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Weather Phenomenon and Elements

Space Weather

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.

Earth-Space Environment
Earth-Space Environment: Art Work Courtesy of NASA

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).

Earth-Sun
Earth-Sun Environment:
Figure Courtesy of US Goddard Space Center
As it passes the Earth, the solar wind interacts with and distorts the Earth's magnetic field. The wind compresses the field in toward the Earth on the sunward side and stretches it out in the anti-sun direction. This gives the Earth's magnetic field a shape similar to a comet and its tail. Indeed, the presence of the solar wind can be seen by observing the tails of comets, which always point away from the Sun. When the comet's particles caught are in the stream of the outgoing solar wind, they are pushed outward into the solar system and beyond.

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.

Sunspots
Sunspots (16 Jan 2000):
Photo Courtesy of SEC/NOAA/US Dept of Commerce
Sunspots appear as dark areas on the solar surface and are, in fact, transient, concentrated magnetic fields. (On the accompanying photo they appear as white spots on the orange solar disk.) They appear dark only in relation to the brightness and heat of the sun around them, their temperature some 2000 Kelvin degrees cooler than the surrounding surface. The dark area at their center, called the umbra, is where the magnetic field is strongest. Sunspots are the most prominent visible features on the Sun, forming and dissipating over periods of days or weeks. And they are large, a moderate-sized sunspot is about as large as Earth. Sunspots rotate with the solar surface, taking about 27 days to make a complete rotation as seen from Earth. Sunspots may form as isolated spots or in groups of two or more. Sunspot pairs have opposing magnetic fields.

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.

Solar Mass Ejection
Solar Mass Ejection:
Photo Courtesy of SEC/NOAA/US Dept of Commerce
Sunspots form when strong magnetic fields push through the solar surface and dissipate over periods of days or weeks. If the complexity of the magnetic field is sufficiently large, the energy can be released in an explosive event known as a solar flare. Along with the production of electromagnetic radiation, the solar flare is associated with the ejection of great clouds of charged particles into the solar wind. Groups of sunspots, especially those with complex magnetic field configurations, are often the sites of sudden and violent releases of bubbles of gas and magnetic fields. These large events are called coronal mass ejections. One to four days after a large mass ejection occurs, the solar particles and magnetic fields reach the Earth, interacting with the Earth's magnetosphere.

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.

Aurora
Auroral From Space Photo Courtesy of SEC/NOAA/US Dept of Commerce
Some electrons in the solar wind become trapped in the Earth's magnetic field and are then accelerated toward the magnetic polar regions and down into the upper atmosphere. Here, high in the atmosphere, the solar wind produces the incredible beauty of the aurora borealis around the northern magnetic pole and the aurora australis around the southern pole. [For more on auroras, click here.]

Auroral Oval
Auroral Oval, Northern Hemisphere: Photo Courtesy of SEC/NOAA/US Dept of Commerce
Auroras form in an oval band centered at each magnetic pole. The auroral oval extends outward around 3000 km from the center during quieter solar periods. However, when the magnetosphere is highly disturbed by geomagnetic storms, the oval may extend much further from the magnetic pole and, during high periods of solar activity, may push as far equatorward as 35o to 40o. During an unusually large geomagnetic storm in 1909, an aurora was visible at Singapore located on the geomagnetic equator.

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.


Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
January 23, 2000, revised June 16, 2005


Space Weather © 2000, 2005, Keith C. Heidorn, PhD. All Rights Reserved.
Correspondence may be sent via email to: see@islandnet.com.

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