Magnetospheric Imaging: Understanding the Space Environment through Global Measurements

Magnetospheric Imaging: Understanding the Space Environment through Global Measurements is a state-of-the-art resource on new and advanced techniques and technologies used in measuring and examining the space environment on a global scale. Chapters detail this emergent field by exploring optical imaging, ultraviolet imaging, energetic neutral atom imaging, X-ray imaging, radio frequency imaging, and magnetic field imaging. Each technique is clearly described, with details about the technologies involved, how they work, and both their opportunities and limitations. Magnetospheric imaging is still a relatively young capability in magnetospheric research, hence this book is an ideal resource on this burgeoning field of study.

This book is a comprehensive resource for understanding where the field stands, as well as providing a stepping stone for continued advancement of the field, from developing new techniques, to applying techniques on other planetary bodies.

• Summarizes and reviews significant progress in the field of magnetospheric imaging
• Covers all of the techniques and technologies available, including a basic overview of each, as well as what it can accomplish, how it works, what its limitations are, and how it might be improved
• Details ways for measuring the space environment on a global scale, what physical measurements various technologies can provide, and how they can be effectively used

• Language: English
• Publisher: Elsevier Science
• Release date: Dec 4, 2021
• ISBN: 9780323858144

Book preview

Magnetospheric Imaging

Understanding the Space Environment through Global Measurements

Editors

Yaireska Colado-Vega

NASA Goddard Space Flight Center, Greenbelt, MD, United States

Moon to Mars Space Weather Analysis Office

Dennis Gallagher

NASA George C. Marshall Space Flight Center, Huntsville, AL, United States

Harald Frey

Berkeley's Space Sciences Laboratory, University of California, Berkeley, CA, United States

Simon Wing

Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Table of Contents

Cover image

Title page

Copyright

List of contributors

Introductory chapter: imaging–a new perspective for magnetospheric research

Potential of global imaging

Global context of the chapters

Future missions

Chapter 1. Ground-based all-sky imaging techniques for auroral observations and space weather research

1. Overview

2. Applications of ground-based all-sky imaging technique

3. Auroral processes

4. Auroral imaging of the magnetosphere

5. Multispectral auroral imaging

6. JHU/APL GoIono GBO and all-sky imagers

7. Summary

Chapter 2. Energetic neutral atom imaging of the terrestrial global magnetosphere

1. Introduction

2. History

3. ENA production mechanisms

4. Measurement techniques

5. Ring current and plasma sheet

6. ENA imaging of ionospheric outflow

7. Inversion techniques

8. Summary and future directions

Chapter 3. Making the invisible visible: X-ray imaging

1. Introduction

2. The governing equations and their inputs

3. Considerations

4. Instruments

5. Simulations

6. Extracting information

7. Future missions

8. Summary

Chapter 4. Radio-frequency imaging techniques for ionospheric, magnetospheric, and planetary studies

1. Introduction

2. Radio remote-sensing techniques

3. Applications of radio techniques to imaging

4. Future capabilities

5. Conclusions

Chapter 5. Magnetospheric imaging via ground-based optical instruments

1. Introduction

2. Instrumentation

3. Technique

4. Conclusion

Chapter 6. The future of plasmaspheric extreme ultraviolet (EUV) imaging

1. Introduction

2. Imaging of terrestrial He+ (30.4 nm)

3. Imaging of terrestrial O+ and O++ (83.4 nm)

4. Imaging S++ in the Io plasma torus (near 68 nm)

5. Summary and closing remarks

Chapter 7. Imaging the magnetosphere–ionosphere system with ground-based and in-situ magnetometers

1. Introduction

2. Magnetometers

3. Field-aligned and ionospheric currents

4. Signal processing techniques for quantifying and imaging magnetosphere and ionosphere waves

5. Imaging the magnetosphere with wave power

6. Field line resonances

7. Substorm dynamics

8. Future concepts

9. Summary

Chapter 8. Imaging the plasma sheet from ionospheric observations

1. Introduction

2. Method for inferring plasma sheet ion T, n, and p from ionospheric observations

3. An example of a case study: May 25, 1997 event

4. Examples of statistical studies

5. Discussion and summary

Chapter 9. Imaging Earth's magnetospheric transient and background synchrotron emission with lunar near side radio arrays

1. Introduction

2. Transient emission

3. Synchrotron emission

4. Simulating the performance of a lunar radio array

5. Noise sources

6. Localizing magnetospheric transients with PRIME

7. Localizing magnetospheric transients with FARSIDE

8. Imaging Earth's synchrotron emission

9. Conclusions

Index

Copyright

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

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List of contributors

Robert C. Allen,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

D.D. Allred,     Brigham Young University, Provo, UT, United States

E. Atz,     Department of Mechanical Engineering, Center for Space Physics, Boston University, Boston, MA, United States

Robert F. Benson,     Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, United States

Sarah N. Bentley

Department of Maths, Physics and Electrical Engineering, University of Northumbria, Newcastle upon Tyne, United Kingdom

Department of Meteorology, University of Reading, Reading, United Kingdom

Pontus C. Brandt,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Y. Collado-Vega,     NASA Goddard Space Flight Center, Greenbelt, MD, United States

M.R. Collier,     NASA Goddard Space Flight Center, Greenbelt, MD, United States

H.K. Connor,     Geophysical Institute, The University of Alaska Fairbanks, Fairbanks, AK, United States

M. Davis,     Southwest Research Institute, San Antonio, TX, United States

Robert DeMajistre,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Gerard Fasel,     Physics Department, Pepperdine University, Malibu, CA, United States

G. Fletcher,     Southwest Research Institute, San Antonio, TX, United States

Shing F. Fung,     Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, United States

Ivan A. Galkin,     Space Science Laboratory, University of Massachusetts, Lowell, MA, United States

D.L. Gallagher,     Marshall Space Flight Center, Huntsville, AL, United States

Malamati Gkioulidou,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

J. Goldstein

Southwest Research Institute, San Antonio, TX, United States

University of Texas San Antonio, San Antonio, TX, United States

James L. Green,     NASA Headquarters, Washington, DC, United States

E. Gullikson,     Lawrence Berkeley National Laboratory, Berkeley, CA, United States

Alexander M. Hegedus,     Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, United States

Syau-Yun Hsieh,     Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Jay R. Johnson,     Andrews University, Berrien Spring, MI, United States

K.D. Kuntz

The Henry A. Rowland Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, United States

NASA Goddard Space Flight Center, Greenbelt, MD, United States

Kan Liou,     Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

John Mann,     Physics Department, Pepperdine University, Malibu, CA, United States

David M. Miles,     Department of Physics and Astronomy, University of Iowa, Iowa City, IA, United States

Donald G. Mitchell,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

P. Molyneux,     Southwest Research Institute, San Antonio, TX, United States

Frank Morgan,     Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Kyle R. Murphy,     Department of Astronomy, University of Maryland, College Park, MD, United States

Romina Nikoukar,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Charles W. Parker,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

F.S. Porter,     NASA Goddard Space Flight Center, Greenbelt, MD, United States

Bodo W. Reinisch,     Lowell Digisonde International, LLC Lowell, MA, United States

Edmond C. Roelof,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

B.R. Sandel,     University of Arizona, Tucson, AZ, United States

Jasmine K. Sandhu

Department of Maths, Physics and Electrical Engineering, University of Northumbria, Newcastle upon Tyne, United Kingdom

Mullard Space Science Laboratory, University College London, Dorking, United Kingdom

D.G. Sibeck,     NASA Goddard Space Flight Center, Greenbelt, MD, United States

Fred Sigernes,     University Centre in Svalbard, Longyearbyen, Norway

Andy W. Smith,     Mullard Space Science Laboratory, University College London, Dorking, United Kingdom

Paul Song,     Space Science Laboratory, University of Massachusetts, Lowell, MA, United States

Vikas Sonwalkar,     Department of Electrical and Computer Engineering, University of Alaska, Fairbanks, AK, United States

R.S. Turley,     Brigham Young University, Provo, UT, United States

T. Veach,     Southwest Research Institute, San Antonio, TX, United States

B.M. Walsh,     Department of Mechanical Engineering, Center for Space Physics, Boston University, Boston, MA, United States

D. Windt,     Reflective X-Ray Optics LLC, New York, NY, United States

Simon Wing,     The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Introductory chapter: imaging–a new perspective for magnetospheric research

Potential of global imaging

For more than 60 years we have flown instruments throughout near-Earth space measuring particles, fields, and waves. These observations have been used to create our understanding of the terrestrial magnetospheric environment and its response to the Sun's influence through its outflowing solar wind and extended magnetic field. The last half of this time has seen research fully transition from a strong focus on characterizing the magnetized plasma environment around Earth to now appreciating that we cannot complete our understanding without adopting a system perspective about the regions and plasma that comprise our highly coupled space environment.

As the science of magnetospheric research has matured, so has the technology to carry it beyond past achievements enabled by our probing touch of space. Technology has now been proven capable of delivering routine global multimessenger sight of our space environment. This book details most of the ways we can now measure our space environment on a global scale, what physical measurements these technologies can provide, and how they can be used. The efforts to understand our local space environment have already given insights to our exploration of the solar system and beyond. Our local astrophysical plasma environment has been a unique natural laboratory accessible to direct measurement. It is also now the only one where we can combine global and local observations to complete our understanding of a planet's interaction with its parent star.

During the night on March 12–13, 567 BC., a red glow was observed in the sky and recorded on a wet clay tablet in the Babylonian astronomical diaries (Stephenson et al., 2004). This is the earliest available recorded auroral observation documenting what must have been a deeply moving experience for humans. It is no less a wondrous visual experience today. It wasn't until Frank et al. (1982), more than 2500 years later that an imager was flown on a spacecraft for the first time that would see the whole of a hemisphere's auroral display. Only following the first USA orbiting spacecraft, Explorer 1, were Earth's magnetically trapped radiation belts found and recognized to be part of an organized plasma region separate from and interacting with the Sun's solar wind outflow. Many decades of ground auroral and magnetic field observations fueled and guided satellite charting of the plasma constituents of the magnetosphere, the electric and magnetic wave interactions with plasma ranging from less than 1 eV to more than 10 MeV, and regions and conditions where they were found. Almost 20 years later, the Imager for Magnetosphere-to-Auroral Global Exploration Mission (Burch, 2000) pioneered system-level multimessenger imaging of magnetospheric plasma populations. These observations coincide with an ongoing shift in emphasis for magnetospheric research from empirical and modeling studies of distinctive domains of plasma energy and spatial regions to coherent system-level studies that incorporate the strong coupling between these domains. Today, another 20 years after the IMAGE Mission, technology is poised to provide the qualitative and quantitative context required to understand the magnetospheric system in concert with comprehensive in situ measurement of the plasma states and processes that underpin system behavior. The chapters of this book document most of the techniques and technologies available today to meet the challenges of this next phase of planetary magnetospheric research. This content also reviews the past achievements of these experimental methods and the promising future offered by their continued use.

Global context of the chapters

Optical imaging

- Gerard Fasel et al.: Magnetospheric Imaging via Ground-Based Optical Instruments

- Syan-Yun Hsieh et al.: Ground-based All-sky Imaging Techniques for Auroral Observations and Space Weather Research

Ground-based all-sky cameras have been used for more than a century in meteorology and astronomy. These two chapters describe different techniques and applications of such cameras for the detailed investigation of magnetospheric research. One chapter (Fasel) describes a technique that enhances features on the auroral arcs taken with the all-sky cameras by a clearer identification of auroral boundaries. The technique is illustrated with a particular example that demonstrates how enhanced images can be used to identify poleward moving auroral forms (PMAFs), which can then help to understand the effects of magnetic reconnection on the dayside magnetopause. The next chapter (Hsieh et al.) focuses on the enhanced capabilities of all-sky cameras enabled by new sensor technologies and demonstrates how ground-based all-sky imaging techniques complement space-based missions and together answer fundamental science questions. Multispectral imaging of the aurora enables studies into the processes that control the flow of solar wind mass, energy, and momentum through the magnetospheric system. A particular new all-sky system is described with some examples providing crucial information concerning the magnetospheric processes that drive ionospheric and thermospheric responses.

ULF wave imaging

- Kyle Murphy et al.: Imaging the magnetosphere-ionosphere system with ground-based and in-situ magnetometers

The dynamics of the interaction of the solar wind and the Earth's magnetic field can be directly analyzed with in-situ observations and the use of ground-based magnetometers at the footpoints of the ionospheric magnetic field. This chapter discusses the basic concepts of search coil and fluxgate magnetometers and also emerging magnetometer technologies. It also describes different measurement and data analysis techniques, and the approach of evaluating currents and waves from the retrieved data. They describe a few examples and how imaging the magnetosphere with waves plays a crucial role in understanding energetic particle dynamics, field line resonances, and substorm dynamics. It also includes a description of future missions that are relevant to magnetometer research and that will play a crucial role on the continued use of such instrumentation and techniques.

Plasma sheet imaging from particle precipitation

- Simon Wing et al.: Imaging the plasma sheet from ionospheric observations

The plasma sheet is an extensive region on the Earth's magnetotail. Many studies have shown that the plasma sheet is highly variable, and the changes are strongly correlated with the geomagnetic activity and solar wind parameters. This chapter presents a technique (Wing and Newell, 1998) to infer plasma sheet ion densities, temperatures, and pressures from LEO satellite observations in the ionosphere. The technique uses an automated algorithm to identify the equatorward ion isotropy boundary (b2i). This is the only way the technique works since the density, temperature, and plasma pressure are approximately conserved along the magnetic field line. The technique then fits the ion spectrum to distribution functions (kappa and Maxwellian) and the best fit is selected. Sometimes ions cannot perfectly fit the distributions and the limitations of the technique are described.

Ultraviolet imaging

- Jerry Goldstein et al.: The Future of Plasmaspheric Extreme Ultraviolet (EUV) Imaging

Extreme ultraviolet (EUV) observations have proved to be a very powerful technique to image regions with a comparably high content of ionized He+ and O+, namely the plasmasphere and the dayside ionosphere. This chapter summarizes the achievements of the past with the EUV instrument on the IMAGE spacecraft, but primarily focusses on the future developments and promises of EUV imaging for the investigation of plasmasphere erosion and refilling and the dense oxygen torus around Earth by new instrument designs and viewpoints. The chapter does not stop with observations of Earth, but also describes the imaging of the Io plasma torus around the planet Jupiter.

Energetic Neutral Atom (ENA) imaging

-Brandt, et al.: Energetic Neutral Atom (ENA) Imaging of the Terrestrial Global Magnetosphere

Energetic neutral atoms (ENAs) are generated during the charge exchange between energetic trapped, singly charged ions, and ambient cold neutral atoms and molecules. Their detection can be used as an imaging technique to understand the dynamics and distribution of energetic ion populations in magnetospheres. The chapter presents the history behind such technique, and scientific results from the missions IMAGE and TWINS that include observations of the ring current, plasmasheet, and ion outflow, an inversion technique to retrieve the parent ion distribution, and the future direction with improved angular resolution for lower energy neutral atoms.

X-ray imaging

- Kip Kuntz et al.: Making the Invisible Visible: X-Ray Imaging

Although X-ray images have been used to study distant astrophysical objects for quite some time now, the idea of using X-ray images to study the Earth's magnetosheath is relatively new. This chapter introduces the instrumentations, modeling, and data processing techniques involved in imaging the magnetosheath with X-rays. X-ray imaging can capture the large-scale structures of the magnetosheath and the location of the magnetopause. Together with in-situ measurements and other imaging such as FUV and ENA, X-ray imaging can likely lead to ground-breaking studies of the bow shock, magnetosheath, and magnetopause and their driving under various solar wind conditions. The X-ray images can also facilitate model-data comparisons of large-scale structures with global MHD models. Finally, the chapter describes the high-level concepts and designs of several future satellite missions that will carry X-imagers on board, namely CuPID, LEXI, SMILE, and STORM.

Radio-frequency imaging

Shing Fung et al.: Radio-Frequency Imaging Techniques for Ionospheric, Magnetospheric, and Planetary Studies

Radio-Frequency Imaging Techniques for Ionospheric, Magnetospheric, and Planetary Studies brings to us a nontraditional remote sensing form of imaging. Long-wavelength electromagnetic radio waves not only enable the viewing and locating of objects at a distance but also the capability to look into them to measure densities, temperatures, and motions. Radio-frequency (RF) imaging is inherently multispectral, often involving complex interactions between naturally and artificially generated waves and the medium through which the waves propagate. Through RF transmissions we discovered the ionosphere, how to track real-time space weather interference in geolocation technology, water ice below the surface of Mars, and much more.

Magnetic field imaging

Alex Hegedus: Imaging Earth's Magnetospheric Transient and Background Synchrotron Emission with Lunar Near Side Radio Arrays

This chapter summarizes the use of space-based radio arrays for low frequency imaging. A technique that will help us acquire not only the magnetospheric emission but also the source of such emission. The chapter describes how the lunar surface would be a great location to obtain the spatial resolution needed to obtain an overall picture of the local and global energetic electron activities in the Earth's magnetosphere. It is a technique that could also help capture the emission from distant galaxies and exoplanets.

The scientific imaging technologies discussed within these pages are not all new, but they remain young with much yet to offer on an ever-broader scale as you will see. These chapters by experts in their respective experimental fields explain imaging techniques, technologies, what these have achieved, and what will come next as they add to our scientific knowledge. The light these works shine forward from which we will later see the unknown future path is no less the light within which we stand now as we see our path ahead.

Future missions

Several new missions are being developed that will increase the capability of utilizing imaging techniques to understand the dynamics involved with the energy transfer from the solar wind into the Earth's magnetosphere. The CuPID (Cusp Plasma Imaging Detector) Cubesat Observatory is a 6U spacecraft mission supported by NASA that it is scheduled to launch in late 2021 to a circular, highly inclined, sun-synchronous, low Earth orbit. It is designed to measure soft X-rays emitted from the process of charge-exchange, a process that occurs when solar wind plasma interacts with neutral atoms in the Earth's atmosphere. It will use a wide field-of-view soft X-ray telescope. The LEXI (Lunar Environment Heliospheric X-ray Imager) is also a NASA soft X-ray instrument that will be deployed to the lunar surface where it will take images of the global interaction of the solar wind and the Earth's environment. The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) is a NASA Small Explorer (SMEX) mission, and it is a pair of spacecraft that will observe particles and fields at the Earth's northern magnetic cusp region to study magnetic reconnection.

The Solar-terrestrial Observer for the Response of the Magnetosphere (STORM) was one of five mission proposals selected to proceed to Phase A concept studies as part of the 2019 NASA Heliophysics Medium (MIDEX) Class Explorer. The main scientific goal is to provide a global understanding of the energy transfer by combining global imaging of the Earth's magnetosphere with in-situ measurements. Other missions also starting phase A studies are HelioSwarm and Auroral Reconstruction CubeSwarm (ARCS). HelioSwarm will use a swarm of nine spacecraft to study in a wide range of spatial and temporal scales, the process of turbulence in space plasmas. ARCS will use 32 CubeSats and 32 ground-based observatories to study the relationship between the auroral system and the dynamics of the ionosphere and thermosphere. A mission undergoing Phase C is the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) which a joint mission between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS). This mission aims to understand the Sun-Earth connection by imaging the Earth's magnetosphere with its X-ray and Ultraviolet cameras and with in-situ measurements.

This book fosters a comprehensive examination of the various technologies and techniques available today for obtaining global measurements of the magnetospheric environment that does not otherwise exist anywhere in the field. The book is a stepping stone for the continued advancement of magnetospheric imaging, of the development of new techniques, and for use of these techniques at other planetary bodies in the solar system and beyond.

References

1. Burch J. IMAGE mission overview.  Space Sci. Rev.  2000;91:1–14. doi: 10.1023/A:1005245323115.

2. Frank L.A, Craven J.D, Burch J.L, Winningham J.D. Polar views of the Earth's aurora with Dynamics Explorer.  Geophys. Res. Lett.  1982;9:1001–1004. doi: 10.1029/GL009i009p01001.

3. Stephenson F.R, Willis D.M, Hallinan T.J. The earliest datable observation of the aurora borealis.  Astronom. Geophys.  2004;45(6) doi: 10.1046/j.1468-4004.2003.45615.x Pages 6.15–6.17.

4. Wing S, Newell P.T. Central plasma sheet ion properties as inferred from ionospheric observations.  J. Geophys. Res. Space Phys.  1998;103:6785–6800. doi: 10.1029/97JA02994.

Chapter 1: Ground-based all-sky imaging techniques for auroral observations and space weather research

Syau-Yun Hsieh, Kan Liou, and Frank Morgan     Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States

Abstract

Ground-based all-sky imaging techniques have been used in many research and science applications to study the magnetosphere, upper atmosphere, and space weather. Rapid advances in sensor technology in recent years have greatly improved sensor performance, which allows ground-based imagers to capture very weak nightglow and auroral emissions at high spatial and temporal resolution. Ground-based auroral imaging provides a wealth of observations concerning key magnetospheric and ionospheric phenomena and is an essential partner for many space-based missions. Magnetospheric and ionospheric physicists seek to understand geospace phenomena from micro- to global-scales. Geomagnetic storms and substorms represent two of the most important scientific topics. Studies of these two topics often deal with the processes that control the flow of solar wind mass, energy, momentum through the magnetospheric system. We will discuss how ground-based all-sky imaging techniques complement space-based missions and together answer fundamental science questions. This chapter discusses ground-based all-sky imaging techniques, recent progress, and salient scientific contributions. We will introduce the GoIono multispectral all-sky imaging array of the Johns Hopkins University/Applied Physics Laboratory (JHU/APL) installed at the Poker Flat Research Range and research facility of the High-Frequency Active Auroral Research Program (HAARP) in Alaska that have been operating since October 2018.

Keywords

Airglow; Aurora; Imaging; Ionosphere; Magnetosphere; Space weather

1. Overview

Coupling of the Earth's magnetosphere with the solar wind plays the major role in space weather. The ceaseless solar wind shapes the Earth's magnetosphere. When the solar wind and its embedded magnetic field is turbulent or exhibits a unique structure, the magnetosphere becomes disturbed. It is generally accepted that magnetic reconnection on the dayside magnetopause for southward interplanetary magnetic field orientations facilitates the interaction and initiates global magnetospheric convection (Dungey, 1961). Strong coupling between the magnetosphere and the high-latitude ionosphere enables electric currents and charged particles associated with magnetospheric disturbances to enter the ionosphere along Earth's magnetic field, perturb geomagnetic fields, and produce auroras.

The intensity and spectrum of auroral emissions contain information about precipitating particles and thermospheric neutral constituents. Therefore, observations of auroral brightness at certain emission lines or bands allow scientists to study processes associated with magnetospheric disturbances. While satellite-based imaging in visible or far ultraviolet wavelengths can be and has been used to observe auroras on a global scale, ground-based all-sky imaging can offer higher spatial and temporal resolution at much lower cost. This chapter focuses on ground-based imaging technology and its implementation, along with a brief introduction to auroral physics, observations, and applications.

2. Applications of ground-based all-sky imaging technique

Ground-based imaging has long been used to record nighttime auroral displays from all-sky imagers (ASIs) installed at different high latitude locations. Akasofu (1964) deduced the different phases of the substorm cycle by examining the morphology of auroral features seen in films recorded by all-sky cameras located in Alaska. He proposed a working model for global auroral activity over the polar region and combined all the major features seen in auroral observations from a single station into a global picture. The significance of this proposed model was immediately recognized and led to the implementation of the THEMIS ASI imaging array and its numerous scientific contributions in auroral and magnetospheric physics during the THEMIS era. In this section, we discuss some magnetospheric and space weather topics suitable for ground-based imaging. We highlight the implementation of THEMIS all-sky imaging array and some of its scientific contributions.

2.1. Auroras, geomagnetic storms, and substorms

Magnetospheric substorms are perhaps the most important dynamic feature in Earth's magnetosphere. During individual substorms, energy captured from the solar wind is subsequently released from the magnetotail through the explosive growth of instability and the Earth's magnetic field undergoes a morphological change from a stretched to a dipolar configuration. Concurrently, particle acceleration and precipitation produce bright auroras at auroral latitudes. Geomagnetic storms are often caused by solar coronal mass ejections or high-speed streams. Just before substorms occur, the addition of open flux to the magnetotail causes the auroral oval boundary to move equatorward. At onset the nightside auroral oval boundary brightens and moves poleward. The temporal and spatial evolutions of substorms and precipitating particles in auroras have been long studied, but due to the complexity of the interactions of charged particles and the energy transport, questions concerning how the magnetosphere works, how charged particles interact with the Earth's upper atmosphere, and how substorms develop and evolve through difference phases remain unanswered. Tracking auroral dynamics provides us essential information concerning how the magnetosphere works. For instance, observations of the proton aurora will allow us to identify times and locations where the inner magnetospheric magnetic field is stretched so much that ions cannot conserve their adiabatic moments and precipitate. Auroral observations enable us to follow the ion plasma sheet as it moves earthwards and retreats in response to convection electric fields and to determine when and where wave activities cause particle precipitation.

2.2. Wave–particle interactions

Particle energization and loss processes are critical to understanding the dynamics of the radiation belts. The effects of geomagnetic storms on radiation belt fluxes are a delicate balance between particle acceleration and loss processes, and storms do not simply energize the radiation belt (Reeves et al., 2003). Radiation belt particle populations result from a dynamic equilibrium between particle loss via precipitation into the upper atmosphere and/or magnetopause outflow and refilling process from the external injections, transport, and acceleration (Lyons et al., 1973). Recent observations from Van Allen Probes suggest that right-hand polarized whistler waves (Horne et al., 2005; Thorne et al., 2010, 2013) provide the means to energize electrons locally as observed in the radiation belt (Mozer et al., 2014; Reeves et el., 2013). The same whistler-mode waves can also cause precipitation of relativistic electrons into the atmosphere via pitch angle scattering (Lyons et al., 1973; Li et al., 2007; Horne et al., 2007; Miyoshi et al., 2003, 2007). A multievent study using observations from THEMIS spacecraft and ground-based all-sky images (Nishimura et al., 2011) shows that whistler-mode chorus waves interacting with energetic particles play an important role in driving precipitating pulsating auroras. The auroral patches pulsate in phase with lower-band chorus intensity modulation, enabling accurate mapping of magnetic field lines from the ionosphere to the magnetosphere.

Substorm particle injections are critical to the processes that energize the radiation belt electrons. Substorms release magnetic energy and provide a seed population for the subsequent transport and energization that form the radiation belts (Baker et al., 1979, 1981; Fok et al., 2001). As it drifts around the Earth, this seed population of medium energy ions can become unstable to electromagnetic ion cyclotron (EMIC) waves, whereas the seed population of medium energy electrons can become unstable to whistler/chorus mode waves (Meredith et al., 2002). Both kinds of waves can interact with higher energy electrons, causing them to precipitate or become even more energized.

Particle interactions with left-hand polarized EMIC waves in the magnetosphere are also an important process contributing to the scattering loss of relativistic electrons in the radiation belts as well as energetic ions in the ring current (Cornwall et al., 1970; Thorne, et al., 1971; Jordanova et al., 2007; Meredith et al., 2003; Millan et al., 2007). Magnetospheric ions with energies of tens of keV are scattered by the wave resonances and precipitate into the ionosphere, creating proton auroras observed at subauroral latitudes (Sakaguchi et al., 2007; Jordanova et al., 2007). Close correlations between localized proton auroras and Pc1 waves, the result of the ion cyclotron instability (Sakaguchi et al., 2007, 2008), have been reported both from space-based and ground-based observations (Yahnina et al., 2000; Yahnin et al., 2007). Miyoshi et al. (2008) reported observations from POES-17 satellite, a ground-based magnetometer, and a ground-based imager that showed coincident precipitation of relativistic electrons, 10   s of keV ions in the form of a proton aurora, and EMIC waves. Recent observations of a geomagnetic storm on January 17, 2013 from the Relativistic Electron–Proton Telescope (REPT) of the Van Allen probes showed that left-hand polarized EMIC waves can produce significant and fast losses of the 4.2   MeV electron population to the atmosphere while accelerating 1.02   MeV electrons (Shprits et al., 2016). The spatial and temporal extents of regions of EMIC wave activity in the inner magnetosphere are important to determine the significance of these waves. Information about their extent was published using observations from the Van Allen Probes (Blum et al., 2017) and ground-based observations (Nomura et al., 2012; Sakaguchi et al., 2012).

2.3. Space weather and ionospheric scintillation

Space weather impacts the Earth's environment, particularly during geomagnetic storms. Radiation belt relativistic electrons can cause significant damage to spacecraft and their onboard sensors. Ionospheric scintillations, one of known space weather phenomena, cause rapid fluctuations in the received amplitude and phase of radio frequency signals that pass through the small-scale plasma density irregularities in ionosphere (Yeh et al., 1982; Conker et al., 2003; Kintner et al., 2007). The intense storms can cause ionospheric scintillation at high latitudes which can adversely affect the operations of communication and navigation system. The global navigation satellite systems (GNSSs), including GPS, Galileo, GLONASS, BeiDou, QZSS, and SBAS, provide precise positioning and time keeping information. Ionospheric disturbances can cause rapid changes in the amplitude and phase of GNSS signals. The signal fluctuation can significantly impact GNSS positional accuracy and performance of communication and navigation systems (Oksavik et al., 2015). Degraded receiver tracking performance caused by both phase and amplitude scintillations has been reported at high latitude polar and auroral regions (Skone et al., 2000). Several studies show that the significant phase scintillations are frequently observed at auroral and subauroral regions, but few amplitude scintillations were observed (Doherty et al., 2000; Skone et al., 2005). Auroral substorms are one of most dynamic ionospheric disturbances at high latitudes. Several studies indicate that GPS signal phase scintillations are strongly correlated with substorm-related auroral disturbances at high latitudes (Aarons et al., 2000; Spogli et al., 2009; Li et al., 2010; Alfonsi et al., 2011; Tiwari et al., 2012; Van der Meeren et al., 2015). Although several statistical studies have been performed concerning GPS phase scintillation at high latitudes, the changes in GPS scintillations that occur during the different phases of substorms remain poorly characterized and more studies are required. Several studies provide a clue of what to look for. Prikryl et al. (2010, 2013, 2015) reported detection of moderate GPS scintillations at auroral latitudes during auroral breakups whereas (Hosokawa et al., 2014) discovered that phase scintillations increased dramatically after the onset of the expansion phase as seen in discrete aurora.

3. Auroral processes

Precipitating electrons and ions can generate a wide spectrum of emission lines and bands. Due to the atmospheric UV cutoff, ground-based photographic imaging has focused mainly on visible and near infrared wavelengths. A detailed review of the history of auroral spectral studies is given by Chamberlain (1961). The auroral spectrum is dominated by atomic lines and molecular bands from oxygen and nitrogen and their ions (Bates, 1960; Chamberlain, 1961; Vallance Jones, 1974). Atomic lines are single lines. On the other hand, molecules and their ions produce vibrational bands, which consist of numerous rotational lines. Among these, the 557.7   nm green and 630.0   nm red forbidden emission lines dominate and often can be seen visually by the naked eye. A minor but prominent emission in the blue region of the visible spectrum comes from the N2 + 427.8   nm emission.

The 557.7   nm green line is in general the brightest emission in a variety of auroral forms. It is emitted by atomic oxygen transitioning from the OI metastable ¹S0 state to the ¹D2 state (McLennan et al., 1925). There are three main sources of excited OI:

(1) Energy transfer: N2+O→N2+O(¹S)

(2) Dissociative recombination: O2++e→2O (³P2, ¹D,¹S), and

(3) Direct electron impact excitation: O+e→O(¹S)+e.

The 630.0   nm emission is sometimes seen and dominates in the upper (higher altitude) part of auroral arcs. It is emitted by atomic oxygen in its transition from the lowest excited state O(¹D) to the atomic ground state O(³P2). There are two major sources of excited O(¹D) (Solomon et al., 1988; Link et al., 1988):

(1) Direct electron impact excitation of atomic oxygen: O(³P2)+e→e+O(¹D)

(2) Dissociative recombination of O2+: O2++e → O+O(¹D) (e.g., Meier et al., 1989)

Since the excited O(¹D) state is quenched at lower altitudes, the ratio of 630.0   nm to N2 + 427.8   nm intensity has been taken as an indicator of the characteristic energy of the precipitating electrons (Rees et al., 1974, 1986). It is estimated that ∼2% of the N2 ionization results in emission of 427.8   nm (Vallance Jones, 1974). Hβ emission (486.1   nm) has been used to study temporal variations and spatial distributions of proton aurora (see review from Galand et al., 2006), proton aurora associated with substorms (e.g., Vallance Jones et al., 1982; Samson et al., 1992; Deehr et el., 2001; Takahashi et al., 2001), and dayside proton aurora associated with magnetic impulse events (Ebihara et al., 2010). However, quantitative assessments of proton energy flux using 486.1   nm   Hβ emission measurements can be problematic (e.g., Lummerzheim et al., 2001; Galand et al., 2004).

4. Auroral imaging of the magnetosphere

The aurora is a result of electromagnetic coupling between the dynamic magnetosphere and the ionosphere. Auroral observations provide a window of opportunities to study this coupling from the ground. The discovery of the auroral substorm (Akasofu, 1964) accelerated the adoption of collaborative ground-based auroral imaging in space physics. The thickness of auroral arcs varies substantially, ranging from tens of meters to a few tens of kilometers (e.g., Kim et al., 1965; Maggs et al., 1968). Space-based observations from DE, Polar, and IMAGE indicate that auroral structures such as arcs can stretch over many hours in magnetic local time, thereby forming the instantaneous auroral oval. Since the magnetosphere maps to the entire auroral oval, observations of the aurora from ground using ASIs requires not only longitudinal but also latitudinal chains of observation sites to cover the nightside part of dark oval. This approach has been adopted and implemented by NASA's THEMIS mission.

4.1. THEMIS all-sky imager array

The ground-based ASI array is part of the THEMIS mission Ground-based Observatory (GBO) network to provide the global context of magnetospheric dynamics from the ground. The objective of this imaging array is to provide the timing and location of substorm onset. The THEMIS GBO consists of 20 all-sky white-light imagers that were installed across the North American continent (Mende et al., 2008). Along with ground-based magnetometers, the GBO array is able to monitor the large part of the nighttime geomagnetic and auroral activities across a large section of auroral oval continuously over several hours of local time. The THEMIS GBO imager sensors are monochromatic and suitable for measurements of weak and fast events. In conjunction with the THEMIS probes in the nightside magnetosphere, the GBO has provided auroral images of small and large scales at a cadence of 3   s. It enabled studies of magnetotail dynamics such as substorms (e.g., Zou et al., 2009; Nishimura et al., 2010, 2016; Frey et al., 2010; Lyons et al., 2013; Mende et al., 2009, 2011, 2012) and magnetic reconnection (e.g., Leda et al., 2016).

A number of new auroral features associated with substorm onset have been identified. For example, observations of weak auroral streamers prior to and their connection to auroral breakup has led to a new theory for the substorm onset triggering mechanism (Nishimura et al., 2010; Lyons et al., 2010, 2018). The theory invokes the concept of new plasma intrusion into the inner plasma sheet by enhanced azimuthally-limited flows. The enhanced flows associated with auroral streamers have been confirmed using incoherent scatter radars at Sondrestrom and Poker Flat (Lyons et al., 2010). Another new feature that ties to substorm onset is the auroral beads (Nishimura et al., 2016). The high sensitivity and high cadence of the GBO allows fast imaging of weak and fast auroral forms that have not previously been observed. The auroral beads are east-west aligned periodic auroral structures. Because the beads can move eastward (Motoba et al., 2012; Gallardo-Lacourt et al., 2014) or westward (Nishimura et al., 2014), they can be mistakenly thought as an east-west arc by low cadence all-sky cameras. Auroral beads that occur prior to onset are generally considered onset waves and are linked to plasma instabilities in the magnetosphere and may be responsible for the triggering of substorm onset (e.g., Rae et al., 2010; Nishimura et al., 2016).

Another ASI imaging array worth mentioning is the Red-line Emission Geospace Observatory (REGO) (Liang et al., 2016). Designed and operated by the Auroral Imaging Group (AIG), REGO is part of the Canadian Space Agency's Observatory initiative to support Resolute Bay Incoherent Scatter Radar's studies of the coupling between the near-Earth space environment and upper atmosphere. The REGO array consists of nine red line (630.0   nm) ASIs deployed in Canada in 2014 and still in operation. The REGO all-sky imaging array is specially designed to monitor auroral activity in 630.0   nm associated with soft (<∼1   keV) electron precipitation. In conjunction with other auroral instruments, REGO images have been used to study certain auroral forms (e.g., Liang et al., 2016, 2019; Gillies et al., 2018), polar cap patches (Zou et al., 2015), and some optical phenomenon known as Steve (e.g., Gallardo-Lacourt et al., 2018; Archer et al., 2019).

While the THEMIS GBO array has met and exceeded all its goals and demonstrated its endurance (it is still operating to date), it has some limitations for auroral and magnetospheric research. Due to its monochromatic sensors, information about precipitating electrons cannot be inferred. As mentioned in the previous section, current advances in multispectral auroral imaging have enabled derivations of the energy flux and characteristic energy of precipitating electrons from a few auroral emission lines. A global chain of ASI that incorporates multispectral sensors would naturally be the next generation of GBO. We will discuss the multispectral imaging technique and applications in the next section.

5. Multispectral auroral imaging

Ground-based all-sky cameras provide prospects for magnetospheric research as their numbers are growing and they can provide synoptic view of the ionospheric image of the magnetosphere at high time resolution. The energy spectra of auroral particles are closely related to the type of aurora. The discrete aurora can be associated with two distinct types of electron precipitation: monoenergetic and broadband (e.g., Newell et al., 2009). The monoenergetic electrons result from plasma sheet electrons undergoing a potential drop along the magnetic field line. The diffuse aurora (Lui et al., 1973) is associated with direct precipitation of plasma sheet electrons into the loss cone of Earth's magnetic field due to wave scattering (Horne et al., 2000; Ni et al., 2011a,b). These particles have energy spectra similar to those observed in the plasma sheet, in particular, the central plasma sheet (Meng et al., 1979). Some auroral arcs are associated with electron acceleration events that occur over a wide range of energies (Johnstone et al., 1982; Lotko, 1986). Broadband precipitation is associated with substorm onset (Newell et al., 2009) and is believed to be caused by dispersive Alfven waves (Ergun et al., 1998; Chaston et al., 2003a,b). The differing energy spectra reflect the differing magnetospheric source regions. Work to identify magnetospheric source region using in situ particle measurements from low-Earth orbit satellite has been performed (Winningham et al., 1974, 1975; Newell et al., 1988, 1991a,b,c). Attempts to map the ionosphere to the magnetosphere exist (Newell et al., 1992, 1994, 2004). These climatology maps are useful to characterize the average structure of the magnetosphere for given controlled parameters. On the other hand, they do not provide the instantaneous state of the magnetosphere.

As new observation techniques improve, such as multispectral auroral imaging, ground-based auroral observations may provide continuous measurements of auroral energy input over a large area simultaneously. Photometric measurements of emission ratio for several emission lines have been explored to infer the characteristic energy of precipitating electrons. Attempts using photometric observations from the ground to infer precipitating electron energy began with Gattinger et al. (1972) and Rees et al. (1974) and further developed on both theoretical (Mende et al., 1975; Strickland et al., 1983; Gustafsson et al., 1985; Ono et al., 1987; Rees et al., 1988; Meier et al., 1989) and experimental (McEwen et al., 1981; Vallance Jones et al., 1987; Solomon et al., 1988; Steele et al., 1990) grounds. For example, the emission rate ratios, 630.0/427.8, 557.7/427.8, and 630.0/557.7 (McEwen et al., 1981), together with the absolute emission rate of the 427.8   nm radiation, may be used to infer a characteristic energy of the precipitating electron flux (Rees et al., 1974). This characteristic energy and the 427.8   nm intensity then determine the total electron flux and the energy deposition rate. More recently, Aryal et al. (2018) used high-resolution brightness measurements of N2 +(427.8   nm), OI (557.7   nm), and OI (630.0   nm) emissions to derive energies and energy flux of precipitating electrons in an auroral event study, in conjunction with the GLOW model (Solomon, 2017, and references therein), to examine procedures that improve the inversion technique. These studies often require knowledge of the atmospheric parameters, which are difficult to obtain. Hecht et al. (1989) showed that one may combine measurements of I(630.0)/I(427.8) with high resolution of the 777.4   nm OI line to obtain information on the atmospheric composition and the incident electron energy simultaneously. Semeter et al. (2001) developed a simplified framework using only two auroral emission lines (427.8   nm and OI 732.5   nm) to derive precipitating electron parameters for auroral arcs in 2D images. Grubbs et al. (2017) used multifiltered (557.7   nm, 427.8   nm, and 844.6   nm) 2D auroral images to derive the energy flux and average energy of precipitating electrons. The relationships between electron population characteristics, emission line intensities (427.8   nm, 557.7   nm, and 844.6   nm), and emission line ratios were examined and compared using three electron transport models (GLOW, ETRANS, and B3C) and in situ data (Grubbs et al., 2018). While much more work is required, these results are encouraging and are potentially useful to diagnose magnetosphere and magnetospheric processes.

5.1. Potential application in future missions

The successful experience and demonstration of THEMIS GBO network yield a new possibility that an imaging array employing multispectral imaging technique can be considered and included in support of future missions. One potential opportunity is provided by the proposed Solar-Terrestrial Observer for the Response of the Magnetosphere (STORM) mission for NASA MIDEX program. The STORM mission was selected for a NASA MIDEX Phase A study from October 2020 to July 2021 with a site visit in October 2021. The main objective of the STORM mission, led by David Sibeck of NASA Goddard Space Flight Center, is to quantify the global circulation of energy in the solar wind–magnetosphere interaction that powers space weather in Earth's environment. The STORM mission proposes to implement a dedicated multispectral all-sky imaging array across Alaska and Canada. Imaging auroral emissions of 557.7 and 630.0   nm simultaneously from each observatory locations, this imaging array is tasked to produce high spatial and temporal auroral images to capture the microscale auroral structures to quantify their role in substorm onset and the energization of the inner magnetosphere during various