Aurora

1. Introduction.- 1.1. Basic Ideas.- 1.2. Brief History of Auroral Studies.- 1.3. Aurora and the Magnetosphere.- 2. Techniques of Observation.- 2.1. Observations of Total Intensity.- 2.1.1. Visual Observations.- 2.1.2. All-Sky Camera Observations.- 2.1.3. Television and Image Intensifier Camera Systems.- 2.1.4. Ground Based, Rocket and Satellite Auroral Photometry.- 2.1.5. Height Finding Systems.- 2.2. Spectroscopic Techniques.- 2.2.1. Grating Spectrographs.- 2.2.2. Grating Spectrometers.- 2.2.3. Fabry-Perot Spectrometers.- 2.2.4. Interference Filter Photometers.- 2.2.4.1. Multiple Channel Systems.- 2.2.4.2. Use of Tilting-Filter Systems.- 2.2.4.3. Spatial Scanning Techniques.- 2.2.4.4. Wedge Interference Filters.- 2.2.4.5. High Order Interference Filter Systems.- 2.2.5. Fourier Spectroscopy.- 2.2.5.1. Field Compensated Michelsons.- 2.2.6. Birefringent Filter Photometers.- 2.3. Particle Measurements.- 2.3.1. Particle Detectors.- 2.3.2. Measurement of Particle Energy Distribution.- 2.4. Radio Reflection Techniques.- 2.4.1. Pulse Radar Systems.- 2.4.2. Continuous Wave Reflection Techniques.- 2.4.3. Ionospheric Sounders.- 2.5. Magnetic Field Measurements.- 2.5.1. Classical Methods.- 2.5.2. Fluxgate Magnetometers.- 2.5.3. Zeeman Effect Magnetometers.- 2.5.4. Rotating Coil Systems.- 2.6. Other Observing Techniques.- 2.6.1. Radio Emissions.- 2.6.2. Auroral Cosmic Noise Absorption.- 2.6.3. Electric Fields Associated with Aurora.- 2.6.4. Detection of Infrasonic Emissions from Aurora.- 2.6.5. Detection of X-Rays from Aurora.- 3. Occurrence and Morphology.- 3.0. Introduction.- 3.1. Occurrence of Visible Aurora.- 3.1.1. The Auroral Zones.- 3.1.2. The Auroral Oval.- 3.1.3. Conjugacy of Aurora.- 3.1.4. Longitude Effects.- 3.2. Individual Displays — Auroral Substorms.- 3.3. Detailed Morphology of Auroral Forms.- 3.3.1. Classification of Visual Auroral Forms.- 3.3.2. Detailed Structure of Auroral Forms.- 3.3.3. Intensity Indices.- 3.3.4. Height Distribution of Normal Aurora.- 3.3.5. Visual Types of Aurora.- 3.3.6. Variations in Aurora with Geomagnetic Time and Latitude.- 3.3.7. Rapid Time Variations — Pulsing Aurora.- 3.3.7.1. Pulsating Aurora.- 3.3.7.2. Flickering Aurora.- 3.4. Proton Aurora.- Height of Proton Aurora.- Intensity and Intensity Fluctuations.- Geomagnetic Time-Latitude Variation — Proton Auroral Oval.- Proton Precipitation in Auroral Substorms.- 3.5. Relation of Aurora to Solar Events.- 3.5.1. Periodicities and Recurrences.- 3.5.2. Correlations with Solar Events.- 3.5.3. Relation of Aurora to Solar Wind and Interplanetary Magnetic Field.- 3.6. Magnetic Disturbances and Aurora.- 3.6.1. Magnetic Storms and Equivalent Current Systems.- 3.6.2. Real Current Systems in Aurora.- 3.6.3. Electric Fields in Aurora.- 3.7. Relation Between Auroral Substorms and the Magnetosphere.- 3.8. Particle Fluxes in the Auroral Oval.- 4. Optical Emissions from Aurora.- 4.1. Optical Transitions and the Auroral Spectrum.- 4.1.1. The Observed Spectrum.- 4.1.2. Excitation and Ionization Cross Sections.- Excitation Cross Sections.- Ionization Cross Sections.- Partial Cross Sections for Vibrational Levels.- 4.1.3. Optical Transition Probabilities.- Atomic Lines and Multiplets.- Electronic Band Systems.- Rotational Fine Structure of Electronic Bands.- 4.1.4. Quenching and Energy Transfer Processes.- 4.2. Electron Aurora.- 4.2.1. Interaction of an Energetic Electron Beam with the Atmosphere.- Semi-Empirical Method of Rees.- Extended Fokker-Planck Method.- Model of Stolarski and Green.- 4.2.1.1. Empirical Energy Deposition Function.- 4.2.1.2. Calculated Energy Deposition Function.- 4.2.1.3. Ionization Rate Height Profiles.- 4.2.1.4. Production Rates of Individual Ions.- 4.2.1.5. Production Rate of Secondary Electrons as Function of Energy.- 4.2.1.6. Flux of Secondary Electrons.- 4.2.1.7. Primary Electron Flux.- 4.2.1.8. Total Electron Fluxes.- 4.2.2. Excitation of Atmospheric Atoms by Primary and Secondary Electrons.- 4.2.2.1. Excitation and Ionization by Primary and Secondary Electrons.- 4.2.2.2. Excitation and Ionization by Secondary Photons.- 4.2.3. Indirect Excitation Processes and Quenching.- 4.2.3.1. Dissociative Recombination.- 4.2.3.2. Energy Transfer.- 4.2.3.3. Quenching.- 4.2.3.4. Thermal Excitation.- 4.2.3.5. Cascading.- 4.2.4. Predicted Electron Auroral Spectrum and Comparison with Observation.- 4.2.4.1. 1N N2+ Bands.- 4.2.4.2. Meinel N2+ System.- 4.2.4.3. N2 Triplet and Triplet-Singlet Systems.- 4.2.4.4. N2 Singlet Systems.- 4.2.4.5. O2 Systems.- 4.2.4.6. O2+ Systems.- 4.2.4.7. NO ? System.- 4.2.4.8. OH Vibration-Rotation Bands.- 4.2.4.9. [Oi] Forbidden Transitions.- Analysis of pulsing aurora.- 4.2.4.10. [Ni] Forbidden Transitions.- 4.2.4.11. [Oii] and [Nii] Forbidden Transitions.- 4.2.4.12. Oi Higher Level Transitions.- 4.2.4.13. Ni Higher Level Transitions.- 4.2.4.14. Oii and Nii Higher Level Transitions.- 4.2.4.15. Other Atomic Lines.- 4.2.5. Variations in the Auroral Spectrum.- 4.2.5.1. Introduction.- 4.2.5.2. Simple Height Effects.- 4.2.5.3. Simple Energy Spectrum Effects.- 4.2.5.4. Complex Energy Spectrum Effects.- 4.2.5.5. Atmospheric Temperature Effects.- 4.3. Proton Aurora.- 4.3.1. Interaction of a Proton Beam with the Atmosphere.- 4.3.1.1. Ionization Equilibrium of a Proton Beam.- 4.3.1.2. Ion Production Rate.- 4.3.1.3. Secondary Electron Energy Distribution.- 4.3.2. Excitation of Hydrogen Emission Lines and Their Doppler Profiles.- 4.3.2.1. Hydrogen Line Emission.- 4.3.2.2. Ionization.- 4.3.2.3. Hydrogen Line Profiles.- 4.3.2.4. Height Profiles of Balmer Lines.- 4.3.3. Spectrum of Proton Aurora.- 4.3.3.1. Theory.- 4.3.3.2. Observed Spectrum and Comparison with Theory.- 4.3.4. Polar Cap Glow.- 5. Aurora and the Ionosphere.- 5.1. Auroral Ionization and Its Effects.- 5.1.1. The Ion and Electron Continuity Equation in Aurora.- 5.1.2. Ion Chemistry in the Auroral Atmosphere.- 5.1.3. Effects of Transport on Auroral Ionization.- 5.1.4. Observations of Electron Concentrations in Aurora.- 5.1.5. Comparison of Theoretical and Observed Ion Concentrations in Aurora.- 5.2. Electrical Conductivity, Electric Fields and Currents.- 5.2.1. Conductivities.- 5.2.2. Currents and Electric Fields in the Auroral Ionosphere.- 5.3. Heating Effects in Aurora.- 5.3.1. Heating of the Neutral Atmosphere by Auroral Particle Fluxes.- 5.3.2. Heating of Electrons by Auroral Particles.- 5.3.3. Heating of Ions by Auroral Particles.- 5.3.4. Heating of Ions and Electrons by Electric Fields.- 5.3.5. Heating of Neutral Particles by Electric Fields.- 5.3.6. Transport Effects in the Electron and Ion Gas.- 5.3.7. Cooling of the Electron Gas.- 5.3.8. Cooling of Ion Gas.- 5.3.9. Electron Temperatures in Aurora.- 5.3.10. Ion Temperatures in Aurora.- 5.3.11. Effects of Neutral Particle Heating in Aurora.- 5.4. Reflection of Radio Waves from the Auroral Ionosphere.- 5.4.1. Reflection Mechanisms.- 5.4.2. Fine Structure of Auroral Ionization.- 5.4.3. Comparison of Theory and Observations of Radio Aurora.- 5.5. Radio Absorption.- 5.5.1. Theory of Absorption.- 5.5.2. Application to Aurora.- 5.5.3. Results of Riometer Studies.- 5.6. X-Rays from Auroral Electrons.- 5.6.1. Theory of X-Ray Production.- 5.6.2. X-Ray Propagation.- 5.6.3. Observations of Auroral X-Rays.- 5.7. Radio Emission from Aurora.- 5.7.1. Theory.- 5.7.2. Comparison with Observations.- 5.8. Infrasonic Emission from Aurora.- 5.8.1. Observations.- 5.8.2. Theory of Propagation.- 5.8.3. Theory of Generation.- 6. Mechanisms of Precipitation of Auroral Particles.- 6.0. Introduction.- 6.1. The Steady State Magnetosphere.- 6.1.1. Shape and Structure.- 6.1.2. Convective Circulation.- 6.1.3. The Effect of the Ionosphere.- 6.1.4. The Effect of Merging on Form.- 6.1.5. Particle Entry and Energization.- 6.1.5.1. Origin of Auroral Particles.- 6.1.5.2. Adiabiatic Energization.- 6.1.5.3. Neutral Sheet Energization Processes.- 6.2. Transient Effects and Substorms.- 6.2.1. Stable Transient Tail Growth.- 6.2.2. Instability in Tail Growth.- 6.2.3. Additional Acceleration Processes.- 6.2.4. Particle Trapping and Pitch Angle Scattering.- 6.2.5. Proton Precipitation.- 6.2.5.1. Plasma-Wave-Proton Interactions.- 6.2.5.2. Energization of Protons and the Ring Current.- 6.2.5.3. Other Acceleration Processes for Protons.- 6.2.6. SAR- or M-Arcs.- 6.3. Fine Structure of Auroral Precipitation.- 6.3.1. Formation of Auroral Arcs.- 6.3.2. Deformation of Auroral Arcs.- 6.3.2.1. Charge-Sheet Instability.- 6.3.2.2. The Current Sheet Instability.- 6.3.3. Temporal Structure — Pulsing Aurora.- 6.4. Conclusion.- Appendices.- 2A. Use of Filter Photometers for Measurement of Absolute Total Intensity of Emission Bands.- 3A. Geomagnetic Latitude and Time.- 3B. Magnetic Disturbance Indices.- 6A. Trajectories of Particles Trapped in a Dipole Field.- References.