Societal aspirations for a secure, prosperous, and technologically sophisticated future are increasingly influenced by Earth’s near-space environment. Central to the decadal survey’s strategy is the intent to achieve scientific results that will be useful to society; this chapter briefly reviews societal needs for improved knowledge of solar and space physics phenomena related to the effects of space weather.1
The Sun’s photon energy is the main source of heat for the entire Earth system. Slight variations in this light with the Sun’s 11-year activity cycle (Figure 3.1), and possibly on longer timescales, continuously affect Earth’s climate and atmosphere, at times temporarily masking the effects of changing concentrations of greenhouse gases. Earth’s atmosphere protects us from biologically damaging shorter-wavelength solar ultraviolet emissions. These emissions fluctuate by orders of magnitude more than variations in the total brightness, altering the ozone layer significantly. Solar-induced photochemical and dynamical changes in Earth’s middle atmosphere may affect the climate at lower altitudes and in the upper atmosphere and ionosphere through dynamical coupling of changing wave structures.
Precipitating electrons from the aurora affect the atmospheric chemistry in the polar regions. During major magnetic storms, significant enhancements of nitric oxide concentrations occur in the auroral zone and can literally propagate downward from space into the stratosphere. Nitric oxide plays an important role in the chemistry of stratospheric ozone. The downward transport of auroral products is difficult to trace and varies with the general circulation of the polar atmosphere, but it is clear that major space storms can potentially modify stratospheric composition and reduce ozone densities for a period of time following their occurrence.
FIGURE 3.1 Total solar irradiance (TSI) observed over the past three solar cycles (since 1978), varying between 1,357.5 and 1,363.5 W/m2. This composite time-series plot is based on the lower TSI level established with new laboratory calibrations of TSI instruments (see Figure 10.5 in Chapter 10). Differences between levels of irradiance during the solar minimum epochs (1986, 1996, 2008) are not significant because of instrument uncertainties. SOURCE: Replotted courtesy of Judith Lean, Naval Research Laboratory, after G. Kopp and J.L. Lean, A new, lower value of total solar irradiance: Evidence and climate significance, Geophysical Research Letters 38:L01706, doi:10.1029/2010GL045777, 2011.
Satellites orbiting Earth support essential societal infrastructure and now form the basis for a total global economy in excess of $250 billion per year.2 To inform our daily activities and decisions, we rely on weather predictions based on measurements from satellites. Satellites serve as communication relays and platforms for direct broadcasts of data and television signals. The nation’s military protects U.S. strategic interests around the world through continuous surveillance from satellites and depends on satellites for global communication, continuous situational awareness, and geolocation related to national security. Although a relatively new technology, the use of signals from Global Positioning System (GPS) satellites is pervasive, facilitating everyday activities that range from navigation to financial transactions.
The magnetosphere is the domain of nearly all Earth-orbiting satellites, affecting those in low, medium, and geostationary orbits, as well as those in high-apogee orbits. It is a region filled with charged particles, including the intense radiation belts that vary continuously in response to changes in the solar wind and to the solar disturbances that strongly affect the space environment. (See Figure 3.2.) Charged particles affect space technology in a variety of ways: at their most benign they cause surface charging and discharging,
2 See Report on the Space Economy Symposium, March 13, 2009, available at http://spaceeconomy.gmu.edu/ses2009/symposiumreport2009.pdf.
FIGURE 3.2 A diagram of the Van Allen radiation zone surrounding Earth. This cutaway image shows the weak inner zone, the “slot” region that is relatively devoid of trapped radiation, and the more intense and highly variable outer Van Allen belt. The two spacecraft of the Radiation Belt Storm Probes mission are shown schematically. SOURCE: Courtesy of NASA.
and at their most destructive they damage electronics components, including the temporary (single-event) upset of spacecraft commanding. Furthermore, upper atmospheric heating associated with the dynamics of the space environment can dramatically change drag effects on low-Earth-orbiting satellites, notably the International Space Station (ISS).
The specification and forecasting of ionospheric scintillation (i.e., radio propagation fluctuations due to plasma density irregularities) is a high priority for both civilian and military space operations. These scintillations are prevalent at low geographic latitudes as well as in the auroral regions, disrupting radio communications in critical geographical locations. They occur more frequently and extend to higher altitudes during times of high solar activity. Changes in ionospheric total electron content during geomagnetic storms compromise the performance of GPS technology vital for aviation and many other commercial and defense applications (see Figure 3.3). The participating electrons from the aurora also charge space systems, such as the ISS, generating the danger of arcing associated with discharges.
Humans venturing into space are vulnerable to damage caused by episodic radiation in the form of energetic particles from the Sun and from cosmic rays that constantly impinge on the solar system from
FIGURE 3.3 Total electron content (TEC), a measure of column-integrated electron density, derived from dual-frequency Global Positioning System (GPS) receivers for the November 20, 2003, geomagnetic storm. The tongue of enhanced electron density (red hue) is a signature of storm-time circulation and reflects the interplay of ionosphere-magnetosphere dynamics during active periods. Gradients in TEC (shown here as variations in hue) can cause GPS receivers to lose location lock, affecting many GPS-dependent systems. SOURCE: A. Coster and J. Foster, Space weather impacts of the subauroral polarization stream, Radio Science Bulletin 321:28-36, 2007; copyright 2007 Radio Science Press, Belgium, for the International Union of Radio Science (URSI); used with permission.
the galaxy. These dangers are well recognized3 if not yet predictable: astronauts on the ISS must retreat to protected areas in the event of high-energy solar radiation. Space radiation from all sources will pose important hazards for space systems and astronauts on long-duration flights away from Earth’s magnetic field.
The electric power grid, the backbone of modern society, is particularly vulnerable to space environmental effects. As the geomagnetic field changes as a consequence of impacts from solar eruptions, large currents are generated and guided into the ionized layers of Earth’s upper atmosphere, where they contribute substantially to the outward expansion of the outer atmosphere, causing satellite drag effects. Even more importantly, temporal changes in the currents induce voltages in the ground. The ground conductor of the power grid is connected to this source of voltage, and consequently currents other than those associ-
3 National Research Council, Managing Space Radiation Risk in the New Era of Space Exploration, The National Academies Press, Washington, D.C., 2008.
ated with power transmission can flow on the grid. These geomagnetically induced currents (GICs) have the potential to overload transformers, causing, at a minimum, reductions in efficiency. Large-amplitude GICs can age, or even destroy, a transformer.
Owing to the critical place electric power has in the maintenance of society, a number of recent studies4,5,6 have emphasized the need for further research with the objective to understand, and then prevent and mitigate, deleterious GIC-based hazards. Ground-based assets such as long-distance pipelines are also susceptible, responding to the strong currents induced by geomagnetic storms. Some of the many manifestations of disturbances from the Sun that now have the potential to disrupt society are illustrated in Figure 3.4.
Powerful solar flares (e.g., see Figure 3.5) and their accompanying ejections of mass and energetic particles occur episodically. In an extreme event in 1859, a large solar eruption triggered a geomagnetic storm that sparked fires in telegraph offices across the United States and triggered aurorae as far south as Central America. Such a powerful event directly striking Earth today could severely affect the power grid, destroying transformers and causing widespread outages.
Although during the space age a direct hit of this magnitude has not yet occurred, severe solar storms have nevertheless damaged spacecraft and power grids, producing, for example, widespread power outages in Quebec in 1989 and in South Africa in 2003. Very energetic solar particles (SEPs) that are accelerated in solar flares and Earth-ward-propagating interplanetary shocks penetrate along open magnetic field lines into Earth’s polar ionosphere, where they degrade high-frequency communications over the poles. This interference forces the airline industry to reroute transpolar flights, at a significant cost in time and fuel. European flight crews on shorter high-latitude routes are categorized as “radiation workers” and are monitored by film badges because of their increased exposure to SEPs. Lacking the knowledge for predictive mitigation of severe solar storm impacts, operators currently simply assume (and hope) that the rarity of extreme events and the vastness of space will protect against the most deleterious consequences.
Although particularly intense events occur about once per solar cycle and strong to extreme particle storms can occur about 15 times per cycle, somewhat less intense geomagnetic storms occur even more often. Even during these weaker geomagnetic storms, large changes in ionospheric currents threaten transformers in long-distance east-west power lines in North America and northern Europe.
Science cannot now reliably predict, with sufficient warning, the disturbances from space that might threaten society at any particular time. The physical processes that control space weather differ in complex ways from those that control the weather of the neutral atmosphere of Earth. Within the entire system numerous phenomena have to be addressed on a wide range of physical scales—for example, the gas density can vary from 1019 in Earth’s atmosphere to just a few particles per cubic centimeter in the solar wind, and the relevant scale lengths can vary from centimeters to astronomical units (AUs). The many different and complex interactions include electromagnetic forces that accelerate and control the flow of
4 J. Kappenmann, Metatech Corporation, Low-Frequency Protection Concepts for the Electric Power Grid: Geomagnetically Induced Current (GIC) and E3 HEMP Mitigation, prepared for Oak Ridge National Laboratory, Oak Ridge, Tenn., January 2010.
5 MITRE Corporation, Impacts of Severe Space Weather on the Electric Grid, JASON report, McLean, Va., November 2011.
6 North American Electric Reliability Corporation, 2012 Special Reliability Assessment Interim Report: Effects of Geomagnetic Disturbances on the Bulk Power System, February 2012, available at http://www.nerc.com/files/2012GMD.pdf.
FIGURE 3.4 Examples of effects of space weather on critical infrastructure. SOURCE: NASA, Heliophysics: The New Science of the Sun-Solar System Connection. Recommended Roadmap for Science and Technology 2005-2035, NP-2005-11-740-GSFC, NASA, Greenbelt, Md., February 2006, available at http://sec.gFsficg.nuaresa3.g-o4v/Roadmap_FINALpri.pdf.
FIGURE 3.5 Solar Dynamics Observatory (SDO) image from August 9, 2011, X7 class flare. An X-class flare began at 3:48 a.m. EDT on August 9, 2011, and peaked at 4:05 a.m. The flare burst from active region AR11263 before it rotated out of view from Earth. The image here was captured by NASA’s SDO in extreme ultraviolet light at 131 Å and at the beginning of the event, just before the satellite sensors were overwhelmed by energetic particles. SOURCE: NASA/Solar Dynamics Observatory/Atmospheric Imaging Assembly; available at http://www.nasa.gov/mission_pages/sunearth/news/News080911-xclass.html.
electrically charged particles, supersonic flows and shock waves, explosive release of magnetic energy, and solar-driven winds and tides in Earth’s atmosphere.
Moreover, the Sun, the heliosphere, Earth, and the planets together constitute a coupled and intertwined system. It is a formidable challenge to understand the detailed individual processes that control the space environment, while also accounting for the global couplings among the various interacting members of the Sun-heliosphere-Earth system and their subelements, such as the neutral atmosphere and ionosphere. Significant progress has accrued during the past few decades from observations made by space missions and ground-based observatories and from theories and models developed to explain the observations. However, owing to the complexity of this variable, coupled system, scientists have not yet achieved a sufficiently
reliable predictive capability for when and in what direction major disturbances will be emitted by the Sun; or for how disturbances from the Sun, coupled with inputs from Earth, affect the space environment near Earth; or for what the radiation environment through which astronauts might fly will be; or for exactly how changes on the Sun may affect Earth’s climate, atmosphere, and ionosphere.
Despite these challenges, prediction of the space environment is, in principle, a decipherable problem. New ground- and space-based measurements are adding considerable knowledge to enhance understanding of the space environment and its governing processes. In parallel, increasingly sophisticated comprehensive physical models are being developed that run on ever more powerful computers. Given an adequate investment of effort in fundamental scientific research and modeling, the research community should be able to leverage advances in computing capability to develop the predictive models required to specify the extended space environment in order to protect society and advance growing aspirations for the use of space.