Transcript Auroras - MIT Haystack Observatory

Auroras
Lights in the Sky… Why??
Gallery of photos
Photo credit: Background photo and example photographs – Jan Curtis
http://www.geo.mtu.edu/weather/aurora/images/aurora/jan.curtis//
Auroras …
• Seem to happen near the poles
• Seem to happen after Solar Flares and
Coronal Mass Ejections (CMEs)
• Have many different characteristic
patterns which can indicate what is
happening
• Have amazed everyone, the world over,
for MANY years. Check out this video!
• So, we’ll look at what happens, and why?
What is an Aurora?
• Auroras are:
– Patterns of light in the night sky usually seen
in northern latitudes.
– Related to Sunspot activity
– Related to Coronal Mass Ejections by the sun
• Auroras can:
– Look like curtains, ribbons, or point sources of
light emanating from a place near the celestial
poles.
What causes an Aurora
• A little Technical
background:
– Earth has a magnetic field
with “lines of force” , it’s
Magnetosphere, extending
(roughly) from North pole to
the South pole.
– The Sun creates a Solar
Wind of subatomic particles
(protons & electrons)
which, when it gets to the
earth, is diverted by the
earth’s magnetosphere.
“Cutting to the chase…”
So when do Auroras Occur?
• When there are CME’s or high sunspot activity:
– Solar wind pressure creates an electric voltage between the
magnetosphere and the earth’s poles, like the voltage between
the two terminals of a battery.
– It can reach about 10,000 volts!
– The voltage pushes electrons (with little mass) toward the
magnetic poles
– Accelerates the electrons to high speeds, much like the
electrons in a TV picture tube that accelerate to hit the screen.
– Electrons zoom along the field lines towards the ground to the
north and south,
– Huge numbers of electrons are pushed down into the upper
layer of the atmosphere, the ionosphere.
Finally, In the Ionosphere…
– The speeding electrons collide violently with
gas atoms.
– This gives the gas atoms energy.
– This causes them to release both light and
more electrons.
– So, the gases of the ionosphere glow and
conduct flowing electric currents into and out
of the polar region.
– Check out this video of an aurora viewed from
space.
What about the colors? Where do
the colors come from?
• An Analogy… Neon
signs glow different
colors due to the
concentration of neon
atoms inside the
glass tube
• Colors in the Auroral
display are formed by
high energy photons
hitting gas particles in
the upper atmosphere
Accelerated particles move along
the magnetic lines of force into the
poles energize gas particles
What do specific colors mean?
• Red Auroras are caused by
high-altitude oxygen in the
ionosphere
• Yellow-Green is caused by
Oxygen at lower altitudes.
(This is the most common and
brightest color of auroras)
• A light blue is caused by
ionized nitrogen molecules
• Neutral nitrogen glows in a
weak tint of red
• Nitrogen creates the purplishred lower borders and ripple
edges of an aurora.
When can I see
auroras?
•
•
•
Auroras are usually coupled with
Sunspot activity
Auroras will follow Coronal Mass
Ejections (CMEs)
So when you hear of Coronal Mass
Ejections, Auroras will probably follow
within a day or two.
A gallery of Auroras
Bands with ray structure (at left)
A gallery of Auroras
Yellow-Green (Oxygen at lower altitudes)
is the most prominent type of Aurora
A gallery of Auroras
Homogeneous Bands
A gallery of Auroras
Draperies – still yellow green (oxygen)
A gallery of Auroras
Comet Hale-Bopp (1997) with luminous glow near horizon