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Radio and Plasma Waves at the Outer Planets 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. Ó 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 Ó 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 À 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 À fcefci þðfce=2Þ Þ À 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 Á À1=2 account in their dispersion relations. À1 2 2 À1 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 <c) above fpe, quencies versus the distance to the planet in the Earth’s and also called L–X mode below fpe. Various theories magnetosphere are displayed on Fig. 1. Due to the have been proposed for its generation, among which coupling between the waves and the plasma, wave en- coupling to the R–X mode at the edges of auroral ergy is able to propagate only along specific curves in the plasma cavities (where e 1), because in this case Z and P. Zarka / Advances in Space Research 33 (2004) 2045–2060 2047 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 À k m À nx ¼ 0; ð1Þ tense auroral hiss (see below).
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