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Solar Maximum

12 X-ray images of the Sun's atmospher
This image is comprised of 12 X-ray images of the Sun's atmosphere taken between 1991 and 1995 in 120-day increments. This composite image clearly demonstrates how the Sun changes during the solar cycle.
Scientists became aware that the Sun went through cycles and changes by observing sunspots, the darker, relatively cooler areas of the Sun. The number of sunspots can be an indication of the degree of solar activity. The average number of visible sunspots varies over time, increasing and decreasing on a regular cycle of between 9.5 to 11 years, on average about 10.8 years. An amateur astronomer, Heinrich Schwabe, was the first to note this cycle in 1843. The part of the cycle with low sunspot activity is referred to as "solar minimum," the portion with high activity is known as "solar maximum." The year 2000, was the last solar maximum for the current solar cycle.

Solar Flares and CMEs

While sunspots have been used historically to indicate levels of solar activity, other solar features increase in number and intensity along with the fluctuations in the Sun's magnetic field structure. Coronal mass ejections (CMEs) and emissions of high-energy solar flares become more common and more intense during the period of solar maximum. This increase in solar activity can affect us here on Earth (and in Earth's orbit) by what is known as space weather.

Earth's magnetosphere
This NASA illustration shows the Earth's magnetosphere and its interaction with the Sun. (Please note this illustration is not to scale.)
Effects on Earth

The Earth's own magnetic field protects it from most of the direct effects of the Sun's particle emissions, including the solar wind. But during periods of intense solar activity geomagnetic storms can produce heightened, spectacular displays of the Aurora Borealis and Aurora Australis, known as the Northern and Southern Lights. Geomagnetic storms can also disrupt radio transmissions and affect power grids. Energetic electromagnetic bombardments can interfere with the transmission of radio waves and the flow of electric current in wires. Radio operators are familiar with solar maximum and have to deal with an increase in static on the airwaves. Occasionally, radio signals can be completely drowned out by interference from solar radio noise. Power grids can be overloaded by these same bombardments. In 1989, during the last solar maximum, the power grid that supplies Canada's Quebec province was knocked out by a geomagnetic storm.

Satellite Disruption

Satellite operators may have trouble locating and communicating with spacecraft through the screen of the disturbed ionosphere. Users of GPS (Global Positioning Satellites) and other satellite-based navigation aids may find the errors in their position estimates significantly increased. Since satellites are outside the protection of the Earth's atmosphere, they are particularly vulnerable to the severe geomagnetic storms that can result from solar activity. Satellites can be damaged by exposure to the increased electromagnetic radiation from the active Sun, as well as energetic particle radiation from either solar activity sources (flares or CMEs) or the enhanced Van Allen belts resulting from geomagnetic storms. Their surfaces can also experience damaging discharges.

Satellite drag
"Satellite drag" caused by the previous solar max, helped bring down Skylab early.

In addition to these polarity changes which can damage sensitive electronics in satellites, the increased solar emission also causes the Earth's atmosphere to "puff out," creating increased drag on orbiting satellites. This increased drag can cause satellite orbits to decay more rapidly than predicted. The 100+ ton Skylab station is a good example. Launched in 1973, the station was supposed to remain in orbit until the 1980s. The purpose of Skylab was, among other things, to study the Sun. Ironically, due to increased solar activity, Skylab re-entered Earth's atmosphere in 1979—raining debris over the Indian Ocean and parts of Western Australia.

Solar Max 2000-2001 and Future Maxima

Scientists still don't completely understand all of the aspects of the solar cycle and it's difficult to predict just how strong each solar maximum will be. The 22 solar cycles studied since the one Heinrich Schwabe noted in 1843 have varied in intensity. This current solar cycle, #23, has been the most closely observed ever. There are nearly one dozen space-based observatories watching the Sun during this solar cycle.

globe icon Find out more about Solar Maximum at the Solar Max 2000 Web site.

Story courtesy of The Exploratorium ©2002 -
Images courtesy of NASA.

Related to chapter 6 in the print guide.
Related Materials

For an overview of how the Sun effects the Earth see the Sun-Earth Connection.

For a more on the history of human observation of sunspots, see the Galileo Debate.

See video of a Coronal Mass Ejection taken from the SOHO spacecraft.

Glossary Terms

Click for the definitions of the following words that are used on this page: (Definitions appear in a pop-up window.)

coronal mass ejection
geomagnetic field
geomagnetic storm
solar cycle
solar maximum
solar minimum

View the full, printable version of the glossary.

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