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Extending out from the surface of the Sun to the Earth is a region of space known as the Sun-Earth environment or interplanetary medium. Although nearly a complete vacuum, the harsh Sun-Earth environment is dominated by strong electromagnetic radiation and fast-moving electrically charged particles ejected from the Sun. Changing conditions within the Sun-Earth environment are termed space weather.
Day after day, year after year, our sun flings out into space high-energy streamers of ionized gas, composed mainly of hydrogen nuclei and electrons, at the rate of about 1 million tons of matter every second. This ejected mass produces the main feature of space weather: the solar wind. This wind sweeps particles and their magnetic fields across interplanetary space toward Earth at supersonic speeds, ranging from 300 kilometres per second to over 1000 km/s (675,000 to 2,250,000 mph).
Variations in space weather result from the blowing and gusting of the solar wind, primarily from changes in the speed or density of the wind. When solar storms rage on Sol's surface, as they do during solar cycle maxima, they send out large and active bursts of particles (and associated magnetic fields) toward Earth. The main variable feature on the Sun's surface that causes solar storms is sunspot activity.
Over the last 300 years, the number of sunspots has generally waxed and waned regularly with an approximately 11-year cycle. The Sun, like Earth, has its four seasons but its "year" equals 11 of ours. The last sunspot maximum came in 1989 and the last minimum in 1995. The next peak will occur sometime in 2000 or early 2001.
The peak sunspot number within observed solar cycles has varied greatly. The greatest, annually-averaged daily sunspot number observed was 201 in 1957. Extended periods of very low sunspot numbers (an annual average of less than 10 sunspots per day) have also been observed. The most famous of these to solar scientists, a period known as the Maunder Minimum, lasted from 1645 to 1715. A similar period of low sunspot numbers, which lasted from 1460 to 1550, is known as the Spörer Minimum.
Daily sunspot numbers may very from zero to 355 or more. On the day I wrote this article, January 24, 2000, 133 sunspots dotted the face of the sun, and the solar wind was blowing with a velocity of 335.8 km/s (751,200 mph), a rather mild breeze. Some solar scientists predict that the cycle peak in 2000 will have an average daily number of sunspots of 160. If so, it will be the third greatest year in the historical record of observations which began in 1755.
The magnetosphere is a region of rarified, ionized gases caught in the Earth's magnetic field and located from 150 km to 70,000 km in altitude on the sunward side of earth and out to 300,000 km on the side of the planet away from the sun. A gusty, stormy solar wind will causes extraordinary variations in Earth's magnetic field, producing rapid changes in its direction and intensity. As this solar wind storm buffets the magnetosphere, and portions of its energy are transferred to the magnetosphere, a geomagnetic storm results on Earth.
When geomagnetic storms, natural hazards like hurricanes and tsunamis, are severe, they may disrupt local and global communications and abruptly increase drag on spacecraft and satellites, thus altering their orbits. The storms may also induce surges in electric power lines and cause equipment failures in a power grid. Such incidents may result in electric utility blackouts over a wide area. This happened on March 13, 1989 when 6 million customers of Quebec Hydro in Montreal were without commercial electric power for 9 hours. Some areas in the northeastern U.S. and Sweden also lost power. Geomagnetic storms can last several hours or even days, waxing and waning several times a day.
The Earth's magnetosphere acts as a protective barrier preventing energetic solar particles and radiation in the hot solar wind from reaching the planet's surface. Most of these energetic particles are deflected around the Earth as they stream by, like a stone diverts the water in a fast flowing stream. The presence of these deflection streams, known as the Van Allen belt, was discovered in the late 1950s by the first earth satellites.
With the high vulnerability of our orbiting satellites, space missions, the coming space stations and ground-based electric power grids to the magnetic and particle fluxes produced in geomagnetic storms, the US Government has established the Solar Environment Center within the National Oceanographic and Atmospheric Administration (NOAA). Its role is to monitor the sun, forecast its level of activity and warn of impending geomagnetic storms so that communications systems, space agencies, electric utilities and satellite operators can prepare for possible disruptions or equipment damage. If you wish to keep your eye on space weather, the Solar Environment Center has a website at www.sec.noaa.gov which provides information on solar activity, observations and forecasts. See also, the site of Dr. Sten Odenwald, an astronomer at the NASA-Goddard Space Flight Center, Space Weather at http://www.spaceweather.us/ which looks at impacts of space weather on us.
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