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About 70 to 400km above the Earth's surface is a layer of the atmosphere called the ionosphere. In the ionosphere air molecules are ionised by sunlight. The ionosphere is very important for radio communications. The lowest part (called the D-region) is strongest during the day when it blocks medium-wave radio signals. Continental MW radio stations can be heard at night the signal can pass through the D-region and reflect off the upper layers of the ionosphere. It is expected that the shadow of the moon will cause the ionosphere to behave as if it is night during the eclipse.

As well as absorbing radio waves transmitted from Earth the D-region also absorbs radio waves from space. Like our sun other stars also emit radio waves. A riometer records the level of the cosmic noise to measure the amount of ionisation in the D-region. We expect to see the cosmic noise increase during the eclipse.

The riometer we will be using for the eclipse is a very simple device. More advance riometers can take multiple measurements to `see' what is happening in the ionosphere. We have an advanced instrument called IRIS located in Kilpisjärvi, Northen Finland.

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