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Dayside and Polar Cap Aurora

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The auroral emissions in the upper atmosphere of the polar regions of the Earth are evidence of the capture of energetic particles from the Sun, streaming by the Earth as the solar wind. These auroral emissions, then, are a window to outer space, and can provide us with valuable information about electrodynamic coupling processes between the solar wind and the Earth's ionosphere and upper atmosphere. Studying the physics of these phenomena extends our understanding of our plasma universe.
Ground-based remote-sensing techniques, able to monitor continuously the variations in the signatures of aurorae, in combination with in-situ satellite and rocket measurements, promise to advance dramatically our understanding of the physical processes taking place at the interface of the atmospheres of the Earth and the Sun. Decoding their complexity brings us closer to reliable prediction of communication environments, especially at high latitudes. This understanding, in turn, will help us resolve problems of communication and navigation across polar regions.
Aurorae have been the object of wonder and scientific curiosity for centuries. Only recently, however, have we been able to detect, with sensitive instrumentation, noontime aurorae, and persistent aurorae deep within the polar cap. This book is the first to provide a morphological and theoretical framework for understanding these dayside and polar cap aurorae.
The book also communicates the excitement of discovery, as it details the nature of these newly revealed auroral displays. It is a fascinating voyage of exploration, one appropriate for students of nature, wherever and whoever they may be.

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Contents
Preface Acknowledgements 1Auroral Physics: The Preamble 1.1A Look at History 1.2TwentiethCentury Pioneers 1.3The Origins of Aurora 1.4Observing Aurora 1.5Concepts of the Auroral Oval and SunAligned Arcs 1.6New Discoveries 2NearEarth Space and Dayside Aurora 2.1Introduction 2.2Magnetospheric Boundary Layers 2.3Dayside and Polar Cap Auroras 2.4Convection Patterns 2.5Responses to IMF Transitions: Aurora and Convection 2.6Remarks on Major Results
3Optical Aurora 3.1Introduction 3.1.1 Altitude Profiles 3.1.2Useful Relationships 3.1.3 Auroral Emissions from Dayside Aurora and SunAligned Arcs 3.1.4 Atomic vs. Molecular Emission 3.2Forbidden Atomic Lines in the Auroral Emissions 3.3Permitted Atomic Lines 3.4Thermal Excitation in General 3.5Atmospheric Temperatures and Auroral Emissions 3.6The Hydrogen Lines — Proton Auroras 3.7Characteristics of Auroral Emissions 3.8Auroral IntensitiesUnits of 3.9Summary 4Dayside Auroral Forms and Activities 4.1Introduction 4.2Case 1: December 3, 1997 4.2.1Auroral Observations
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[ xii ]
CONTENTS
4.2.2Magnetic Observations 4.2.3Case Review 4.3Case 2: November 30, 1997 4.3.1Solar Wind and IMF Observations 4.3.2Auroral Observations 4.3.3 Magnetic Observations 4.3.4Combined Ground and Satellite Observations 4.3.5Case Review 4.4January 11, 1993Case 3: 4.4.1Case Review 4.5January 3, 1995Case 4: 4.5.1 IMF Observations 4.5.2Auroral Observations 4.5.3Magnetic Observations 4.5.4Observations of Particle Precipitation 4.6Case Review 4.7Case 5: January 12, 1997 4.7.1Solar Wind and IMF Observations 4.7.2Auroral Observations 4.7.3Case Review 4.8Case 6: November 22, 1995 4.8.1 Solar Wind and IMF Observations 4.8.2Auroral and Magnetic Observations 4.8.3Ionospheric Ion Drift Observations 4.8.4Observations of Particle Precipitation 4.8.5 Case Review 4.9Case 7: November 20, 1995 4.9.1 Solar Wind and IMF Observations 4.9.2Auroral Observations 4.9.3Plasma Convection 4.9.4Aurora and Particle PrecipitationGround and Satellite Observations: 4.9.5Case Review 4.10December 16, 1998Case 8: 4.10.1 Solar Wind and IMF Observations 4.10.2Auroral Observations 4.10.3Ionospheric Ion DriftRadar Observations: 4.10.4Satellite Observations: Particle Precipitation 4.10.5 Case Review 4.11 Summary of Observations 4.11.1Classification of Dayside Forms 4.11.2 Cusp Aurora and Plasma Convection in Relation to the IMF 4.11.3Dayside Auroras in Relation to Particle Precipitation and Boundary Layer Plasma Sources 4.11.4 Cusp Aurora in Relation to Solar Wind Density/Dynamic Pressure
5Morphology of PolarCap SunAligned Arcs 5.1Introduction 5.1.1 Why this Interest in SunAligned Arcs? 5.1.2Presentation Plan 5.2Historical Background and IMF Context
61 63 66 66 68 74 77 78 83 88 89 89 89 91 94 95 97 97 99 101 107 107 107 110 114 115 120 120 120 123 123 127 129 129 129 133 135 138 141 141 149
158 162
167 167 168 169 170
5.3
5.4
5.5
5.6
5.7
5.8
5.9
CONTENTS
5.2.1Historical Context 5.2.2First Sighting of SubVisual SunAligned Arc Aurora 5.2.3The Two States of the Polar Cap 5.2.4GroundBased Signatures of Southward vs. Northward IMF 5.2.5Improved Sensitivity 5.2.6Improved Physical Insights Arc Electrodynamics 5.3.1OhmsLaw Arcs 5.3.2Verification and Calibration Using Satellite Data 5.3.3IncoherentScatter Radar Verification and Calibration Studying Arc Events Using Satellite Overflights 5.4.1Supporting ASIP Images 5.4.2In Situ Measurements by Dynamics Explorer, DE2, Satellite 5.4.3In Situ Signatures 5.4.4Geophysical Noise in Arc Detection 5.4.5SmallScale Flow Reversals 5.4.6SmallScale and LargeScale Flow Studying Arcs with IncoherentScatter Radar (ISR) 5.5.1Imaging 5.5.2Mapping 5.5.3Deriving Cross Sections 5.5.4Mesoscale Dynamics 5.5.5DirectlyObserved ISR Parameters 5.5.6Derived Parameters 5.5.7NonParticle Heating 5.5.8Polar Thermospheric Thermal Balance 5.5.9Variability Along Arcs 5.5.10 Summary of ISR studies Studying Arcs with Rockets 5.6.1Ion Composition 5.6.2Lower Hybrid Frequency 5.6.3Plasma Instabilities Statistical Behavior of SunAligned Arcs 5.7.1Methodology for Statistical Studies 5.7.2Coordinates for SunAligned Arc Statistics 5.7.3Detecting Arcs 5.7.4Arc Orientation 5.7.5DawnDusk Velocity 5.7.6Arc Motion vs. IMF 5.7.7Arc Lifetime Particle and Emission Spectra 5.8.1Satellite Study of Electron Precipitation Energy Spectra 5.8.2Spectroscopic Measurements Response Time and Dynamics 5.9.1ReversalsResponse Time 5.9.2Simultaneous Arcs and Patches 5.9.3Arcs at the Auroral Oval Boundary
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170 171 173 177 182 182 183 184 185 186 189 189 189 191 192 192 193 194 195 197 198 200 204 204 206 209 210 210 212 213 213 213 215 216 218 220 223 224 225 225 225 226 228 228 228 230 232
[ xiv ]
CONTENTS
6Theory of PolarCap SunAligned Arcs 6.1Introduction 6.1.1 Elements of Theory and Processes 6.1.2EnergyFlow Framework 6.2Anatomy of a PolarCap SunAligned Arc 6.2.1 Ionospheric Altitudes 6.2.2NearEarth Altitudes 6.2.3 Out to the Solar Wind Interface 6.3Electrodynamics of SunAligned Arcs 6.3.1 OhmsLaw Arcs 6.3.2Rest FrameNeutral Gas 6.3.3Electric Fields, Plasma Drift, and Currents 6.3.4SelfConsistent Calculation of Arc Conductivities 6.3.5 A New Research Tool 6.3.6Chemical Lifetime 6.4Current Sheets 6.4.1 Optical Emission in and Near SunAligned Arcs 6.4.2Thermal Excitation of Arc Emission 6.4.3IR and UV Auroral Emissions 6.4.4Hydrogen Emissions 6.5The Polar Ionosphere 6.5.1 Polar lonization 6.5.2Polar Plasma Motion 6.5.3Rest Frame for ElectrodynamicsThe Thermosphere as a 6.5.4Plasma Instabilities 6.5.5General Instability Theory for FRegion Ionosphere 6.5.6 GroundBased Scintillation Studies 6.5.7Plasma IrregularitiesSatellite Direct Measurement of 6.6Energy Estimates 6.6.1 ParticleEnergy Deposition 6.6.2Poynting Flux 6.7Electron and IonGas Thermal Balance 6.7.1 General Considerations for Plasma Thermal Balance 6.7.2Electron Gas Thermal Balance 6.7.3 Ion Gas Thermal Balance 6.7.4Overall Energy Flow Within the Arc 6.8Thermospheric Heating and Momentum Transfer 6.8.1 Thermospheric Heating 6.8.2Thermospheric Winds 6.9Composite Essential Arc Features, Properties, and Processes 6.9.1 Signatures of SunAligned Arcs 6.9.2NearEarth Physics 6.9.3FarEarth Physics 6.10Further Physical Insights From Statistical Studies
Bibliography
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