Session J5: Invited Session: The Dynamics of Waves and Energetic Particles: Observations

Radio emissions from planetary magnetospheres

Since the discovery of intense radio emissions from Jupiter by Burke and Franklin in 1955, it is now known that the magnetospheres of all the strongly magnetized planets emit intense non-thermal radio emissions. This talk will review the progress that has been made in understanding these radio emissions during the more than fifty years since their discovery. It is now known that two basic radio emission processes are involved: cyclotron maser radiation from precipitating auroral electrons, and mode conversion from electrostatic waves driven by the anisotropy of magnetically trapped magnetospheric electrons. Of these, the cyclotron maser radiation is by far the most intense. Since the gaseous outer planets have no visible surface and since the magnetic field which controls the motion of the electrons is linked to the deep interior, the rotational modulation of cyclotron maser radiation provides the primary method of determining the rotation rates of these planets. Cyclotron maser radiation has also been detected from certain strongly magnetized stars, and serious efforts are now underway to try to detect cyclotron maser radiation from extra-solar system planets.   

Energetic Particle Transport by Instabilities in Fusion Plasmas

Recent experimental confirmation of various mechanisms of fast-ion transport by instabilities in magnetically-confined plasma is presented. Energy transfer depends on $\oint \vec v\cdot\vec E$, where $\vec v$ is the fast-ion velocity, $\vec E$ is the perturbed electric field produced by the instability, and the integral is over the orbit. For instabilities that do {\it not} coincide with frequencies of orbital motion (non-resonant instabilities), the large orbits of fast ions reduce transport via phase-averaging of the electric field. Drift waves with small spatial structure cause less transport than drift waves with large structure, and coherent waves cause less transport than turbulent waves. For a sawtooth instability with a large, transient electric field, trapped particles with large drift orbits that decouple the ions from flux surfaces suffer less transport than passing particles. Resonant instabilities are qualitatively different. The standard resonance condition for modes with frequencies $\omega\ll\omega_{ci}$ is $\omega=n\omega_\zeta+p\omega_\theta$. Here $\omega_{ci}$ is the cyclotron frequency, $n$ is the toroidal mode number of the instability, $p$ is an integer, and $\omega_\zeta$ and $\omega_\theta$ are the toroidal and poloidal frequencies of the orbital motion. Coherent resonant losses occur when Alfv\'en waves push fast ions onto loss orbits. A large fraction of the fast-ion population is expelled via coherent losses when large global modes maintains the resonance condition across much of the plasma. Multiple resonances often produce diffusive transport. Many small-amplitude Alfv\'en eigenmodes cause diffusive flattening of the fast-ion profile. For large mode amplitudes, other resonant transport mechanisms become operative. Above a certain threshold, an avalanche of different Alfv\'en waves can be excited, causing substantial transport. Large amplitude waves can nonlinearly excite new resonances at harmonics and subharmonics of the orbital frequencies, as recently observed for energetic-particle driven geodesic acoustic modes.   

Pulsar Radio Emission Mechanisms: The Crab Enigmas

The Crab pulsar, which resulted from a supernova explosion in year 1054 A. D., has been studied intensely over a wide range of wavelengths, yet it continues to reveal new phenomena that challenge explanation. The emission structures in the radio regime are complex and some may be unique among pulsars. The standard models for pulsar geometry and radio emission physics are reviewed briefly. Their predictions are then compared with observations of the Crab pulsar radio emission and the observations are used to critique the theoretical models from an observer's point of view. The models must explain the extremely short and bright nanopulses (0.4 ns duration implying an equivalent brightness temperature of $10^{42}$ K), the wide bandwidths of radio emission (at least 0.02 to 46 GHz), the regular banded nature of the high frequency interpulse emission, the complex polarization structure, and the phases of pulsar rotation where emission occurs. So far no comprehensive model satisfies all of the observational discriminants.