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Aurora can affect propagation conditions over the polar routes and is often used
to enhance VHF propagation, especially at 50 MHz and sometimes the 144 MHz Amateur
bands. At times of auroral displays, HF radio signals traveling over the
polar route will develop a distinctive warbling sound. You can often hear these
effects on 14 MHz SSB signals that have followed the great circle route over the
polar region. During intense auroral activity, HF propagation down to the 3.5 MHz
band can be affected. And if you are lucky enough to be in the right place, you
can watch the spectacular visible auroral displays in the night-time sky.
Taken from the article on Basic Radio Wave Propagation
by E. C. Jones, AE4TM. Click the link to view the entire report.
A- and K-index: Geomagnetic activity indices, high indices (K:>5 or A:>20) means stormy conditions with an active geomagnetic field. The more active, the more unstable propagation with possible periods of total propagation fade-out. Especially around the higher latitudes and especially at the polar regions, where the geomagnetic field is weak, propagation may disappear completely. Extreme high indices may result in aurora propagation, with strongly degraded long distance propagation at all latitudes. Sporadic-E is strongest during low indices. Low indices result in relative good propagation, especially noticeable around the higher latitudes, when transpolar paths may open up. Maximum K-index is 9, and the A-index can exceed well over 100 during very severe storm conditions, with no maximum. The ARRL often reports the K-index from the Alaskian station where this index is known as the College K-index. Other stations reporting K-indices include Planetary and Boulder. In contrast, the A-indices are usually reported for the Planetary station only.
The higher the K-index, the more unstable propagation becomes, the effect is stronger at high latitudes, but weaker near low latitudes. When storm level is reached, propagation strongly degrades, possibly fade out at high latitudes.
Classification of K-indices are as follows:
K8=Very severe storm
K9=Extremely severe storm
As with the K-index, the higher the A-index, the more unstable propagation becomes.
Classification of A-indices are as follows:
A0 - A7 = quiet
A8 - A15 = unsettled
A16 - A29 = active
A30 - A49 = minor storm
A50 - A99 = major storm
A100 - A400 = severe storm
Solar Flux: This flux number is measured from the amount of radiation on the 10.7cm band (2800MHz). It is closely related to the amount of ultraviolet radiation, which is needed to create an ionosphere. The lowest possible number for this solar flux is 63.75. Single hop propagation already starts at 70 in lower latitude areas. Worldwide long distance propagation (DX) may turn up already with a solar flux at 120. From experience, an average solar flux of 170 seems to be ideal for 10m-20m bands QRP DX with good possibilities during these conditions to reach every possible part of the globe with a simple dipole running as low as 5 Watts!
Taken from the article on Propagation by Glenn Hauser. Click the link to see the whole article
Sporadic E skip (Es)
Es has a set of characteristics which, when taken all together, set it apart from all other forms of DX propagation. It builds up from low frequencies to a certain maximum usable frequency (MUF) which may vary widely from minute to minute, and opening to opening. Es always hits the lower frequencies first. It may or may not get above channel 2. Of course, if the skip is coming from a sparsely populated area, there may be no channel 2 transmitter --- so check channels 3 and 4 as well. A good opening will not stop at channel 6, but may continue upward into the FM band which begins immediately above channel 6. An extraordinary opening may even continue into the aeronautical band above 108 MHz, through the "2 meter" (144 MHz) ham band, past the heavily-populated 2-way mobile bands, even up to channel 7 --- 180 MHz! An Es opening reaching channel 7 is a rare treat; high-band (TV channels 7-13) Es may even poke a channel or two above 7. Usually, highband Es can be ruled out when the skip is not extending above channel 6 into the FM band.
In general, as Es distances shorten on the low band and FM, the opening is becoming more intense and the MUF goes up. As experienced Dxers can attest, Es at 1000 miles is much more common than Es at 600 miles. The usual minimum distance for Es is about 500 miles. In fact, a study we made of hundreds of Es receptions revealed that 950 miles is the optimum distance on channel 2, with distances lengthening slightly with each higher channel, up into the FM band.
Es occurs when patches in the E layer of the ionosphere, about 65 miles above ground, become ionized. This layer normally refracts shortwave and mediumwave signals but is transparent to VHF radiation. The cause of Es ionization is not precisely known; some researchers connect it with low pressure areas and thunderstorms. However, it has no direct connection with surface weather, and the novice's association of Es with regular clouds in the sky is completely erroneous.
For once thing, the Es patch must be at approximately the midpoint between the transmitter and receiver, far beyond the visible horizon. Sometimes the patches remain fairly stationary, but usually they move at speeds up to several hundred miles per hour, more or less in a straight line. This means that one station after another will come in, with quite a lot of interference as they overlap.
They will probably be in a rough geographical progression, but not in a straight line. One can plot the midpoints on a map, and by correlating observations with other DXers viewing at the same time from other angles, pin down the Es patch with a degree of accuracy. This can prove useful in determining probable target areas (PTAs).
Long single hops of Es can reach about 1500 miles. Double-hop or cloud-to-cloud hop Es often occurs during the summer when more than one Es patch may be active simultaneously, in different parts of the continent. The two patches, the station, and you must all be along the same line. However, not too many stations are identified by double-hop Es for several reasons:
(1) Interference: the TV and FM bands are so congested in North America that there are usually stations on the air near the double-hop path midpoint, severely interfering with further stations.
(2) The earth is a rather poor reflector of VHF signals, but this it must do at the midpoint. Double-hop Es where the midpoint is water (an ocean or Great Lake) is much more efficient.
(3) The patch with the lower MUF is the controlling factor. For this reason, there's much more double-hop Es on channel 2 than on channel 6 or FM.
Es is very unpredictable, but we do know this much! Es is very much a summertime phenomenon in the temperate latitudes, with peaks in June and July; very good openings also in May and August; and a sprinkling in late April and early September. It can occur on any day of the year; these are known as off-peak openings. The winter solstice also brings a minor peak in December and January, as if some of the Es' fury were "bleeding over" from the southern hemisphere where, of course, the summer peak is in progress. The winter and off-season openings are most likely in the early evening hours. During the main "season", Es may start early in the morning and continue all day, into the night, but it likes to take a breather around mid-morning and mid-afternoon, and seldom lasts much past midnight. If you don't want to miss an Es opening, try checking once or twice an hour just before ID time, or tune a VHF radio paging channel.
Es can be very strong with lots of fading and interference. But strong signals may rival those of local stations and even interfere with them. Es may build up rapidly, over the course of a few minutes, but usually it decays more slowly. Weak openings in which the MUF hovers around TV channel 2 may tantalize you as stations fade in and out.
Es is more likely in southern areas during the off-season, but northerners should not assume that subzero temperatures or snowstorms rule out any DX! Closer to the equator, Es becomes more and more a year-round, daily phenomenon. "Diurnal Es" may provide a weak, scattery signal virtually every day over an Es-distance path. Other strange things happen, such as Es reception at double-hop distances but with the signal bouncing from one path to another without touching ground in between.