DAWN OF THE NORTH
The terms “Aurora Borealis” and “Aurora Australis” meaning “dawn of the north” or “south” are believed to have
been coined by Galileo Galilei after watching them in fascination. For thousands of years, these colored curtains
of shimmering, shifting light have been a source of stories and superstition. They foretold omens of destruction
and prompted myths to be created. Later, scientific theories were developed displacing mythical ideas of auroral
creation, like those saying it was reflected sunlight off the arctic ice. We know a lot more about the aurorae
today, but still have a lot of questions. Scientists, like those involved in the FAST satellite program, are gathering
key pieces of the aurora “puzzle” and new pictures are beginning to emerge.
Aurorae are related to disturbances on the Sun which increase the number and speed of ions and electrons in the
solar wind. The solar wind carries with it the Sun’s magnetic field, which may connect and join with the Earth’s
magnetosphere. The magnetosphere is a large magnetic envelope surrounding the Earth and its atmosphere.
Electric fields and currents result from the Sun and Earth magnetic fields joining and from the solar wind flow
around the Earth’s magnetosphere. These electric fields accelerate solar wind particles to high energies where
they enter the Earth’s ionosphere. The ionosphere is an area in the uppermost fringes of the Earth’s atmosphere
that contains free electrically charged particles. Some of the solar wind particles are guided down to the Earth’s
polar regions by its magnetic field. These magnetic field lines encircle the Earth’s poles, forming concentric
rings of similar magnetic field stregnth. The particles interact with the ionosphere along these rings producing
an auroral oval around the Earth’s magnetic poles (see upper right inset on front). The glow we see as shimmering
curtains of light is caused by processes associatied with the collisions of these particles with the atoms and
molecules in the ionosphere.
Scientists have known for a long time that there are some regions above the poles where electrons flow downward
through the ionosphere towards the poles with high energy. This is where the “visible” aurora can be seen. FAST
has found that there are some regions where electrons move up away from the Earth. This is where the “inverse”
aurora occur. Inverse aurora cannot be seen. What is interesting to scientists in this region is its connection
to the visible aurora and the fact that positively charged particles are heated.
Auroral displays are quite eerie and beautiful. From the ground, an aurora can appear as “discrete” or “diffuse”
bands or areas of light. Discrete aurora may appear as a stable east-west arc, or a distinct curve of light that
trails away toward the horizon (see lower right inset on front), while diffuse aurora may be evident as hazy light
covering the entire sky. Viewed from space, however, the aurora rings the Earth with a fiery crown. |
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 The FAST satellite is one of many tools that scientists are
using to investigate the aurora and plasma processes occurring in the region where aurora producing particles are
accelerated. Airborne observatories flying below, sounding rockets passing through, and satellites like FAST and
POLAR, orbiting above the auroral zones help give a more complete picture of the complex processes which create
the auroral displays.
 The visual image of the
aurora in the photograph at left was taken with an all-sky camera from a high-altitude airplane. This photograph
is aligned with data taken from the FAST satellite. Superimposed on the visual aurora is a projected path of the
FAST satellite which simultaneously collected electron measurements in space above the visible aurora. The electron
data from FAST in the bottom plot shows the number and energies of electrons that are streaming down the Earth’s
magnetic field. It is these particles that create the aurora. Notice that the peaks in the electron measurement
data correspond to the visible auroral arcs.
The FAST satellite was designed to investigate the plasma physics of auroral phenomena in extremely minute detail.
Data from FAST is collected by “smart” onboard processors that automatically turn on when auroral acceleration
phenomena are encountered. Thus, FAST has been able to collect measurement unattainable by sounding rockets and
other satellites |
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FOR THE CLASSROOM
At right is an analogy which can be used in the classroom to explain some of the auroral formation mechanisms.
A more detailed versions of of this analogy, lesson plans, and educational resource materials associated with the
aurora and FAST can be found at the fast Education and Outreach WebSite at UC Berkeley:
http://cse.ssl.berkeley.edu/fast_epo/ |
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| TELEVISION IN THE SKY |
| A CATHODE RAY TUBE (CRT) ANALOGY |
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Viewing the aurora is like looking at a giant television screen in the sky. The images produced
on the screen of a television or computer monitor are formed in a manner similar to the way aurora are formed,
except that the television or computer cathode ray tube (CRT) which produces the aurora on our atmospheric screen
extends 93 million miles (150 million km) to the Sun’s electron gun. Here’s how it works!
Let’s start with the Sun. The Sun acts like an electron gun and boils off electrons and ions. These particles travel
with the Sun’s magnetic field into space by the solar wind (see section A in diagram at right). Similarly, the
cathode in a CRT acts like the Sun’s surface boiling off electrons which are attracted to the positive anode to
form an electron beam in the CRT’s electron gun. |
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Most solar wind particles which encounter the Earth’s magnetosphere are deflected around the
Earth back into space. Some penetrate it and travel into the Earth’s magnetic and electric fields. While in these
fields, the particle paths are bent and aimed toward the Earth’s polar regions (see section B in diagram). Similarly,
in a CRT, the electron beam emerging from the electron gun passes through an electric and magnetic modulating system
which bends and aims it to the front screen of the television or computer monitor.
The CRT’s electron beam collides with colored phosphorescent dots on its screen, which when added together produce
the colors you see (see section C in diagram). Similarly, accelerated solar wind particles traveling down the Earth’s
magnetic field lines collide with ionospheric molecules and atoms. The amount of energy and the type of atom or
molecule hit creates the auroral light on our atmospheric screen in the sky.
ANALOGY DEMONSTRATIONS
1) Using a cathode ray tube, have students vary the voltage and orientation of the plates at the back of the tube
and watch the trace of the electron beam change on the screen. Have students discuss how this is like the aurora.
2) Working in cooperative learning groups, have students cover a strong bar magnet with a piece of white paper.
Sprinkle iron filings onto the paper. Have students observe the pattern made by the iron filings. Have students
draw their observations and compare them with a diagram of Earth’s magnetosphere. |
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