Sunspots, Aurora, and Magnetic Disturbances
||Aurora Borealis (Northern Lights)
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.
|Plot of Magnetic
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
Data from the past three hundred years provide a wealth
from which we can draw cause and effect relationships.
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
Only during periods of high auroral activity do people
in the warmer land areas well away from the poles begin to see
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?
- Have you ever seen the aurora? Where were you?
What were the main colors visible in the sky?
- Some people believe that small changes on the sun have caused
big changes in climate on the earth.
In very old trees,
analysis of the growth rings shows the same pattern as that of sunspots.
Do you think that the sun, 93 million miles away
and seemingly constant in its energy production,
could be responsible for some of the drastic climatic changes
in the Earth's history?
Aurora and Magnetic Disturbances
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
Scientists became interested in the connection between
sunspots and the Earth's magnetic 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.
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.
- The plot shows a regular pattern of highs and lows.
We call this behavior periodic.
Make a sketch in your Science Journal.
- Count the number of years that pass between any three
selected low points.
Find the average value or period.
Magnetic disturbances on the Earth for the past 120 years show a
Summarize this observation.
This figure shows the yearly sunspot number over the last two centuries.
- As in the plot of magnetic disturbances, this plot of sunspot
number shows a regular pattern of highs and lows.
Make a sketch in your Science Journal.
- What is the length of the period in this repeating
- Look at the year 1890 in the plot of magnetic disturbance
and in the sunspot plot.
Is there is a correlation between sunspot number and magnetic
variation at the Earth?
- Given the general pattern of sunspots, what year in
the future would you expect to have a peak?
- Summarize the correlation between magnetic disturbances at
the Earth and sunspots.
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.
- Sketch the plot. Record any observations that you may not
already have noted.
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 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
This is the subject of solar-terrestrial relations.
Sunspots are one of the signs of solar activity.
The white light image of the Sun is from the
Big Bear Solar Observatory
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,
We thank them for their assistance.
Written by: Hugh R. Anderson and
Tell us what you think about this page.
Mon July 12th 1999