Page 82 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

46 part I The energy–Atmosphere system
Solar Activity and Solar Wind
Telescopes and satellite images reveal solar activity to us in the form of sunspots and other surface distur- bances. The solar cycle is the periodic variation in the Sun’s activity and appearance over time. Since tele- scopes first allowed sunspot observation in the 1800s, scientists have used these solar surface features to de- fine the solar cycle. Solar observation has recently im- proved significantly through data collected by satellites and spacecraft, including NASA’s SDO (Solar Dynam- ics Observatory) and SOHO (Solar and Heliospheric Observatory) (Figure 2.2). (Find real-time SOHO im- ages at sohowww.nascom.nasa.gov; information on all space weather is at spaceweather.com/.)
Sunspots The Sun’s most conspicuous features are large sunspots, surface disturbances caused by magnetic storms. Sunspots appear as dark areas on the solar sur- face, ranging in diameter from 10000 to 50000 km, with some as large as 160000 km, more than 12 times Earth’s diameter.
A solar minimum is a period of years when few sun- spots are visible; a solar maximum is a period during which sunspots are numerous. The solar maximum peak was not reached in 2013, but greater sunspot activity oc- curred through 2014. Over the last 300 years, sunspot occurrences have cycled fairly regularly, averaging 11 years from maximum to maximum (Figure 2.2a). A mini- mum in 2008 and a forecasted maximum in 2013 roughly maintain the average. (For more on the sunspot cycle, see solarscience.msfc.nasa.gov/SunspotCycle.shtml.) Scientists have ruled out solar cycles as a cause for increasing tem- perature trends on Earth over the past few decades.
Activity on the Sun is highest during solar maxi- mum (Figure 2.2b). Solar flares, magnetic storms that cause surface explosions, and prominence eruptions, outbursts of gases arcing from the surface, often occur in active regions near sunspots (for videos and news about recent solar activity, go to www.nasa.gov/mission_ pages/sdo/news/solar-activity.html). Although much of the material from these eruptions is pulled back toward the Sun by gravity, some moves into space as part of the solar wind.
Solar Wind Effects The Sun constantly emits clouds of electrically charged particles (principally, hydro- gen nuclei and free electrons) that surge outward in all directions from the Sun’s surface. This stream of ener- getic material travels more slowly than light—at about
(a) Sunspot maximum in July 2000 and minimum in March 2009.
(b) Solar eruption, December 31, 2012.
▲Figure 2.2 Image of the Sun and sunspots. The prominence eruption rising into the sun’s corona was captured by nAsA’s Solar Dynamics Observatory. This relatively minor 2012 eruption was about 20 times the diameter of earth, shown for scale. earth is actually far smaller than the average sunspot. [(a) nAsA/SDO/steele hill, 2012.
(b) SOHO/eIT Consortium (nAsA and esA).]
50 million km a day—taking approximately 3 days to reach Earth. This phenomenon is the solar wind, origi- nating from the Sun’s extremely hot solar corona, or outer
Relative size of Earth
Georeport 2.2 Recent Solar Cycles
In recent sunspot cycles, a solar minimum occurred in 1976 and a solar maximum in 1979, with more than 100 sunspots visible. Another minimum was reached in 1986, and an extremely active solar maximum followed in 1990–1991, with more
than 200 sunspots visible at some time during the year. A sunspot minimum occurred in 1997, followed by an intense maximum of more than 200 in 2000–2001. The forecasted 2013 maximum was expected to have about 69 sunspots, the smallest maximum since 1906. The actual total for 2013 was 65, but the solar maximum prediction was updated to early 2014. The present solar cycle began in 2008 and carries the name Cycle 24.