Rob’s Web: 160 meter Propagation – Unpredictable Aspects

Are you likely to work 160-meter DX today? It’s very difficult to say. Come learn why.

The sun is the main driving source of the ionosphere, through its ultraviolet(UV) radiation, which illuminates an entire hemisphere. A specific force is the solar wind, which interacts with the outer reaches of the geomagnetic field to form the magnetosphere. The geomagnetic field, however, is a major factor in organizing the ionosphere at lower altitudes. It controls the motions of ionospheric electrons on release by photo-ionization; and by its configuration, it shapes the global distribution of ionization, particularly at low latitudes.

The nighttime ionosphere results largely from geomagnetic control of ionization that remains after sunset, because of its low rate of recombination at high altitudes. There is also a forcing factor from the intermittent occurrence of aurora, with magnetospheric electrons accelerated to high energies going down the high-latitude field lines; those create ionization at E-region altitudes. At lower latitudes, F-region ionization decreases at night, but is maintained at low levels between the E-region and F-region peak because of UV in starlight, galactic cosmic rays and solar UV radiation scattered by the geocorona.

Fig 1 – Global W2 map for 0600 UTC, March 1979. (SSN-137, after Davies, Note 9).

Communication on the upper-HF bands of the Amateur Radio spectrum is largely under direct solar control. Propagation predictions are made using refraction calculations based on global ionospheric maps for the F and E regions, as in Figs 1 and 2. The F-region map shows geomagnetic control by the fact that the critical frequencies foF2 from the ionization are asymmetrical with respect to the geographic equator at the equinoxes. Also, there is the unusual distribution of critical frequencies in the F region, which shows that ionization extends into the hours of darkness at low and equatorial latitudes.

Fig 2 – Global foE map at local noon, March 1958. (SSN-201, after Davies, Note 9).

Fig 2 shows the daytime critical frequencies that result from the distribution of ionization about the sub-solar point at E-region altitudes. That is of little consequence to propagation on the higher bands of the spectrum, as the operating frequencies there are large compared to the critical frequencies foE, as well as the electronneutral collision frequencies Fc. The result is that signals go through the E region with little deviation and only small amounts of ionospheric absorption along their paths.

All in all, HF propagation can be dealt with using only a few parameters: the sunspot number or a suitable surrogate, the planetary geomagnetic K-in-dices or their short-term estimates and the announcements of bursts of solar and magnetic activity. The first two of those parameters essentially give what can be expected on average and the update material provides additional guidance when conditions depart significantly from predicted averages.

The situation is quite different on the lowest band of the Amateur Radiospectrum, 1.8-2.0 MHz. There, signals propagate at altitudes around the E region, but operations occur only at night because of the heavy ionospheric absorption during daytime. Beyond that, there is more than enough ionization overhead to propagate signals in that frequency range, so critical frequencies or MUFs-so important at the top of the HF spectrum-are of no concern. Instead, signal propagation is considered limited largely by absorption and noise.

Even at the solar minimum, sunspot numbers are sufficient to guarantee propagation on the lowest band of the spectrum. There are second-order effects that result from the sunspot number: A small increase in the radiation angle that RF must have to penetrate past the E region to permit longer F-hops and ducting. In addition, there is a small increase in D-region ionization, which increases ionospheric absorption. Both of these effects are quite within the realm of prediction and thus are easily understood and dealt with.

Nonetheless, consideration of average parameters is something of an oversimplification of the situation, as it does not recognize various propagation modes possible at low frequencies for given parameters. They, in turn, can be affected by the dynamics of the neutral atmosphere. So, with propagation being a geometrical affair, rays are refracted as they travel through the ionosphere and anything that affects the geometry of ionospheric regions relative to the earth will have an impact on signal modes.

In that regard, the phrase “relative to the Earth” has a lot of meaning as the atmospheric motions are relative to the Earth, while the ionizing radiation comes from well outside the propagation region-say, auroral electrons spiraling down relatively fixed magnetic-field lines, or solar photons on their straight-line paths from the distant sun. Thus, the level of ionization is more related to the amount of matter traversed by the incoming radiation and the neutral density, as distinct from geodetic altitude.

The atmosphere, being a target for such radiation, will present a different geometry relative to the Earth’s surface for wave refraction as air parcels in the target region are moved about by highaltitude winds. Those can be seen in the motions of visible trails and radar reflections from meteors, or expected from heating and vertical expansion of the atmosphere at sunrise. In addition, auroral energy will be transferred to the atmosphere as heat with the incidence of auroral ionization, say, during magnetic storms. Thus, levels of constant ionization density may move up or down or may even become tilted. All of those have an effect on the refraction process by bending rays vertically, to increase or decrease the lengths of paths, or horizontally, to skew them one way or the other, but away from regions of greater ionization. All those aspects of refraction can be expected to occur in the nighttime propagation of 160 meter signals, around the E region where E-hops, E-F-hops, F-hops and ducting can take place.

In addition to density changes at a given geodetic altitude from mass motions, there is the question of atmospheric composition, particularly the role of some minor constituents that are man-made. Among others, those include nitric oxide (NO), which is a byproduct in the exhaust of jet engines and carbon dioxide (CO₂), which results from the widespread use of fossil fuels.

Those minor constituents are created in specific locations, but their presence is related to transport through atmospheric circulation, making them highly variable in their concentrations. Water vapor and ozone are two other trace constituents that are highly variable because of transport, but they are produced continually by the effects of solar radiation: heating of the oceans in the first instance and photodissociation of molecular oxygen, a major constituent of the atmosphere, in the second. Beyond its importance to atmospheric processes, ozone is of particular interest concerning the lower ionosphere, as it is transparent to visible radiation but opaque to UV. Thus, it limits the UV photo-ionization of the neutral atmosphere ‘and photo-detachment of electrons, firmly bound to negative ions, at low altitudes around sunrise and sunset.

Ionospheric Variability

While 160 meter signals are propagated at heights between the bottom of the D region and the lower F region, it is of interest to first look at the variability found in measurements of parameters throughout the ionosphere and how they affect hop lengths and critical frequencies in the spectrum. Then we can consider how often observations are made related to those results in the ionosphere and compare them to observations for those aspects of the neutral atmosphere that affect 160 meter propagation. First, we note the principal ionospheric measurements that bear on Amateur Radio operations are critical frequencies, foF2 for the F region and foE in the E region. Concerning the F region, ionospheric variability is found in the records of ionosondes, as in Fig 3, which shows foF2 values for the hours of the day from ionosonde observations at Slough, England, in January 1969.(1) That figure shows the location of the median (50%) value of foF2 for each hour and the IONCAP prediction program makes use of the lower (90%) decile and upper (10%) decile values, which come from similar observations. Those are used with paths to predict the frequencies above which a path is open 27 days of the month (FOT) or only 3 days of the month (HPF), respectively, while the median (50%) value is used to find the MUF for the path.

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