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?

(+)

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)

Press down arrow to view animation

Press up arrow to reset animation

(+)

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 down arrow to view animationPress up arrow to reset 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

Daytime Nighttime

wind wind

How do we study the ionosphere

• Incoherent Scatter

• Heating facilities

• Rockets & Spacecraft

Incoherent Scattering

Reflecting photons off of electrons

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

= 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 )

Press down arrow to view animationPress up arrow to reset animation

Plasma frequencyThe 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)

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

Press down arrow to view animationPress up arrow to reset animation

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

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).

Press down arrow to view animationPress up arrow to reset animation

Heating Facilities

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):

Cool Ions

Hot Ions

m v2 = 3 k T

m – mass of ionv – velocity of ion

k – Boltzmann’s constant (1.38 * 10-23 J/K)T – temperature in Kelvins

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 DriftElectrons 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.

Right handed

left handed

Vertical

Horizontal