The Northern Lights are
auroras. Auroras look like
curtains of light in the night sky around the north and south
magnetic poles. Sometimes these can be seen closer to the
equator in more populated areas. In the
northern hemisphere, they are often called the Northern Lights or
Aurora Borealis, in the
southern hemisphere they are known as
Aurora Australis or the
Southern Lights. They are caused by an interaction of the
Solar Wind and the
Earth's Magnetic Field.
The solar wind is a stream of
plasma (
electrons and
protons) which constantly flows from the Sun out past all of the
planets in the
solar system. When these charged
particles meet the Earth's magnetic field, it directs them to the north and south magnetic poles, just like the
electromagnets in a
television or
oscilloscope guide the beam of electrons across the screen. The particles are directed to areas known as the
auroral ovals. These
ovals are permanent features of our
upper atmosphere and constantly have
visible auroras. The
majority of us don't tend to see them though because of their location. The southern auroral oval usually resides over
Antarctica and the northern oval is frequently over areas like
Fairbanks and
Anchorage. The ovals are not uniform shapes and bulge on the side of the poles opposite to the Sun. This means that the larger areas of auroral activity occur at local
midnight which conveniently makes them easier to see. Relative to the Sun, these bulging areas stay put while the planet rotates beneath them.
When there is a
solar flare, the solar wind picks up, we get more plasma travelling at faster speeds. When this hits the magnetic field, the force of the extra particles causes the ovals to expand and spread out over a larger area, the lower limits getting closer to the
equator. The northern oval can at times spread as far down as
Europe or the southern
states of
America.
Electrons from the plasma enter our
atmosphere, strike
atoms or
molecules and pass on
energy, the energy is soon released again as
visible light in the form of a
photon. The
colour of the light released depends on what they hit and how much energy is passed on. Electrons hit
oxygen atoms above 200km and release red light, between 100 and 200 km they hit
nitrogen and release blue light. This also gives out a secondary electron which when it hits an oxygen atom gives out green light. Below 100km, nitrogen molecules glow crimson. The green light is easier to see than the reds which are in turn easier to see than the blue light. This means that to the observer, there seems to be a lot more green and red in the mix.
The colour is usually the first thing people notice about auroras, the second is the
patterns that form and shift. The plasma of the solar wind contains both
electric and magnetic fields which interact with the Earth's magnetic field. Because the solar wind is not a constatnt stream of plasma but
gusts like
wind here on
Earth, it causes ripples and distortions in the Earth's magnetic field. The colours are caused by particles of plasma and the particles are channelled to Earth by our magnetic field so when you see the auroral
arcs or "curtains", you're looking at the Earth's magnetic field lines in motion.
Curls form as small
kinks develop in straight auroral arcs. Huge
spirals that fill the sky form as auroral
currents twist the magnetic field lines. As the currents increase, auroral arcs brighten and begin twisting into the spiral form, when the current decreases, the spiral fades and unwinds again.
Many other types of movements are seen in a good auroral display. The movement of a charged particle creates a magnetic field around the particle itself. As vast numbers of these particles follow the Earth's magnetic field lines towards the upper atmosphere, their magnetic fields interact with each other and with the Earth's magnetic field, creating the complex effects that are often seen.
Because auroras are intensified by solar flares, we can to an extent predict their arrival. The
SOHO satellite constantly observes the Sun & sees solar flares coming. We get about 2-3 days warning before their effects are felt here on Earth. To receive email telling you when there's a flare headed our way, you can sign up to the
space weather mailing list at www.space-weather.com. you can also find out up to the minute scientific info on the Sun-Earth environment.
info gained from:
http://www.spaceweather.com
http://science.nasa.gov
http://www.pfrr.alaska.edu - the
Poker Flat Research Range