VLF Signal Propagation - A Discussion by SID Observers

Why do VLF signals strengthen at night instead of getting weaker? If propagation is basically via the waveguide effect, why doesn't the signal drop DOWN at night when the waveguide disappears with the D layer? Is there some kind of reduced absorption at night? if so, where is it taking place and why. Also, what accounts for the big fluctuations in signal strength at night, apparently more or less at random. Any ideas would be appreciated.

The strength of the received signal depends on the effective reflection coefficient of the region from which the radio wave reflects in its multi-hop path between earth and the ionosphere. In daytime the reflecting region is lower, the air density is higher, and the free electron density is controlled strongly by the solar radiation, etc. At nighttime the reflecting region is higher, the air density is lower, and the free electron density is controlled by variable ambient conditions as well as variable influences from electron "precipitation" from above, etc.

My interpretation is that the lower air density in the higher reflection region at night results in a higher reflection coefficient with less absorption whereas the higher air density in the lower daytime reflection region results in a smaller reflection coefficient with more absorption. In the daytime lower levels the free electrons suffer more collisions per second than do the free electrons at the higher nighttime levels so that the nighttime "mirror" is brighter (less absorption of the VLF radio signal). Electron collisions with heavier air particles result in absorption of the VLF radio signal and the absorption depends on the collision frequency. Nighttime fluctuations in signal strength are especially noticeable during and for sometime after geomagnetic storms when electrons somewhat erratically precipitate into the upper atmosphere from the earth's radiation belts.

Thanks so much for the very helpful explanation of VLF propagation. The free electron day vs. night difference makes sense to me. But I still gave a hang-up about the D layer. If this disappears at night then the nighttime VLF propagation is not "waveguide" mode? Some other mode? Does the "waveguide" adopt a night boundary where ionization is more persistent at night?

It may be better to speak in terms of "regions" rather than "layers". I am looking at a paper by C.A. Schoute-Vanneck (1973) called "VLF Radio Transmissions at Sunrise". An example is given which shows electron density at night and noon versus altitude.

At NOON the electron density is about 10 electrons/cm3 at an altitude of 40 km, 100 electrons/cm3 at an altitude of 60 km, 1000 electrons/cm3 at an altitude of 80 km and 10,000 electrons/cm3 at an altitude of 85 km.

At NIGHT these figures become 10 electrons electrons/cm3 at 85 km, 100 electrons/cm3 at 88 km and 1000 electrons/cm3 at 95 km and then remains somewhat the same up to at least 140 km.

So yes, at night the electron density in the lower part of the D region pretty much disappears. At 40 km the electron collision frequency is about 1,000,000,000 collisions per second whereas at 80 km the collision frequency drops to 1,000,000 collision per second. The reflection coefficient depends on ( among other things) the number density of free electrons, the collision frequency, and the frequency of the radio signal. It is found by a mathematical integration throughout the entire D-region and of course the result depends on what time of the 24 hour day one performs the integration. The paper mentions reflection coefficients of the order of 0.6 at night and of the order of 0.4 for noon. The paper calculates what the sunrise VLF signal strength signature should look like for certain transmission paths and compares them with actual signatures. The results were very good.

Thanks very much for your very interesting and enlightening letter answering Jim's question about D-Layer propagation. There were some other interesting papers written back in the 1970s by a theoretician at the National Bureau of Standards whose name was Wait (not sure of the spelling). He developed the wave guide theory of D-Layer propagation of VLF signals. The waveguide mode as I remember, applied only to daytime D-Layer propagation and said nothing about nighttime propagation. Presumably we can think of the E-Layer propagating the signal at night. Then the prominent sunrise pattern we see is a shift from E-Layer propagation back to D-Layer as the sun rises and forms the daytime D-Layer. The sunset pattern is the reverse. An interesting feature of waveguide mode propagation was that the signal was split into two components which can form an interference pattern. I believe it was the Earth's magnetic field that caused the splitting. The interference pattern was discovered way back in the 1930s and is described in a paper published by the Cavendish Laboratory in England. They measured the signal strength of GBR on 16 kHz with two identical receivers each with a loop antenna. One loop antenna was oriented so its plane was perpendicular to the direction of GBR and therefore it placed the GBR signal in its null. The weakened signal, they called the ground wave. The other loop was oriented to receive what they called the down-coming wave which was reflected from the D-Layer. The receiver outputs were displayed on an oscilloscope; One output fed the horizontal axis and the other the vertical axis. They set the gains so the scope displayed a circle. Then the whole business was placed on a truck and driven away from GBR and set up at various distances. As the distance increased the circle turned into an ellipse. As they continued farther from GBR, the ellipse returned to being a circle and then at still farther distance it became an ellipse again but with its axis rotated 90° This paper also shows a strip chart recording of GBR at the time of a solar flare and the signature is the same as the way we record flares as SESs. We sometimes see inverted SESs which are probably due to this interference pattern between what the Cavendish Laboratory called the ground wave and the down coming wave that was reflected from the D-Layer. I'm not sure but I believe the waveguide mode theory calls them the ordinary wave and the extraordinary wave, terms also seen in books on optics.

Thanks again for your helpful (and patient) ideas on this propagation issue. I've been doing a bit of reading with what references I have around here, and I think my failure to understand was based mainly on my forgetting that there is another (and very important) VLF propagation mode besides the "wave guide." That, of course, is good ‘ol ground (surface) wave!! At night the wave guide effect is largely lost. BUT: surface wave propagation is so enhanced due to lower absorption that it can provide a higher signal even than the wave guide does during the day. The daytime wave guide mode provides us with a smooth curve because of its inherent stability (except with solar flare effects, of course.) The nighttime surface wave is much more sensitive to random changes in electron density and the like. That being the case, one would expect to be able to see some kinds of incoming ionizing events at night (perhaps things like gamma ray bursts.) The problem, of course, is recognizing them among all the clutter.

Some further thoughts on inverted SESs are that they are much easier to understand following Al's advice and thinking of a D-Region rather than a D-Layer. Layer suggests reflection more like a mirror when in fact the free electrons density varies by a factor of 1000 over the 45 km thickness that probably does most of the reflecting. The density gradient can be thought of as being in balance with ambient solar ultraviolet radiation and the recombination rate of the free electrons. The balance produces a straight line on our SES charts. An M4 flare like the one yesterday disturbs the delicate balance when it emits x-rays and additional ultraviolet radiation. These increase the ionization in the D-Region and therefore the density of free electrons throughout the D-Region. More free electrons make it a better reflector and also lowers its effective height. Being a better reflector enhances the signal strength to produce the SES. Lowering the effective height changes the phase relationship between the ground wave and the reflective sky wave if you think of it the way the Cavendish Laboratory explained it back in the 1930s, as described in my letter yesterday. This is easier to understand than the waveguide theory. What happens next depends on how far you are from the signal. Here in Florida my distance from NAA on 24 kHz in Maine, USA must be such that the two phases first become more out of phase to cause the signal strength to decrease. As the effective height is lowered more, they pass through being 180° out of phase and the signal then increases as the sky and ground wave start to become more in phase. Jerry, A-50, in Texas is farther from NAA than I am and he seldom sees inversions in his SESs.