Aurora

Over the past two decades auroral science has developed from a somewhat mysterious and imprecise specialty into a discipline central in the study of the ionosphere and magnetosphere. The investigation of aurora unites scientists with very different backgrounds and interests so that it is difficult to write a self-contained account of the field in a book of reasonable length. In this work I have attempted to include those aspects of theory which I have found valuable in predicting the effects on the atmosphere of auroral particle precipitation. In addition I have attempted to describe the techniques of observation with particular emphasis on optical methods which have been useful. While the aeronomy of aurora has been regarded as central, the mechanisms by which particles are accelerated and precipitated into the atmosphere is of no less interest. This aspect of the subject has however been treated in a briefer fashion since it is a part of the immense and rapidly developing field of magnetospheric science. Generally I have attempted to provide a coherent introduction to auroral science with an emphasis on relatively simple physical interpretations and models. References are given to enable the reader to find more extensive or rigorous discussions of particular topics. A fairly complete, quantitative atlas of the auroral spectrum is included.

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 Acc