# Measuring Temperatures and Composition in the Ionosphere

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How does a radar work?

### Measuring Temperatures and Composition in the Ionosphere

The ionospheric plasma is composed of electrons and ions, freed from each other by ultraviolet energy from the sun. When scientists make a theory of how this plasma functions, they find that under conditions seen by the Arecibo radar the motions of the electrons and ions are coupled. The electrons are very light; therefore they respond to the radar wave and do the reflecting. The ions are much heavier; therefore they do not reflect the radar waves, but the ions tend to hold the electrons nearby. This means that the speed of the ions shows up in the overall motion of the electrons. Thus the spectral width that we see is related to the speed of the ions; if there are more than one type of ion present, then we see a complicated spectrum that has more than one width. The spectrum below shows the widths associated with the hydrogen ions and the oxygen ions, assuming that they have the same temperatures.

The ionospheric temperatures can be quite complicated. The ultraviolet energy of the sun goes mostly into the electrons, and so their temperature rises above that of the ions. This has a definite effect on the spectrum. In the black and gray spectrum below, the oxygen ion and electron temperatures are the same. The red and green spectrum placed on top has an electron temperature that is twice as high. Notice that the area under the curve is reduced, and the little peaks on top are higher. Thus we see that the radar can also measure the electron temperature.

The Arecibo radar is the only one in the world which can measure these things all at once from a single spectrum in part of the ionosphere:

• temperatures: electron, oxygen ion, and hydrogen ion
• ion content: oxygen, helium, and hydrogen
• velocities: oxygen ion and hydrogen ion.

Getting numbers for these quantities from the spectrum is hard. We consider how to do it on the next page. Consider one problem illustrated below. The spectra we have drawn here are smooth and accurate.

Real measurements consist of that have errors; that is, they are "noisy".