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Sunspots, Aurora, and Magnetic Disturbances

Active Sun
Aurora Borealis (Northern Lights)
(Note the
"Dark Spots"
or sunspots)
Plot of Magnetic Disturbances
What is the connection between the phenomena in these pictures and plots? Finding a detailed answer to this question has been one of the major scientific goals of the space program.

Sunspots

Using one of the earliest telescopes, Galileo recorded dark spots on the Sun in the seventeenth century. Chinese astronomers had also reported the dark spots some two centuries earlier. These sunspots have strong magnetic field and are cooler than the rest of the Sun's surface. Since before 1700, astronomers world-wide have accumulated sunspot numbers. Data from the past three hundred years provide a wealth of information from which we can draw cause and effect relationships.

The Aurora

The aurora borealis ("Northern Lights") and aurora australis ("Southern Lights") have been a source of myth and awe since ancient times. When sunspots are plentiful, the aurora become more visible from populated regions on the earth. There are always aurora in rings around the north and south magnetic poles. Only during periods of high auroral activity do people in the warmer land areas well away from the poles begin to see them. Then the Northern Lights are visible to much of the northern United States and Europe.

Astronomers in the 18th and 19th centuries made the association between sunspots and unusually large aurora. Satellite measurements in the last 30 years outside the earth's atmosphere allow us to understand the cause and effect relationship between sunspots and the aurora. Do high sunspot numbers cause other effects?

Science Journal


Aurora and Magnetic Disturbances

Earth's dipole field The Earth's magnetic field or geomagnetic field is similar to the field from a bar magnet. The magnetic poles are near, but not the same, as the geographic poles. The geomagnetic north pole is near the north end of Hudson Bay in Canada.
In the mid 1700's, when sensitive compass needles showed changes in direction of the magnetic lines of force, observers discovered a big aurora overhead means disturbances in the magnetic field. Scientists became interested in the connection between sunspots and the Earth's magnetic field.

At the Earth's surface, magnetometers measure the geomagnetic field magnitude and direction. This figure shows the AA Yearly Index, a way of quantifying the magnetic disturbances, over the last 120 years.

Science Journal


This figure shows the yearly sunspot number over the last two centuries.

Science Journal



This figure shows the magnetic disturbance index and sunspot number plotted as a function of time. The correlation is most clear when the plots can be superimposed.

Science Journal


The correlation between sunspots, aurora, and magnetic disturbances strongly suggests that changes in sunspot number somehow affect the Earth. But the visible light that comes from the sun is very constant and cannot produce magnetic variations. Besides visible light, what does the sun emit that could affect the Earth?

The launching of Explorer-1 into Earth orbit as part of the International Geophysical Year (IGY) in 1958 and the associated research began to answer the questions of how changes in sunspot number cause the aurora and affect the Earth's magnetic field. This is the subject of solar-terrestrial relations. Sunspots are one of the signs of solar activity.


Image Credits

The white light image of the Sun is from the Big Bear Solar Observatory Latest Images page (October 26, 1995).

Most of the other figures are from the National Geophysical Data Center. The National Oceanographic and Atmospheric Administration (NOAA) operates the National Geophysical Data Center (NGDC) in Boulder, Colorado. We thank them for their assistance.


Written by: Hugh R. Anderson and Kathee Terry

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Last Modified Mon July 12th 1999