Transcript Ionosphere as Plasma - MIT Haystack Observatory

the Ionosphere as a Plasma
Begin
Questions
• Why is it called the ionosphere?
• How do we study the ionosphere?
• What can we measure in the ionosphere?
Why the Ionosphere?
Press down arrow to view animation
(+)
Press up arrow to reset animation
UV light from the sun hits atoms in Earth’s upper
atmosphere. The energy from this light knocks an electron
off the atom, leaving a free electron and an Ion. This type
of ionized gas is called a plasma. Unlike other gases, it
can conduct an electric charge and is affected by magnetic
fields. (learn more)
How high up are the
ions?
Upper altitudes have fewer
atoms to ionize. Less
radiation reaches lower
altitudes, and there ions
recapture electrons more
easily.
Most ions are created at an
intermediate height.
Once ionized, the ions
distribute themselves
according to their weight
with the heaviest closest
to the Earth.
Just as
temperatures
define the
layers of the
atmosphere,
electron
concentrations
define the
different layers
of the
ionosphere.
Each layer
plays an
important
role in
absorbing
some of the
sun’s
radiation.
Ions and Electrons move
around in the Ionosphere
•
•
•
•
Above 80 km, ions are pushed around by collisions with a wind of neutral
particles. Electrons are smaller and are able to avoid these collisions. Instead,
they spiral around magnetic field lines. This causes a current at this altitude
known as the electrojet (learn more).
Above 200 km, the neutral wind is too sparse to push ions around. They too
begin to spiral around the magnetic field lines.
The frequency of these rotations is the plasma’s cyclotron frequency (learn
more).
As a charged particle rotates about a field line, it emits radiation.
Press
Press
down
up arrow
arrowto
toreset
view animation
this motion contributes to day night differences in the ionosphere
Daytime – In the northern
hemisphere, winds blow from the
hot south to the cool north and
push the spiraling ions down.
Nighttime – Winds blow from north
to south and push the spiraling
ions up
wind
wind
Daytime
Nighttime
How do we study the
ionosphere
• Incoherent Scatter
• Heating facilities
• Rockets & Spacecraft
Incoherent Scattering
Reflecting photons off of electrons
Press
Pressdown
up arrow
arrow
to to
reset
viewanimation
animation
Some of the energy from the photon is converted to the
electron’s kinetic energy. The reflected photon has less energy
and therefore a longer wavelength.
The change in wavelength of the photon is due to the Compton
Effect (learn more). For reflection, the change in wavelength is
Dl = 2 h / c me
h = Planck’s constant (6.63 * 10-34 J sec)
c = speed of light (3.00 * 108 m/s)
me = mass of electron (9.11 * 10-31 kg )
Plasma frequency
The electron in the previous slide was pushed to the right. As it moves from
its original position, it is attracted back by the surrounding positive ions.
(learn more)
Press
Press
updown
arrowarrow
to reset
to view
animation
animation
It will oscillate around its equilibrium position. The frequency of these
oscillations is called the plasma frequency.
freqplasma = 9 √Ne
Ne = number of electrons per cubic meter
Reflections can be used to measure the
height and density of the ionosphere
• The photon is reflected when its frequency equals the plasma
frequency.
• The longer it takes for the wave to be reflected back, the
higher up the plasma layer.
• Current research is being conducted at Millstone Hill (learn
more)
Lower frequency
Higher frequency
Heating Facilities
Press
Press
down
up arrow
arrowto
toreset
view animation
These programs pump the ionosphere with
so much energy that the temperature of a
small part of it increases. Radio telescopes
then study the plasma as it cools down.
Current research is being conducted by
EISCAT (learn more).
Rockets and Spacecraft
Since the 1950’s, scientists have been able to
probe the ionosphere directly with rockets.
They have also been able to
look at it from space with
man-made satellites.
Although it may look like the
astronauts in the space shuttle
are in “outer space” they are still
in the ionosphere. There is plenty of matter and
plenty of activity going on outside the shuttle.
Current research is being conducted by NASA
(learn more).
What can we measure?
The incoherent scattering technique uses a radio
telescope to fire radiation at the ionosphere. The
telescope then listens for the echo. It records the
reflected signal which typically looks something like this:
With it, we can measure
density
ion temperature
electron temperature
electron drift (current)
velocity of neutral particles (wind)
electric field
Density
The density of a
region of plasma is
proportional to the
total power reflected
back. The top graph
was made of a denser
region than the
bottom graph
Ion temperature
The velocity of the ions
are proportional to the
width of the signal. This is
due to Doppler
broadening. An ion’s
temperature can be
calculated from its velocity
and its mass with the
following equation (learn
more):
m v2 = 3 k T
m – mass of ion
v – velocity of ion
k – Boltzmann’s constant (1.38 * 10-23 J/K)
T – temperature in Kelvins
Cool Ions
Hot Ions
Electron temperature
Electrons are generally hotter than ions. Their relative
temperatures are proportional to the relative heights of the
peaks and valley on the graph
Cool electrons
Hot electrons
Electron Drift
Electrons are much
lighter than ions and
can therefore move
much faster. If a group
of electrons are all
moving in the same
direction, an electric
current is established.
The reflected signal will
then be Doppler
shifted.
neutral wind velocity
Below 200 km, the
ionosphere is so dense that
ions and neutral particles
collide frequently enough to
reach thermal equilibrium –
the same temperature.
The incoherent scattering
techniques described
above can be used at
these heights to measure
the temperature of the
neutral gases and thus
their velocities.
Electric Field
An electric field in the ionosphere can be
measured by the relative polarization of
the collected radiation.
Vertical
Right handed
Horizontal
left handed