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Radar Reflections From the Ionosphere

When the Arecibo incoherent scatter radar transmits into the ionosphere, each of the millions of electrons in the ionosphere illuminated by the beam acts as a very small target. The receiver sees a signal composed of the part of the sum total of all the reflections that falls on the 1000 foot dish. Shown below is a typical spectrum of the transmitted signal and the reflections.

Let us examine some of its properties:

• The received spectrum appears on both sides of the transmitted spectrum.This is because the reflections are from millions of moving particles going in all directions, and so some are going away from the radar and have lowered frequencies, while others are going towards it and have increased frequencies. This tells us about the temperature in the ionosphere. (more on this on the next page)

• The more electrons there are, the more power we receive. Thus the sum of all the power underneath the spectrum (gray) is a measure of the number of electrons in the ionosphere.

• ·Although it is not easily visible in the spectrum, the whole spectrum can be shifted slightly up or down in frequency. This measures the motion of the whole rather than the individual particles.

Consider another spectrum; it has a wide part and a narrow part. Why is this?

This is a an interesting question which we will consider on the next page, but here are some clues:

• The upper atmosphere is composed of various kinds of gases.

• The ultraviolet energy from the sun removes the electrons from some of them; the resulting ion has a positive charge, while the free electron has a negative charge.

• Positive and negative charges attract, and so the motions of the electrons and ions interact.

Next page: Measuring temperatures and composition in the ionosphere