Invisible magnetic fields emanate from the Earth, as well as the Sun. As early as 1600, William Gilbert, physician to Queen Elizabeth I of England, demonstrated that our planet is itself a great magnet, which explains the orientation of compass needles. It is as if there was a colossal bar magnet at the center of the Earth, with magnetic fields that emerge out of the south geographic polar regions, loop through nearby space, and re-enter at the north polar regions (Fig. 8.1). Since the geographic poles are located near the magnetic ones, a compass needle always points north or south. The magnetic fields are produced by electrically conducting currents in the Earth’s molten core, so the acts like it has a magnet buried at its center.
The dipolar (two poles) magnetic configuration applies near the surface of the Earth, but further out the magnetic field is distorted by the Sun’s perpetual wind. The energy-laden, electrically-charged solar wind blows out from the Sun in all directions and never stops, carrying with it a magnetic field rooted in the star. Although it is exceedingly thin, far less substantial than a terrestrial breeze or even a whisper, the solar wind is powerful enough to mold the outer edges of the Earth’s magnetosphere into a changing asymmetric shape (Fig. 8.2), like a tear drop falling toward the Sun.
The solar wind pushes the magnetic field toward the Earth on the day side that faces the Sun, compressing the outer magnetic boundary and forming a shock wave. It is called a bow shock because it is shaped like waves that pile up ahead of the bow of a moving ship. The Sun’s wind drags and stretches the terrestrial magnetic field out into a long magnetotail on the night side of Earth. The magnetic field points roughly toward the Earth in the northern half of the tail and away in the southern. The field strength drops to nearly zero at the center of the tail where the opposite magnetic orientations lie next to each other and currents can flow (Fig. 8.2).
Thus, the Earth’s magnetosphere is not precisely spherical. It has a bow shock facing the Sun and a magnetotail in the opposite direction. The term magnetosphere therefore does not refer to form or shape, but instead implies a sphere of influence. The magnetosphere of the Earth, or any other planet, is that region surrounding the planet in which its magnetic field dominates the motions of energetic charged particles such as electrons, protons and other ions. It is also the volume of space from which the main thrust of the solar wind is excluded.Yet, some of the energetic particles in space do manage to penetrate the Earth’s magnetic defense.
The merging between the magnetic fields of the solar wind and the Earth is most effective if they are pointing in opposite directions. With this orientation, the two fields become linked, just as the opposite poles of two toy magnets stick together, and the solar wind particles can enter the magnetosphere.
The wind’s magnetic field will be dragged by the flow of the wind behind the Earth into its magnetotail, wrapping and clinging around the magnetosphere like saran wrap (Fig. 8.3). The magnetosphere can then be punctured in the tail, providing a back door entry that funnels some of the wind into the magnetosphere. The passing solar wind is slowed down by the connected fields and decelerates in the vicinity of the tail. Energy is extracted from the solar wind and drives a large-scale circulation, or convection, of charged particles within the magnetosphere (Fig. 8.3).
When solar wind electrons and protons enter the Earth’s domain, they also become trapped within it and cannot easily get out. In fact, the inner magnetosphere is always filled with a veritable shooting gallery of electrons and protons, trapped within two torus-shaped belts that encircle the Earth’s equator but do not touch it (Fig. 8.4). These regions are often called the inner and outer Van Allen radiation belts, named after James A. Van Allen who discovered them in 1958. Van Allen used the term “radiation belt” because the charged particles were then known as corpuscular radiation; the nomenclature is still used today, but it does not imply either electromagnetic radiation or radioactivity.
More than half a century before the discovery of the radiation belts, Carl Størmer showed how electrons and protons can be trapped and suspended in space by the Earth’s dipolar magnetic field. An energetic charged particle moves around the magnetic fields in a spiral path that becomes more tightly coiled in the stronger magnetic fields close to a magnetic pole. The intense polar fields act like a magnetic mirror, turning the particle around so it moves back toward the other pole.
Thus, the electrons and protons bounce back and forth between the north and south magnetic pole (Fig. 8.5). It takes about one minute for an energetic electron to make one trip between the two polar mirror points. The spiraling electrons also drift eastward, completing one trip around the Earth in about half an hour. There is a similar drift for protons, but in the westward direction. The bouncing can continue indefinitely for particles trapped in the Earth’s radiation belts, until the particles collide with each other or some external force distorts the magnetic fields.
Large, sporadic geomagnetic storms
We have known about significant variations in the Earth’s magnetic field for almost three centuries. They are detected by irregular movements in the direction that compass needles point, with typical fluctuations lasting seconds to days. These variations are caused by invisible geomagnetic storms that rage in the magnetic fields far above our atmosphere.
Solar wind disturbances driven by fast coronal mass ejections are now thought to produce the most intense geomagnetic storms, at least during the maximum in the Sun’s activity cycle. Slow coronal mass ejections do not produce such events because they lack the strong magnetic fields and high speeds required to stimulate intense magnetic activity on Earth. The Earth intercepts about 70 coronal mass ejections per year when solar activity is at its peak, and less than 10 will have the punch needed to produce large, geomagnetic storms.
The solar wind generally moves slower than coronal mass ejections, so the ejection plows through the solar wind on its way into interplanetary space, driving a huge shock wave far ahead of it (Fig. 8.6, 8.7). When directed at the Earth, these shocks ram into the terrestrial magnetic field and trigger the initial phase, or sudden commencement, of a large geomagnetic storm a few days after the mass ejection leaves the Sun.
Strong interplanetary magnetic fields are also generated by fast coronal mass ejections (Fig. 8.6). It is their intense magnetism and high speed that account for the main phase of a powerful magnetic storm, provided that the magnetic alignment is right. The Earth’s field is generally directed northward in the outer day side magnetosphere, so a fast coronal mass ejection is more likely to merge and connect with the terrestrial field if it points in the opposite southward direction. The rate of magnetic reconnection, and hence the rate at which energy is transferred from the solar wind to the magnetosphere, increases with the strength and speed of the interplanetary magnetic field. The energy gained drives currents that make the intense magnetic storm.
Moderate, 27-day recurrent geomagnetic activity
Unlike the great sporadic storms, moderate geomagnetic activity does not exhibit a well defined connection with sunspots or any other indicator of solar magnetic activity. Indeed, these weaker events, sometimes referred to as substorms, can occur when there are no visible sunspots. A solar connection is nevertheless indicated by their 27-day repetition period, corresponding to the rotation period of the Sun at low solar latitudes when viewed from the moving Earth. Centuries.
The recurrent activity is linked to long-lived, high-speed streams in the solar wind that emanate from coronal holes. When the Sun is near a lull in its 11-year activity cycle, the fast wind streams rushing out of coronal holes can extend to the plane of the solar equator. When this fast wind overtakes the slow-speed, equatorial one, the two wind components interact, like two rivers merging to form a larger one. This produces shock waves and intense magnetic fields that rotate with the Sun (Fig. 8.8). Such Co-rotating Interaction Regions, or CIRs for short, can periodically sweep past the Earth, producing moderate geomagnetic activity every 27 days. Near solar maximum, at the peak of the 11-year activity cycle, coronal mass ejections dominate the interplanetary medium, producing the most intense geomagnetic storms, and the low-level activity is less noticeable.
The northern or southern lights, named the aurora borealis and aurora australis in Latin, are one of the most magnificent and earliest-known examples of solar-terrestrial interaction. They illuminate the cold, dark Arctic and Antarctic skies with curtains of green and red light that dance and shimmer across the night sky far above the highest clouds, pulsating and flickering for hours to days.
The reason that auroras are usually located near the polar regions is that the Earth’s magnetic fields guide energetic electrons there. The auroral lights form when high-speed electrons rain down along the Earth’s magnetic field lines into the upper atmosphere in the polar regions, like electricity making the gas in a neon light shine. The cascade of electrons collides with oxygen and nitrogen in our atmosphere, boosting them to higher energies and causing them to glow. It is something like the beam of electrons that strikes the screen of your color television set, making it glow in different colors depending on the type of chemicals (phosphors) that coat the screen.
Sun-driven space weather endangers humans whenever they venture into space. Down here on the ground, we are shielded from the direct onslaught of the raging solar wind by the Earth’s atmosphere and magnetic fields, but out in deep space there is no place to hide. Harmful high-energy particles, carried by gusts and squalls in the solar wind, can wipe out unprotected astronauts and destroy satellite electronics. They are of serious concern to future astronauts who might construct the International Space Station and explore the Moon or Mars.
Satellites can also be disabled by stormy weather in space. Powerful blasts from coronal mass ejections can compress the Earth’s magnetic field and send energetic particles into the magnetosphere, providing threats to Earth-orbiting satellites. Intense radiation from solar flares can change the electrical properties of our atmosphere, disrupting radio navigation or communication systems, and making the atmosphere expand farther into space than usual. Friction can develop between the expanded atmosphere and satellites traveling in it, slowing down the satellites, altering their orbits, and bringing them to a premature end. Forceful coronal mass ejections can also generate strong currents in out atmosphere, overloading transmission lines on the ground and producing power surges that can blackout entire cities.
Our technological society has become so vulnerable to the potential devastation of these storms in space that national centers employ space weather forecasters, and continuously monitor the Sun from ground and space to warn of threatening solar activity.
The hazards of space travel
The ultimate vacation, a trip into deep space, is fraught with danger, primarily from energetic particles. Even in the comparative safety of low-Earth orbits, beneath the protection of Earth’s magnetic field, astronauts have reported flashing lights inside their eyes. Energetic protons, perhaps trapped in the Van Allen radiation belts, pass through the satellite walls and the astronaut’s eyelids, striking their retinas and making their eyeballs glow inside.
Once outside the Earth’s magnetosphere, astronauts are exposed to the full blast of the ever-flowing solar wind. They could then suffer serious consequences from solar energetic particles even within their spacecraft, resulting in cataracts, skin cancer or even lethal radiation poisoning.
Energetic protons hurled out from intense solar explosions are especially hazardous. The largest events could inflict serious radiation damage on any astronaut caught in space without adequate shielding (Fig. 8.10, Fig. 8.11). Several of these proton events, each lasting 1 to 3 days, occur each year on the average. The high-speed solar protons could even kill an unprotected astronaut that ventures into space. Astronauts walking on the lunar surface in 1972 had at least one close call involving potentially deadly solar-flare events.
Fast coronal mass ejections plow into the slower-moving solar wind and act like a piston that drives shock waves ahead of them, accelerating electrons and protons as they go, much as ocean waves propel surfers. The mass ejections move straight out of the Sun and flatten everything in their path, like a gigantic falling tree or a car out of control. They energize particles on a grand scale that covers large regions in interplanetary space.
The crucial information is how strong the storm is and if and when it is going to hit us. The exact warning time will depend on the type of solar hazard, since they travel with different velocities and on various trajectories in space (Fig. 8.13). Intense radiation from powerful solar flares moves from the Sun to the Earth in just 8 minutes, traveling at the speed of light. Energetic particles, accelerated during the flare process or by the shock waves of coronal mass ejections, can reach the Earth within an hour or less (for energies above 10 MeV). A coronal mass ejection arrives at the Earth as a dense cloud of magnetic fields, electrons and protons one to four days after leaving the Sun.
Satelites in danger
Energetic charged particles from solar explosions can seriously damage satellites. When an energetic flaring proton, above 10 MeV in energy, strikes a spacecraft, it can destroy its electronic components. Metal shielding and radiation-hardened computer chips are used to guard against this persistent, ever-present threat to satellites, but nothing can be done to shield solar cells. Since they use sunlight to power spacecraft, solar cells must be exposed to space. Energetic solar protons scour their surface and shorten their lives. They have destroyed the solar cells on at a least one weather satellite.
Increases in the dynamic pressure of the Sun’s winds during solar activity compresses the magnetosphere and puts high-flying satellites at risk. When a coronal mass ejection slams into the Earth, the force of impact can push the bow shock, at the day side of the magnetosphere, down to half its usual distance of about 10 times Earth’s radius. Geostationary spacecraft, that stay over the same spot on Earth, orbit our planet at about 6.6 Earth radii, moving around it once every 24 hours or at the same rate that the planet spins. When the magnetosphere is compressed below their geosynchronous orbits, these satellites are exposed to the full brunt of the gusty solar wind and its charged, energized ingredients.
Turning off the lights
During an intense geomagnetic storm, associated with a colliding coronal mass ejection, strong electric currents flow in the auroral ionosphere. They induce potential differences in the ground beneath and produce strong currents in any long conductor such as a power line (Fig. 8.14). Up to 100 Amperes of Direct Current, or DC, surge through long-distance power lines designed to carry Alternating Current, or AC, blowing circuit breakers, overheating and melting the windings of transformers, and causing massive failures of electrical distribution systems.
A coronal mass ejection can thereby plunge major urban centers, like New York City or Montreal, into complete darkness, causing social chaos and threatening safety. It is capable of permanently damaging multi-million dollar equipment in power generation plants, and producing hundreds of millions of dollars in losses from unserved power demand or disruption of factories. The threat is greatest in high-latitude regions where the auroral currents are strongest, such as Canada, the northern United States and Scandinavia. In fact, one great magnetic storm in March 1989 put the entire Quebec electric power system out of operation, turning off the lights in a large part of the area for 9 hours.
Forecasting space weather
Space weather is here to stay, and the dangers blowing in the Sun’s winds are not going away. In tens of minutes, intense explosions hurl out energetic particles that can endanger humans in space and destroy satellites. Forceful solar mass ejections can also damage or destroy Earth-orbiting satellites, and create power surges that can blackout entire cities. Recognizing our vulnerability, government agencies post forecasts that warn of threatening solar activity.
The Space Environment Center (SEC) of the National Oceanic and Atmospheric Administration collects and distributes the relevant data, using satellites and ground-based telescopes to monitor the Sun and relay information about conditions in interplanetary space. Its Geostationary Operational Environmental Satellites, or GOES for short, monitor threatening activity as it nears the Earth, including the powerful X-ray emission of solar flares (Fig. 8.15), and high-speed electrons and protons.
Predictions about space weather events, based on data from the SEC and NASA satellites are given at
Solar astronomers are now looking back at the source of it all, developing methods of predicting solar explosions based on the Sun’s magnetic contortions or the growth of active regions on the invisible back side of the Earth. Space scientists are extending this effort, studying the vital links and dynamic interplay between the Sun and the Earth and viewing them as an interconnected whole. A variety of spacecraft are making coordinated, simultaneous measurements of the Sun, the solar wind, and the Earth’s magnetosphere, providing a new global perspective of the intricate coupling between the Sun and Earth under the auspices of an International Solar Terrestrial Physics (ISTP) program. For the first time ever, we can now track every move of possibly destructive events from their beginning on the Sun, to their passage through space, and their ending impact on Earth (Fig. 8.17).