How do the numbers in an eAlert (Kp, Ap, SFI, and so forth) tell us what is going on with propagation?
First, let's look at the Ap and Kp numbers. The more active the geomagnetic field, the more unstable propagation is, with possible periods of total propagation fade-out. This is especially true at higher latitudes and especially at the Polar Regions, where the geomagnetic field is weak. At these high latitudes, propagation may disappear completely long before total degradation of signals over low- and mid-latitude paths. Extremely high indices may result in aurora propagation, with strongly degraded long-distance propagation at all latitudes. Low indices result in relatively good propagation, especially noticeable around the higher latitudes, when transpolar paths may open up.
The 10.7-cm Solar Flux index (SFI) is a number obtained from the amount of radiation on the 10.7-cm band (2800 MHz). It is closely related to the amount of ultraviolet radiation, which is needed to create an ionosphere. We want high SFI numbers because this would translate to higher levels of ionization, which in turn would provide more stable refraction of radio signals, higher in frequency. Solar Flux readings are more descriptive of daily conditions than the Sunspot Number. The higher the Solar Flux, the stronger the ionosphere becomes, supporting refraction of higher frequencies.
The Sunspot Number (SSN) is a number related to the observable sunspots on the solar face. Sunspots are magnetic regions on the Sun with magnetic field strengths thousands of times stronger than the Earth's magnetic field. Sunspots appear as dark spots on the surface of the Sun.
The sunspot number is calculated by first counting the number of sunspot groups and then the number of individual sunspots. The "sunspot number" is then given by the sum of the number of individual sunspots and ten times the number of groups. Monthly averages (which are updated monthly) of the sunspot numbers show that the number of sunspots visible on the sun waxes and wanes with an approximate 11-year cycle.
We look at the Planetary A (Ap) index to get a picture of how conditions have been and to discover a trend. The Planetary K (Kp) index, on the other hand, indicates the overall current state of the geomagnetic field. When the Ap has been rising, or has been high over several days, we expect that the ionospheric propagation will be degraded. If we see the Ap falling, or remaining low for a number of days, we can expect that shortwave propagation will be good to excellent, even possibly over the high latitude and transpolar paths. On the other hand, if we see a quick rise in the Kp index, we might be alert for sudden fading and loss of signals, and even possible Aurora.
If the Kp index rises above 5, it is quite possible to have visual sighting of Aurora in mid- and even low-latitude locations. Some recent aurora events in the last several years have been viewable as far south as Mexico. Propagation was shut down on the high frequencies during these periods, but aurora-mode propagation on VHF and above was quite active. When we see the Kp index rapidly falling, or staying low for a period of time, we expect great conditions on the high frequencies.
We look at the sunspot and 10.7-cm activity because these numbers have a direct correlation to the level of ionization during the period in question. Over many years of careful observation and exploration, scientists have been able to model the way the ionosphere works, and how it is influenced by the solar activity. Using software tools, even radio hobbyists may now take the sunspot and flux numbers and make an analysis of propagation over various paths through the ionosphere.
The general rule of thumb is that the higher the solar activity, as shown by higher solar sunspot numbers and higher solar flux numbers, the higher the frequencies that will propagate via the ionosphere. So, when we see a trend of rising flux levels over several days, we can expect openings on higher frequencies, while a dive in flux levels warn of the closing of higher frequencies.
Your example of the 41-meter station - and that you want to know when you will be able to hear it, and not struggle with tuning it in - is addressed by looking at the typical propagation characteristics of the 41-meter band frequencies.
During the day, some propagation of 7 MHz signals is possible off of the F layer (and perhaps the E layer), for paths up to 2500 miles away. However, because the daytime D layer of the ionosphere is highly ionized by the direct radiation from the sun, a great amount of the signal energy (perhaps all of it) is absorbed by the lowest of the ionoshperic layers. Ths signal just does not get through the D layer. So, signals tend to be "close in" on the band. But, as the gray line approaches, and night conditions arrive, the D layer nearly dissapears, allowing the 41-meter signals to punch through to the higher E and F layers. There, the signals are refracted, making DX possible on these frequencies far away, even over multiple hops.
So, if the K (and A) indexes are high, why would the 41 meter band shut down? This would happen during minor to major geomagnetic storms because the geomagnetic influence on the chemistry of the ionosphere causes a recombination of the ions with the atoms. This causes the Maximum Usable Frequency (that frequency which is the highest that would be refracted, rather than able to "punch through") to decrease by as much as 50% of the normal MUF. If the MUF was, say, 15 MHz, and a major geomagnetic storm occurs (Kp > 7), 41 meters will barely be under the MUF - causing signals to fly out into space, rather than be refracted back to earth at a distant point.
It is useful to use one of the software tools that allow you to plug in these numbers and get a "generalized" picture of propagation. Several great offerings exist which are built on the IONCAP engine. I'll be posting more about those in another message, later.
I hope this helped answer your questions.