Geomagnetic Storms

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Although the gusty solar wind never reaches the Earth's surface, it can cause dramatic changes in the Earth's magnetic field. George Graham (1674-1751), a London watchmaker, first noted the magnetic variation in 1722 as a pronounced swing and rapid fluctuation in the direction that compass needles point. Graham and the Swedish astronomer and physicist Anders Celsius (1701-1744) independently noticed in 1741 that irregular deflections of the compass needle occur when intense auroras are present, tten in the mid-nineteenth century the German explorer and naturalist Baron Alexander von Humboldt (1769-1859) measured variations in the magnetic field during his global trips, giving them the name magnetic storms.

Nowadays we often attach the appellation "geo" to the name, using the term geomagnetic storms to stress their Earthly nature. Unlike localized stormy weather on land or sea, a geomagnetic storm is invisible and silent, undetectable by the human eye or ear, but like our daily weather, geomagnetic storms can sometimes have devastating effects.

tte intense geomagnetic storms vary in tandem with the 11-year sunspot cycle. Already in 1852, Colonel Edward Sabine (1788-1883), superintendent of four of the magnetic observatories in the British colonies, was able to show that global magnetic fluctuations are synchronized with this cycle. In response to a letter from the astronomer John Herschel (1792-1871), that called attention to Samuel Heinrich Schwabe's (1789-1875) discovery of the sunspot cycle, Sabine wrote:

With reference to Schwabe's period of 10 years having a minimum in 1843 and a maximum in 1848, it happens that by a most curious coincidence (if it be nothing more than a coincidence) that in a paper now waiting to be read at the Royal Society, I trace the very same years as those minimum and maximum of an apparent periodical inequality which took place in the frequency and magnitude of the [terrestrial] magnetic disturbances and in the magnitude of the mean monthly range of each of the 3 magnetic elements shown concurrently in the two hemispheres.37

Earlyin the 20th century, the Norwegian physicist Kristian Birkeland (1867-1917) argued that beams of electrons are sent from the Sun to the Earth, causing both auroras and geomagnetic storms. But it was soon realized that the electrical repulsion between the electrons would cause such a solar electron beam to disperse into space before reaching Earth. So in the 1930s, the English geophysicist Sydney Chapman (1889-1970) and his young colleague Vincent Ferraro (1907-1974) reasoned that geomagnetic storms are caused when an electrically neutral plasma cloud is ejected from the Sun and envelops the Earth, generating currents in the magnetosphere and distorting its magnetic field.

We now know that Chapman and Ferraro were close to solving the mystery of geomagnetic storms, tte most intense ones are associated with coronal mass ejections, great magnetized bubbles of plasma hurled from the Sun, which can generate currents in the Earth's magnetosphere and energize charged particles in it (Fig. 8.11).

If averaged over both a yearly and global scale, intense geomagnetic storms do vary in step with the sunspot cycle. When the Sun shows more spots, the terrestrial magnetic field is more frequently disturbed by violent storms. But it is not the sun-spots themselves that bring about the changes on the Earth. Great magnetic storms are

FIG. 8.11 Interplanetary CME shocks As it moves away from the Sun (top left:) a fast coronal mass ejection (CME, top right) pushes an interplanetary shock wave before it, amplifying the solar wind speed, V, and magnetic field strength, B (bottom). The CME produces a speed increase all the way to the shock front, where the wind's motion then slows down precipitously to its steady, unperturbed speed. Compression, resulting from the relative motion between the fast CME and its surroundings, produces strong magnetic fields in a broad region extending sunward from the shock. The strong magnetic fields and high flow speeds commonly associated with interplanetary disturbances driven by fast CMEs are what make such events effective in stimulating geomagnetic activity.

FIG. 8.11 Interplanetary CME shocks As it moves away from the Sun (top left:) a fast coronal mass ejection (CME, top right) pushes an interplanetary shock wave before it, amplifying the solar wind speed, V, and magnetic field strength, B (bottom). The CME produces a speed increase all the way to the shock front, where the wind's motion then slows down precipitously to its steady, unperturbed speed. Compression, resulting from the relative motion between the fast CME and its surroundings, produces strong magnetic fields in a broad region extending sunward from the shock. The strong magnetic fields and high flow speeds commonly associated with interplanetary disturbances driven by fast CMEs are what make such events effective in stimulating geomagnetic activity.

caused by coronal mass ejections that occur more often when the Sun is more spotted and active.

tte consequences of an exceptionally intense magnetic storm are truly awesome. Several times every solar cycle, a solar eruption of extraordinary energy creates a brief, violent gust in the solar wind that sets the entire magnetosphere reverberating with catastrophic impact. When the high-velocity shocks arrive at the Earth, followed by the magnetic fields, they can compress the dayside magnetosphere down to half its normal size confusing geomagnetic navigational and detection sensors in satellites and disorienting homing pigeons and other migratory animals that depend on the Earth's magnetic field for guidance. High rates of connection between the solar-wind magnetic field and the terrestrial one increase the size of the magnetotail that connects to the poles, and as a result the aurora oval intensifies and spreads eerily beautiful auroras across the sky to tropical latitudes in both hemispheres.

Shortly after the turn of the twentieth century, E. Walter Maunder (1851-1928), a sunspot expert from the Royal Observatory at Greenwich, England, found that many moderate geomagnetic storms occur at intervals of 27 days, corresponding to the apparent rotation period of the Sun as viewed from the Earth. He concluded that this recurrence could be explained:

by supposing that the Earth has encountered, time after time, a definite stream, a stream which, continually supplied from one and the same area of the Sun's surface, appears to us, at our distance, to be rotating with the same speed as the area from which it rises.38

As emphasized by the German scientist Julius Bartels (1899-1964), these recurrent geomagnetic storms sometimes occur when there are no visible sunspots. In 1932 Bartels referred to the sources of geomagnetic storms as M regions and noted that while M regions were sometimes associated with sunspots, in many other cases they seemed to have no visible counterpart, tte M probably stood for magnetic, but it might have denoted mysterious; for several decades the elusive source of the moderate, recurrent geomagnetic storms remained one of the great-unsolved mysteries in solar physics.

tte recurrent activity is linked to long-lived, high-speed streams in the solar wind that emanate from coronal holes and periodically sweep past the Earth. 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. Ms produces shock waves and intense magnetic fields that rotate with the Sun, 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.

ttus, geomagnetic activity shows two components, both connected with the Sun. One, associated with intense geomagnetic storms and coronal mass ejections, varies in tandem with the sunspot cycle of solar magnetic activity, tte other, related to moderate geomagnetic storms and recurrent high-speed solar wind streams, is out of phase with the solar cycle, dominating geomagnetic storms when the solar activity cycle is at a minimum.

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Renewable Energy 101

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