Particles from the sun produce the aurora. They do not simply
collide with Earth’s atmosphere to produce the glow, if
they did we would not see many aurorae because on their own the
solar wind particles are not energetic enough. Instead, the interaction
is more subtle and involves the combined magnetic fields of the
sun and the earth to trap and accelerate the particles.
The particles are mainly ionised hydrogen
(protons) and electrons plus a few helium and heavier ions. This
is the 'solar wind' and it flows continuously*.
coronal mass ejections (CMEs) and coronal holes that are directed
towards Earth substantially strengthen the solar wind
and these are the real aurora makers.
The wind is a ‘plasma’, it is an overall electrically
neutral mixture of positive and negatively charged particles.
Plasmas trap magnetic fields and the
solar wind carries not only particles to
earth but also the sun’s magnetism.
Earth’s magnetic field, generated in its rotating
and turbulent outer core of molten iron, is close to sea level
like that of a bar magnet. Further out the
impact of the solar wind and sun's magnetism distorts it.
The solar wind is supersonic and on Earth’s sunward side
it produces a shock wave, the bow shock, where it first impacts
Earth’s magnetic field and is abruptly slowed. Inside
the bow shock the now subsonic wind particles flow around the
earth’s magnetic field but the sun’s magnetism still
trapped in the plasma interacts strongly. It pushes the Earth’s
sunward field inwards and drags the night side out into a tail
more than a million kilometres long. The region dominated by
the sun’s magnetism is the magnetosheath. The inner region
where Earth’s magnetism rules is our magnetosphere. The
boundary is the magnetopause.
In the steady wind from a quiescent sun particles cannot get
inside the magnetosphere. A few leak in but that is all. Aurorae
need stronger magnetic interactions - reconnections.
When the sun’s trapped field points southwards** it is opposite in direction
to that of the sunward field of the earth. When strengthened by
a flare or CME it can interact strongly with Earth's field. The opposed fields
snap and reconnect to a lower energy state. The sun’s and earth’s
fields are now linked and there is a hole in the sunward side of the magnetosphere
- an open path through which solar particles can flow inside.
The joined sun-earth magnetic field is still trapped in the
moving solar wind plasma and it is swept with its particles onwards
to the earth’s night side and into the tail of the magnetosphere. There
the magnetic filed is increasingly stretched and the trapped particles fill a
long inner tail of the magnetosphere called the plasma mantle.
Another magnetic filed reconnection or rearrangement ultimately
produces the aurora. The long stretched field of the magnetotail
is unstable. The stretched field lines snap like elastic bands
and rearrange to form two regions, one controlled purely by the
earth’s field and one that is part of the solar wind again.
The earth's part contracts rapidly accelerating its trapped
particles towards us.
The final twist, or rather corkscrew, is that the trapped particles
follow spiral paths around the magnetic field lines as they accelerate
towards Earth. Some feed the equatorial radiation belts. Others
spiral along field lines joining the polar
ovals. These eventually plunge into the upper atmosphere to make
aurorae. However, because the particle clouds in the contracting fields
can equally go northwards or southwards along a field line connecting both ovals
(and some particles oscillate between the two), the resulting Aurora Borealis
and Aurora Australis are mirror images of each other.
average solar wind average speed in Earth's orbital plane
is about 400 km/s (one million mph). Variations
from 300 km/s up to 800 km/s occur and the speed variations
perturb the earths magnetosphere.
||Reconnections occur with
other solar magnetic field directions. However, a strong
southward solar field is best for them and subsequent aurorae.