If you are standing in Alaska, Canada,
or the Northern United States on a clear dark night and looking
up into the sky, you may see a bright greenish-white band of
light that stretches across the sky from the East to the West.
You are seeing the Northern Lights, also known as the Aurora
Borealis. These types of lights also occur near the South Pole,
where they are known as the Southern Lights and Aurora Australis.
On a lucky night, you may watch this auroral band continue to
brighten as it moves toward the South. Then, within minutes,
you may see the band of light break into many bands of light some
of which will move back overhead and to the North, dancing rapidly
and turning red, purple, and white. If you've been lucky to see
this sequence of events, you have witnessed the beginning phases
of an auroral substorm.
When we see these dancing auroral lights evolve suddenly from
a slowly moving auroral arc across the sky, we know that there
are two distinct processes occurring in Earth's magnetosphere.
But which one triggered the abrupt change from a calm aurora
to a dancing aurora?
Themis, the goddess of justice, wisdom and good counsel, the guardian of oaths
in Greek mythology, represents the THEMIS mission. She will confirm without prejudice,
as implied by her fame, one of the two competing theories for auroral eruptions.
THEMIS, with her sword (representing instruments) and scales (representing science
discoveries), has both power and impartiality.
| Using
an "All-Sky" Camera |
All-Sky cameras are special "movie" cameras
used to study the aurora. These cameras look up into the sky
and take black and white movies of the aurora from horizon
to horizon. A THEMIS All-Sky camera has captured this beginning
sequence of an auroral substorm. You can find out more about
All-Sky cameras in the Gallery
and Activities Page, where we show many different
movies of All-Sky cameras located in Canada and Alaska.
Below is a sequence of images that follow the beginning substorm phases: the
brightening of an auroral band and then the sudden splitting of the arc into
many forms of light dancing overhead, expanding towards both the North and the
South. All images have been provided by the University of Calgary.
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| A
slowly moving auroral arc is seen to the North (top of
image). |
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The
auroral arc brightens
suddenly and begins to move more rapidly. |
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| The
arc begins to split into many arcs which move toward the
South (toward the bottom of the image). |
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| The
arcs now move rapidly, continuing to move toward the South. |
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The
arcs change into
even smaller features and into auroral patches that pulsate on and off every
3
seconds or so. The aurora continues to move toward the South. |
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| Most
of the aurora has become patches that pulsate and no more
auroral arcs can be seen. |
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