Daniel N. Baker
Laboratory for Atmospheric and Space, University of Colorado, Boulder, CO 80303, USA
Abstract. Adverse space weather is one of the principal threats to modern human technology. Solar coronal mass ejections, large solar flares, and high-speed solar wind streams often lead to sequences of damaging disturbances within the Earth's magnetosphere, in the atmosphere, and even on the Earth's surface. Powerful and long-lasting geomagnetic storms can develop following solar disturbances and enhancements of the highly relativistic electron populations throughout the outer terrestrial radiation zone can also result. High-energy protons and heavier ions arriving in near-earth space - or trapped in the magnetosphere and having clearest effect in the South Atlantic Anomaly (SAA) - can damage satellite solar power panels, confuse optical trackers, and deposit harmful charges into sensitive electronic components. Recent international space science programs have made a concerted effort to study activity on the Sun, the propagation of energy bursts from the Sun to near-Earth space, energy coupling into the magnetosphere, and its redistribution and deposition in the upper and middle atmosphere. Extreme solar, geomagnetic and solar wind conditions can be observed by a large array of international satellites and ground-based sensors. Many types of space weather-related problems have been identified in recent years. This chapter presents examples of space weather-induced anomalies and failures and discusses community efforts to propose technical and operational solutions to space weather problems now and in the future.
Above the thin layer of Earth's atmosphere where normal weather occurs (the troposphere), there is a vast region extending into interplanetary space that is permeated by highly fluctuating magnetic fields and very energetic particles. The collective, often violent, changes in the space environment surrounding the Earth are commonly referred to as "space weather". For several decades now, humans have increasingly used space-based assets for navigation, communication, military reconnaissance, and exploration. New observations, numerical simulations, and predictive models are helping to make important strides to deal with (if not alter) space weather (National Space Weather Program Strategic Plan, 1995 ; Baker, 1998 ).
As shown by Fig. 1 (taken from NASA "Roadmap" documents), the Sun and its interaction with the Earth is a prototype for much of our understanding of cosmic plasma physics. The upper chain of insets suggest that our understanding of the fundamental elements of magnetospheric physics,
D.N. Baker, Introduction to Space Weather, Lect. Notes Phys. 656, 3-20 (2005) http://www.springerlink.com/ © Springer-Verlag Berlin Heidelberg 2005
our approach to comparative planetary environments, and ultimately our understanding of the plasma universe springs from our studies of Sun-Earth connections.
The lower chain of insets in Fig. 1, however, makes another important point: The space environment that we study for its intrinsic science value (as just described) is also an environment that has crucial practical importance. The effects of the space environment on humans in space, spacecraft operations, communications systems, power systems, and even (possibly) on climate make the understanding of Sun-Earth connections a manifestly important subject from a very pragmatic standpoint. Thus, the space weather "branch" of Fig. 1 is highly important much as is the basic science "branch".
Figure 2 shows in a schematic way the linked Sun-Earth system. It is known from several decades of research that the Sun is the overwhelming driver of space weather effects in near-Earth space. The solar wind emanating from the Sun - and the embedded interplanetary magnetic field (IMF) -provides the momentum, the energy, and much of the mass that fills and powers the Earth's magnetosphere. The Earth's ionosphere and atmosphere responds to this solar wind driving in complex ways. The ionosphere can also supply particles (mass) to populate the terrestrial magnetosphere and, of course, the neutral atmosphere responds strongly to solar irradiance (photons) as well as to plasma interactions with the solar wind.
The magnetosphere-ionosphere-atmosphere system is immensely complicated and constitutes a high-coupled system (see Fig. 3). There are several
large-scale current systems and there are key regions of distinct plasmas (often separated - at least conceptually - by boundary layers). Trapped energetic particles constitute the van Allen radiation belts and a cold plasma region in the inner magnetosphere is called the plasmasphere. These plasma regions extend along magnetic field lines and couple into the ionosphere and the neutral atmosphere. Other chapters in this book will treat many aspects of the Sun-Earth system in great detail.
A point to bear in mind from Figs. 2 and 3 is that virtually all human technological systems operate on (or near) the Earth's surface or else in near-Earth space. Thus, power grids, communications systems, navigation satellites, and military space assets are all within - and are very much affected by - solar and magnetospheric disturbances. In this sense, space weather is an ever-present set of factors for advanced human technological resources. This chapter provides an overall introduction to space weather consequences and mitigation strategies.
As shown in Fig. 4, the Sun can emit giant clouds of ionized gas (coronal mass ejections, CMEs) which contain upwards of 1016 grams of hot plasma. These CMEs can move outward from the Sun's surface at speeds of 1000 km/s (or more) and can have embedded within them strong magnetic fields and highly energetic particle fluxes. The active Sun is also the source of powerful solar flares and streams of high-speed solar wind flows. As these solar disturbances reach the Earth and its vicinity, they can give rise to long-lasting and disruptive disturbances called geomagnetic storms. High-energy ions and electrons produced during geomagnetic storms, as well as fluctuating magnetic fields themselves, can have detrimental effects on Earth-orbiting spacecraft and on humans in space (Lanzerotti, 2001 ).
As shown in Fig. 5a, high-energy protons and heavier ions arriving in near-Earth space can interact with spacecraft in several damaging ways. The ionization track that energetic ions can leave in microminiaturized electronics can upset spacecraft computer memories and can otherwise disrupt sensitive space electronics. The result can be damage to satellite solar power panels, confusion to optical tracker systems, and scrambling of spacecraft command and control software. Even more worrisome is the fact that high-energy solar particles can be damaging, or even potentially deadly, to astronauts who are in space at the time of major solar particle events (Turner, 2000 ).
Another aspect of the space environment that can be quite harmful to spacecraft is very energetic ("relativistic") electrons. As shown in Fig. 5b, these energetic electrons can penetrate through even thick spacecraft shielding and can bury themselves within dielectric (insulating) materials deep within spacecraft systems and subsystems. When sufficient charge has built up within dielectric materials such as coaxial cables or electronics boards, a
powerful internal electrical discharge can occur (Baker, 1998 ; Robinson, 1989 ). This is very much like a miniature lightning strike within sensitive spacecraft electronics. Numerous recent spacecraft failures have been laid at the feet of this "deep dielectric charging" mechanism (Vampola, 1987 ; Baker, 1998 ; Baker et al., 1998 ).
Yet another space weather phenomenon of concern, known as "surface charging", is illustrated by part (c) of Fig. 5. Electrical charges coming from 10-100 kilovolt electrons within Earth's magnetosphere can accumulate on insulating surfaces of satellites. As with interior spacecraft insulators, if enough charge builds up on a region of surface dielectric material there can be a powerful, disruptive discharge. This can generate electrical signals in the spacecraft vicinity that can scramble and disorient the satellite and its subsystems (Robinson, 1989 ).
It is becoming increasingly understood and appreciated that continental-scale power generation and distribution systems are also vulnerable to the effects of space weather (Kappenman, 2001 ). Space storms can impact the operational reliability of electric power systems. For example, a major storm in 1989 shut down the Hydro Quebec power system in Canada for many hours. Space storms can disrupt power grids by introducing geomagnetically-
induced currents (GICs) into the transmission network. The GICs which flow through transformers, power lines, and grounding points can sometimes disrupt large portions of the power distribution system and such disruptions can occur within remarkably short periods of time (Kappenman, 2001 ). There are many other effects of space weather that manifest themselves in both subtle and very obvious fashions. A major space storm can modify the ionosphere of the Earth and therefore change the wavelength at which high-frequency (HF) radio communication is possible. This is a problem to the military and to airlines that are attempting to communicate with aircraft flying transpolar routes. Space weather can also cause sudden, unexpected heating of the Earth's upper neutral atmosphere. This heating causes an expansion of the upper atmospheric layer (the thermosphere) which can suddenly increase the drag force on low-altitude spacecraft (Lanzerotti, 2001 ; Singer et al., 2001 ).
As illustrated in Fig. 5b, very high-energy electrons can penetrate through spacecraft walls and through electronics boxes to bury themselves in various dielectric materials (e.g. Robinson, 1989 ). This can, in turn, lead to electric potential differences in the region of the buried charge. In some instances, intense voltage breakdowns can occur leading to surges of electrical energy deep inside circuits. This can cause severe damage to various subsystems of the spacecraft.
Many examples of such "deep-dielectric charging" have been presented by various authors (e.g., Vampola, 1987 ; Baker et al., 1987 ). An interesting case study presented by Baker et al. (1987)  is shown in Fig. 6. In this figure, smoothed daily averages of E = 1.4 — 2.0 MeV electron fluxes at geostationary orbit are plotted versus time (late 1980 through early 1982). Also shown by bold vertical arrows are some of the main occurrences of star tracker anomalies onboard this geostationary operational spacecraft. The star tracker upsets were normally associated with high intensities of relativistic electrons. However, some high intensity electron events did not produce star tracker anomalies (see Baker et al., 1987 ) so there are more subtle controlling factors as well. Figure 7 shows how electrons must build up in dielectric materials for quite some time before a harmful discharge can occur. Thus, it is both the intensity of relativistic electron irradiation and its duration that is important. During some intense events in late 1981, the star trackers were actually turned off and so no operational "anomalies" could be recorded. The anomalies tended quite clearly to occur only during relatively long-duration events. Thus, it was not only the peak intensity of electrons, but also the duration of exposure that proved to be important.
Numerous previous studies (e.g., Reagan et al., 1983 ; Robinson, 1989 ; Wrenn, 1995  have shown the clear role-played by high-energy elec-
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