Download Lecture notes 100805

A SCIENCE OF AURORA.
Lecture notes 100805
http://www.irf.se/~christer/
Christer Juren (IRF)
Preface.
The picture illustrates ---
--- passive experience will sometimes give nothing!
You have to play with aurora in order to understand it. Whistle at it and see if it
reacts! Look through different filters. Listen to it!
You need some methods to start with. They will later be corrected or changed
probably completely. After some time you have the basics?
Generally you have to be active, but it can be difficult in space physics!
Objectoriented what ever
From Norman Maclaren’s “Rytmics”
We need a structure for putting all the different parts together, methods, which
will make up an understanding of the aurora. We need components, complex or basic
building blocks. We need languages to bring them together and let them talk with
each other and at the end -- us..
Understanding Colours?
Interact and get some understanding of colours in generally or as in this case in
aurora!
Colours are interesting. Aurora has a lot of different colours, but it is enough with
four to get a real understanding of the physics and its behaviour in space and
time!
The colour are also interesting in the way that there are a lot of theories for it,
some close to physics some close to art. In our case we think about Göthe and his
scientific ideas in which he has a colour theory.
He talks about blackness and lightness as fundamental components. Conflict
between them results in colours!
It exists black aurora, but only in weak aurora. There is symmetry between black
and normal aurora.
The above reasoning’s are very common in the development of science. One don’t
know at the outset how to go! But one has to enjoy the research procedure.
Anyhow we can think of four fundamental colours in the aurora, blue, green, red
and red. With them we can mix and get almost all types of colours, even purpure,
which the rainbow doesn’t have.
Animation of the different timescales for the light emissions
.
In Aurora the main four colours have different timescales, which means that the light
will have specific duration depending of the kind of atom or, iono or molecule, which
send out the light. This is very important. It explains why the green colour can’t be
very detailed in time and in space. But the blue and one of the reds can.
Chaotic movements and global interactions
Water movements have chaotic behaviours, global interactions and little dissipation.
Dissipation means energy turning into heat. Dissipation is very uncommon in ocean
water waves. The same is the case for movements in the magnetosphere. After a hard
storm on the Baltic Sea there will be very high water waves, which will interact with
each other and make up a turbulent system, in which almost everything can happen,
giant waves for example. After the storm it will take some time for the waves to
disappear and the ocean will behave smoothly and finally be calm and very little bit
warmer.
Early Radio
From Paul J. Nahin’s “Science of Radio”
An instable receiver with restore so the next pulse can be detected! This can be
associated to the trigging of the magnetosphere of the earth, but without the
restore. The aurora act as a registrator!
The solar wind blows continuously out from the sun and our magnetosphere
collects charged particles from the wind. The particles will be trapped by the
magnetic field of the earth. If the solar wind is steady the magnetosphere will be
loaded with particles and by them it contains a large amount of energy. It is like
the radio receiver above. It will be enough for a small disturbance in the solar
wind to get the whole magnetosphere in vibration. It normally happens by an
extra magnetic field following the solar wind.
A trapped particle
How to lie with statistics
The illustrated proportions are ment to be 1:2, but they look as and will be
remembered as 1:8.
In the beginning of the 60th a lot of papers were produced where one discussed
the statically behaviour of auroral arcs. In that way one didn’t get any feeling for
special event. One only got an overview how the arcs behaved in the mean. The
physical theory which could be associated was thermodynamics, which is about
heating and temperature. In the middle of the 60th Akasofu turned up on the
scene and he pointed at special events and he could make characteristics of them
and get what he called substorms. Substorms are now generally accepted as basic
blocks!
How to lie with pictures
The illustration can give the idea that the sun is completely controlling the
earth’s magnetosphere.
Lord Kelvin, a well known physicist, concluded that the sun had no impact on
the earth magnetic field. The distance was too big. He thought about a current
on the sun, which generate a magnetic field but only strong close to the sun.
He couldn’t think of a collection of something starting at the sun, leave it and
move to the earth. He was also involved with a theory for the first Atlantic cable.
In that theory he mainly thought of Ohm’s and the capacitor law. The electric
field in the European end of the cable controlled through the cable the electric
field in the American end. He couldn’t think of an electromagnetic pulse emitted
in Europe, travel along the cable and finally arrive undisturbed in America. He
didn’t understand the induction!
A solar flare will initiate a plasma bulb with magnetic field inside. The bulb will
loose its contact with the sun and travel and eventually reach the earth very little
disturbed!
The Sun, the solar wind and the magnetosphere of the earth.
How a rocket can blow a gas with supersonic speed.
The gas from a rocket will first be compressed and then expand by itself. In the
latter phase it gets a supersonic speed, relative to the rocket. A similar situation
is on the sun: the gas leaves the sun but it will experience a gravitational force
back which will act as a nozzle in a rocket. The solar wind will be created as a
supersonic flow and looses almost all contacts back to the sun! We have two
independent objects: the sun and the solar wind.
The bowshock at the earth.
When the solar wind with its supersonic speed, the radial relative velocity
between the sun and the earth is quite low, reaches the earth’s magnetosphere a
supersonic-shock will appear, the bow shock. That will act as a separation
between the solar wind and the magnetosphere of the earth.
Three main objects are identified: the Sun, the Solar wind and the
magnetosphere of the earth
The solar flare
Besides the continues flow of plasma from the sun some emissions appear from
time to time which send out a cluster of plasma including a self contained
magnetic field. Such clusters will sometimes move out from the sun with very
high intensities and velocities and if it reach the earth it can destroy the normal
structure of the magnetosphere and very speculative effects can occur, lets call it
Skansk Aurora, because sometimes one can see powerful mostly red Aurora in
south of Sweden. We will not worry about these happenings here.
If as normally the emissions are weak there magnetic field will moderately
change the magnetic field in the magnetosphere and the trapped particle in it
will change there orbits and the magnetosphere will be instable and the aurora
will appear.
A solar flare is the fourth object.
A theory for such aurora is possible and can be arranged by the substorm
concept.
The size relation between the earth and our sun.
The “SiliconWalley ball” is 12 cm in diameter, if it represents the sun the little
head of the upper insect needle will represent the earth: the diameter of the sun
is 100 times larger than the diameter of the earth. The ball has a kernel which
can be noticed in the picture and it will represent the fusion part of the sun,
hydrogen bomb. The rubber bands can be considered as magnetic field lines,
which can be dropped from the ball and then represent a solar flares emitting a
plasma cloed!
A distance of 12 meter between the ball and the head of the needle represents the
distance between the sun and the earth.
The Magnetosphere.
Above the ionosphere about 700 km in height is the lower border for the
Magnetosphere, which besides the Earth’s outer magnetic field also has electrons and
protons trapped by that field. The “individual” particles move as such over very large
distances. Some physicists want to incorporate the magnetosphere in the ionosphere,
but I think the two regions have so dramatic different physical happenings that it is
not to be preferred. The magnetosphere is a place for chaotic processes responsible
for the geometrical structures and movements of the aurora displays. Chaotic
processes are described by reversible classical physics. The ionosphere on the other
hand is a place for processes which also can be described by thermodynamical type of
physics. At high latitudes the ionosphere is very much more impulsive than at low
latitudes due to its close connection with regions in the magnetosphere generating
chaotic bursts of electrons. These can penetrate downwards, disturb the distribution of
the electrons in the ionosphere and generate aurora. Finally the atoms and molecules
of mostly atmospheric origin in the ionosphere act as light transmitters for aurora.
Opened magnetosphere shaped by the solar wind.
The magnetosphere described as a sum of three fields:
the earth magnetic field plus a field caused by the streaming of the solar wind plus
the interplanetary magnetic field.
The effect from the streaming from the sun goes through the trapped particles.
The interplanetary magnetic field, in this case, points to the south
The connection of aurora and the magnetic field of the earth.
Via the website:
http://www.irf.se/~ionogram/ionogram/javascaling/viewMag.html
It is possible to study aurora and the magnetic field in Kiruna from 2002 up
to now.
Sound from or more correct in connection with aurora
Low frequency waves in the atmosphere in connection with the ionosphere
Atmospherics and a very nice Tweek, they are always there.
But here an example of risers, caused by electromagnetic fields in the
magnetosphere above Aurora: