Near Vertical Incidence Skywave
Near Vertical Incidence Skywave (NVIS) propagation is one of the mechanisms of propagation of RF in the lower HF amateur bands (specifically 160m, 80m and 40m). NVIS provides fairly reliable propagation beyond ground wave distances (say 30 - 50km distance) out to about 400km. Beyond that distance, normal skywave propagation via refraction from the E and F-layers of the ionosphere applies; however, those normal low incidence angle propagation modes produce a so-called skip-zone - an area between the edge of ground wave coverage and the start of the first refracted signals from the E or F layer.
For a given state of the ionosphere (which varies according to the time of day, the tilt of the earth (season), the 11 year sunspot cycle, magnetic storms and other disturbances, and normal statistical variations), there is a frequency at which a vertical signal will be reflected back to earth by each layer (E, F, F1 or F2) of the ionosphere - this frequency is called the Critical Frequency. For locations around the world, the Critical Frequency is measured using ionospheric sounding (in Australia by the Department of Science - Space Weather Services) and incorporated into propagation predictions (such as the SWS GRAFEX prediction, as discussed in the PARG presentation on HF Ionospheric Prediction (Workshop/Technical)).
At frequencies above the Critical Frequency for a particular layer of the ionosphere, vertical signals pass through that layer - but as the angle if incidence decreases from 90 degrees to low angles, refraction starts to occur, and the signal starts to be reflected (I suggest, refracted... but that's my own theory) and skywave propagation over long distances past the Skip Zone occurs. However, below the Critical Frequency - signals at high angles of incidence with the ionosphere are refracted/reflected back to earth, close to the transmitter (typically from 40km to 400km). The Critical Frequency varies during the day - typically rising from around 3MHz to around 10MHz after dawn and dropping back around dusk. This is the mode of propagation that we might typically use on 40m during the day and 80m at night for 40 - 400km contacts.
In the attached paper, (Link) Ben A Witvliet a PhD student of the Netherlands' University of Twente discusses NVIS propagation from the perspective of its use for emergency communication (for cases where the existing infrastructure is damaged); the dissertation includes a number of published papers (by the author with other participants) in addition to the normal literature review. Although the entire document is 200 pages long, the individual articles stand alone and can be read with only limited reference to the others.
For example, one of the papers demonstrated a very novel way of measuring the angles from which the NVIS signals were typically received. A short-duration amateur radio contest in Holland on 40m and 80m was used to gather statistical data on the received propagation angles, based on measured angle of reception from amateur stations across the Netherlands. Clever!
The other articles provide useful information on transmitting and receiving antennas for NVIS. Yes, I understand about Maxwell and reciprocity.... but the articles articulate that what drives practical receive antenna performance is received signal to noise ratio.... and antennas that pick up less noise will work better on receive than antennas that may have gain (or less loss) at high angles of radiation. The articles also allude to why vertically polarised (and mobile whip / wound squid pole) antennas are not useful for NVIS propagation. Interestingly, the articles demonstrate that a horizontally polarised transmit antenna will produce two separate NVIS signals at the receiver - one right hand circular polarised, and one left hand circular polarised - as a result of the influence of the earth's magnetic field (NB - the direction of polarisation may be different in the Southern Hemisphere, and the shape of the polarisation changes from the equator to the poles).
Now don't fret when you open the very large file (it could take a long time to load) - you'll see that the first few pages are in Dutch! However, the articles themselves are all in English. I suggest that you use the index to navigate (unfortunately it's not hyperlinked, but you might be able to Ctrl-F and insert the name of the Chapter into the search that opens).
For those of us who use 80m for the Tuesday arvo/evening CQ QRS Slow CW practice sessions, the PARG monthly 80m net, the PARG WinLink net, or for general rag-chewing - the thing not emphasised in the articles is the effect of the day-time D-layer absorption on the lower frequencies. So for us, on 80m although NVIS exists during the day, the mode usually can't be used because of absorption of signals by the D-layer. At night on 80m, the solar-radiation-driven D-layer dissolves (and the F-layer splits into F1 and F2 by the way), but most of the time at our latitudes, the critical frequency stays above 3.5MHz, so NVIS dominates at night.