Large, sporadic geomagnetic storms
We have known about significant variations in the Earthís 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 Sunís 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). 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 Earthís 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. 8.7). 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.
Forceful coronal mass ejections can generate exceptionally intense auroras when solar activity is at its peak. When their magnetic field are swept around and next to the Earthís magnetotail, the interaction can create an opening in the Earthís magnetic barrier, allowing solar wind particles and energy to pour into the plasma sheet at the center of the magnetotail. The energy gained during this process not only produces intense geomagnetic storms; it also accelerates the infiltrating solar wind particles and local particles already in the magnetosphere. At such times, the accelerated electrons hurtle along magnetic conduits connected to the upper atmosphere, or ionosphere, in both polar regions, generating spectacular auroras. Today spacecraft look down on the auroras from high above, showing them in their entirety (Fig. 8.8). They form an oval centered at each magnetic pole, resembling a fiery halo.
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 (Fig. 8.9), pulsating and flickering for hours to days.
The reason that auroras are usually located near the polar regions is that the Earthís magnetic fields guide energetic electrons there. The auroral lights form when high-speed electrons rain down along the Earthís 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 Earthís 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 Earthís 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 Earthís 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 astronautís eyelids, striking their retinas and making their eyeballs glow inside.
Once outside the Earthís 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. 8.12). 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 Sunís 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 Earthís 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. 8.13). 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 Sunís 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. 8.14), 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 Sunís 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 Earthís 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. 8.16).
The Sun is a magnetic variable star
Our lives depend on the Sunís continued presence and steady output. It illuminates our days, warms our world, and makes life on Earth possible.The total amount of the Sunís life-sustaining energy is called the ďsolar constantĒ, perhaps because no variations could be detected in it for a very long time. Yet, as reliable as the Sun appears, it is an inconstant companion. Its luminous output varies in tandem with the Sunís 11-year magnetic activity cycle.
Stable detectors placed aboard satellites above the Earthís atmosphere have been precisely monitoring the Sunís total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant (Fig. 8.17). It is almost always changing, in amounts of up to a few tenths of a percent and on time scales from 1 second to 20 years. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.
The Earthís varying Sun-layered atmosphere
Not only does the atmospheric pressure decrease as we go upward, the temperature of the air also changes, but it is not a simple fall-off. It falls and rises in two full cycles as we move off into space (Fig. 8.18).
The temperature decreases steadily with increasing height in the lowest region of our atmosphere, called the troposphere from the Greek tropo for turning. Visible sunlight passes harmlessly through this region to warm the ground below. The temperature above the ground tends to fall at higher altitudes where the air expands in the lower pressure and becomes cooler. The temperature increases at greater heights within the next atmospheric layer, named the stratosphere. The Sunís invisible ultraviolet radiation is largely absorbed in the stratosphere, where it warms the gas and helps make ozone.
The threat of dangerous and even lethal ultraviolet rays caused world-wide concern when it was discovered that everyday, man-made chemicals are punching a hole in the ozone layer (Fig. 8.19). The chemicals, called chlorofluorocarbons or CFCs for short, were therefore completely banned by international agreement in 1990. Still, the ozone layer is not expected to regain full strength until well into the latter half of the twenty-first century.
The mesosphere, from the Greek meso for intermediate, lies just above the stratosphere. The temperature declines rapidly with increasing height in the mesosphere, reaching the lowest levels in the entire atmosphere. The main reason for the decreasing temperatures is the falling ozone concentration and decreased absorption of solar ultraviolet.
The temperature then begins to rise again with altitude in the ionosphere, a permanent, spherical shell of electrons and ions, reaching temperatures that are hotter than the ground. The ionosphere is created and heated by absorbing the extreme ultraviolet and X-ray portions of the Sunís energy. This radiation tears electrons off the atoms and molecules in the upper atmosphere, thereby creating ions and free electrons that are not attached to atoms.
Solar X-rays and extreme ultraviolet radiation both produce and significantly alter the Earthís ionosphere. Their greater intensity near the maximum of the 11-year magnetic activity cycle produces increased ionization, greater heat, and expansion of our upper atmosphere. At a given height, the temperature, the density of free electrons, and the density of neutral, unionized atoms all rise and fall in synchronism with solar activity over its 11-year cycle (Fig. 8.20). This Sun-induced change in the content and structure of the ionosphere affects its ability to mirror radio waves.
The Sunís radiation and global warming
The land temperatures have been correlated with the length of the solar cycle. The yearly mean air temperature over land in the Northern Hemisphere has moved higher or lower, by about 0.2 degrees Centigrade, in close synchronism with the solar-cycle length during the past 130 years (Fig. 8.21). Short cycles are characteristic of greater solar activity that apparently warm our planet, while longer cycles signify decreased activity on the Sun and cooler times at the Earthís surface. These temperature variations might be attributed to solar-driven changes in cloud cover, caused by the Sunís 11-year modulation of the amount of cosmic rays reaching Earth.
At times of enhanced activity on the Sun, the solar wind is pumped up with intense magnetic fields that extend far out into interplanetary space, blocking more cosmic rays that would otherwise arrive at Earth. The resulting decrease in cosmic rays means that fewer energetic charged particles penetrate to the lower atmosphere where they may help produce clouds, particularly at higher latitudes where the shielding by Earthís magnetic field is less. The reduction in clouds, that reflect sunlight, would explain why the Earthís surface temperature gets hotter when the Sun is more active.
Many of the temperature changes on Earth during the first half of the 20th century could be directly related to brightening and dimming of the Sun. Solar variability provides a reasonable match to the detailed ups and downs of the temperature record during this period (Fig. 8.22).
To fully understand the temperature measurements, scientists have examined historical records of the variable brightness of the Sun and other stars. Their reconstruction of the varying solar irradiance of Earth (Fig. 8.23) show that the Sunís changing brightness dominated our climate for two centuries, from 1600 to 1800. Cooling by hazy emission from volcanoes next played an important role, but the Sun noticeably warmed the climate for another century, from 1870 to 1970. After that, heat-trapping gases apparently took control of our climate.
The Earth is now hotter than it has been any time during the previous 1,000 years (Fig. 8.24). Global warming by the greenhouse effect is probably responsible for this recent, unprecedented rise in temperature. Minor ingredients of the atmosphere, such as carbon dioxide and water vapor, absorb the groundís infrared radiation, holding it close to the planetís surface and elevating the temperature there. Methane and nitrous oxide also act as greenhouse gases, but they are less abundant than carbon dioxide and water vapor.
The continued accelerated burning of fossil fuels will someday cause great damage to the environment, so both the developing and industrial nations should now do more to stop it. The Sunís activity can nevertheless substantially enhance or moderate this warming, and there isnít very much we can do about the Sunís changing temperament except monitor it. Moreover, there may be relief on its way when the next ice age begins.
Cooling the Earth down
Spacecraft observations of the varying solar brightness over the past two decades indicate that it has varied by about 0.1 percent. The observed brightness and magnetic variations of stars, with masses and ages close to those of the Sun, indicate that more substantial variations of the Sunís luminosity are possible. They may be associated with dramatic changes in the Earthís climate on time scales of hundreds, thousands, and hundreds of thousands of years.
Profound Sun-driven transformations in climate are suggested by past solar activity recorded in sunspot observations, tree rings and ice cores. An example is the period from 1645 to 1715, now known as the Maunder Minimum, when sunspot activity dropped to unusually low levels and the world experienced one of the coldest periods of the Little Ice Age in Europe. Comparisons with the brightness variations of Sun-like stars indicate that the Sun was approximately 0.25 percent dimmer at the time of the Maunder Minimum then currently, and was therefore capable of explaining the estimated drop of about 0.5 degrees Celsius in global mean temperature.
The radiocarbon records confirm that the Maunder Minimum corresponded to a dramatic reduction in solar activity, and show that such are a fairly common aspect of the Sunís behavior. During the past two thousand years, the Sun has spent nearly a third of the time in a relatively inactive state (Fig. 8.25). Extended periods of solar inactivity must therefore be considered to be a permanent feature of the Sun, and can be expected to occur again in the future.
The changing Sun has been drastically altering the climate for thousands of years. The Little Ice Age (1400-1800), for example, overlaps the SpŲrer Minimum (1400-1530) and Maunder Minimum (1645-1715) in solar activity. During this long period of unusual cold weather, alpine glaciers expanded, the Thames River and the canals of Venice regularly froze over, and painters depicted unusually harsh winters in Europe (Fig. 8.26).
Further back in time, during the past one million years, our climate has been dominated by the recurrent ice ages, each lasting about 100 thousand years. At the height of each long ice age, the great polar ice sheets advance down to lower latitudes. These glaciations are punctuated every 100 thousand years or so by a relatively short interval of unusual warmth, called an interglacial, lasting 10 or 20 thousand years, when the glaciers retreat. We now live in such a warm interglacial interval, called the Holocene period, in which human civilization has flowered. Still, the die is cast for the next glaciation, and the ice will come again.
The rhythmic alteration of glacial and interglacial intervals is related to periodic alterations in the amount and distribution of sunlight received by Earth over tens of thousands of years. When less sunlight is received in far northern latitudes, the winter temperatures are milder there, but so too are summer temperatures. So, less polar ice then melts in the summer, and over time the winter snows are compressed into ice to make the glaciers grow.
Three astronomical cycles combine to alter the angles and distance at which sunlight strikes the far northern latitudes of Earth, triggering the ice ages. This explanation was fully developed by Milutin Milankovitch from 1920 to 1941, so the astronomical cycles are now sometimes called the Milankovitch cycles. They involve periodic wobbles in the Earthís rotation and changes in the tilt of its axis and the shape of its orbit, occurring over tens of thousands of years (Fig.8.27).
Cores extracted from the glacial ice in Greenland and Antarctica provide the longest natural archive of the Earthís past climate. They strongly support the idea that changes in the Earthís orbit and spin axis cause variations in the intensity and distribution of sunlight arriving at Earth, which in turn initiate natural climate changes and trigger the ebb and flow of glacial ice.
Air trapped in the polar ice cores indicates that the Antarctica air-temperature changes are associated with varying concentrations of atmospheric carbon dioxide and methane. The temperatures go up whenever the levels of carbon dioxide and methane do, and they decrease together as well (Fig. 8.28). Scientists cannot however, yet agree whether the increase in greenhouse gases preceded or followed the rising temperatures.
Still, we should not discount recent global warming. The concentrations of carbon dioxide and methane have now risen to unprecedented levels in our air, vastly exceeding those at any time during the past 420 thousand years. The warming produced by their greenhouse effect might counteract the cold of the next ice age.