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HF Propagation tutorial

by Bob Brown, NM7M, Ph.D. from U.C.Berkeley

Distribution of ionospheric electrons (III)

In the previous page, it was pointed out that further progress on propagation requires knowledge of how ionospheric electrons are distributed. Of course, that will be different, day and night, as well as with seasons and sunspot cycles. Again, it would be easy way to fall back on something in the introduction, say the night-time ionosphere and continue the discussion from there. But that would involve a tremendous leap over distance and logic that's not too productive. So let's talk/walk our way up to higher altitudes, starting from where we are now, the D-region.

For one thing, the D-region involves a lot of familiar ideas and we can work from there. For example, below the 90 km level, our atmosphere is pretty well mixed, about 78% nitgrogen molecules and 21% oxygen molecules, by volume. The remaining 1% is made up of permanent constituents, like the noble gases as well as hydrogen, methane and oxides of nitrogen. Of course, every schoolboy knows about the variable constituents, like water, carbon dioxide, ozone and various bits of industrial debris, smog, that are found in around heavily populated regions.

Global weather systems keep the lower atmosphere all stirred up, in a mechanical sense, but that is not to say that convection from solar heating is the only influence of the sun. Indeed, as was discussed earlier, there are electrons and positive ions in the lower D-region, released by solar EUV and X-rays. When the sun sets, one might think that all the ionization disappears by recombination and the region becomes de-ionized and neutral.

At left structure of the neutral atmosphere et the ionosphere. At right closeup on the ionosphere layers where charged particles (electrons and ions) of thermal energy are present, which are the result of ionization of the neutral atmospheric constituents by electromagnetic and corpuscular radiation. At night the D-, E-, F1 and F2-layers vanished and are replaced by the F-layer located higher in altitude. The lower boundary of the ionosphere coincides with the region where the most penetrating radiation (generally, cosmic rays) produce free electron and ion pairs in numbers sufficient to affect the propagation of radio waves (D-region). The upper boundary of the ionosphere is directly or indirectly the result of the interaction of the solar wind with the earth upper atmosphere. On the night-side the ionosphere can extend to greater distances in a tail-like formation, representing the solar wind shadow. In the tail the extent of the ionosphere is limited by the condition for ion escape. 

Of course, the ionosphere is always electrically neutral, with the equal numbers of positive and negative charges, but recombination lowers their numbers. Still some ionization does remain, produced by other sources; those include UV and X-ray photons in starlight, sunlight scattered by the gas envelope (geocorona) surrounding the earth and even charged particles, the energetic protons in the galactic cosmic ray beam.

So it follows that ionospheric absorption would be greatly reduced after dark but does not go to zero. There is good news in this discussion, however, as some electrons are taken out of the absorption loop at night by becoming attached to oxygen molecules. Those negative ions are so massive that they can't be budged by RF going by and just do not participate in the absorption process.

And at night, the number of negative ions of molecular oxygen in the lower D-region grows to large numbers in going downward from the 85 km level. That is the very reason that those solar proton or PCA events mentioned previously show much less absorption when the sun sets. But when the sun comes up, solar photons detach electrons from the negative ions and absorption goes back to the daytime level again. That does not happen for auroral events and that is another story, about another region higher in the ionosphere. More on that later.

In any event, the frequency dependence is still in effect for whatever absorption occurs, taking a heavy toll on low frequency signals. But that is still not fatal to propagation, even on the low bands. Thus, everyone knows about broadcast stations coming in better after dark and those signals can be heard across very great distances, as many SWLs will testify. And even with more limited power, 160 meter operators can still work great DX. But in the last analysis, both SWL and low-band DXers run up against the same problem, noise. That also has its origins down at low altitudes so we can deal with that right now, while in the region.

Noise[1] is described as broad-band radiation from electrical discharges, either man-made or natural in origin. Whatever the case, being a radio signal, noise will be propagated like any other signal on the same frequency. That means, for one thing, that noise signals that are below the critical frequency of the F-region overhead will be confined to the lower ionosphere, dissipate down there and not escape to Infinity. By the same token, noise signals above the critical frequency are lost and won't bother us very much on the higher HF bands. But the lower bands do have a problem; so let's talk aboutit.

Noise of atmospheric origin comes from lightning strikes and will be seasonal and originate in fairly well-defined areas. Among the powerful sources of noise are low-latitude regions of South America, South Africa and Indonesia. But the U.S.A. have their own noise source too, the southeastern states during the summer months. So broad-band noise originates from those regions and is propagated far and wide through regions in darkness. But once the sun comes up, ionospheric absorption takes over and the only noise heard is of local origin, static crashes from nearby lightning strikes.

The above points are not news to domestic DXers; they are quite familiar with their own situation and can work within its limits. But those going on DXpeditions often go into unfamiliar territory and don't always think about the atmospheric noise problem. So 160 meter operators on DXpeditions have been known to be greeted by S-9 noise the first time the receiver was turned on. That evokes instant panic and sets in motion efforts to ameliorate the problem, say trying different antennas and such. Those don't work every time and hindsight often proves the problem could have been avoided, in large measure, by planning the DXpedition for a time on the winter side of an equinox, not the summer side.

Of course, the other source of noise is quite local, man-made in origin and coming from various electrical devices. While the global dimensions of atmospheric noise have been investigated extensively over the last 50 years or so, the same is true of man-made noise and it can be categorized as to origin and even given a frequency dependence. 

As for origins, the worst situation is an industrial setting and then lesser problems are found with residential, rural and remote sites, in that order. In that regard, the IONCAP propagation program allows one to select the receiver siting and then takes that, as well as the bandwidth (in Hz) of the operating mode, into consideration in calculating the signal/noise ratio that would be expected for a path.

Of course, an operating frequency is put in for each calculation, giving results for noise power similar to the rough sort of frequency variation shown at left.

It should be realized that those values for the noise power are averages throughout a day and subject to considerable variation, with changes in human activity.

So low-band DXers sitting there in the wee hours of the morning will not hear the buzz of chain saws or weed-eaters but they might have to put up with other noise, say sparking heaters in fish tanks or hash from computers, TVs or various forms of consumer electronics in nearby homes.

 Last of all, there are extraterrestrial sources of noise too, from the galaxy, as noted in regard to riometers, and solar noise outbursts. Galactic radio noise is quite weak and reception requires very sensitive receivers at sites well-removed from sources of man-made noise. But solar noise is another thing and it can be quite strong at times when solar flares are in progress.

As you'd expect, solar noise can pass through the F-region if its downward path has an effective vertical frequency that is greater than the critical frequency of the F-region. Thus, solar noise would be heard more often at the top of the amateur spectrum, especially when the sun is at a high angle in the sky. And it can be quite strong at times, whooshing sounds that rise and fall in intensity, even capable of overpowering CW and SSB signals on the higher bands. By way of illustration, solar noise was discovered by British scientists during WW-II and was first thought to be a new form of German radar jamming. OK?

Extraterrestrial noise sources are getting a bit far afield so we'd better get back down in the D-region and move on from there, going above 90 km and seeing how matters start to change.

Now we have to move up from the D-region, going above 90 km into greater heights. In doing that, it is necessary to not only talk about the ionosphere but also the underlying neutral atmosphere.

A few words about the ionosphere will do for starters since that is something we've already covered. For example, the collision frequency of electrons with their neutral surroundings is quite important in discussing ionospheric absorption. And I mentioned that falls off with increasing altitude. The same is true of the collisions between the neutral constituents. So neutral-neutral collision frequency goes from about 6.9x1010/sec at sea level to 1.2x104/sec at 90 km, dropping about six orders of magnitude. The same is true of the number density, going from 2.5x1025 particles per cubic meter at sea level to 5.9x1019 particles/m3 at 90 km.

Clearly, things thin out as we go up and collisions become much more infrequent. Of course, you suspected all that but now you know some of the numbers. But you may have not suspected how those changes would affect DXing on HF, even VHF. So stay tuned as I go a bit further; then I will get to the "nuts and bolts".

To go on, I mentioned the atmosphere is lightly ionized and I also pointed out that recombination was the fate of electrons and positive ions, especially after dark. But it does go on even in the sunlight and one process involves recombination of positive molecular ions of oxygen (O++) with electrons. When that happens, the neutral molecule (O2) is re-formed but with excess energy; so it flies apart, into two oxygen atoms (O). But considering how lightly ionized things are in the ionosphere, that can hardly be considered as a strong source of oxygen atoms. OK?

But during the day, the atmosphere is bathed by energetic solar photons; some, as we know, ionize oxygen molecules and thus can contribute to the ionosphere. Others dissociate oxygen molecules into two atoms. But with such a low collision frequency at 90 km, an oxygen atom can linger around for about a week before finding another oxygen atom and recombine to form molecular oxygen again.

So the long and short of it is that by the steady illumination of the atmosphere by the sun, atomic oxygen can build up to become an important constituent of the atmosphere above 90 km. One step further tells us the atomic oxygen ions, O+, will be created too by all those solar photons going by. So how long will those ions last?  Good question; it depends on which process is considered, perhaps recombination with an electron to form a neutral atom. It turns out that if recombination were the only possible fate for O+ ions, they'd linger around a long time too. Something else seems to happen but before getting to that, let's look a bit deeper into the O+ situation up above 90 km. OK?

The recombination of O+ with an electron is a radiative process, the excess energy being given off as a photon while the atom recoils to conserve momentum. But it is slow , I mean VERY SLOW in the scheme of things. And that seems to be the case for other similar radiative processes, like with metallic ions. It just seems to take forever for an electron and metallic ion to get it together and recombine. But now comes the PUNCH LINE; there are metallic ions in the upper atmosphere, meteoric debris that has drifted down and been ionized by solar photons.

Ionospheric effects on radio waves propagation. Document IRPG.

And recombination being a slow process, they linger around a long time. In fact, they can linger around and be caught up in the occasional weather activity up around 100 km, wind shears. And being tied, as it were, to field lines, wind shear can compress them into a thin layer. But their electrons are not far away so that makes for a thin layer of electrons too. So now you guessed it; I'm talking about sporadic E layers up around 100 km or so.

The electron population, being squeezed into a thin layer, looks sort of metallic too when it comes to wave propagation so RF is really reflected by those layers, the sort of thing we talked about in the introduction, tilted reflecting layers. In the present case, the tilt would be that of the magnetic field lines that hold the charges. But the tilt is not so important to DXers; it's the presence of a strong, reflecting layer around 100 km altitude.

Sporadic E is known to be a nuisance for HF propagation. By its presence, it can RF cut off from long paths via the F-region up around 300 km and thus disrupt long-haul communications. And the reflecting properties can be so great as to not only reflect RF from the top of the HF spectrum, to the annoyance of 28 MHz DXers, but also reflects RF in the VHF portion of the amateur spectrum, to the joy of the 50 MHz and 144 MHz DXers. I should add that some contestors love sporadic E as they can go to higher bands and make many short-haul contacts on bands that would be quite dead otherwise. All that from the fact that recombination is so slow for atomic oxygen and metallic ions.

Still speaking about the importance of atomic oxygen in the atmosphere above the D-region, its build-up by photo-dissociation of oxygen molecules serves to add it to the "targets" for the various forms of incoming radiation, photons or charged particles, that pass through the upper atmosphere. And just to make my remarks rather "timely", if you saw any bright aurora a couple weeks ago, at the end of September, the green color you saw was the 5577 Angstrom spectral line from atomic oxygen. How about that?  I should add that the green aurora "washes out" to become gray aurora at great viewing distances. That's a property of the eye, they tell me.

And speaking of great viewing distances, the best atomic oxygen story I know of has to do with the early days of Rome. It seems a red glow was seen in the northern sky and the Romans figured it was the Huns, pillaging villages up north. So they saddled up, got in their chariots and roared off in the night. No Huns were found but the sky glowed again the next night. More riding, still no Huns. Nowadays, we know they were fooled by the red line of atomic oxygen, 6300 Angstroms found up around 1,000 km. You can do a simple graphical calculation to find the distance of the aurora from the Romans. (Using 6,371 for the radius of the earth and my plastic ruler/compass, I get about 3,300 km; that works out to about 30 of latitude, putting the aurora up over the northern coast of Norway. Sounds right to me!)

But back to the ionosphere and the O+ ion. As I indicated, its recombination with electrons goes very slowly, meaning that it could undergo other, more likely processes.

To make a long story quite short, an ion-atom interchange can take place in nitrogen molecules with oxygen ion displacing a nitrogen atom and forming a positive nitric oxide ion, NO+.

So now we have all the principal players in the ionospheric drama, electrons and negative ions of molecular oxygen as well as all the molecular ions, oxygen, nitrogen and, now we add, nitric oxide. It is the physics and chemistry of those ions, in the presence of the neutral atmosphere, that we have to look to to understand all the mysteries of HF propagation.

But now, we have to work our way up above 90 km. So the next stop will be the E-region, up around 105 km. During the day, it is one of the levels of the full electron distribution shown at right which density is ranging from 1 to more than 1000000 electrons/cm3 near 300 km aloft.

Reference Notes

A brief discussion of the occurrence of sporadic E layers is given in Section 3.5 of McNamara's book and a detailed discussion of the mechanisms related to sporadic E, complete with references, can be found in the October/November '97 issues of QST.  

The Roman aurora story as well as other interesting tales about the geomagnetic field may be found at the end of the second volume of "Geomagnetism" by Chapman and Bartels, Oxford University Press, 1940. Great reading!

We pick up where we left off, going up to the E-region. You will recall it is the first "step" in the ionosphere that lies above the D-region, essentially an inflection point in the curve that outlines the vertical distribution of electrons:

In the early days of ionospheric sounding, that inflection was enough to give an echo, making it stand out in the records like the peak of the F-region. And it is there all the time, the most well-known and studied part of ionosonde records. But there were also surprises in the same range of the records, sporadic E layers. But those are known for their irregular and unpredictable behaviour and make a separate study that will not concern us here.

But those sounders were calibrated in frequency, not electron density, and thus they provided data on critical frequencies. If one does a bit of ionospheric theory, the electron density and critical or plasma frequency are found to be related as follows:

fc (MHz) = 9.10-6 x N

where fc is the critical frequency and N the electron density expressed in electrons/m3

Going to the curve above, the electron density at 100 km is roughly 8.104 electrons/cc or 8.1010 electrons/m3, yielding a critical frequency of 2.6 MHz.

 The electron density profile given above is for daytime conditions so signals incident on the bottom of the ionosphere would pass on to the F-region overhead if their effective vertical frequency were above 2.6 MHz. As an illustration, 7 MHz RF launched at 30 would have an effective vertical frequency of 3.5 MHz and make it through to the F-region easily while at 15, the effective vertical frequency would only be 1.8 MHz and RF would be blocked or "cut-off" from the F-region. I'm sure you've heard that term before in connection with propagation programs.

 Now I made a couple of points about the positive ion of atomic oxygen (O+): that its recombination rate is quite low and that it can undergo ion-atom interchange with molecular nitrogen to yield a positive ion of nitric oxide (NO+). Just to come up with some numbers, I checked on the situation here at my QTH, using the International Reference Ionosphere (IRI) program at local noon for the recent equinox. The atomic oxygen ion proved to be less than 1% of the positive ions at the 100 km level; also, using some rate coefficients from ion-chemistry, it turned out that the molecular ions recombine with electrons at a rate which is 150 time faster than that for the atomic oxygen ion. OK? See what I mean?

The relative rates will remain the same with solar zenith angle so that means that at low altitudes in the D-and E-region, the slow loss rate of O+ by recombination is not important and ionization largely disappears as molecular ions recombine with electrons when the sun sets. Put another way, the level of ionization in the E-region is really controlled by the zenith angle of the sun, being the greatest when the sun is highest angle in the sky and quickly disappears by electron recombination when the sun sets.

Of course, the phase of the solar cycle plays a role too so the experimental studies show that the critical frequency foE of the E-region during daytime hours is given by the following expression:

foE (MHz) =  0.9 x [(180 + 1.44 x SSN) x cos(Z)]0.25

where Z is the solar zenith angle and SSN is the solar sunspot number.

It should be noted that this expression does not apply at high latitudes where auroral ionization in the same altitude range is common and would be added to that of solar origin. And it does not apply at night where there are special conditions just above the E-region. More on that later.

But beyond those caveats, it should be borne in mind that the data on which that algorithm is based had some experimental uncertainty associated with it, say 5%-10% for individual foE entries from the raw ionosonde records. So it would be a mistake to give any reliance on the predictions that are inconsistent with the data input. This holds true throughout all of ionospheric work; the ionosphere is not a High-Q device and though results derived from the databases can be given to a large number of figures, not all of them are really significant. OK?

Next chapter

Critical frequency maps of the E- and F-regions

Page 1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 -

[1] A detailed discussion of radio noise, both atmospheric (QRN) and man-made 5QRM), is found in Section 12.2.4 of Davies book, "Ionospheric Radio". In addition, McNamara shows how to calculate noise power for the various categories of sites on p.143 of his book, "Radio Amateurs Guide to the Ionosphere"; in addition his Appendix A goes on to show how to find field strengths and S/N values on any path.

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