Neutral-atom Astronomy: Plasma Diagnostics From The Aurora To The Interstellar Medium

Description

Astronomy has been associated with the detection of electromagnetic waves or photons from within and beyond the solar system, ranging from Radio to Gamma-ray Astronomy. Particle Astrophysics, including Neutrino and Dark-Matter Astrophysics today, started with the discovery of cosmic rays in 1911. The Space Age expanded particle observations to in-situ studies of lower energy electrons and ions with a variety of charge states in space plasmas traversed by spacecraft. Remote observation of space plasmas became possible only after the discovery of energetic neutral atoms (ENAs) in space in 1950. This book is a primer for those who wish to learn more about the origins of ENAs, related detection techniques, and how ENA images and spectra can be used to study space plasmas beyond the reach of spacecraft. It tells a comprehensive story from the first encounters with ENAs in the Earth's magnetosphere to Neutral-Atom Astronomy of the edge of the heliosphere and the interstellar medium. This story includes how ion mass spectrographs evolved into ENA imagers, overcoming the technical challenges, how to extract information from ENA data, and a variety of diagnostic applications on the magnetosphere, interplanetary space, other solar-system objects, the heliospheric boundary, the local interstellar medium, and a glimpse into the future of Neutral-Atom Astronomy. The authors hope to inform and inspire readers to further enrich this field of study. Contents Preface List of Tables List of Figures List of Acronyms Symbols and Units for Useful Quantities Chapter 1. Introduction 1.1. Members of the Cast 1.2. Earth’s Magnetosphere 1.2.1. Earth’s exosphere 1.3. The Heliosphere 1.3.1. The interstellar neutral-atom flow in the heliosphere 1.3.2. Global view of the heliospheric boundary with ENAs References Chapter 2. First Encounters with ENAs 2.1. The Discovery: From the Telescope to the Accelerator 2.2. First Diagnostic Use of ENAs in Space 2.3. Invoking ENAs to Explain New Satellite Observations 2.4. First ENA Images from Space References Chapter 3. Remote Sensing of Space Plasma Through ENA Observations 3.1. ENA “Imaging”: Determination of the Spatial, Energy & Elemental Distributions of Remote Ion Populations 3.2. Ion Distribution Functions and Plasma Processes 3.2.1. Velocity distributions in thermal equilibrium 3.2.2. Non-thermal velocity distributions 3.2.3. Moving distributions and frame transformations 3.3. Relation Between the Observed ENA Flux and the Remote Ion Distribution 3.4. Propagation of ENAs and Observational Limits 3.4.1. ENA trajectories in gravitational fields 3.4.2. Extinction of ENAs from the source to the observer 3.4.3. Effects of elastic collisions on the ENA diagnostics 3.4.4. PUIs as inevitable companions of ENAs 3.5. Summary References Chapter 4. ENA Instrumentation: General 4.1. Choosing Vantage Points for ENA Observations 4.1.1. Viewing the heliosphere: SOHO and IBEX 4.1.2. Viewing Earth’s magnetosphere: IMAGE and TWINS 4.2. General Considerations for Space-Borne Particle Instruments 4.2.1. Particle selection by analyzers 4.2.2. Particle detectors 4.2.2.1. Electron multipliers 4.2.2.2. Solid-state detectors 4.2.3. Principles of TOF measurements 4.2.3.1. Combination of TOF and SSD measurements 4.2.3.2. Combination of TOF and ESA measurements 4.2.3.3. Intrinsic noise-suppression capability of TOF sensors 4.2.3.4. TOF spectrographs for neutral atoms 4.3. ENA-Specific Observational Challenges 4.3.1. Suppression of charged particles 4.3.2. Attenuation of EUV 4.3.3. Conversion of ENAs into ions before their analysis 4.3.3.1. Conversion of ENAs to positive ions 4.3.3.2. Conversion of low-energy ENAs or ANAs to negative ions References Chapter 5. ENA Sensor Implementations 5.1. The Road to ENA Instrumentation 5.1.1. Common root: two non-ENA instruments of relevance 5.1.2. First ENA instrument in space 5.2. Non-TOF ENA Sensors 5.2.1. The first ANA instrument: GAS on Ulysses 5.2.2. ENA sensors with MCP single-pulse detection 5.2.3. ENA sensors with SSD single-pulse detection 5.3. ENA Spectrographs with TOF Measurement 5.3.1. ENA cameras for the 10–200 keV range 5.3.2. ENA cameras for the 0.3–20 keV range 5.3.3. Neutral atom cameras for the 5–2000 eV range References Chapter 6. ENAs from Magnetospheres and Small Bodies in the Solar System 6.1. Earth’s Magnetosphere 6.1.1. The outer magnetosphere 6.1.1.1. The magnetosheath 6.1.1.2. The polar cusp 6.1.1.3. The magnetotail 6.1.2. The inner magnetosphere 6.1.2.1. ENA images and their intrinsic challenges 6.1.2.2. Deconvolution of ENA images 6.2. Magnetospheres and Ionospheres of Other Planets 6.2.1. Jupiter and Saturn 6.2.1.1. Jupiter seen in ENA images 6.2.1.2. Saturn seen in ENA images 6.2.2. SW-induced magnetospheres 6.2.2.1. ENAs at Mars 6.2.2.2. ENAs at Venus 6.2.3. Mercury 6.3. Small Bodies in the Solar System 6.3.1. ENAs from the Moon 6.3.2. Smaller bodies and dust References Chapter 7. ENA Diagnostics of the Heliosphere-Interstellar Medium Interaction 7.1. Getting Acquainted with the Heliosphere 7.2. Interstellar Neutral Wind through the Heliosphere 7.2.1. Overview of in-situ diagnostic methods 7.2.2. Connecting ISN flow observations with the ISM 7.2.3. Observational challenges and opportunities for the ISN flow 7.3. The ENA Ribbon, a “Compass” for the Interstellar Magnetic Field 7.3.1. Ribbon overview 7.3.2. The Secondary-ENA model as likely explanation 7.3.2.1. Simplified analytical model 7.3.2.2. Tests of the Secondary-ENA model 7.3.3. Challenges and opportunities with the ENA Ribbon 7.3.3.1. Effectiveness of Ribbon generation and turbulence 7.3.3.2. Determination of the interstellar magnetic field 7.3.3.3. Reach of the Solar Wind into the ISM 7.4. Secondary Interstellar Neutrals 7.4.1. Extraction of secondary interstellar neutrals 7.4.2. Challenges and opportunities with secondary interstellar neutrals 7.5. Globally Distributed ENAs 7.5.1. Extraction of the globally distributed ENAs 7.5.2. Challenges and opportunities with the GDF of ENAs References Chapter 8. What Have ENAs Revealed and What is Next? 8.1. Understanding the Cross-Scale Coupling of the Magnetospheric Systems 8.2. Surveying Exposed Surfaces in the Solar System: Planets to Dust 8.3. Understanding the Sun-ISM Interactions and Particle Acceleration on Large Scales 8.4. Bridging the Temporal and Spatial Divide References Appendix A. Geometrical Factor and Angular Response A.1. Geometrical Factor A.2. Angular Response A.2.1. Example of a calibrated angular response A.2.2. Ignoring the angular response, a potential problem References Appendix B. ENA-Beam Calibration Facilities B.1. Requirements and General Principles of Operation B.1.1. Neutral-atom beams through physical contact B.1.2. Neutral-atom beams through photo detachment B.1.3. Comparison of different schemes References Index