Skywave operation is really the most relevant here, line of sight might be effected somewhat by noise floor effects from sun but that is only relevant some of the time.

Reflection Layers

There are two main factors from the sun that affect operation. One is radio interference, this would be caused by flares and ejections directed at earth. In extreme cases it can cause an EMP but thats very rare.

The other is ionizing radiation in the form of UV (a much shorter wavelength version of UV than what reaches the earth), ionized particles, solar wind and similar. This ionizes the ionosphere in a specific way that causes radio waves to be blocked and/or reflected. Basically there are two regions in the ionosphere both in whats called the F-region.. These are F1 which is at 200 km above sea level, and F2 which is at 300 km. The higher F2 layer allows for radio signals at a particular angle to be reflected, this allows radio operators to bounce their signals off this part of the sky and reach distant receivers. Since F2 is much closer to the ground this particular region actually blocks long range communication and thus significantly reduces the distance a radio signal can reach to mostly line of sight.

Usually, when the radiation is high enough, we see the F2 layer ionize first in the morning (basically when the sun is just coming up for people who live near your horizon), which makes the morning the best time to transmit. By afternoon the F1 layer is ionized by sun radiation and thus the signal is blocked again.. the reverse happens in the evening. So early and late day propagation is best. This is called gray-line propagation.

There is also E layer propagation which operates at much lower frequencies and at steeper angles. This is what is used for very short distance transmissions within the 100’s of km. This is called NVIS (Near Vertical Incidents Skywave).

Reading the screencap

Now with this said, it only works when the radiation from the sun is just right. Basically there needs to be enough radiation from the sun to actually fully ionize the layers.

SFI

SFI on the chart stands for “solar flux index” this is a measure of the quantity of ionized particles and solar wind measured. This is usually in the range of 0 - 400 with 0 - 100 being poor for propagation, 0 - 200 being marginal, and 200+ being ideal/good.

SN

SN stands for sunspot numbers, these effect different layers selectively. sunspots reflect the intensity of the sun’s magnetic field. It ranges from about 0 to 400 as well.

Lower values here show a preference to ionize lower levels of our ionosphere. 0 to about 150 will preferentially ionize the E-region and be ideal for low frequency propagation (160m wavelength to 80M) in the NVIS configuration, so very short distance (100’s of km) propagation only which is all these low frequencies can ever do.

Higher values, above 200 means the F-regions are preferentially ionized. That means low frequencies like 160m and 80m will not propagate at all and only work line of sight (10’s of km), but higher frequencies ~20m and higher in frequency will propagate via skywave. These frequencies now can propagate 1000’s of km around the world in these conditions (assuming SFI and other factors are good).

K index

The next line is K-index just labeled K. This one is rather complicated.. it basically looks at the horizontal component of the earth’s magnetic field and how it is disturbed (which is an indirect way of measuring the solar winds and its interaction of the earth).

This doesn’t effect the ionosphere itself so much as the other measures since its only partly effected by solar winds. This is used as a measure of expected band noise and thus how high the noise floor will be. the max value is 9 and indicates significant noise. 5 is about the cutoff where geomagnetic storms are present.

The K-index is not linear and is calculated from the a-index (lowercase a, different from A-index below).

A-index

is really just another way of measuring K-index. Or to be more precise both A-index and K-index are calculated from the underlying a-index (a-index is different from A-index). The A-index is the average of the last 8 a-index, and thus has a much simpler interpretation.

Think of A-index as a long-term rolling average of the K-index in a different scale.

Generally A-index is even less linear than K-index with 0-50 being low noise and 100 - 400 being high noise.. lower is better.

304A

304A stands for “304 Angstroms” which is the wavelength of UV light measures. Basically its the strength of UV radiation from the sun as measured from space (different than earth UV levels). the “@ SEM” part refers to the instrument on the satellite used to record it, called SEM, SOHO and EVE are other possible instruments used to get this measurement and it changes depending on the instrument available at the time.

In this case higher is better as it means more of that F-layer ionization I mentioned.below 80 is poor, 150 and up is good, 250 and up is amazing.

Ptn/Elec Flux

This is Proton and Electron flux. These have a similar effect as UV except they ionize the E-layer more so than the F-layer. So they harm long distance short wavelength propagation but improve short distance long wave-length propagation.

Aurora

This is just the predicted chance of aurora. Not directly relevant for radio.

Aur Lat

This tells us the largest lattitude likely to see the aurora.

Bz and SW

This is the interplanetary (in space) magnetic field vector (B-field means magnetic field). This is the magnetic field that is incoming and striking the earth from space.

The Bz part is the intensity, the SW part is the direction in degrees. When it lines up with the earths magnetic field it strengthens it, when it doesnt it weakens it. Positive values strengthen it, negative weaken it.

other values

Everything else is self explanatory I suspect. “solar flare prob” is the percentage change of a solar flare, which we dont tend to know until just a few minutes before their ejections strike.

“MUF” stands for “Maximum Usable Frequency”. It indicates the highest frequency (shortest wavelength) that is likely to be capable of bouncing off the ionosphere (f-layer or e-layer) and therefore the highest frequency capable of skywave propagation.

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