Are you interested?

[KL7AJ]

Now that we have an actual ionosphere, the ionogram is more useful (and interesting) than ever. Is anyone here interested in a comprehensive course on how to interpret ionograms, particularly the Lowell Digisonde?

Eric

[W4SUL]

Sure, I enjoy all of your educational posts. It's one of the reasons I'm here at all. Now if your friends at HARP would just use that big array to ionize the ionosphere artificially that would be great too!

[KL7AJ]

Sure, I enjoy all of your educational posts. It's one of the reasons I'm here at all. Now if your friends at HARP would just use that big array to ionize the ionosphere artificially that would be great too!

Cool! Okay....I'll get started 'afore long.

eric

[AF6LJ]

Sure, I would love that.

[KD7EIR]

Sounds great!

[KL7AJ]

All righty then. Here we go!


Well, as people say, everyone talks about weather, but nobody does anything about it. The same could be said about the ionosphere. But we can at least predict a few things about it.

The best tool for this is the ionosonde....there is one near you. The one I rely on is the HAARP ionosonde. Here's a recent ionogram from HAARP.

http://137.229.36.30/cgi-bin/digison...563&pick.y=223

This ionogram is fairly textbook....stable sky as indicated by the THIN traces. Later in the evening when the ionosphere recedes, you'll see a lot fatter traces.

The bottom scale (x axis) is frequency. An ionosonde is nothing more than a radar that sweeps through the radio spectrum and looks at the time-of-flight return at each frequency. This time of flight is indicated by the Y-axis, which correlates to ionospheric height...with a couple of subtleties.

The black line (with two gray "outer markers") is the electron density profile. It shows the relative concentration of free electrons at various heights. Its magnitude is the horizontal axis as well, (but there's no scale for it) We generally aren't interested in absolute numbers, but rather at what altitude we have the MOST electron density.


We pause now for questions.


Eric

[KL7AJ]

No questions. So I think we'll move on.


Notice that the maximum height is around 650 km. This is about 400 miles, which is about the outer limit of the usable ionosphere. In this ionogram, the second set of fuzzy traces up near 650 kilometers is a double reflection. If you see multiple reflections, it means the ionospheric ABSORPTION is very low...which is a good thing. I've seen up to 5 reflections. Three reflections is not uncommon. However, for actual measurement, it's only the first reflection that counts.

We pause again for "reflection"

Eric

[AF6LJ]

Kool!!
Now that graph is beginning to make sense instead of looking something the tom cat marked.

[KL7AJ]

Kool!!
Now that graph is beginning to make sense instead of looking something the tom cat marked.

Great....well now at least SOMEONE is paying attention.

Let's look at the traces themselves. We see two main traces, one in RED and one in GREEN. The Red trace is the Ordinary mode trace (O wave) and the green is the extraordinary mode trace (X wave). When you launch a linear polarized wave into the ionosphere, it splits into two separate waves that follow two different paths. They each have a different reflection height and a different CRITICAL FREQUENCY, that is, the frequency at which it just launches out into space with no reflection.

If you put together a simple circular-polarized antenna, such as a crossed-dipole or "turnstile" antenna, you will see about a 3 S-unit difference between clockwise and counterclockwise polarization, regardless of your location. If you happen to be in a location where you receive ONLY the X or ONLY the O wave, you can get nearly INFINITE discrimination by swapping the circularity. More on this later.

For now, let's look at the O-mode, which is generally the stronger wave and the one for which we have more crucial information.

Notice the critical frequency. This is the frequency where the curve goes straight up. See that it corresponds with the MAXIMUM electron density. This is important fact to recognize when we look at ionospheric physics. In this particular iongram, the critical frequency is 5.725 Mhz. (The X mode critical frequency is 6.5 MHZ)


We now pause for questions.


Eric

[AF6LJ]

So far that makes perfect sense.