Friday, 7 May 2010

Remote Sensing of the Ionosphere: Part 1

Now that the hectic period of writing up my thesis is over, resulting in somewhat of a blogcation, I thought that it was high time I shared some of the research that went into my thesis.

The ionosphere is a region of plasma in the upper atmosphere, it extends from about 50 km on the lower end to more than 1000 km at the top. Mostly the plasma is created by radiation from the sun, which breaks up (dissociates) the atoms, i.e. ionises them, into electrons and positive ions. A small part of the ionisation is created by cosmic rays, and particularly at the lower altitudes where the neutral atmospheric density is higher the electrons can collide with the neutral atmosphere to form negative ions.

The ionisation density depends both on the rate of input from the sun or other sources (dissociation rate) and on the rate at which the ionisation decays by recombining to form neutral atoms (recombination rate).

Now the ionosphere is not really a layer in the atmosphere like the troposphere and the stratosphere which are defined by temperature, but rather is a region in which the plasma is overlayed over the top of these temperature variation defined layers. So the ionospheric altitudes are the same as those covered by the mesosphere and the thermosphere.

Different frequencies of solar radiation interact with different molecules or atoms in differing regions of the atmosphere to create several layers within the ionosphere. The peak density of ionisation is in the F layer, above 200 km in altitude, which is due to extreme UV solar radiation ionising atomic oxygen (O). Below this there is the E layer, around 90-120 km in altitude, due to soft X-rays and UV ionising molecular oxygen (O2). Below this is the D region, not a true layer like the E and F, but more of a bump in the slope of the electron density profile. The D region is 50-90 km in altitude and is mostly due to Lyman-α ionising nitric oxide (NO).

Since most of the ionosphere is due to the sun, we see variation between day and night and between the seasons as well as over different latitude ranges. In the image you can see the difference between a summer ionosphere and a winter ionosphere, with the electron densities displayed for Corsica (summer, solid line) and Dunedin (winter, dashed line) in late July.

By having all these electrons (and ions) up in the atmosphere, the physical properties of the atmosphere are altered. In particular the electrons are very good at doing what they do in copper wires, conducting electricity. This presence of conducting layers in the atmosphere reflects radio waves, forming a "leaky" or partial mirror. In fact the first direct evidence for the existence of the ionosphere came in mid-December 1901 when Guglielmo Marconi informed the world he had received radio signals at Newfoundland, Canada, sent across the Atlantic from a station he had built in Cornwall, England.

Differing frequencies reflect off differing electron densities (and hence conductivities) at differing altitudes in the ionosphere. My work was focused on VLF (Very Low Frequency, 3-30 kHz) radio waves which reflect of the D-region. Higher altitude regions can be studied using higher frequency radio waves.

VLF radiation, as well as some other frequencies, reflects not only off the ionosphere but also off the surface of the Earth (very well off sea water, not so well off land, and very poorly off ice, depending on the conductivities of these surfaces). The result of this is that the radiation can travel long distances (>10000 km for powerful transmitters) reflecting between the Earth and the ionosphere, like it was travelling in a waveguide.

This allows us to observe radio waves at a receiver and infer the condition of the ionosphere between the transmitter and the receiver. Changes in the received signal are caused by changes in the ionosphere (or extremely rarely the ground), and by comparing the observed changes to expected changes from computational modelling, you can get a indication of what processes are occurring in/effecting this altitude range and the relative importance of the processes.

The ability to remote sense the 50-100 km region of the ionosphere using VLF radio waves is very useful, since this altitude range is too high for direct observation by plane or balloon, and too low for in situ measurements by satellites. Rockets have been used to study this region but those measurements are transitory and relatively expensive.

Of course operating large, powerful VLF transmitters is also expensive (especially since they usually have low transmission efficiencies ~10-20%), but fortunately for the world of science, many governments have undertaken to operate such transmitters (usually for the purpose of maintaining communication channels with submarine fleets). The US, Russia, Japan, China, India, France, Germany, Italy and the UK all have (or used to have) such transmitters. Radio waves in this frequency range can penetrate tens of meters into seawater allowing communication (even if simple communications with a low baud rate, due to the VLF carrier frequency) with submerged submarines, so that the submarines do not have to surface and give away their position.

This network of transmitters in conjunction with similar networks of scientific receivers allows simultaneous coverage of much of the Earth's ionosphere (at least at D region altitudes).