Dynamics of the Earth's Radiation Belts and Inner Magnetosphere

By Danny Summers and D. N. Baker

Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 199.

Dynamics of the Earth's Radiation Belts and Inner Magnetosphere draws together current knowledge of the radiation belts prior to the launch of Radiation Belt Storm Probes (RPSP) and other imminent space missions, making this volume timely and unique. The volume will serve as a useful benchmark at this exciting and pivotal period in radiation belt research in advance of the new discoveries that the RPSP mission will surely bring. Highlights include the following: a review of the current state of the art of radiation belt science; a complete and up-to-date account of the wave-particle interactions that control the dynamical acceleration and loss processes of particles in the Earth's radiation belts and inner magnetosphere; a discussion emphasizing the importance of the cross-energy coupling of the particle populations of the radiation belts, ring current, and plasmasphere in controlling the dynamics of the inner magnetosphere; an outline of the design and operation of future satellite missions whose objectives are to discover the dominant physical processes that control the dynamics of the Earth's radiation belts and to advance our level of understanding of radiation belt dynamics ideally to the point of predictability; and an examination of the current state of knowledge of Earth's radiation belts from past and current spacecraft missions to the inner magnetosphere. Dynamics of the Earth's Radiation Belts and Inner Magnetosphere will be a useful reference work for the specialist researcher, the student, and the general reader. In addition, the volume could be used as a supplementary text in any graduate-level course in space physics in which radiation belt physics is featured.

• Language: English
• Publisher: Wiley
• Release date: May 9, 2013
• ISBN: 9781118704370

Book preview

Section I

Historical Perspective

Space Weather: Affecting Technologies on Earth and in Space

Louis J. Lanzerotti

Center for Solar Terrestrial Research, New Jersey Institute of Technology, Newark, New Jersey, USA

Beginning with the era of development of electrical telegraph systems in the early nineteenth century, the space environment around Earth has influenced the design and operations of ever-increasing and sophisticated technical systems, both on the ground and now in space. Newfoundland had key roles in important first events in communications, including the landing of the first working telegraph cable in Heart’s Content (1866), the first reception of trans-Atlantic wireless signals at Signal Hill in St. John’s (1901), and the North American location of the first trans-Atlantic telecommunications cable in Clarenville (1956). All of the systems represented by these firsts suffered from effects of space weather. This paper reviews some of the historical effects of space weather on technologies from the telegraph to the present, describing several events that impacted communications and electrical power systems in Canada. History shows that as electrical technologies changed in nature and complexity over the decades, including their interconnectedness and interoperability, many important ones continue to be susceptible to space weather effects. The effects of space weather on contemporary technical systems are described.

1. INTRODUCTION

The aurora has been observed and marveled at for as long as humans have existed. The aurora has been viewed with awe not only in polar regions but also has been visible at various times at low geomagnetic latitudes such as Hawaii, Cuba, Rome, and Bombay. An understanding of the origins of the aurora only began to become scientifically rigorous in the late nineteenth century. Slowly, the aurora became to be understood as somehow related to the Sun, and therefore, solar activity might influence the Earth. That is, the Sun could influence the space environment around the Earth [Chapman and Bartels, 1940; Soon and Yaskell, 2003]. The Sun, in fact, does influence the weather in the space environment around the Earth, although the terminology of space weather did not become commonly employed until near the end of the twentieth century.

Any noticeable effects of the Sun and the Earth’s space environment on human technologies had to wait until the first large-scale electrical technologies began to be deployed and used. This first large-scale technology was the electrical telegraph, first put into use some 160 or so years ago, several generations ago, and yet short in the course of human existence. W. H. Barlow, the company engineer for the Midland Railroad in England reported spontaneous deflections of the needles of the telegraph lines running aside the railroad tracks [Barlow, 1849]. Barlow’s data for the Derby to Birmingham line is shown in Figure 1 for about 2 weeks of measurements in May 1847. The hourly variations in the deflections are clearly evident, as is an approximately daily variation throughout the interval. Barlow [1849, p. 66] further noted that . . . in every case that [came under his] observation, the telegraph needles [were] deflected whenever aurora [were] visible.

Less than two decades after Barlow’s observations, the white light solar flare observed by Carrington [1863] on 1 September 1959, ejected what we now know to be a coronal mass ejection (CME) into the interplanetary medium. Less than 20 h later, the Earth’s space environment was struck. Huge changes in the geomagnetic field were observed wherever measurements were being made: During the great auroral display . . . disturbances of the magnetic needle [at] Toronto, in Canada, the declination of the needle changed nearly four degrees in half an hour [Loomis, 1869, p. 12]. Auroras were observed on Earth from the north to as low latitude as Hawaii.

Figure 1. Galvanometer deflections on the telegraph cable along the Midland Railroad line from Derby to Birmingham for a 2 week interval in May 1847.

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In the years since Barlow, the telegraph made large strides in its development and deployment across many locales on Earth, greatly enabling faster communications across large distances. Carrington’s solar event produced greater disturbances in telegraph systems than Barlow had experienced. As reported by Prescott [1875, p. 322], on the telegraph line from Boston to Portland (Maine) on Friday, 2 September 1859, . . . the line was worked [without batteries] more than two hours when, the aurora having subsided, the batteries were resumed. Between South Braintree and Fall River (Massachusetts; a distance of about 40 miles) such was the state of the line . . . when for more than an hour [the operators] held communication over the wire with the aid of the celestial batteries alone.

Prescott [1875], in his treatise on the electric telegraph, records the observations and experiences of many eastern U.S. telegraph operators during the Carrington event (as well as observations of effects on telegraphs from other auroral events in Europe and the United States). Prescott subtitles one section of his chapter on Terrestrial Magnetism as Working Telegraph Lines with Auroral Magnetism. Shea and Smart [2006] published a compendium of eight contemporary, published U.S. articles attributed to, or written by, Elias Loomis (Professor at Yale) related to the aurora and magnetic observations during the Carrington event.

The decades following the Carrington event found much work by electrical and telegraph engineers in attempts to understand the origins of the spontaneous electrical currents in their systems and to mitigate against them. In parallel, scientists worked to attempt to understand how an event on the Sun, such as the one Carrington reported, could affect the Earth so far away.

Newfoundland in the late nineteenth and early twentieth century was the site of many firsts in electrical communications technologies. The first trans-Atlantic telegraph cable from Valencia, Ireland, to Bull Arm (Trinity Bay) failed after 1 month. Cyrus Field, using the huge Great Eastern ship, was successful in establishing the first operating cable from Valencia, landing it in Heart’s Content (Trinity Bay) in 1868 (Figure 2). This cable was operational for a century, until 1965. Each year, a celebration is held in the small village of Heart’s Content on the anniversary day of the landing, 27 July. The 145th anniversary party occurred the week following the 2011 Chapman Conference on Dynamics of the Earth’s Radiation Belts and Inner Magnetosphere in St. John’s (Figure 3). As recounted by Rowe [2009, p. 45], … the beginning of cable service was far from reliable … Earth’s magnetic currents, lightning, the aurora borealis … sent the [galvanometer] into wild and rapid gyrations. The Earth’s magnetic currents and the aurora borealis were evidence of space weather, although not known to be such at the time.

Figure 2. Still existent trans-Atlantic telegraph cables landing from Trinity Bay, Newfoundland, with Heart’s Content cable station (original station in red) in the background. The station is now a Canadian Provincial Historical Site.

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The year 1956 saw the inaugural call on the first trans-Atlantic voice telephone cable (TAT-1) across the Atlantic. The cable, from Oban, Scotland, was landed in Clarenville, Newfoundland (Trinity Bay), and saw service until 1978. TAT-2, from Penmarch, France, to Clarenville, was placed in service in 1959 and was retired in 1982. The large magnetic storm of February 1958 produced havoc on TAT-1. As John Brooks wrote in the New Yorker magazine At almost the exact moment when the magnetograph traces leaped and the aurora flared up, huge currents in the earth … manifested themselves not only in power lines in Canada but in cables under the north Atlantic [Brooks, 1959, p. 56].

Axe [1968] reported that an induced voltage swing larger than ~1.5 kV was measured at Oban, Scotland, during the most intense portion of the geomagnetic storm that affected TAT-1. Voltage excursions larger than 1 kV were measured on the telephone cable from Clarenville to Sydney Mines, Nova Scotia, at the peak of the storm [Winckler et al., 1959].

2. ADVANCES IN ELECTRICAL TECHNOLOGIES

Over the following century and a half, to today, as humans continued to develop electrical technologies for communications, electric power, and other uses, the effects of the Sun and the Earth’s space environment continued to be felt and had to be dealt with. Engineers and scientists did not understand solar-terrestrial phenomena, or believe, until the early twentieth century, that the Sun could actually disturb the Earth in the ways that were implied by the coincidences observed between solar activity and geomagnetic storms, aurora, and disturbances on electrical systems.

In 1885, Guglielmo Marconi began experimental studies of wireless transmissions on his father’s estate near Bologna, Italy. This work evolved into extensive experiments on land, shore-to-sea, and between-ship transmissions, much carried out in England. In 1901, Marconi established transmitting stations at Poldhu and The Lizard in Cornwall, and receiving stations at Wellfleet on Cape Cod, Massachusetts, and on Signal Hill, St. John’s in Newfoundland (there are many books and articles relating to Marconi’s life and work, e.g., the work of Bussey [2001]). He encountered many hurdles, including wind damage and destruction of transmission and receiving towers, in his attempts to cross the Atlantic with a radio wave.

On 12 December 1901, Marconi had success, receiving the Morris Code letter S at Signal Hill (Figure 4) as transmitted from Poldhu. For this, he was awarded, with Karl Braun of Germany, the Nobel Prize in Physics for 1909. The beginnings of communications through the air had begun. Such communications had the potential to provide more bandwidth, did not require the laying of long cables across a deep ocean, and could evade the pesky spontaneous currents in the telegraph cables.

The Marconi Company established a wireless station at Cape Race, Newfoundland, in 1904 (Figure 5). It was at this station that the distress signals from the Titanic were received on 14 April 1912.

While the anomalous (and often large) electrical currents experienced in the telegraph wires were avoided by Marconi’s wireless innovation, the solar-terrestrial environment had surprises for the new electrical technology. As Marconi [1928] himself wrote

… times of bad fading [of the wireless signals] always coincide with the appearance of large sun-spots and intense aurora-boreali usually accompanied by magnetic storms…. These are … the same periods when cables and land lines experience difficulties or are thrown out of action.

Figure 3. Celebration cake for the 145th year of landing of the first successful trans-Atlantic telegraph cable, 27 July 2011.

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Figure 4. Cabot Tower, Signal Hill, St. John’s, Newfoundland, August 2011, a Canadian National Historic Site.

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Figure 5. Cape Race, Newfoundland, on the Avalon Peninsula, the site of the Marconi Company wireless station established in 1904.

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In the early days of wireless, and as the understanding of the relationship of solar activity to successful operations of the technology increased, the need for better understanding of the causes of wireless fading and other anomalies became more important. In fact, the data shown in Figure 6 (reproduced from the work of Lanzerotti [2004]) of the relationship between daylight trans-Atlantic signal strength (15–23 kHz) and sunspot numbers shortly after the AT&T company began trans-Atlantic transmissions represent one of the earlier efforts at what might today be called space weather predictions. It is clear that the field strength of the signal appears to be correlated with sunspot number, with higher strength when there was more solar activity during these two solar cycles (numbers 15 and 16).

Figure 6. Trans-Atlantic daylight wireless transmission signal levels (15–23 kHz) measured by the AT&T Company during sunspot cycles 15 and 16.

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The experience of unexpected effects of the Earth’s space environment on new electrical technologies (such as cable and then wireless) is a theme that persists to the current day. As new technologies are introduced, their successful operations can often be impeded by surprises from the solar-terrestrial environment. The characteristics of the environment must often be used in the making of design decisions. Since some of the most intense space environmental changes occur infrequently, past events of operational failure can be forgotten; design and operation decisions might then be made on the basis of more benign assumptions as to possible space environmental impacts. The fact that the most intense space environmental effects occur so infrequently also means that design decisions have to be made on imperfect knowledge of the solar-terrestrial environment [e.g., Riley, 2012]. All of these types of considerations have been operative throughout the twentieth century as one after the other of electrical technologies have been introduced and employed, for civilian and for national defense purposes.

While not recognized until their discovery by James Van Allen in 1958, the trapped radiation belt particles are centrally involved in producing many of the foregoing historical impacts on electrical technologies. In particular, the depletion of the trapped radiation under geomagnetic storm conditions greatly increases the conductivity of the ionosphere at the foot points of the magnetic field flux tubes that contained the formerly trapped particles. These changes in ionosphere conductivity, and the spatial differences in the conductivity that are produced because of different intensities of trapped particle loss, are responsible for large changes in magnetic fields at the Earth’s surface. These Earth surface magnetic field changes are those that give rise to the telluric currents that flow in and disrupt long conductor systems. The enhanced ionosphere conductivities also produce the anomalous propagation conditions for wireless signal transmissions.

3. CONTEMPORARY SPACE WEATHER EFFECTS

Table 1 (adapted from the work of Lanzerotti [2004]) lists a majority of the solar terrestrial processes that are understood today that can affect contemporary technical systems. These processes, and some of the impacted technologies, are illustrated in Figure 7. Many of these physical processes are coupled. For example, the magnetic field variations as measured on the Earth’s surface that can affect systems consisting of long conductors (such as the first telegraph cables and contemporary electric power grids and transocean cables) are produced by variations in the electrical currents flowing in the ionosphere. This, in fact, is the physical basis behind the observation made by Marconi in the quote above from one of his publications. Different types of ionosphere disturbances (such as the plasma bubbles in Figure 7), and confined principally to some regions on the Earth such as near the equator and in the auroral zones, are the source of disturbances (signal fading and signal scintillations) on other wireless signals such as modern-day navigation and satellite-to-ground signals.

3.1. Magnetic Field Variations

The magnetic field variations in Table 1 that produce electrical currents in the Earth that can affect electricity grids, long communications cables, and pipelines primarily result from geomagnetic storms. The largest of these storms, those most likely to produce the largest currents in the Earth, usually result from CME events striking the Earth’s magnetosphere. These geomagnetic storms produce large changes in the electrical currents in the ionosphere. These fluctuating ionosphere currents produce fluctuating magnetic fields at the Earth’s surface, which in turn cause electric currents to flow in the Earth’s crust. These telluric currents seek the highest conductivity paths in which to flow, and long grounded conductors such as power grids, communications cables, and pipelines provide such paths [e.g., Lanzerotti and Gregori, 1986].

Table 1. Solar-Terrestrial Processes and Their Consequencesa

Figure 7. Illustration of many space weather effects and some of the modern-day electrical technologies and systems that can be adversely affected by them.

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The magnitude and locations of the flowing telluric currents depend very much on the locations of the currents in the ionosphere and the conductivity of the underlying Earth. The ionosphere currents vary in location from geomagnetic storm to geomagnetic storm; current models of geomagnetic activity cannot predict these locations very precisely. The conductivity profile of the Earth is not known well in many locales. A given ionosphere current variation (if there was a standard variation) could produce very different telluric currents in regions that had different Earth conductivities.

The telluric currents that flow in the long conductors can produce damage, and even failure, of power system transformers, overwhelm constant current powering systems on long communication cables, and render inoperative pipeline corrosion protection circuits. Mitigation procedures depend upon the particular technical system and are implemented in some systems. There are costs for mitigation, and business decisions are made as to the cost/benefit results that might be achieved by implementations.

Some spacecraft use the Earth’s magnetic field for orientation and guidance. A very intense solar wind shock wave, as from a strong CME event, can push the Earth’s magnetopause inside geosynchronous orbit. The magnetic field just outside the magnetopause is of opposite polarity to that inside; a magnetically oriented spacecraft that crosses the magnetopause will thus suddenly be misoriented, and operators will likely have to intervene.

3.2. Ionosphere Fluctuations

The fluctuating ionosphere currents can cause havoc in the propagation of radio signals over a wide bandwidth. At HF and VHF frequencies, frequencies used, for example, by civil emergency agencies and by commercial airlines flying over the north polar region, signals transmitted from a location on the Earth can be absorbed or reflected anomalously from the ionosphere and, thus, not reach their destinations. At higher frequencies, such as those used for satellite-to-ground (and ground-to-satellite) transmissions and for GPS navigation signals, the radio waves can be severely distorted in phase and amplitude, affecting severely communications and navigation.

3.3. Solar Radio Noise

A scientific area that has become of more importance in recent years is that of the effects of solar radio noise on navigation technologies in the form of GPS. Solar radio bursts produced by solar flares were discovered in 1942 when British radars were rendered inoperable by jamming signals. These radars were being used to warn of enemy aircraft launched from continental Europe. The initial thinking was that the enemy was purposefully jamming the radars. J. S. Hay, a British scientist, identified the source as actually coming from the Sun, and not from across the English Channel [e.g., Hey, 1975].

While radars are still susceptible to solar radio noise, the vast proliferation of technologies that operate in the near- GHz and GHz frequency ranges, such as GPS and cell phones, means that this aspect of solar phenomena becomes of more significance in terms of its influence on technical systems [e.g., Cerruti et al., 2006, 2008]. Solar radio noise, as well as bursts of solar X-rays, arrives at the Earth at the speed of light. Thus, there is no warning of their occurrence as there is for a possible encounter of a CME with the Earth to produce a magnetic storm. Accurate predictions of solar flares are still rather rudimentary, and predictions of how intense a radio burst or (an X-ray burst) might be are nonexistent. Radio noise and bursts, and X-rays are produced by electrons trapped and propagating in the Sun’s magnetic fields. Until the intensities of the trapped electrons and of the magnetic fields can be readily measured and/or predicted, the forecast of the occurrence and intensity of solar radio and X-ray events remains a major unsolved problem.

3.4. Particle Radiation

When Sir Arthur Clark and John Pierce proposed Earth-orbiting communications satellites (at geosynchronous and low Earth-orbiting altitudes, respectively), they did not anticipate that the space environment around the Earth was not benign. However, the charged particle environment in space determines the design of space systems in many important ways. These charged particles are those trapped in the Earth’s magnetosphere (the radiation belts), solar energetic particles outside the magnetosphere and those that penetrate into the magnetosphere, and galactic cosmic rays.

Even though the spatial extent and intensities of the trapped radiation were not delineated for a number of years following Van Allen’s discovery in 1958, it was, nevertheless, recognized that the radiation presented a formidable environmental constraint to system designs and to human occupation of space. The design and build of the first active telecommunications satellite Telstar 1, conceived and promoted by John Pierce of Bell Laboratories, could not provide substantial shielding for many reasons, including the launch vehicle available (a Delta), and the size and weight of the small spacecraft (about 77 kg; 87.6 cm in diameter).

Telstar 1 was built at Bell Laboratories of largely discrete components including transistors (no integrated circuits or microprocessors available in those days), paid for by AT&T, including the launch costs reimbursed to NASA. The spacecraft carried several transistor solid-state detectors with different front aperture thicknesses to measure the radiation environment encountered by the satellite. Telstar was launched on 10 July 1962 into a low Earth orbit (perigee 952 km; apogee 5933 km). On the prior day, the United States had conducted the Starfish Prime high-latitude nuclear test over the Pacific. In addition to large disturbances in the electric grids in Hawaii and New Zealand, the test injected into the low-altitude magnetosphere fluxes of electrons more than 100 times the natural background radiation. Less than 8 months after launch, Telstar succumbed to the radiation environment. The electron data obtained from the radiation detectors on Telstar are still referenced today for the information obtained on electron lifetimes at those altitudes.

The radiation environment at geosynchronous altitude was unknown at the time of launch (July 1963) of the first operational satellite at that location, Syncom 2. The NASA Applications Technology Satellite ATS-1, launched to geosynchronous orbit in December 1966, carried particle detector instruments from three U.S. institutions: Aerospace Corporation, University of Minnesota, and Bell Laboratories. Discovery data from these instruments showed that the radiation environment at this altitude, where almost all communications satellites reside today, is highly variable in time and location along the orbit. These data also provided the first evidence that in some solar-produced events, the magnetopause can be pushed inside geosynchronous, exposing space assets to the interplanetary environment.

Table 1 lists many of the ways that charged particles can affect space systems. The lowest-energy particles, in the hundreds of eV to KeV energy range, are the sources of charging on spacecraft surfaces and solar arrays. If spacecraft surfaces are not adequately grounded to one another, discharges (similar to lightning) can occur between these areas, discoloring surface materials and producing electromagnetic noise interference in electronic systems. Very low energy neutral atoms of oxygen and nitrogen, as encountered in low Earth orbits, can sputter and discolor spacecraft surfaces.

At hundreds of keVand MeVand higher energies, charged particles, dominantly the trapped population, can damage solar cells in the arrays (thus, decreasing over time the electrical output of the arrays), damage and cause upsets in semiconductor components, and produce charging in dielectric materials deep within a spacecraft. Dielectrics that experience sufficient charging will also discharge, with the resultant damage to materials and the emission of electromagnetic noise interference.

Charged particles will also produce noise and, at times, complete obscuration of optical instruments such as star trackers and scientific telescopes. The loss of lock from a temporarily unavailable star tracker signal will affect spacecraft control.

As is clear from the first days of the space age, radiation trapped in the Earth’s magnetosphere is of central importance to the design and operations of space systems, from communication satellites at geosynchronous orbit, to elliptical and low circular orbits of national security systems, to low orbit communication and navigation systems. Deeper understanding of the dynamics of these trapped populations is required as space systems become more complex and as component parts decrease in size and increase in density. Better and more comprehensive measurements will result in deeper understanding that will then result in better models for making wise design decisions and for potential forecasting of the occurrence of deleterious space weather conditions.

In addition to the trapped populations, low-energy magnetosphere particles, from ambient plasma conditions to the ring current population, are also critical for describing and modeling the radiation environment that space systems encounter. Much better understanding of the temporal and spatial distributions of these populations are necessary in their own right for design and modeling purposes and, importantly, as seed populations for the trapped particles. The acceleration mechanism(s) of these seed populations to the high energies that can penetrate deeply into space systems is(are) yet to be understood. These acceleration mechanisms are, thus, important from both the aspect of fundamental cosmical plasma physics and for their importance in applications to practical engineering problems.

3.5. Micrometeoroids and Artificial Space Debris

Solid materials in the space environment, natural or artificial, will produce physical damage to space vehicles and will cause disturbances to spacecraft orientation. The micrometeoroid environment at geosynchronous orbit, where communications satellites operate, is poorly known, and better understanding could aid in the operations of such spacecraft.

3.6. Atmosphere

The neutral atmosphere in low Earth orbit varies appreciably over a solar cycle. The atmosphere density increases with increasing altitude during solar maximum conditions when the enhanced solar EUV and UV emissions heat the atmosphere, causing it to rise. Therefore, the drag from the atmosphere on low Earth-orbiting satellites will increase during solar maximum years, with the resultant that orbits for the International Space Station and for any other satellites that have active orbit control will need to be raised more frequently than they have to be at other times in the solar cycle. Other, not actively controlled, spacecraft in low Earth orbit will see their orbit altitude decrease with time.

The increased atmosphere density also causes increased drag on artificial space debris, causing pieces to descend and burn up in the ionosphere. This cleansing effect helps to lower the danger from some debris objects (but not those at altitudes too high to be affected by the atmosphere).

It is well understood that water vapor and rainfall can seriously affect the propagation and transmission of microwaves through the atmosphere. It is now acknowledged that there is coupling between clouds and the ionosphere under at least some (not well understood) meteorological and ionosphere conditions. While it is only speculation at the moment as to whether space weather conditions that change the ionosphere might somehow be related to cloud nucleation and electrification processes, and thus to atmospheric weather that can affect microwave signals, the idea warrants continued examination.

4. CONCLUSION

Over the last 160 years, there have been striking advances in electrical technologies. Many of these advances are related in one way or the other to communications, since the time of the recording of space weather disturbances on the first telegraph lines, initially on short distances, and then on the first trans-Atlantic cable that landed in Hearts Content, Newfoundland. The historical record demonstrates that space weather processes often provide surprises in the implementation and operation of new electrical technologies, such as the wireless receiver that Marconi employed at Signal Hill, to early radar receivers in Great Britain, to spacecraft. The historical record also demonstrates that as the complexity of systems increase, including their interconnectedness and interoperability, they can become more susceptible to space weather effects. This is especially true for electrical grids in developed nations, upon which depends the energy for most aspects of modern society.

Central to understanding many physical processes in the solar-terrestrial environment that can affect technical systems are the trapped and lower-energy plasma populations of the Earth’s magnetosphere. Deeper understanding, at fundamental physical levels, are required of these populations if more reliable models are to be achieved and if more accurate forecasting of potentially damaging conditions are liable to occur following a solar event.

REFERENCES

Axe, G. A. (1968), The effects of Earth’s magnetism on submarine cables, Electr. Eng. J., 61, 37–43.

Barlow, W. H. (1849), On the spontaneous electrical currents observed in the wires of the electric telegraph, Philos. Trans. R. Soc. London, 139, 61–72.

Brooks, J. (1959), A reporter at large: The subtle storm, New Yorker, 19 Feb.

Bussey, G. (2001), Marconi’s Atlantic Leap, Radio Soc. of G. B., London, U. K.

Carrington, R. C. (1863), Observations of the Spots on the Sun From November 9, 1853, to March 24, 1861, Made at Red Hill, Williams and Norgate, London, U. K.

Cerruti, A. P., P. M. Kintner, D. E. Gary, L. J. Lanzerotti, E. R. de Paula, and H. B. Vo (2006), Observed solar radio burst effects on GPS/Wide Area Augmentation System carrier-to-noise ratio, Space Weather, 4, S10006, doi:10.1029/2006SW000254.

Cerruti, A. P., P. M. Kintner Jr., D. E. Gary, A. J. Mannucci, R. F. Meyer, P. Doherty, and A. J. Coster (2008), Effect of intense December 2006 solar radio bursts on GPS receivers, Space Weather, 6, S10D07, doi:10.1029/2007SW000375.

Chapman, S., and J. Bartels (1940), Geomagnetism, Clarendon Press, Oxford, U. K.

Hey, J. S. (1975), The Evolution of Radio Astronomy, Watson Intl., London, U. K.

Lanzerotti, L. J. (2004), Solar and solar radio effects on technologies, in Solar and Space Weather Radiophysics, edited by D. E. Gary and C. U. Keller, pp. 1–16, Springer, Heidelberg, Germany.

Lanzerotti, L. J., and G. P. Gregori (1986), Telluric currents: The natural environment and interactions with man-made systems, in The Earth’s Electrical Environment, pp. 232–258, Natl. Acad. Press, Washington, D. C.

Loomis, E. (1869), The aurora borealis or polar light, Harper’s New Mon. Mag., 39, 1–21.

Marconi, G. (1928), Radio communications, Proc. IRE, 16, 40–49.

Prescott, G. B. (1875), History, Theory and Practice of the Electric Telegraph, Osgood, Boston, Mass.

Riley, P. (2012), On the probability of occurrence of extreme space weather events, Space Weather, 10, S02012, doi:10.1029/2011SW000734.

Rowe, T. (2009), Connecting the Continents: Heart’s Content and the Atlantic Cable, Creative Book, St. John’s, Newfoundland, Canada.

Shea, M. A., and D. Smart (2006), Compendium of the eight articles on the Carrington Event attributed to or written by Elias Loomis in the American Journal of Science, 1859–1861, Adv. Space Res., 38, 313–385.

Soon, W. W.-H., and S. H. Yaskell (2003), The Maunder Minimum and the Variable Sun-Earth Connection, World Sci., Singapore.

Winckler, J. R., L. Peterson, R. Hoffman, and R. Arnoldy (1959), Auroral x-rays, cosmic rays, and related phenomena during the storm of February 10–11, 1958, J. Geophys. Res., 64(6), 597–610.

L. J. Lanzerotti, Center for Solar Terrestrial Research, New Jersey Institute of Technology, Newark, NJ 07102, USA. ([e-mail])

Section II

Current State of Knowledge of Radiation Belts

SAMPEX: A Long-Serving Radiation Belt Sentinel

Daniel N. Baker

Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA

J. Bernard Blake

SSL, The Aerospace Corporation, Los Angeles, California, USA

The near-Earth region of the magnetosphere responds powerfully to changes of driving forces from the Sun and the solar wind. The Earth’s radiation belts and inner magnetosphere show substantial differences in their characteristics as the Sun’s magnetic field and solar wind plasma properties change over the approximately 11 year solar activity cycle. Solar coronal holes produce regular, recurrent fast solar wind streams in geospace, often enhancing highly relativistic electrons and causing recurrent geomagnetic storms. These phenomena are characteristic of the approach to sunspot minimum. On the other hand, major geomagnetic disturbances associated with aperiodic coronal mass ejections occur most frequently around sunspot maximum. Such disturbances also can often produce significant radiation belt enhancements. We describe the observational results that characterize the differences throughout the inner part of geospace during the course of the solar activity cycle. We place particular emphasis on long-term, homogeneous data sets from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission. The NASA SAMPEX spacecraft launched in 1992 is expected to end its mission by December 2012. This space platform has revolutionized our views of the dynamic radiation belt environment. We conclude that SAMPEX has been a most successful and impactful mission for radiation belt studies.

1. INTRODUCTION

In the summer of 1992, a small spacecraft called the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) was ready to be launched from NASA’s Western Test Range into a low-Earth, high-inclination orbit on board a Scout launch vehicle. SAMPEX was the first mission in a new line of Small Explorer (SMEX) projects that were initiated by NASA in the late 1980s [Baker et al., 1991]. The primary expressed goals of the SAMPEX program were to study (anomalous) galactic cosmic rays and solar energetic particle (SEP) enhancements [Baker et al., 1993]. The SAMPEX payload was tailored for high and medium energy ion measurements. Little was it imagined that the tertiary objectives of measuring magnetospheric electron populations with the SAMPEX instrument complement would establish the spacecraft in a position of key prominence as one of the finest NASA radiation belt missions.

Despite the modest role initially envisioned for SAMPEX in the magnetospheric particle detection part of the program, it quickly became apparent after launch on 3 July 1992 that SAMPEX had remarkable potential to study radiation belt processes. The 600 km, nearly circular 82° inclination orbit was ideal for sampling essentially all magnetic field lines threading the inner and outer Van Allen belts [Baker et al., 1993]. The instrument payload included sensors that measured high-energy (E >1 MeV) electrons with two separate systems, namely, the Proton-Electron Telescope (PET) [Cook et al., 1993] and Heavy-Ion Large Telescope (HILT) [Klecker et al., 1993]. There was also an important ability to measure medium-energy electron populations with a portion of the HILT investigation, as well as with the low-energy ion composition analyzer (LICA) [Mason et al., 1993].

From the outset of the SAMPEX mission, the large-area, highly sensitive electron (and proton) detectors on board the spacecraft gave new insights into the dynamics of the Earth’s radiation belts [e.g., Baker et al., 1994a]. Issues of acceleration, transport, and loss of relativistic electrons could be assessed with high time-resolution ability [e.g., Blake et al., 1996], and the continuous monitoring of the radiation belts gave a valuable new space weather analysis tool [e.g., Baker et al., 1994b].

For the purposes of this paper (and in the context of this radiation belt monograph), we have chosen to review some of the considerable contributions that SAMPEX has made to magnetospheric particle physics. The SAMPEX spacecraft, which was launched in 1992, is now close to termination and is expected to reenter the Earth’s atmosphere in December 2012 at the latest. Available space does not allow an exhaustive assessment or documentation of all that the mission has done to advance the discipline of radiation belt science. When the range and depth of magnetospheric studies are further considered in the context of all SAMPEX has contributed in solar and galactic cosmic ray studies, it becomes clear that this small and relatively unprepossessing spacecraft has been one of the most successful programs (especially when considered dollar for dollar and pound for pound) that NASA has ever flown.

2. RADIATION BELT STRUCTURE AND DYNAMICS

Figure 1 shows a schematic diagram of the Earth, its strong dipolar magnetic field region, and the regions of trapped energetic particles known as the inner (blue) and outer (purple) Van Allen radiation zones. Also shown in Figure 1 is a schematic diagram of the SAMPEX orbit. As is evident from Figure 1, the SAMPEX spacecraft samples essentially all of the relevant radiation belt regions from the vantage point of low-Earth orbit by cutting through various magnetic field lines. In a single ~100 min orbit, SAMPEX would be expected to cut twice through all field lines threading the northern and southern extensions of the inner and outer radiation belts.

Figure 2 shows an approximately 12 year record of electron (E = 2–6 MeV) measurements made by the SAMPEX/PET sensors [adapted from Baker et al., 2005a]. The format of the data display, commonly used in radiation belt studies, shows intensity of electron fluxes color coded according to the logarithmic scale to the right. The vertical axis of Figure 2 is L value (i.e., the geocentric distance scaled in Earth radii, RE, at which magnetic field lines would cross the magnetic equatorial plane). An L value of 1.0 would be near the Earth’s surface, while an L value near 6.6 would be the region of space where geostationary Earth orbit spacecraft operate. The horizontal axis in Figure 1 is time measured in years: SAMPEX launched in July 1992 and the data are shown through early 2004.

Figure 2 clearly illustrates the double-belt structure of the Earth’s electron radiation zones. The inner belt (1 < L < 2) is relatively weak in electron flux strength and varies (generally) over long time scales (an exception is the period in late 2003 known as the Halloween Storm period; see below). The electron slot region (2 < L <3) is usually devoid of relativistic electrons [see, e.g., Lyons and Thorne, 1973; Schulz and Lanzerotti, 1974]. The outer electron belt (L ~3 to L > 6) is highly variable and often shows electron intensities (E >2 MeV) that are 5 orders of magnitude higher than the inner zone population.

Figure 1. A schematic cross-sectional diagram of the Earth’s inner and outer Van Allen radiation belts superimposed on dipolar magnetic field lines shown as white curves emanating from the Earth. Also shown is the SAMPEX low-altitude orbit and (in yellow) the trapped belt of anomalous cosmic rays.

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Figure 2. Color-coded flux levels of E = 2–6 MeV electrons measured by SAMPEX spacecraft instruments from July 1992 through March 2004.The color bar is shown to the right and is logarithmic in electron intensity. The vertical axis in the figure is magnetic L shell parameter (as described in the text), and the horizontal axis is time (in years). Adapted from Baker et al. [2005a].

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Figure 2 also demonstrates clearly that outer zone electrons are much higher in certain intervals of time that are about a decade apart (~1994 and ~2004 in the data shown). This will be discussed further below.

3. INNER ZONE PROPERTIES

Figure 2 makes clear that the inner zone electron fluxes were relatively elevated in 1993 through 1995 and then were at quite weak levels for much of the period from 1996 into the year 2000. An obvious enhancement of inner zone electrons occurred again in late 2003 in association with the Halloween Storm period [see Baker et al., 2004a]. It was the established position of the SAMPEX team [Blake et al., 2001b; Baker et al., 2004a, 2005a, 2005b] that the higher 1992–1995 fluxes around L ~ 1.5 were residual effects of the famous March 1991 geomagnetic storm that was observed by the CRRES [Blake et al., 1992].

A key fact to recall is that the weak electron inner zone population is spatially commingled with a high-intensity proton (ion) population to form the inner Van Allen belt. SAMPEX was designed innovatively to use the Earth’s magnetic field as a charge-state analyzer to study high-energy heavy ions and to confirm the source of the so-called anomalous cosmic ray (ACR) component [e.g., Fisk et al., 1974; Blake and Freisen, 1977]. As shown in Figure 3, SAMPEX was able to measure with high precision both the interplanetary flux of ACR ions (such as oxygen nuclei and nitrogen nuclei) on open field lines over the Earth’s polar caps as well as the trapped ACR ions confined on closed magnetic field lines in the Earth’s inner Van Allen zone (see Figure 1). The work completed by the SAMPEX team [e.g., Klecker et al., 1998; Mewaldt et al., 1996] confirmed the interplanetary acceleration mechanism for the ACRs [Fisk et al., 1974] as well as the magnetospheric trapping and flux concentration mechanism proposed by Blake and Freisen [1977].

4. OUTER ZONE ELECTRON DEPENDENCE ON SOLAR WIND FORCING

Figure 4 shows the record of slot and outer zone electron (E = 2–6 MeV) behavior from the time of SAMPEX launch (July 1992) through the middle of 2009 (when the PET sensor on SAMPEX ceased returning data). Figure 4 [from Li et al., 2011] is a record of continuous, homogeneous data for nearly two solar sunspot cycles. As was hinted in Figure 2 above, and as discussed in detail by Li et al. [2011], the intensity and spatial extent of outer zone electrons is controlled in clear ways by solar and solar wind forcing. Figure 4 (top) shows smoothed sunspot number (black curve) and plots solar wind speed (red spiky curve with axis to the right) for this extended period. Obviously, times of higher solar wind speed (e.g., 1994–1995 and 2003–2004) tended to be times of elevated electron flux throughout the outer zone [Paulikas and Blake, 1979; Baker et al., 1994a, 1994b, 1997a]. But the interplanetary magnetic field also plays a key, indispensable role [Blake et al., 1997], and this point has been clarified in recent papers [e.g., Li et al., 2011].

Figure 3. Directional differential fluxes of 17 MeV (nucleon oxygen cosmic rays)−1 measured from 1992 to 2000. The lower trace is from the Climax neutron monitor, while the middle set of data points is the interplanetary flux levels (as shown by the colors indicated) of the anomalous cosmic ray (ACR) component. The highest trace is the trapped ACR flux measured by SAMPEX. From Selesnick et al. [2000].

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Figure 4. (bottom) A 17 year record of high-energy electrons measured by SAMPEX similar in format to Figure 2. (top) A trace of smoothed sunspot number (black line) and solar wind speed (red line) for the same period from July 1992 through July 2009. The black trace in the bottom plot shows the estimated plasmapause location at any given time. From Li et al. [2011].

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Note in Figure 4 the excellent correlation of the inward extent of the outer zone electrons and the concurrent location of the plasmapause boundary shown by the superimposed black trace (scale to the right of bottom plot). As described by Li et al. [2006, 2011] and as seen in extreme events such as the Halloween Storm period of 2003 [Baker et al., 2004a], the coldest plasmas in the Earth’s magnetosphere and plasmasphere really do exert a significant influence on the radiation belts.

5. TRANSIENT SOLAR DISTURBANCES AND OUTER BELT RESPONSES

Figure 5 shows results from an early event analysis in which SAMPEX data played a key role [Baker et al., 1998a]. Figure 5a is the familiar L-versus-time color spectrogram plot for SAMPEX 2–6 MeV electrons during the first 200 days of 1997. On day of year (DOY) 135 (15 May 1997), a powerful coronal mass ejection (CME) struck the Earth and produced a quite abrupt acceleration of the Earth’s entire outer zone electron population. Baker et al. [1998a] were able to study this event in detail and place it into the context of solar, interplanetary, and other magnetospheric measurements. In particular, as shown by Figures 5b and 5c, the orbit-by-orbit data of SAMPEX and the corresponding detailed data from the Polar spacecraft High-Sensitivity Telescope [Blake et al., 1995] were able to show the coherent and nearly simultaneous global acceleration of electrons throughout much of the outer radiation belt on time scales of minutes to hours.

This kind of work using correlated SAMPEX and Polar measurements was subsequently pursued in other related studies [Baker et al., 1998b, 2000, 2001; Blake et al., 2001b]. The idea of remarkable global coherence in radiation belt acceleration was especially well established by the studies of Kanekal et al. [2001]. Figure 6 (top) [from Kanekal et al., 2001] shows data from SAMPEX and closely analogous data from Polar (Figure 6, bottom) for all of 1998. Virtually every feature seen by Polar at high altitudes (relatively close to the magnetic equator) was also seen, quite comparably, by SAMPEX at low altitudes near the foot of corresponding magnetic field lines.

Figure 5. (a) L sorted electron fluxes measured at low altitudes by SAMPEX instruments for 2 < E< 6 MeV. Data are coded according to the color bar to the right. c03_image005.gif values from 1 to 8 are shown for the first 200 days of 1997, and several electron enhancement events are seen, notably one commencing on Day 135 (May 15). (b) Similar to Figure 5a but showing orbit-by-orbit data from SAMPEX for Day 133 (May 13) through Day 137 (May 17) of 1997. Data are for Southern Hemisphere, duskside portions of the SAMPEX orbit. (c) Similar to Figures 5a and 5b but for E >2 MeV electrons measured by the Polar spacecraft. From Baker et al. [1998a].

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6. ACCELERATION MECHANISMS

SAMPEX data have proven to be important to study the systematics of electron acceleration throughout the outer radiation belt. Figure 7 is an example of such data. Figure 7 (top) is the usual format of electron data for 1 year from mid-2000 through to mid-2001. As can be seen, many specific enhancements of outer-zone electrons can be identified throughout late 2000 and again in the spring season of 2001. Figure 7 (bottom) shows a higher PET energy channel of SAMPEX (E = 3.5–16 MeV). It is seen from this channel that only a few of the events that were so clear in the E ~ 2 MeV channel of the top plot were really prominent in the multi-MeV energy range of the bottom plot. This means that some magnetospheric electron acceleration events produce very hard energy spectra, but most do not.

Elkington et al. [2004, 2005] have used sophisticated methods to study high-energy electron transport and acceleration in the Earth’s magnetosphere. This work starts with a global simulation of the solar wind-magnetosphere system using the Lyon-Fedder-Mobarry (LFM) numerical simulation code. Elkington et al. then push energetic electrons in the self-consistent electric and magnetic fields of the LFM code. The result is an ability to simulate specific solar-driven geomagnetic storms and to see by direct simulations how the magnetosphere methodically transports and accelerates relativistic electrons within the inner magnetosphere.

An example of these results is shown in Figure 8 [see Elkington et al., 2005 and Baker et al., 2005b]. The powerful geomagnetic storm of 31 March 2001 [see Baker et al., 2002] was simulated by the LFM model. (This storm and the resulting relativistic electron enhancement are, it might be noted, seen to be quite prominent in Figure 7, top). Figure 8 shows snapshots of the inward transport (Figure 8a), the radial diffusive acceleration (Figure 8b), and ultimately the strong trapping (Figure 8c) of E > 1 MeV electrons during the course of the March 2001 storm [see Baker et al., 2005b].

The test-particle MHD simulation codes of Elkington et al. [2004, 2005] and other authors produce electron acceleration by means of earthward radial transport, which essentially equates to the betatron mechanism. Energy diffusion due to gyroresonant interaction with VLF chorus waves can also be effective in generating relativistic (>1 MeV) radiation belt electrons [Summers et al., 1998, 2002, 2007; Roth et al., 1999; Varotsou et al., 2005; Horne et al., 2005]. While radial diffusion and transport is particularly effective for energizing electrons outside geosynchonous orbit, there is considerable evidence that an additional local acceleration mechanism (e.g., VLF chorus diffusion) is required to explain observed relativistic electron flux increases inside geosynchronous orbit [e.g., Miyoshi et al., 2004; Iles et al., 2006; Shprits et al., 2006]. Since geomagnetic storms can result in both net acceleration and net loss of energetic radiation belt electrons [Reeves et al., 2003], it is important to incorporate electron loss mechanisms in radiation belt electron dynamical models. An important radiation belt loss process is due to pitch angle scattering into the atmospheric loss cone due to electron cyclotron resonance with VLF chorus, ELF hiss, and electromagnetic ion cyclotron waves [Summers et al., 1998; Summers and Thorne, 2003; Summers et al., 2007]. Construction of realistic 3-D time-dependent radiation belt models requires comprehensive observational data on the spatiotemporal properties of these waves. While such data sets are currently limited, it is expected that the imminent Radiation Belt Storm Probes mission [Kessel, this volume] will provide valuable new data on these and other plasma waves that control radiation belt electron behavior.

Figure 6. (top) Data similar in format to Figure 2 from SAMPEX for the year 1998. (bottom) Similar data from the Polar/High-Sensitivity Telescope instrument showing remarkable global coherence of electron throughout the outer radiation belt. From Kanekal et al. [2001].

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7. HIGH-SPEED SOLAR WIND STREAM ACCELERATION

Immediately prior sections of this review have emphasized the key and obvious role of aperiodic, transient solar disturbances in accelerating high-energy magnetospheric electrons. This role of CME-driven events is undeniable in many cases. However, the strongest and most methodical acceleration of relativistic electrons in the Earth’s outer radiation belts really is associated with recurrent high-speed solar wind streams [e.g., Paulikas and Blake, 1979; Baker et al., 1990]. SAMPEX observations [Baker et al., 1997a, 1997b] have proven to play a very key role in understanding this aspect (see Figures 2 and 4 above).

Figure 9 shows SAMPEX data for the entire year of 1994 in the L versus time spectrogram format. The data show regular, episodic enhancements of 2–6 MeV electrons for essentially the entire year. The white vertical arrows in Figure 9 delineate 27 day recurrent periods. As is evident, there is a strong tendency for the electron flux enhancements to occur with an obvious 27 day period.

Baker et al. [1997a, 1997b] showed evidence that prototypical relativistic electron acceleration in the Earth’s radiation belts often occurs with several repeatable steps: (1) Highspeed solar wind streams drive strong magnetospheric substorm activity. (2) Substorms produce a large population of seed electrons extending up to several hundreds of keV in energy. (3) The enhanced radial diffusion of the seed electrons associated with high solar wind speeds, ULF waves drivers, and VLF wave heating produced by such streams inside the magnetosphere leads regularly to high-intensity electron radiation belt populations.

Many details of this complex process are not fully understood, however, and construction of realistic 3-D models of radiation belt electron dynamics remains a challenge, as we have implied above.

8. RADIATION BELT CONTENT AND STATISTICAL STUDIES

The continuous, homogeneous radiation belt information provided by SAMPEX has proven to be essential for long-term and statistical studies. This data set has given substantial insight into how the connected solar wind-magnetosphere-atmosphere system works.

A realization [Baker et al., 1999] was that one could use the SAMPEX measurements such as those shown in Figure 2 or Figure 4 and integrate across all outer zone L values (say 2.5 < L < 6.5) to estimate the entire average flux of electrons within the outer zone. One can do this on a daily, monthly, seasonal, or annual basis. Figure 10, for example, shows results from SAMPEX for a study of seasonal flux variations [Baker et al., 1999]. These results, sorted according to equinox and solstice periods, show well that outer radiation belt fluxes around the Spring and Fall equinox periods (roughly March and September) are clearly three or more times higher than are those around the solstice periods (roughly June and December). Close inspection of Figure 2 or Figure 4 above will confirm this result. More recent studies [e.g., McPherron et al., 2009] have confirmed these earlier results and have discussed the phenomenon in terms of the semiannual Russell-McPherron effect.

Figure 7. (top) Data similar in format to Figure 2 showing SAMPEX electron fluxes from July 2000 through June 2001. (bottom) SAMPEX data for the same time period as top plot but showing electron fluxes in the energy range 3.5–16 MeV.

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Baker et al. [2004b] extended the 1999 seasonal-dependence study and introduced the radiation belt content index (often shortened to radiation belt content (RBC)). Figure 11a and 11b show examples of the RBC as described by Baker et al. [2004b]. Figure 11a shows the total estimated number of electrons in the outer radiation belt (2.5 < L < 6.5) as a function of time from 1992 through 2002. The computation of RBC takes into account the entire flux tube content and also integrates the particle spectrum in energy above E = 2 MeV. It is seen in Figure 11a that the peak RBC content was ~5 × 10²³ electrons (mid-1994), and the minimum content of the outer belt (occurring in 1996 and again in 1999) was ~3 × 10²⁰ electrons.

Another use of the RBC, shown by Figure 11b, is to assess probabilities of encountering certain flux levels or certain total content levels [see Baker et al., 2004b]. It is seen in Figure 11b that the content index follows a log-normal Gaussian probability distribution function (over several orders of magnitude). The 50% probability value of the RBC for 1992–2002 was about 10²² electrons contained within the outer belt.

Figure 8. Snapshots of numerical simulation results [Elkington et al., 2004, 2005] shown for a geomagnetic storm that occurred on 31 March 2001. Energetic electrons are pushed in the self-consistent electric and magnetic fields of the Lyon-Fedder-Mobarry (LFM) MHD simulation code (as described in the text). Three different times on 31 March are shown: (a) 04:00 UT; (b) 07:00 UT; and (c) 08:55 UT. From Baker et al. [2005b].

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Figure 9. Data similar in format to Figure 2 but showing electron measurements from the SAMPEX spacecraft for the year 1994. The white vertical arrows are placed at recurrent 27 day intervals. The data show that electron flux enhancements recur with a clear 27 day period.

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Figure 10. Seasonally averaged and normalized fluxes of electron fluxes measured by SAMPEX over the period 1992–1999. The data have been integrated over the entire outer radiation belt (2.5 < L < 6.5) and show a much higher average flux level for the spring and fall equinox periods compared to the summer and winter solstice periods. From Baker et al. [1999].

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Finally, the statistical studies using the RBC index formalism led to some other insights about the radiation belt electron behavior. For example, Figure 12 from Baker et al. [2004b] shows the scatter of daily RBI values versus concurrent solar wind speed values (VSW).

Rather than following a linear or log-linear relationship as might have been expected from earlier work [e.g., Paulikas and Blake, 1979; Baker et al., 1979], the scatter of points in Figure 12 is quite triangular. Baker et al. [2004b] argued that enhanced solar wind speeds clearly increase the probability of higher radiation belt fluxes, but fluxes can remain quite elevated even after the solar wind forcing speed has diminished again. Thus, in many cases, the RBC index remained high for days or even weeks after the solar wind speed had decreased. The substance of this kind of correlation work has been revisited recently using geostationary orbit data [see Li et al., 2011, and references therein; Kellerman and Shprits, 2012]

9. RADIATION BELT ENHANCEMENT AND DECAY RATES

As has been noted in several ways above, the sudden appearance of high-energy radiation belt electrons is an important and intriguing scientific problem. It is also quite clear that such electron enhancements can cause significant spacecraft operational problems [Baker, 1987, 2002] through the mechanism of deep dielectric charging. Obviously, the flux of electrons at a given point in the magnetosphere is a delicate balance of source strength and local loss rates. Many important space weather incidents related to radiation belt enhancements have been studied using the SAMPEX data sets [e.g., Baker et al., 1994b, 1998c].

Figure 11. (a) Daily values for the radiation belt content (RBC) index for the period 1992 through 2002. Adapted from Baker et al. [2004b]. (b) Cumulative probability curve for the radiation belt content index as discussed in the text. Modified from Baker et al. [2004b].

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Figure 12. A scatter diagram of the RBC index versus solar wind speed (VSW) based on daily average values. From Baker et al. [2004b].

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One of the largest and most studied radiation belt enhancement events occurred in late October and early November 2003. This Halloween Storm event (or set of events) was touched on several times previously in this paper. Figure 13 shows an expanded record of SAMPEX data (and solar wind speed data in the top plot) from a paper by Baker et al. [2007]. The period of time covered is 2003 to the end of 2005. Figure 13 (bottom) shows quite obviously the sudden appearance (or at least, powerful reemergence) of a new belt of electrons associated with the Halloween Storm [Baker et al., 2004a]. Over at least the next 2 years, the inner zone belt decayed away. But superimposed on the gradual decay were punctuated episodes of further enhancement of the inner zone associated with the outer zone events (such as around DOY 600 and again around DOY 700).

The original powerful acceleration event in late October 2003 was analyzed in detail by Baker et al. [2004a]. As shown in the interpretive sketches of Figure 14 (from that paper), the Earth’s outer Van Allen zone was virtually annihilated by the Halloween Storm. The complete disappearance of multi-MeV electrons at L ~ 4 was quickly followed by the new generation of MeV electrons in the heart of the slot region (which usually is devoid of such electrons). Baker et al. [2004a] showed that the plasmasphere was scoured away in this event, and this allowed a complete (if temporary) reconfiguration of the radiation belts.

Figure 13. Daily averaged data for the years 2003–2005 inclusive. (a) Solar wind speeds measured by instruments onboard the ACE spacecraft upstream of the Earth’s magnetosphere. (b) Relativistic (2–6 MeV) electron fluxes for the range 1 < L < 8 in a logarithmic color-coded format as shown by the color bar to the right. Data in Figure 13b were obtained from instruments onboard the SAMPEX spacecraft (E = 2–6 MeV electrons). From Baker et al. [2007].

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Following the formation of the new inner zone belt of electrons, we were able to study episodic losses of electrons from low L shells for the next several years. Figure 15 is from Baker et al. [2007]. It shows cuts through the data portrayed in Figure 13 at several fixed L values (indicated by the different colors). The result is traces of electron flux levels versus time. Baker et al. [2007] were able to use the exponential decay rates of each flux spike in Figure 15 to deduce the inner zone electron lifetimes from L ~ 1.5 to L ~ 2.2. Such estimates of lifetime were made possible by the continuous SAMPEX monitoring.

10. ELECTRON LOSSES: ATMOSPHERIC COUPLING

Figure 16 is another form of display of electron flux measurements from SAMPEX. These global polar maps were developed largely by Callis et al. [1996a, 1996b, 1998] to show the regions of the Earth’s atmosphere that would be affected by electrons being precipitated and lost during radiation belt events. What is fascinating about Figure 16 is how dramatically the precipitating electron flux can alter from just 1 day to the next. As shown by Callis et al., the energy into the Earth’s middle atmosphere could easily change by 5 or more orders of magnitude in a day. SAMPEX has been