A ROARAing Through the Night

The daytime sky is clear. The sun shines, calm and steady - we walk our planet without heeding the star so close to us.

But the sun isn’t the constant, placid object we assume it is; it is a seething, erupting nuclear furnace of unimaginable power. And we are up-close. And aurora dance because the sun’s energy breathes against our planet’s magnetic field.

The Sun

The sun cycles through an 11 year-long period of low to high activity and back to low activity that takes about 11 years. Sunspot activity increases from a ‘solar minimum’ to a ‘solar maximum’. We know the sun is active when we see bright, frequent auroral displays.

Sunspots

Take the opportunity to look at the sun through a properly filtered telescope (see Solar Safety article). What looks like the surface of the sun is a layer called the PHOTOSPHERE.and peppered on that surface are dark SUNSPOTS.

The sunspots appear dark because they are cooler (3800 Kelvin) than the photosphere (5800 Kelvin).

The dark inner area is the ‘umbra’. The lighter ‘penumbra’ spreads its threads out into the surrounding ‘surface’

Sunspots look tiny in a telescope but, don’t let what you see fool you, the spots are huge. You could probably drop the earth (with a diameter of 12 756 km) into one of those black specks - a really large sunspot can grow to 50,000 km across.

Sunspots reveal areas where the sun’s magnetic field twists and arches out of the photosphere. The energy locked in these magnetic loops can slowly flow back into the sun or the energy may explode into a flare or a CORONAL MASS EJECTION.

Solar Wind

The thin ‘chromosphere’ overlays the photosphere and above that is the sun’s "atmosphere", the CORONA. The corona is HOT - over a million degrees Kelvin - and it’s rays stream millions of kilometres into space. At this high temperature electrons and protons break free of each other in an "electrified" gas called a PLASMA. The plasma flows away from the sun as a wind of charged particles - the SOLAR WIND.

It takes the solar wind 2 to 6 days to reach the earth - it travels along at a leisurely 450 km/s.

Coronal Mass Ejection

A coronal mass ejection is an unbelievably huge bubble of high-energy plasma blasted away from the sun’s active sunspot regions. The expanding bubble of charged particles speeds along at 1300 to 1800 km/s into space

When one of these explosions comes from a sunspot facing the earth we have a very good chance of seeing a spectacular auroral display.

After traveling 2-5 days through space, the plasma reaches the earth's magnetic field. The earth's magnetic field blocks the solar wind and protects us from the solar particles.

The Earth and the Magnetosphere

The solar particles stream around earth’s magnetic field and shape the magnetic field into a tear-drop shaped cavity called the MAGNETOSPHERE. The narrow end of the magnetosphere stretches out into earth’s night side. This narrow end is the MAGNETOTAIL. A very conductive plasma sheet divides the magnetotail in two.

The solar wind and earth’s magnetic field interact on the magnetosphere’s surface. Here the solar wind flowing across the magnetosphere induces currents in the magnetosphere.

A large coronal mass ejection strengthens the solar wind that lengthens the night side magnetotail. The tail’s stretched magnetic field lines pinch together and "short-circuit" about 60 000 km away on earth’s night side. The magnetic field lines from both sides of the plasma sheet now conduct particles and act as a generator that eventually powers the AURORA with up to ten million megawatts of electrical power.

Adjacent magnetic field lines with opposite polarities connect inside the plasma sheet. Intense electric fields created by this magnetic reconnection launch plasma toward the Earth's north and south magnetic poles where it strikes the IONOSPHERE.

Ionosphere

The outer atmosphere contains a thin and partly ionized layer called the ionosphere.

Here the thin gases are composed of ionized particles and act as an electrical conductor. It is here that the energy from the magnetosphere takes a quantum leap to life as the aurora in a glowing ring called the AURORAL OVAL. An auroral oval crowns both the north and south poles. The night time half of the oval is broader and extends farther away from the poles.

Aurora

If you hit an atom with enough energy, one or more electrons will jump to an orbit a little farther away from the nucleus. The electron now carries that energy you hit it with as ‘potential energy’. However, the new orbit is unstable and the electron wants to drop back to its original orbit. The electron releases that stored energy as light when it drops back.

Billions of energetic particles from the magnetosphere hit billions of atoms in the ionosphere. Billions of quantum leaps happen and billions of times electrons drop back and a package of light - a photon - shines. We see the aurora dance above.

Aurora spread across the sky above us as the electrical current from the magnetosphere surges. The lights dance in the changing flow of the current generated by the magnetosphere.

Sometimes it seems as if you could reach and touch the aurora, but not even a jet plane flies high enough. The aurora start to live about 100 km up and they can extend all the way out to over 400 km. Would the Space Shuttle fly through them?

People in the southern United States can see aurora if the sun sends a really energetic coronal mass ejection towards earth. The oval can grow 600 km wide in its night time portion. Most of the time, however, only people who live in the north see aurora.

The location of auroras on Earth is strongly controlled by the Earth's magnetism.

People are fascinated with the flowing dance and changing shape of aurora. Long arcs and curtains sweep overhead, soft glows change to spectacular light shows and fade away. Magnetic substorms send wave after wave of aurora traveling away from the poles.

Scientists recognize several shape categories: arcs can be homogenous or rayed; arcs or spots can pulse, rays or ‘flames’ can shoot spaceward; the aurora can be a glowing patch or a vast glowing swath across the sky.

We usually see green aurora. However, an intense storm can come alive with colour and beautiful reds, blues and purples billow above. Each kind of gas in our atmospheric glows with a specific color and the colour is also affected by the energy of the particle hitting the atmosphere’s atoms and molecules. And each colour lives at a different altitude.

Oxygen atoms glow green at lower altitudes. Hit an oxygen atom at the top of the aurora with more energy and the atom will glow red.

Nitrogen glows with colours that range from a deep red through blue and ultraviolet. The purple-red nitrogen appears a lower altitude rippling along the lower edge of a display.

The colours blend and ripple- the aurora show off to any sleepless observers.

Observations

Most of the RASCal observing gang were sensibly tucked snugly into their dreams on the morning of August 12th but a couple of guys felt they could try to catch a falling star and headed out to watch the Perseid meteor shower. They didn’t spend too much time watching meteors streak overhead ...

Eric didn’t even try to sleep - a hint of an auroral display drew him out to a dark beach where water lapped at his feet and aurora lapped at his head. He was out there taking pictures while David was snoozing.

See Astrophotographer goes into the night - by Eric Schandall

David almost didn’t make it. He’s a dedicated napper but he keeps forgetting to set his alarm clock. It’s a good thing he found some film. See Another astrophotographer follows ... - by David Lee

You can see today's auroral forecast at the University of Alaska's Geophysical Institute

You can watch the sun’s corona at The Solar and Heliospheric Observatory

More Weblinks on Aurora

Thursday's Classroom -- lesson plans and educational activities from NASA

Newton's Apple: Teacher's Guides

EXPLORATORIUM Auroras: Paintings in the Sky

EXPLORATORIUM Auroras: Links

The Extraordinary Geomagnetic Perseid Meteor Shower

NINE PLANETS The Sun

SIDC Brussels World Data Centre for the Sunspot Index

Solar saga

Space Weather: Basic Facts About Space Weather

Space Weather More Basic Facts

Space Weather Storm Display

ALASKA GI Space Physics and Aeronomy Home Page

Alaska Science Explained - Rocket Scientist, Neal Brown, explains the Aurora, rockets and more!

ALASKA Aurora's Northern Nights

NASA Space Radiation Storm

NASA Earth's Aurora - The Second Step of the Virtual Tour

NASA What is the Magnetosphere?

NASA The Exploration of the Earth's Magnetosphere"

NASA Layers of the Earth's Atmosphere

NASA Goddard Space Flight Center Homepage

The Aurora Page

Aurora Borealis - the northern lights

Nordlys (Norway)

Glossary for Space Weather: A Research Perspective

On-Line Glossary of Solar-Terrestrial Terms

Solar Glossary

An Extended Glossary of Solar Terrestrial Terms

Timo Leponiemi's Aurora Website (Finland)