-07-28-06. magnetospheres, and we describe the various radio E-mail address: [email protected] (P. Zarka). components discovered in planetary magnetospheres. In
0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.07.055 2046 P. Zarka / Advances in Space Research 33 (2004) 2045–2060 the last section, we discuss the non-thermal generation mechanisms which may account for these components, especially at the outer planets. We largely refer to previous reviews dealing with specific parts of the subject, and cite only those recent papers not covered as well as a few key papers.
2. Plasma wave modes
A magnetized plasma exhibits characteristic fre- quencies linked to electron and ion densities, and mag- netic field magnitude. These frequencies represent natural cutoffs or resonances of the various modes which can propagate in the plasma. ‘‘Electronic fre- quencies’’ include: 2 1=2 • The plasma frequency: fpe ¼ð1=2pÞðNe e =e0meÞ / N e1=2. Fig. 1. Frequency variations of characteristic frequencies and ranges of the four main electromagnetic modes versus distance in a magnetized • The cyclotron frequency: fce ¼ð1=2pÞðeB=meÞ/B. plasma representative of the Earth’s auroral regions (Gurnett et al., 2 2 1=2 • The upper hybrid frequency: fUH ¼ðfpe þ fceÞ . 1983). • The low-frequency cutoff of the right-hand polarized/ fast extraordinary (R–X) mode (see below) 1=2 plane ðf ; kÞ – or modes –, with k ¼ 2p=k the modulus of 2 2 fR¼0 ¼ fX ¼ fpe fcefci þðfce=2Þ þ fce=2: the wave vector. Table 1 summarizes the characteristics of the various modes that are observed in magneto- • The low-frequency cutoff of the left-hand polarized/ spheric plasmas. When several ion populations exist in slow extraordinary (Z) mode the plasma, multiple ion cyclotron modes appear at 2 2 1=2 increasingly lower frequencies for increasing ion mass. fL¼0 ¼ fZ ¼ðf fcefci þðfce=2Þ Þ fce=2: pe Electromagnetic (e.m.) waves are generally ‘‘fast’’ In the latter two formulas, the term fcefci must be waves (the group velocity mg is much larger than the neglected when dealing only with electron dynamics. plasma thermal velocity) so that their dispersion can be Equivalent ion frequencies involve the ionic plasma described using the cold plasma formalism. Conversely, and cyclotron frequencies (fpi and fci) in place of their electrostatic (e.s.) waves, often produced by particle electron counterparts. The lower hybrid frequency mixes beams, are generally ‘‘slow’’ waves so that thermal ef- electron and ion frequencies fects are generally important and must be taken into 1=2 account in their dispersion relations. 1 2 2 1 fLH ¼ðfcefciÞ þ fpi þ fci : Fig. 2 displays the cold plasma dispersion curves of e.m. electronic (‘‘high frequency’’) modes when ion When the electron plasma frequency is small com- motions are neglected. Fig. 2(a) corresponds to propa- pared to the cyclotron frequency ðf f Þ, as will be pe ce gation quasi-perpendicular to the static magnetic field, generally the case in the outer planets’ magnetospheres and Fig. 2(b) to quasi-parallel propagation. ‘‘Radio (at least at high latitudes), neglecting ion frequencies 2 waves’’ consist of the L–O and R–X electromagnetic and defining e ¼ðf =f Þ , one gets pe ce free-space modes. As shown in Fig. 1, they are able to fUH fceð1 þ e=2Þ; propagate to infinity in a medium where the character- istic frequencies steadily decrease along the propagation fR¼0 ¼ fX fceð1 þ eÞ; path (i.e. outwards from the radiosources embedded in the magnetosphere), and thus they allow for remote f ¼ f ef ; L¼0 Z ce sensing of their source regions. and also Knowledge of Z mode is limited by the fact that it is trapped within a bounded altitude layer where fZ < fUH fLH fpi: (Fig. 1). When fpe fce, its bandwidth reduces to The typical variations of these characteristic fre- f fpe. It is subluminous (phase velocity Table 1 Basic characteristics of magnetospheric plasma wave modes Mode Spectral range e.m./e.s. Polar Free space L–O mode f > fpe e.m. L Free space R–X mode f > fX e.m. R Z mode fUH > f > fZ e.m. (e.s. near fUH)R> fpe,L< fpe Whistler mode f < minðfpe; fceÞ e.m. (e.s. near fLH)R Electron plasma oscillations (Langmuir waves) f fpe e.s. Quasi-thermal noise 0:5fpe < f < 3fpe e.s. Electron cyclotron waves (ECH/Bernstein modes) f ðn þ 1=2Þfce 6 fUH e.s. Electrostatic ion cyclotron waves (EIC/ICH) f ðn þ 1=2Þfci e.s. Electromagnetic ion cyclotron waves (EMIC) f < fci e.m. L Ion acoustic waves f < fpi e.s. Fig. 2. Cold plasma dispersion curves of electromagnetic electronic modes for fpe=fce ¼ 0:3 with ion motions neglected. (a) Quasi-perpendicular propagation. (b) Quasi-parallel propagation. X modes have the same polarization and the non- Whistler waves are ubiquitous in inner magneto- propagation band between fUH and fX is small spheric regions (Earth’s plasmasphere, Io torus, etc.) ð efce=2Þ. Z mode propagates horizontally in the per- where they are detected as unstructured bands named mitted layer where its propagation is possible, and ‘‘hiss’’ and structured tones named ‘‘chorus’’ (f < a few shows little fine time–frequency ðt–f Þ structure. Its kHz, i.e. so-called VLF range). Their generation is lowest frequencies (f < minðfpe; fceÞ) overlap with the attributed to non-relativistic cyclotron resonance whistler range so that it is often obscured by more in- x k m nx ¼ 0; ð1Þ tense auroral hiss (see below). k k ce In the whistler mode, the wave energy is guided with x ¼ 2pf , n ¼ 1; 2; ...; kk the parallel wave vector, parallel to the static magnetic field. The name of this and mk the particle’s parallel velocity. Wave growth oc- mode originates from the fact that lightning discharges curs when a velocity inversion ðof =om? > 0Þ does exist in produce broadband short duration whistler mode the particle populations. Whistler mode waves can res- emissions, which propagate along magnetic field lines onate with electrons of 100–1000 keV (hiss) or 1–10 keV with group velocity depending on frequency (mg in- (chorus), causing their pitch-angle scattering in the at- creases with f ); after propagation over several planetary mospheric loss-cone. Resonant interaction with hiss is radii, a broadband short-duration burst becomes a tone thus believed to be the dominant loss mechanism for sliding from high to low frequencies; when converted to radiation belts electrons, limiting trapped energetic audio waves, this sliding tone produces a whistling electron fluxes. The unstructured dynamic spectrum sound. of hiss is attributed to quasi-linear wave-particle 2048 P. Zarka / Advances in Space Research 33 (2004) 2045–2060 interactions at equilibrium, generating a fully developed cies depend on the cold and hot plasma densities and turbulent spectrum. The origin of chorus structure is not temperatures), up to fUH. Emission at fUH is particularly well understood: it could involve rapid wave growth and intense when fUH ðn þ 1=2Þfce. These emissions are non-linear saturation producing isolated wave packets described in the literature under the name ‘‘ECH bands’’ with rapidly varying frequency. (electron cyclotron harmonics) and ‘‘UHR’’ waves. At high latitudes, above the auroral regions, whistler They are confined to the vicinity of the magnetic equator mode emissions are observed with a specific V-shaped ð 1 –2 Þ, where fce fpe, because they are generated structure on ðt–f Þ dynamic spectra, and named ‘‘auroral by electrons with 90 pitch angle (i.e. trapped near the hiss’’ and ‘‘VLF saucers’’. Poynting flux measurements equator) and because propagation constrains these have shown that auroral hiss is propagating downwards waves along the equator. Like whistler mode waves, while saucers are propagating upwards. They are both they cause pitch angle scattering of electrons in the at- interpreted as whistler mode waves propagating along mospheric loss cone, being thus a secondary loss process the resonance cone, i.e. at an angle for which the re- for trapped energetic belt electrons. No ECH is observed fraction index n !1 (hence mg ! 0 and the intensity within the Earth’s plasmasphere because the plasma is becomes large); propagation is closely focussed along the too cold there. static magnetic field (the angle increases with increasing Finally, we briefly comments on ion waves: frequency, causing the V-shape in the t–f plane). Ter- • e.s. ICH bands, extending up to fLH, are the ionic restrial auroral hiss is associated with ‘‘inverted-V’’ counterpart of ECH. They are also confined near electron precipitation of 0.1–1 keV, with source at 1RE the magnetic equator. Recent measurements from altitude above the auroral oval. Saucers originate from FAST have shown their association with field aligned lower altitudes (1000–3000 km). Recent measurements currents in upward and downward current regions. by the FAST spacecraft have show that saucers are • e.m. ion cyclotron waves (EMIC) are analogous to mostly e.s. waves near fLH, associated with upgoing su- left-hand polarized whistlers. They can resonate with prathermal electron beams of 50 eV energy (downward gyrating energetic ions (10–100 keV) with loss cone current regions). Their quasi-electrostatic nature allows distribution and cause the loss of radiation belt ions. o o excitation by beams with f = mk > 0 (Landau reso- FAST also detected EMIC in auroral current regions. nance), producing radiation in the direction of the beam. Both EIC/ICH and EMIC are believed to resonate The short duration of saucers implies that the electron with and accelerate auroral energetic electrons. beams involved are narrow and collimated. • Ion acoustic waves are excited by escaping ion beams, Langmuir waves are e.s. electron plasma oscillations e.g. in the ion foreshock. They are analogous to elec- excited in the magnetospheric electron foreshock (up- tron Langmuir waves but are Doppler-shifted to f stream of the planetary bow shock) by backscattered fpi (they can reach 2 kHz at Jupiter, while fpi 100 Hz). electron beams. They are produced in a narrow fre- Details and further references on plasma wave mode quency range about f fpe, with an average frequency observations can be found in Gurnett et al. (1983), decreasing (as the solar wind plasma frequency) as the Gurnett (1993), Kurth and Gurnett (1991), and Kurth inverse of the distance to the Sun, i.e. 30 kHz at Earth, (1991, 1992a) on QTN and Bernstein modes in Meyer- 6 kHz at Jupiter, etc. Vernet et al. (1998), and on FAST measurements in Quasi-thermal noise (QTN) is the ubiquitous inco- McFadden et al. (1999). herent emission of stable, linear, unmagnetized plasmas at non-zero temperature. It arises from the sum of e.s. fluctuations caused by the random motions of thermal 3. Plasma wave observations at the outer planets electrons. Only the electrons in the Debye sphere around an antenna immersed in the plasma contribute to the Table 2 summarizes and compares the occurrence of detected signal. A fully quantitative theory has been plasma waves at Earth and the outer planets. The developed which allows to derive the local cold and plasma wave spectra display considerable overall simi- hot electron densities and temperatures (assuming a bi- larity at all magnetized planets (see Kurth and Gurnett, maxwellian electron distribution), as well as the proton 1991), in spite of very diverse magnetospheric conditions temperature and velocity in the solar wind. The theory and dynamics. It appears also from Table 2 that besides includes the antenna response and is being adapted to dust impacts (the origin of which is straightforward), non-maxwellian distributions. broadband electrostatic noise (BEN) is detected in var- In a magnetized plasma Bernstein modes are excited ious regions of all magnetospheres and does have direct instead of QTN. They consist of cyclotron emissions relation to the wave modes discussed above. As dis- excited by thermal loss cone or ring distributions with cussed below in 3.4, recent work has shed light on the ðof =om? > 0Þ. The emission is concentrated in bands origin of BEN. We will restrict here to a few remarks on centered about half-integers of the electron cyclotron specific or comparable aspects of plasma wave spectra at frequency (f ðn þ 1=2Þfce, n ¼ 1; 2; ... ; the frequen- the four giant planets. P. Zarka / Advances in Space Research 33 (2004) 2045–2060 2049 Table 2 Comparative occurrence of plasma wave components at Earth and at the outer planets Plasma wave type Earth Jupiter Saturn Uranus Neptune Z mode x x x Whistler-mode hiss x x x x x? Whistler-mode chorus x x x x Whistler-mode auroral hiss x x x? Lightning whistlers x x x? x Upstream Langmuir waves x x x x x QT noise x x x? Electrostatic noise (BEN)a xxxxx ECH/UHR waves x x x x x EIC/ICH x x? x? EMIC x x x? Upstream ion acoustic waves x x Dust impacts x x x a at bow shock, magnetosheath, auroras, tail. 3.1. Jupiter 1996a), possibly causing Landau acceleration of elec- trons) and near Ganymede (whose intrinsic magnetic A whole zoo of plasma waves was observed by field and magnetosphere were first revealed by their Voyager (Gurnett and Scarf, 1983; Kurth, 1991, 1992a), plasma wave signatures (Gurnett et al., 1996b)). Ulysses (Stone et al., 1992; Farrell et al., 1993) and Galileo (Gurnett et al., 1996a) in the vicinity of Io’s 3.2. Saturn plasma torus and of the magnetodisk, with possible consequences for Jupiter’s magnetospheric dynamics: At Saturn, our knowledge of plasma waves relies • Intense hiss and chorus with electric field amplitudes exclusively upon the observations by Voyager 1 and 2 of the order of 1 mV/m have been observed in Io’s (Scarf et al., 1984; Kurth, 1991, 1992a): warm torus ðr P 5:7RJÞ. They were estimated to cause • The observed level of hiss was much lower than at through pitch angle scattering 1013 Wof>100 keV Jupiter (with correspondingly weak pitch-angle scat- electron precipitation towards the auroral iono- tering), possibly related to the weakness of the radi- sphere. This precipitated power represents 10% of ation belts due to energetic particle absorption by that required for UV aurora (i.e. the main Jovian the rings. UV aurora, which is to be compared to the Earth’s • Extensive ECH were observed inside 8RS, with discrete auroral arcs generated through precipitation electric field amplitudes three times lower than at Ju- of accelerated electrons). EIC/ICH waves and heavier piter ( 30 lV/m). Their role in electron precipitation ion modes, detected near the torus at higher latitude has not yet been studied. More generally, the origin and possibly coupled to low-latitude whistlers, were of auroral precipitation is not yet elucidated at Sat- suggested to produce additional ion precipitation to urn. It could be different from the Jovian case due high latitudes. to the relatively small extent of the magnetosphere • Strong ECH/UHR waves were observed inside of and current disk ð 20RSÞ, and the role played by 23RJ, with electric field amplitudes of 100 lV/m, the Kelvin–Helmholtz instability believed to develop leading to 1012 W of electron precipitations ( 1% at the magnetopause (Galopeau et al., 1995). of the auroral power). • Auroral sources were not explored, so that no infor- • BEN was observed by Voyager and Galileo at the mation is available on the existence and characteris- edges of the torus, associated with field-aligned ion tics of auroral hiss. flows (and possibly with field-aligned electron beams) • Electron plasma oscillations were detected inside the suggesting the existence of local acceleration regions. magnetosphere, which is unusual. Their origin is The long standing dilemma of the source of auroral not understood. precipitations was eventually solved by Cowley and • The e.s. noise associated with ring dust impacts on Bunce (2001) who attributed aurora to field-aligned the spacecraft (leading to grain vaporization and ion- currents resulting from corotation breakdown at 20–50 ization, and thus to short-lived electric pulses) domi- RJ from the planet. nates the low frequency part of the plasma wave A very large wave activity was also observed by spectrum in equatorial regions; note, however, that Galileo during the fly-by of Jupiter’s moons, especially this is not a ‘‘natural’’ plasma wave emission. near Io (where intense ULF electromagnetic waves were Some of these properties may not be representative of detected in the southern Alfv en wing (Gurnett et al., the average plasma wave activity at Saturn because it is 2050 P. Zarka / Advances in Space Research 33 (2004) 2045–2060 believed that Saturn’s magnetosphere was in a relatively amplitude 100 mV/m over characteristic sizes of 50–100 ‘‘quiescent’’ state at the time of Voyager fly-bys. m, i.e. 5–50 Debye lengths, leading to overall potential Finally, the vicinity of Titan revealed a plasma wave drops DV up to a few Volts), associated with local elec- zoo, the description of which would require a specific tron density depletions (of 50%), propagating upwards paper. It has been presented and analyzed in Neubauer at >100 km/s ( ion sound speed), often associated with et al. (1984), and plays a major role in the assimilation EIC. Similar (weaker) structures are found in the solar by the magnetosphere of new plasma from Titan’s wind. They are interpreted as ion holes convected with exo-ionosphere. the plasma. In auroral regions, these weak double-layers adding to the static potential structure arise from the 3.3. Uranus and Neptune quasi-total absence of cold plasma and hence of current carriers in AKR sources at low altitudes (Hilgers, 1992). Observations at Uranus and Neptune were performed More recently, FAST observed fast solitary waves and only during the Voyager 2 fly-bys, and are thus very weak double layers in downward current regions, asso- scarce. They are described in Kurth et al. (1991) and ciated with upgoing electron beams, propagating up- Gurnett and Kurth (1995). wards at 500–5000 km/s, and of size 2LD. They have At Uranus, strong hiss allows for pitch-angle scatter- been interpreted as electron holes moving with the beam. ing of 3–40 keV electrons, efficient enough to account for In the Earth’s magnetotail and plasma sheet bound- the observed UV aurora. Actually, scattering is so strong ary layer, Geotail resolved various types of e.s. noise that it should precipitate energetic electrons faster than (broadband, narrowband and Langmuir waves) into they are produced. This should lead to sporadic substorm solitary waves, possibly related to electron holes. In the activity with injection events. ECH were observed at magnetosheath, Wind and Geotail observed such waves 12RU, with electric field amplitudes 10 lV/m. They with amplitudes >100 mV/m and sizes 2–7LD. are tightly confined within 2 of the magnetic equator in Similar e.s. solitary waves were also observed by spite of the large magnetic dipole tilt and leading to a Galileo at Jupiter, with amplitudes 100 mV/m and large rocking of the magnetosphere. As for Saturn, au- sizes a few ·10LD, propagating along the static roral sources were not explored and dust impacts were magnetic field at 1000 km/s (timescales of 10–100 ms). recorded near the ring plane. Phase space holes appear to be self-consistently At Neptune, no hiss was detected (perhaps in relation associated with the intense non-linear e.s. waves into with low radiation belt intensities?), but only quite in- which BEN resolves in high resolution observations. tense ECH (with 30 lV/m amplitude) at distances of These holes trap and reflect part of the ambient electron 10RN. These waves could represent an efficient loss and ion populations, and could produce turbulent ac- mechanism for energetic particles if their intensity in- celeration. They may arise from a variety of processes creases towards the planet. The large tilt of Neptune’s including ion acoustic turbulence, electron beam insta- magnetic dipole combined to Voyager 2’s trajectory bility, etc. allowed the spacecraft to fly over high latitude regions. Lightning whistlers, weak (auroral?) hiss, Z mode radi- ation, and ion modes (EIC/ICH and EMIC) were 4. Radio components at earth and in the outer planets’ detected at this occasion. magnetospheres 3.4. Broadband electrostatic noise Remote observations from space (and from the ground in the case of Jupiter) have revealed a large di- As shown in Table 2, broadband e.s. noise (BEN) is versity of radiosources in planetary magnetospheres, ubiquitous at all planets in several magnetospheric re- depending on the plasma conditions, the possible gen- gions: bow shock, magnetosheath, auroral regions, tail. It eration mechanisms, and the available free energy is made of sporadic bursts covering the spectral range sources (see Zarka, 1998, and references therein). Table below a few kHz to a few tens of kHz, with high ampli- 3 lists all the planetary radio components presently tudes (tens of mV/m, decreasing with increasing distance known and attempts at a classification as function of the from the planet). Their generation and mode of propa- magnetospheric source region. Figs. 3(a) and (b) sketch gation is a long-standing problem. Recent high resolution the radio source locations in Earth’s and Jupiter’s (waveform) observations have allowed to resolve BEN in magnetospheres. Fig. 4 shows a dynamic spectrum re- successions of propagating coherent structures (solitary corded by the Cassini-radio and plasma wave science waves, wave packets, weak double-layers – see Salem et al. (RPWS) instrument of several Jovian radio components. (1999), and references therein). It appears thus very The strongly magnetized auroral regions of all the different from a broadband structureless noise. outer planets and of Earth are the source of very intense In the Earth’s auroral regions, observations by S3-3, low frequency radio waves. As it will be seen below, Viking, and POLAR revealed weak double layers (with these are X-mode cyclotron waves, whose frequency fce P. Zarka / Advances in Space Research 33 (2004) 2045–2060 2051 Table 3 Radio components and comparative occurrence at Earth and at the outer planets Source region Earth Jupiter Saturn Uranus Neptune Auroral regions TKR/AKR DAM SKR UKR NKR HF roar HOM (smooth/bursts) (smooth/bursts) bKOM Equatorial regions ‘‘smooth’’ emission ‘‘smooth’’ emission ? ITKR/LF bursts QP-bursts bursts? bursts? Boundaries NTC (escaping/ NTC (escaping/ NTC (?/trapped) NTC (?/trapped) NTC (?/?) trapped) trapped) Foreshock 2fpe 2fpe Radiation belts ? Synchrotron Atmosphere Lightning ? Lightning Lightning Lightning? Satellites Io flux tube Io-DAM Io torus nKOM Ganymede NTC Fig. 3. (a) Sketch of radio source locations in the Earth’s magnetosphere – see Table 3 for details (adapted from Treumann (2000)). (b) Sketch of radio source locations in the Jovian magnetosphere – see Table 3 for details (adapted from Zarka (2000a)). is thus directly proportional to the planetary magnetic Radiation. At Earth, geophysicists use the name ‘‘Au- field intensity. The resulting emissions are in the kilo- roral Kilometric Radiation’’ while planetary radioas- meter wavelength range (f 6 1 MHz), so that the auro- tronomers prefer ‘‘Terrestrial Kilometric Radiation’’. At ral components are named ‘‘xKR’’ where ‘‘x’’ is related Jupiter, the intense magnetic field (up to 14 G at the to the planet’s name and ‘‘KR’’ stands for Kilometric surface) allows auroral emissions to extend up to the 2052 P. Zarka / Advances in Space Research 33 (2004) 2045–2060 Fig. 4. Jovian low-frequency radio emissions as detected by Cassini-RPWS from 1350RJ range from Jupiter (see Table 3 for details). At Earth and Jupiter, peculiar bursts exist who display an abrupt broadband start and dispersed fre- quency-dependent ends: the so-called LF-bursts at Earth (Steinberg et al., 2004), and the ‘‘Quasi-Peri- odic’’ bursts at Jupiter (MacDowall et al., 1993). The latter occur in groups separated by 10–15 min. (QP15 bursts) or 40–45 min (QP40 bursts); intervals. Their source and generation mechanisms are still poorly known, but their dispersed characteristics have been explained by the propagation through the magneto- sheath and solar wind. Radio emission at f 2 fpe has been observed upstream of the bow shocks of Earth and Jupiter (and it does probably exist at the other planets as well) (Treu- mann, 2000). Jupiter’s stable energetic electron belts produce syn- chrotron emission in the decimeter wavelength range Fig. 5. Spectra of Jovian radio components plus auroral radio emis- sions from the other radio planets. Boldface lines emphasize high- (Carr et al., 1983; Zarka, 2000b). In this respect, Jupiter latitude emission spectra. See Table 3 for details (adapted from Zarka, appears unique among the solar system magnetized 2000a,b). planets but we will see below that the absence of syn- chrotron radiation at the other planets is due to different hectometer and decameter ranges (fmax ¼ 40 MHz). The reasons at each planet. HF roar, in the hectometer range, probably comes from Lightning probably exist (and have been detected) in the Earth’s ionosphere (see below). All the auroral the atmospheres of all outer planets (Desch, 1992). components are made up of a mixture of slowly variable Finally, in the Jovian magnetosphere, the presence of (‘‘smooth’’) and sporadic (‘‘bursts’’) components. Only at least one satellite with an ionosphere (Io – being thus at Uranus and Neptune these components have a non- a major plasma source) and one with a magnetosphere simultaneous occurrence and are thus observed as (Ganymede) (Gurnett et al., 1996b), and their interac- function of planetary rotation. Fig. 5 displays spectra of tion with the Jovian magnetic field, give birth to addi- nearly all the Jovian radio components as well as of the tional radio components as discussed in the next section other planetary auroral emissions. (Saur et al., 2004). At Uranus and Neptune, the large tilt of the plane- At various degrees, all the above radio components tary dipole prevents the accumulation of plasma in the are much more intense than the expected level of ther- equatorial regions. These regions are able to generate mal radiation in the corresponding spectral ranges. ‘‘auroral-like’’ radio components, whose observed dy- Their generation mechanisms are thus non-thermal. We namic spectra are featureless (‘‘smooth’’ emissions). For discuss in the next section the relevant mechanisms details about Uranus’ radio emissions see (Desch et al., which may account for these components, especially at 1991), for Neptune see (Zarka et al., 1995). the outer planets. P. Zarka / Advances in Space Research 33 (2004) 2045–2060 2053 5. Non-thermal radio wave generation at the outer planets ðDf f Þ; it is worth noting that the fact that f fce was directly proved at Earth (through in situ Non-thermal coherent generation mechanisms fall in measurements) and at Jupiter (through k-vector de- three categories (see, e.g. Treumann, 2000, and refer- termination – so-called ‘‘direction-finding’’ method ences therein): on Ulysses). • The direct generation of free-space modes. • 100% circularly polarized (elliptically in the case of • Linear mode conversion. Jovian DAM). • Non-linear processes. • Dominant X mode (with weak O and Z modes emit- Direct generation mechanisms can have high efficiency ted from the same source region). and, apart of the requirement of a rarefied (low b) plasma, • Beaming of the radio emission at large angle ( P 30 , can operate in a very broad range of plasma conditions. up to 90 ) of the magnetic field in the source. They will thus be invoked for ubiquitous intense radio • Time variability at various scales: production of emissions. Conversion processes (linear or non-linear) smooth emissions (with characteristic time of minutes have a weaker efficiency due to the intermediate steps to hours) and bursts ( 6 1 s). involved, are more effective in dense (high b) plasmas, and Several direct radiation mechanisms have been pro- require a finer tuning of the plasma conditions to operate. posed, involving spatial or phase bunching of the radi- They are invoked for example in the generation of solar ating electrons, or an electron beam of very large radio emissions: sophisticated models, in particular for temperature anisotropy ðT? TkÞ. But for several years solar type III radio bursts, have been developed in that the best candidate is thought to be the ‘‘Cyclotron- context (Robinson and Cairns, 2000). Maser’’ instability (CMI – see reviews by Louarn (1992) In addition to these mechanisms, non-thermal inco- and Zarka (1998)). herent processes are involved in the production of some In low b plasmas with fpe fce and negligible colli- 2 planetary radio components like the decimeter emission sions, one has fX fceð1 þ eÞ with e ¼ðfpe=fceÞ . Fig. 2 from radiation belts (synchrotron radiation) and atmo- shows that direct excitation of the X mode (and weak O spheric lightning radio discharges (antenna radiation). mode) near its cutoff is possible as fX fce. Energetic As radio emissions are much better documented for electrons gyrating at a circular frequency xce=C can re- the Earth than for the outer planets (including in situ main temporarily in phase resonance with the electric wave and particle measurements in source regions, with field of R–X mode waves with Doppler-shifted fre- high temporal resolution), terrestrial emissions will often quency x kkmk (in the electrons frame), leading to provide the framework in which similar outer planet perpendicular energy transfer and thus direct wave emissions will be studied comparatively. This explains amplification or attenuation and electron diffusion in why the radio components which have no terrestrial velocity space mk; m?. If the electronic velocity distribu- counterpart (nKOM or Io-DAM in Table 3) resist in- tion f ðmk; m?Þ contains free energy, wave growth be- terpretation harder than more ubiquitous components. comes possible. The CMI growth rate can be written 5.1. Direct generation of high latitude radio emissions c ¼ ImðxÞ ZZ Auroral radio emissions are both universal (observed 2 o o / m?ð f = m?Þdðx kkmk xce=CÞdmk dm?; ð2Þ at all the magnetized planets) and extremely intense, which implies that their generation must be a one step process directly converting the free energy of electrons so that wave amplification ðc > 0Þ requires of =om? > 0 into e.m. waves. The common properties shared by all in the domain where resonance is fulfilled (the rela- auroral radio components are (Zarka, 1998, 2000a): tivistic resonance condition x kkmk nxce=C ¼ 0 de- 15 • High intensity: brightness temperature Tb > 10 K; scribes an ellipse in velocity space). Such distributions 1% of the energy of precipitated electrons is re- with of =om? > 0 found in the Earth’s auroral regions, leased through radio waves. as observed by S3-3 and Viking spacecraft, include the • Sources distributed along high latitude magnetic field atmospheric loss cone of upgoing – magnetically lines, where energetic electrons precipitate (with ‘‘in- mirrored – electrons, hollow beams (depletions at verted-V’’ spectrum); they extend from slightly above mk 6¼ 0; m? 0) and quasi-trapped populations mk 0, the ionosphere to a few planetary radii; radio emis- m? 6¼ 0. Using these distributions, growth rates c of up 3 sions appear to be correlated with UV aurorae (espe- to 10 xce are computed for the fundamental X cially discrete auroral arcs). mode, while higher harmonics as well as O and Z • Depleted, strongly magnetized source regions: modes are produced with 20–30 dB weaker amplifi- fpe fce in the sources. cation. Computing the overall gain (up to 100 dB) • Emission at the local electron cyclotron frequency and intensities requires to estimate the source size and f fceðzÞ, covering a broad overall bandwidth source term (taken as the spontaneous emission of hot 2054 P. Zarka / Advances in Space Research 33 (2004) 2045–2060 electrons), and then solve the wave transfer equation 5.2. High latitude radio emissions at the outer planets across the source region. Viking observations inside AKR sources also We briefly discuss in this section a few specific aspects showed that they are actually small-scale filamentary of the auroral radio emissions at the outer planets, as structures devoid of cold plasma (the observed maxi- compared to the Earth case (see Zarka, 1998, and ref- mum fpe=fce is 0.14 while the theory sets an upper erences therein for details). limit 0.4) embedded in a large scale auroral cavity (Hilgers, 1992). Sources are dominated by quasi-trap- 5.2.1. Source locations and origin of energetic electrons ped electron populations with 1–5 keV characteristic The primary location of AKR and aurora at Earth is energy, and X mode emission was found to reach the evening sector ( 22 h LT). This suggests that au- frequencies f 6 fce inside them. The modified relativis- roral activity (including radio wave activity) is very tic – hot plasma – X mode dispersion indeed implies a closely related to substorms, which start with the re- decreased fX which can eventually fall below fce. This connection of stretched magnetic field lines in the tail. allows wave-particle resonance and amplification for Electrons can be accelerated up to a few keV as a result kk ¼ 0, i.e. strictly perpendicular to the source magnetic of these reconnections. However, plasma cavities that field. For these waves, the quasi-trapped population radiate the AKR are also a region of particle accelera- becomes a very efficient source of free energy. A lam- tion. It is thus likely that a complex multi-stage process inar source model was developed taking into account is at work to produce electron acceleration, possibly partial reflection of the waves at the density gradients involving energy conversion from low frequency waves on the edges of the laminar AKR sources (as in a (for example oblique Alfv en waves – see G enot et al. Laser cavity). It predicts the production of weak O and (2001)). At Saturn (Kaiser et al., 1984), auroral radio- Z modes by conversion of X mode energy at the edges sources (indirectly located through occultations and of the cavity, and the generation of fine spectral polarization studies) and UV emissions are found structures through mode selection inside the cavity mainly in the morning-to-noon sector ( 9–12 h LT). (Louarn and Le Qu eau, 1996). Galopeau et al. (1995) accordingly proposed that a CMI ensures fast growth of R–X mode waves and Kelvin–Helmholtz instability develops along the mor- can thus saturate before the waves exit the source ningside magnetopause, where the velocity shear be- region. At saturation (through quasi-linear diffusion or tween the subcorotating magnetospheric plasma and the non-linear trapping for resp. broadband and narrow- tailward magnetosheath flow is maximum. Parallel band emissions), up to a few percent of the total electric fields may be associated to Kelvin–Helmholtz electron energy may have been converted to X-mode waves, leading to electron acceleration. At Jupiter, high waves. latitude radiosources appear to be at least partly coro- FAST high-rate observations inside AKR sources tating with the planet or with Io (Io-DAM), consistent directly confirmed earlier results (AKR sources as fil- with the origin of the auroral oval being due to field- amentary cavities with perpendicular size down to 10 aligned currents resulting from corotation breakdown at km; fpe fce and fX 6 fce in the sources; direct mea- 20–50RJ from the planet (Cowley and Bunce, 2001). surement of the emission mode). They also allowed to Such currents were actually detected by Ulysses at measure electric field amplitudes >100 mV/m in the 20RJ from the planet (Dougherty et al., 1993). Other sources and strong amplification of AKR (·100) at the radiosources like the Jovian QP-bursts seem to come edges of small-scale cavities. FAST particle measure- from very high latitudes, possibly connected with the ments showed that the hollow beams and quasi-trap- magnetopause, so that acceleration mechanisms linked ped populations previously observed in AKR sources to the Kelvin–Helmholtz instability could also play a were actually parts of ring-like distributions (or role there. Magnetopause ULF waves attributed to the ‘‘horseshoe’’ distributions when combined with a loss- same instability have been observed by Voyager 2 at cone) resulting from the adiabatic evolution of parallel Uranus. beams accelerated downwards above the source (Ergun et al., 1998). FAST also identified three distinct auroral 5.2.2. Plasma cavities current regions (upward, downward, and Alfv enic) and Direct evidence of (small-scale) auroral plasma cavi- characterized their plasma wave spectra (McFadden ties exist only for the Earth, where in situ measurements et al., 1999). AKR is observed in upward current re- by Viking or FAST are available. At the outer planets, gions only, together with electron precipitation (in- the rapid planetary rotation (of Jupiter and Saturn) and verted-V), auroral hiss, ion waves and non-linear wave the tilted, precessing magnetic dipoles (of Uranus and structures. Alfvenic current regions (oscillating trans- Neptune) imply large-scale depleted high latitude re- verse E-field with intense field-aligned electron fluxes) gions, from which small-scale cavities may be absent. are more elusive. As discussed below, they could exist Indirect evidence for auroral cavities at Uranus and at Jupiter in or near the Io flux tube. Neptune (Farrell et al., 1991) has been questioned by P. Zarka / Advances in Space Research 33 (2004) 2045–2060 2055 Zarka (1998). However, auroral radio components like major role by triggering magnetic reconnection with the bKOM, HOM or SKR display rapid intensity fluctua- Earth’s magnetic field (reconnection is favoured when tions, and the instantaneous sources of bKOM and the interplanetary magnetic field is oriented south- HOM, as derived by Ulysses’ direction-finding mea- wards). At Saturn, the radio output appears better surements (Ladreiter et al., 1994), appear to flicker with correlated with the solar wind ram pressure. At Jupiter, time, suggesting small-scale sources or sporadic electron the solar wind influence is secondary to rotation but still precipitations. As noted above at Uranus and Neptune, important for most high latitude radio components the large dipole tilt also prevents accumulation of plas- (including QP bursts, but except Io-DAM, see below), ma in the equatorial plane, so that fpe fce and radio and even for the nKOM emission originating from Io’s emission can be generated there through the CMI. plasma torus. During the Galileo orbital tour, ‘‘ener- getic events’’ were discovered, corresponding to ener- 5.2.3. Saturation of radio emission intensity getic particle injections and intensification of bKOM, The theoretical spectrum of an auroral radio emission nKOM and HOM, possibly linked to plasma centrifugal was computed only in the case of SKR under the ejections from Io’s torus (Louarn et al., 1998). Obser- assumption of CMI saturation through non-linear vations performed by Cassini and Galileo during the trapping of electrons in the wave electric field (Galopeau Cassini-Jupiter fly-by late in 2000, demonstrated the et al., 1989). The computed spectrum fits well the correlation between such events and interplanetary observed one at 1% occurrence level (i.e. the level shocks arising from Coronal Mass Ejections (as ob- reached 1% of the time by SKR at each frequency), served by SOHO) and causing compressions of Jupiter’s suggesting that the SKR is marginally or not saturated. magnetosphere (Gurnett et al., 2002). Even if the physics The same is probably true for AKR. In the case of Jo- of the solar wind influence on high latitude radio emis- vian DAM, the radiosource homogeneity, defined as the sions is not understood in every detail, a scaling law has ratio of the characteristic source size (magnetic field been shown to exist between the solar wind input power gradient length in the parallel direction and projected on the magnetospheric cross-section and the auroral auroral arc width in the perpendicular direction) to the radio output at the Earth and the four outer planets. emitted radio wavelength, is two orders of magnitude higher than for AKR or SKR, thus DAM emission 5.2.6. Satellite control should be always saturated. Satellites embedded in the magnetospheres of the outer planets, if they possess a conductive interior or an iono- 5.2.4. Multispectral correlations sphere, strongly interact with the planetary field and ex- At Earth, AKR is correlated with discrete UV arcs, cite Alfv en or slow mode waves (Saur et al., 2004). These which correspond to regions of upward current flow out waves can eventually accelerate electrons toward the of the ionosphere, carried by hot magnetospheric elec- planet, inducing high-latitude radio emissions near trons flowing down the field lines. At Jupiter, the field- the magnetic footprints of these satellites in addition to aligned currents observed by Ulysses map to the UV the ‘‘auroral’’ ones. It is the case for Io which possesses an aurora observed by the Hubble Space Telescope (Clarke ionosphere and Ganymede around which Galileo dis- et al., 2004). The source of bKOM (and probably also of covered a developed magnetosphere. Induced emissions auroral DAM) is also located along field lines in the have intensities comparable (especially in the case of Io) to middle to external magnetosphere (with apex 12RJ the auroral ones, and are probably generated through the (Zarka et al., 2001a)), and a direct correlation was found CMI. The above mentioned scaling law has been gen- between the bKOM observed by Galileo and the UV eralized to any flow-obstacle configuration (magneto- auroral activity observed by the International Ultravi- sphere-solar wind or satellite-magnetosphere): in all cases olet Explorer (Prang e et al., 2001). HOM, by contrast, the radio power output is found to be proportional to the was found to originate from field lines threading exter- magnetic power incident on the obstacle’s cross section nal regions of the Io plasma torus (between 7 and 11RJ), (Zarka et al., 2001b). where intense VLF activity was observed, implying strong pitch angle scattering of electrons and thus pre- 5.2.7. Fine (f ,t) structures cipitations causing diffuse aurora. Thus, while bKOM Fine structures are ubiquitous at all ‘‘radio’’ planets (and DAM?) appear to be the counterpart of Earth’s with different characteristic frequency ranges and time- AKR, HOM could be the first radio counterpart ob- scales. The most prominent are Jupiter’s S-bursts served for diffuse UV aurora. (‘‘S’’ ¼ short), related to the Io-Jupiter interaction, and drifting over several MHz in frequency with character- 5.2.5. Solar wind control of high latitude radio emissions istic times of a few milliseconds (Fig. 6(b)). A statistical At Earth and Saturn, the solar wind influence on study of the drift rates versus frequency using ground- high latitude radio emission is strong. At Earth, the z based observations has allowed to determine the energy component of the interplanetary magnetic field plays a of the electrons emitting the observed radiation to 5 ± 2 2056 P. Zarka / Advances in Space Research 33 (2004) 2045–2060 5.3. Linear mode conversion processes These processes consist in the conversion of plasma waves (e.m. or e.s.) into free space e.m. modes. This can occur when the dispersion curves of these waves coexist in a common frequency range. In dense (high b) plas- mas, several characteristic frequencies (fX, fUH, fZ and the O mode cutoff) all tend towards fpe, allowing, e.g. conversion of e.s. UH waves (generated by electron beams of a low energy loss-cone) into O or X waves. In low density (low b) plasmas (cf. Fig. 2) e.m. whistler or Z mode waves can convert into O mode waves. Con- version is favoured by the presence of relatively steep density gradients (where the wavelength k is small compared to the gradient length LNe) at which charac- teristic frequencies change rapidly, i.e. at magneto- spheric boundaries. As the wave vector k is smaller for free space modes than for plasma wave modes, k, must increase at the conversion point. Mode conversion is not mutually exclusive with CMI, but it has a much lower efficiency in low b plasmas for the fundamental of the X mode. It may compete advantageously for the genera- tion of O mode or of higher harmonics of X mode (Treumann, 2000). One ubiquitous planetary radio emission that can be Fig. 6. Fine structures of radio emissions from the Earth, Jupiter, and attributed to linear mode conversion is the so-called Saturn. (a) Cluster wideband observations of AKR ‘‘O-bursts’’ non-thermal continuum (NTC), a featureless smooth (courtesy R. Mutel). (b) Ground-based observations (in Nancßay) of low frequency radiation partly trapped in and partly Jovian S-bursts. (c) Cassini-RPWS wideband observations of SKR fine structure (courtesy W. Kurth). escaping from the magnetospheric cavities (Kurth, 1992b). The trapped component generally has frequen- cies lower than about twice the plasma frequency of the solar wind at the orbit of the planet, i.e. lower than fpe in keV. Fine structures also exist in bKOM, as observed by the magnetosheath. Its low frequency cutoff matches fpe Cassini-RPWS (Kurth et al., 2001), as well as in SKR in the magnetospheric cavity. These characteristics (although at longer timescales than S-bursts) as ob- strongly suggest that this radiation is on the L–O mode. served by Voyager and Cassini (Fig. 6(c)). Recently, The escaping NTC has similar characteristics, except wideband observations by FAST (Pottelette et al., 2001) that it reaches frequencies higher than the magneto- and by Cluster showed that the fine structure of some sheath plasma frequency, allowing it to escape in the AKR bursts resolves into narrow drifting features solar wind. In the vicinity of magnetospheric bound- (named ‘‘O’’-bursts) strongly reminiscent of Jovian S- aries, NTC was found to be associated with intense bursts (Fig. 6(a)). The possibly common origin of AKR bands of radiation at f fUH. It is thus believed that and outer planet’s radio bursts fine ðf ; tÞ structures and NTC originates from linear conversion of e.s. UH waves of Jovian S-bursts has been investigated by many au- into O mode radio waves at density gradients (plasma- thors (see Zarka, 1998, 2000a). Proposed mechanisms pause, plasma sheet, and magnetopause). This radiation include CMI operation in a laminar cavity (radio ‘‘las- produced in relatively narrow frequency bands then ing’’), gyrating or phase-bunched electron beams, etc. experiences multiple reflections with random Doppler The question is still open, but it must be noted that the shifts at the above density walls inside the magneto- distribution of S-burst intensities has been found to sphere, turning it into a continuum trapped in the cav- follow a power law of index )2 ± 1 (the index varies over ity. Its upper end only, above fpe (magnetosheath), can timescales of minutes) (Queinnec and Zarka, 2001). It escape the cavity. A similar radiation was observed to has been suggested that such a distribution could be the escape from Ganymede’s magnetosphere (Kurth et al., signature of self-organized criticality in the Io-Jupiter 1997). No NTC was observed at Neptune, but it may interaction leading to Io-induced S-burst radio emission simply have been too weak there and below Voyager’s (Cairns et al., 2001). Radio bursts certainly constitute instrument sensitivity. one of the major remaining puzzles in the physics of At Jupiter it was realized that part of the very intense planetary radio emissions. escaping NTC was in fact the merged tails of the P. Zarka / Advances in Space Research 33 (2004) 2045–2060 2057 mysterious QP bursts discovered by Voyager (as Jovian Treumann, 2000, and references therein) suggest that this ‘‘type III’’ bursts) and further studied by Ulysses (Kaiser process is not efficient in the foreshock, but might com- et al., 1992). These bursts are very similar to the Earth’s pete with CMI in auroral regions (where cavitons created so-called ‘‘LF-bursts’’, except that the latter seem to be by LH waves may couple to UH waves to produce nar- completely aperiodic. The low frequency, quasi-isotro- rowband X mode bursts), and could even exceed its effi- pic tail of terrestrial LF-bursts has been recently well ciency in dense plasmas. explained by scattering due to propagation through the solar wind from the point where the radiation exits the 5.5. Incoherent processes magnetosphere (deep in the tail) to the observer (Stein- berg et al., 2004). The origin of the quasi-periodicity of 5.5.1. Synchrotron emission Jovian QP bursts is not known, but similar periodicities The radio emission (in the decimeter range at Jupiter) (especially about 40–45 min.) affect energetic electron from the energetic electron radiation belts is confidently spectra, ULF waves, and auroral X rays (Gladstone attributed to synchrotron emission. The synchrotron et al., 2002). Preliminary direction-finding measure- process is well known and is described in textbooks, so ments by Cassini-RPWS suggest an apparent source far that the main challenge is to deduce the spatial and above northern auroral regions (Ulysses found a spectral distribution of the electrons allowing to best southern equivalent), at a distance where the observed reproduce the observed 2D and 3D maps of radio radi- frequency is well above the local characteristic ation. Recently, Jupiter’s electron belts have been mod- frequencies of the plasma (Cecconi, 2002, private com- elled through the code ‘‘Salammbo-3D’’^ (energy, radial munication). The generation mechanism of QP- or LF- distance, latitude) originally developed for terrestrial bursts is thus not yet identified. radiation belts, and adapted for including all relevant Finally, the narrowband kilometric radiation physical processes at Jupiter (radial diffusion, absorption (nKOM) which originates from Io’s torus is produced by moons and rings, synchrotron losses, etc.) (Santos- near the local fpe (Reiner et al., 1993). Its generation, Costa and Bourdarie, 2001; Santos-Costa, 2001). The still not understood, is probably also related to mode resulting synthetic 2D radio maps provide an excellent fit conversion from, e.s. waves at fpe. to observations (Fig. 7). The following step is to include time-dependence, for example to account for the possible correlation of synchrotron output with solar wind vari- 5.4. Non-linear processes ations (with a lag of several hundred days). Another interesting question concerns the (quasi-?) Non-linear wave conversion processes require an absence of synchrotron radiation at Earth. A tentative intense background of plasma waves (beam excited answer has been recently proposed by Thorne (2002), Langmuir waves, UH, LH, ECH waves, or ‘‘BEN’’), based on a comparison of the diffusion and loss char- co-located in space and time. The basic three-wave acteristic times at Earth and at Jupiter. Inner radiation process writes: L + L ! T, with L is e.s. longitudinal belts are fed by inwards radial diffusion (caused by waves and T is e.m. transverse wave. The resonance conditions are x1 þ x2 ¼ x3 and k1 þ k2 ¼ k3: ð3Þ Again, as k is smaller for free space modes than for plasma wave modes, one must have k3 k1; k2 which implies k1 k2. When L is a Langmuir waves, then x1 ¼ x2 ¼ xpe so that x3 ¼ 2xpe. Another possibility is L being an UH e.s. wave, in which case T is an X or O mode wave at 2fUH. This type of process can account for generation of emissions at 2fpe from the foreshock. Such three-wave processes have a low efficiency and thus a low emissivity (the probabibility of wave interaction is low). In an intense magnetic field, plasma waves with parallel propagation increase their probabibility of in- teraction and thus the overall emissivity. In the presence of an intense plasma wave background (strong turbulence regime), the wave pressure can exceed the plasma pressure Fig. 7. Comparison of (a) a VLA map of Jovian synchrotron emissions and create density holes containing a broad spectrum of k. to (b) the prediction of a model of Jovian electron radiation belts (the Collapse of these ‘‘cavitons’’ can produce bursts of radio observer is in Jupiter’s equatorial plane, at a longitude CML ¼ 20 ) waves at 2fpe,2fce, etc. Detailed calculations (see (from Santos-Costa (2001)). 2058 P. Zarka / Advances in Space Research 33 (2004) 2045–2060 atmospheric winds generating random electric field dE • The weak, structured ‘‘HF auroral roar’’ observed at through charge separation in the ionosphere, which map Earth is also attributed to CMI in the low ionospheric out in the magnetosphere along magnetic field lines), and layers (where fpe < fce and fpe < fcollision); the radiation emptied by losses due to wave-particle interactions (with must then tunnel through ionospheric inhomogeneities hiss, chorus, ECH, EMIC, etc.). Characteristic diffusion to escape to infinity; no counterpart has been detected time is 1 year and Jupiter and 1 day at Earth, but with at the outer planets (probably because of limited sensi- a smaller b, Jupiter possesses a plasma wave background tivity or different structures of their ionospheres). comparatively weaker than at Earth. Losses strength at • NTC, radio emission from Ganymede, and perhaps Earth would require a diffusion time as short as 10 min nKOM are attributed to linear mode conversion pro- for efficiently filling the internal radiation belt from the cesses at magnetospheric gradients and boundaries. external one. As a consequence, the filling of the inner • 2fpe emission, e.g. in the electron foreshock, is attrib- terrestrial electron belts remains too low to produce a uted to non-linear three-wave processes. significant level of synchrotron emission. • LF-/QP-bursts are the class of radio components for which no generation mechanism is yet identified; one 5.5.2. Lightning reason is that the sources, although certainly in the Atmospheric lightning results from electric discharges magnetosphere, are not localized because the com- neutralizing a large-scale charge separation built from plex propagation sustained by the radiation erases various processes including microscopic charging of at- the memory of its source region; it cannot be ex- mospheric constituents by friction or collisions, associ- cluded that LF-/QP-bursts are generated by the ated to convection. The lightning discharge produces CMI along very high latitude field lines possibly e.m. radiation in a very broad spectrum, down to the low opened to the solar wind (cusp region); at Jupiter, frequency radio and whistler range (antenna radiation). QP-burst tails form a large fraction of what was pre- Lightning whistlers have been observed at all the outer viously thought to be an intense escaping NTC. planets except Saturn, and lightning radio emission ev- As auroral radio emissions represent the major erywhere except at Jupiter (Desch, 1992). It appears thus magnetospheric radio output (the origin of the above that lightning do exist at all the outer planets. The non- scaling laws remains to be explained in details), con- detection of whistlers at Saturn has been attributed to the version processes can be considered of secondary im- fact that lightning were mostly equatorial and thus portance when compared to CMI. whistlers confined to low altitude near equatorial field All the above processes have been best studied in the lines ‘‘missed’’ by Voyager. The absence of ‘‘radio- Earth’s magnetosphere. From the point of view of radio lightning’’ at Jupiter has been attributed to two – possibly emissions, the outer planets have brought little addi- complementary – causes: (i) a strong absorption of the tional knowledge of the fundamental microphysics in- emission during propagation through the low-altitude, volved, but they are very important for applying the dense (high-Ne) ionospheric layers observed by Pioneer concepts developed at Earth in different plasma envi- 10 and 11 and attributed to micrometeor impacts (Zarka, ronments (for example the first modelled auroral radio 1985) and/or (ii) a radio spectrum intrinsically limited to spectrum was that of SKR). Understanding generation very low frequency if the discharges are ‘‘slower’’ at Ju- (and propagation) of radio and plasma waves in turn piter than at the other planets, as suggested by Galileo provides efficient tools for in situ studies (e.g. with QT probe in situ electric field measurements (Farrell, 2000). noise or Bernstein modes) and remote sensing of the outer planets magnetospheres. Synchrotron radiation, for example, is a unique tool for remotely studying the 6. Conclusions Jovian electron radiation belts. Similarly, the radio sig- nature (through Jovian auroral radio components and Plasma wave spectra display a remarkable overall nKOM) of the ‘‘energetic events’’ discussed in the sec- similarity at all magnetized planets in spite of the very tion ‘‘Solar wind control...’’ allows to study remotely diverse magnetospheric configurations, emphasizing the the dynamics of the Jovian magnetosphere and its con- importance of similar microphysics processes at all trol by internal (centrifugal ejections of plasma from Io’s magnetized planets, in particular the outer giant planets. torus?) versus external (interplanetary shocks?) pro- But the magnetospheres of the outer planets also give cesses. Nevertheless, only very few works have consid- birth to a large diversity of radiosources, reflecting their ered the quantitative modeling of plasma wave and complex structure (plasma reservoirs, gradients, regimes radio wave generation. Considerable work is still needed of fpe=fce, etc.) and dynamics (locations and distributions to understand the efficiency of the various generation of energetic electrons). The mechanisms of generation of mechanisms. This is a fundamental issue for using radio most planetary components are more or less understood: emissions as a real quantitative diagnostic tool. • CMI appears to dominate for intense, structured au- The only radio component whose study at the outer roral components. planets could bring significant conceptual advances in the P. Zarka / Advances in Space Research 33 (2004) 2045–2060 2059 physics of the generation process is Jovian S-bursts. Only Carr, T.D., Desch, M.D., Alexander, J.K. Phenomenology of magne- these bursts, which can reach 40 MHz (the maximum tospheric radio emissions, in: Dessler, A.J. (Ed.), Physics of the surface gyrofrequency at Jupiter), have frequencies well Jovian Magnetosphere. Cambridge University Press, New York, pp. 226–284, 1983. above the Earth’s ionospheric cutoff (at fpe 10 MHz) Carr, T.D., Reyes, F. Microstructure of Jovian decametric S bursts. J. and can thus be observed with ground-based instrumen- Geophys. Res. 104, 25127–25141, 1999. tation virtually without any limit on data rates, and Cecconi, B. Private communication, 2002. consequently with extremely high time and frequency Clarke, J.T., Grodent, D., Cowley, S., et al. Jupiter’s Aurora, in resolutions. For example, a study of S-bursts waveform Jupiter, in: Bagenal, F., McKinnon, W, Dowling, T. (Eds.), Cambridge University Press, New York, USA, in press, 2004. samples has recently shown that the drifting radio signal Cowley, S.H., Bunce, E.J. Origin of the main auroral oval in Jupiter’s can remain perfectly monochromatic and phase-coherent coupled magnetosphere–ionosphere system. Planet. Space Sci. 49, for several milliseconds (Carr and Reyes, 1999). 1067–1088, 2001. Table 3 suggests that in terms of its radio zoo, Saturn Desch, M.D. Lightning at planets in the outer solar system, in: Rucker, is less complex than the other outer planets. Its mag- H.O., Bauer, S.J., Kaiser, M.L. (Eds.), Planetary Radio Emissions III. Austrian Academic Science Press, Vienna, Austria, pp. 371– netosphere may have a simpler structure (with a quasi- 390, 1992. axial non-tilted field and no internal plasma source Desch, M.D., Kaiser, M.L., Zarka, P. Uranus as a radio source, in comparable to Io), which motivated the theoretical Uranus, in: Bergstralh, J.T., Miner, E.D., Matthews, M.S. (Eds.), modeling of the SKR spectrum. Starting its P 4-year Uranus. Univesity of Arizona Press, Tucson, AZ, 1991. tour in July 2004, Cassini will reveal whether this is in- Dougherty, M.K., Southwood, D.J., Balogh, A., et al. Field-aligned currents in the Jovian magnetosphere during the Ulysses flyby. deed true or if the complexity was hidden in the Voyager Planet. Space Sci. 41, 291–300, 1993. data (Kurth and Zarka, 2001). An interesting check Ergun, R.E., Carlson, C.W., McFadden, J.P., et al. FAST satellite Cassini will make concerns the above generalized radio wave observations in the AKR source region. Geophys. Res. Lett. scaling law: according to it, Dione, to influence signifi- 25, 2061–2064, 1998. cantly the SKR occurrence as it has been suggested in Farrell, W.M. Planetary radio emission from lightning: discharge and detectability, in: Stone, R.G., Weiler, K.W., Goldstein, M.L., the literature, must be an obstacle in Saturn’s magnetic Bougeret, J.-L. (Eds.), Radio Astronomy at Long Wavelength. field much larger than its physical size, and thus possess Geophysical Monograph 119. AGU, Washington, DC, pp. 179– an extended exosphere or an intrinsic magnetic field 186, 2000. forming its own magnetosphere. Farrell, W.M., Desch, M.D., Kaiser, M.L., et al. 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