In the first two installments of this series we have covered the radio and rocket observation techniques used on Es, with some of the more pertinent findings. The diurnal and seasonal variations etc. have not been treated since past articles in the VUD (particularly the Bob Cooper 'Propagation Wand' series in 1970 and later) did an excellent job of it. (See also March 1972 Radio Science, pp. 351-353 and CQ August 1972, p. 66, for 50-MHz Es studies by the author). Before proceeding into some of the more controversial aspects of theory, it should be made clear that the type of Es this series is dealing with is the mid-latitude or temperate variety. The polar or auroral type is generally agreed on to be due to particle injection into the atmosphere from the sun. The equatorial species of Es (which Glenn Hauser became well acquainted with in Thailand) is due to the coming together of the sun-induced E-layer currents at the magnetic dip equator. Until the 1960's just about any theory one could come up with would have been seriously entered on the then long roster of possible Es causes. Then came the wind-shear theory. The reason for the theory was originally to explain the world-wide variations in the occurrence of Es (see NBS Circular 582), in particular why Japan had so much and South Africa so little. A strong geomagnetic influence was inferred there. A wind shear is rather easy to visualize. It represents a wind which has its velocity (i.e., speed or direction) changing with height (in the case of Es). Technically said, a vertical gradient in the horizontal wind. The wind which affects the E-region is known as the west-neutral wind (it blows to the west and has an overall balance of positive ions and free electrons in it). One of the big problems before the wind shear theory came along was how did so much ionization get into so small a region and stay there so long. There was the choice of (a) creation of new ionization or (b) redistribution of the already existing ions/electrons. As a lot of Es occurs at night 'a' seemed very unlikely so 'b' was decided on. Now, when a charge moves across a magnetic field, depending upon the sign and direction, it will be curved in its path. This is known as the Lorentz Force, and, for those with vector knowledge, is given by: F = q (V X B) c where: q, charge; c, speed of light; v, velocity; B, magnetic field. This is a 3-dimensional equation, i.e., F is Fx, Fy, Fz. Putting this all together, collect some positive ions in a magnetic field with a wind shear and they will be driven into thin layers (mostly in regions where the wind passes thru a minima, not necessarily zero, or undergoes an abrupt change in direction). If the ions do not readily combine with the free electrons then high concentrations of the latter will be allowed to build up. The details of the theory get into Tensor mathematics. It relies on the magnitude of the horizontal component of the earth's magnetic field, which is at a high in the area of Japan and a low over South Africa.
Now the thorny problems. By rocket measurements (with released chemical vapor trails being photographed) it has been found that these neutral wind shears show little significant variation during the year. So, why the marked variations in Es with season and time of day? It thus appears that we need several things happening just right to get the formation of Es. The supply of metallic ions is needed. That's easy enough, meteor debris. Except for the showers, the flux is fairly constant for the whole earth -- i.e., again why seasonal and diurnal variations? Dr. Layzer (Mar 1972 Rad. Sci., pp. 385-395) in 1970 came forth with some nice points on the seasonal variations. In brief, it relys on the electrical conductivity in the E-region at the magnetic conjugate point (i.e., where the magnetic force line leaving a point on earth returns in the opposite hemisphere). He expects that the formation of Es is inhibited when conductivity is high there (e.g., summer there, winter here; high solar activity; magnetic disturbances). The theory also goes a long way to explain the two-peak diurnal pattern in summer vs. the single-peak in winter evenings. ______________ _ ________________ The really big contest on which to choose sides now presents itself. Are there really small blobs of intense ionization (plasma-dense, i.e., as given by the formula for Ne in Part II), or do the reflections on ionograms above fbEs (or above the secant law using fbEs on oblique paths instead of fEs) come from a form of scattering due to the steep ionization gradient found in Es by rockets. There have been some 100 rockets penetrate Es patches since the late 1940's. Not one has found our ellusive million electrons/cc density. But if the blobs of very high ionization were very small - on the order of a hundred feet or so - it is not likely that such a small number of shots could have passed through any. This is one big reason we need a method for horizontal probing in Es regions vs. the tiny vertical area a rocket traverses. The background ionization (100,000/cc) might well be imbeded with raisin-like dense (million/cc) blobs. At this point, some personal opinions will creep forth. From some 13 years of seeing 6-MHz wide TV signals being propagated with extremely good resolution (and with 'sharp' ghosting indicating discrete mutli- paths) it is hard to consider Es to be anything else but specular (mir- ror like) reflections from plasma-dense blobs of ionization. In less subjective measurements, it is somewhat difficult to explain a lot of field strength measurements of Es signals that are up to 20 db above a free space level (i.e., line of sight for the distance) if one thinks of a scattering mode as the main ingredient of propagation. Also, from physics there comes the phenomena of dispersion wherein waves of slightly different frequency are propagated at slightly different velocities. Results from rapid-pulse experiments with Es show little of this dispersion effect (as sharp TV video skip confirms when compared to F2 propagation), indicating a very thin region of ionization involved. _________ __ ___________ There are still a lot of unanswered details about Es, particularly the strong suggestion of tropospheric coupling found by several invest- igators (see Wilson, Dec 1970 and Mar 1971 QST). There appear to be a lot of refinements needed in the wind-shear theory to explain the more intense incidences of Es (i.e., in the VHF range). _________ Many involved in Es studies are not at it as an end in itself, but simply as a tool in detecting other parameters and behavior of the upper atmosphere. As will be seen in the following set of figures, Es effects when involved with 'normal' F2 paths can cause a lot of problems, in particular for h.f. users. (The most obvious problem for v.h.f. users is the co-channel interference aspect.)
In Figure 1 below is a propagation mode known as an N-type reflec- tion, whereby the F2 MUF may be extended into regions unexpected for it at a time of day or season. Figure 1 (not to scale) N-refl. While being an advantage at 'r', a receiver at 'm' may suffer interference from signals from 'r' with the 'normal' ones from 'x'. Also, any receivers more distant than 'r' that relied on F2 from 'x' have the paths disrupted to one extent or another. In Figure 2 is an M-reflection mode. The chief benefit for 'r' is that the signal from 'x' did not pass thru the absorbing D-region an additional two times and encounter a ground reflection loss as well. The big problem is for 'm' wondering whatever became of the signal from 'x', being a victim of 'Es shielding'. (topside reflections) With a little imagination, one can conjure up an endless variety of these multiple-layer modes, some even involving a temporary trap for a signal between F2 and Es. With this final installment on Es, it is hoped that many members might have a better understanding, if not just a different perspective, of what sporadic-E is and how it is scientifically studied. Since Es often affects services well above 100 MHz, its study is of value in VHF allocations when interference from normally independent systems is to be considered. Its effect on global F2 circuits (as in the above figures) cannot be ignored, as being below the F2 layer, Es can 'get' the signal first and last. While mathematics was kept to a minimum, some could not be dis- pensed with. On the other hand, a few readers may have felt that too little was used. Like a picture, an equation is often worth a thousand words. For those who may wonder why there wasn't more emphasis on DXing with TV, FM, etc...it is felt that articles in past (or future) VUD issues are better suited to aid the DXer with how to look for and use Es. As it now stands (without field strength measuring equipment) TV and FM DXers can best provide scientific tracking data on Es which when used with other data can often be extremely valuable. Remember there are more DXers active and a multitude of signals than there are regularly running Es experiments.