"Sun Spot numbers" - an email received from from Paul Harden, NA5N 9/18/2006 6:46 AM via qrp-l
> Bob Henning writes:
> Hi Gang!!
> I was wondering if anyone could explain the what Solar flux, Sun Spot
> number, A index and K index actually mean, and how do I use them. I use
> HFProp in my shack, and would really like to know how these numbers can
> help to determine when the best times are to operate.

During the active sun, I often try to post various explanations of solar activity and HF propagation. Now being in the solar doldrums, it seems there's not much to talk about. However, we often forget how newcomers to QRP come into the hobby. So for those getting interested in the solar phenomenon during the minimum, the following is a general description of things you will hopefully find to answer your questions. As others have mentioned, I would also recommend reading my FDIM presentation (.pdf) on the ARCI website, www.qrparci.org.

The sun is basically a thermonuclear reactor at it's core, producing ionizing gamma and x-ray radiation. By the time this radiation reaches the cooler surface of the sun, the wavelength is much longer ... in the radio spectrum. This produces a wideband radio output from the sun, which is measured on earth to represent the overall electromagnetic energy being generated by the sun. It is measured at 2880MHz, or 10cm, as a frequency to use as the standard reference, and a frequency for which our ionosphere is fairly opaque and not generally effected by the effects of solar flares. The solar radiation measured at 2880MHz is the SOLAR FLUX, measured periodically and averaged over the 24-hour day. It seldom changes much hour-to-hour (or even day-to-day). During the quiet sun, solar flux in the 60-100 range is typical. During the active sun, 150-200 is typical. The higher the solar flux, the more ionizing radiation that is striking our ionosphere, producing free electrons that stratify into the D, E and F layers. The more free electrons in the E and F layers, the more reflective they are to HF frequencies and the higher the MUF. Right now, with solar flux in the 60-100 range, the E and F layers are poorly ionized, yielding a lower MUF and not acting as a very good mirror for bouncing HF signals back to earth. Very generally, when the solar flux is around 100, 15M will be open; above 150 10M will be open. Below 100, 20M will usually die shortly after sunset. IMPORTANT: The MUF seldoms drops below 10MHz. Therefore, the solar flux has very little effect on 30, 40 and 80M propagation. These bands are fairly immune from the solar flux and the 11-year solar cycle. Magnetic disturbances on the sun produce sunspots (cooler areas). Occassionally, the magnetic field lines of the disturbance(s) grow to such an intensity that it produces a small hole in the solar surface, allowing hot solar mass to escape. This is a SOLAR FLARE. While this hole is present (usually in the order of minutes to tens of minutes), energetic electrons and ionozing radiation (that is, x-rays and sometimes gamma rays) are allowed to escape. This, of course, quickly increases the overall radiation output of the sun. The ionozing radiation, when it strikes the earth 8 minutes later, will ionize the E and F layers, making them more reflective to HF and raise the MUF, usually for the rest of the day until local sundown. The radiation from especially strong flares can penetrate into our ionosphere to the D-layer. When the D-layer is highly ionized, it becomes very absorptive to HF signals, and in extreme cases, can produce a temporary HF blackout. Most flares will not appreciably increase the daily solar flux; therefore, the solar flux alone is not a good indicator following a flare to increased E and F layer reflectivity (and hence, good skip DX). As the number of sunspots increases, there is a higher chance of solar flares, and the daily solar flux tends to increase. However, there is *no* direct mathematical relationship between sunspot count and the solar flux. They follow the same trend when plotted, but no one can say 10 sun spots equals xxx solar flux units. The solar flux will vary from a minimum to maximum value over 28-days, related to the solar rotation. It also varies from minimum to maximum over the 11-year solar cycle. Thus, it is a slowly varying indicator that is used to show the general trend of the sun for the current 28-day cycle, and for the current solar cycle. It is not used for an hourly or daily predictor. Propagation programs use solar flux values primarily for calculating the MUF and what bands will be open, or closed, at different times of the day. When a solar flare occurs, it often produces a shockwave carrying electrons and other solar mass away from the sun. This is called a coronal mass ejection or CME. If the solar flare is located towards the center of the sun (as opposed to the limbs or edges), the trajectory of the shockwave will intercept with the earth, usually about 50-55 hours later. When this happens, the shockwave will compress the Earth's geomagnetic field, triggering a GEOMAGNETIC STORM, generating huge electric currents flowing along the Earth's magnetic field lines, causing increased noise levels. IMPORTANT: This effect is more pronounced on the lower frequencies, such that 30M, 40M, 80M are more effected by the "noise storm" than is 20, 15 and 10M. The amount of "wiggling" or disturbance to our magnetic field is the K-Index. It is measured every 3 hours to show what the present state of our geomagnetic field is. K=1 to 3 is fairly quiet to unsettled. Higher numbers (K>4) is a geomagnetic storm. K>7 is a severe to extreme storm. The K-Indices throughout the day are averaged over the UTC day to form the A-Index. It basically tells you what our geomagnetic field did YESTERDAY. It is usually expressed as the "Ap," or planetary A-index, being averaged over 24 hours and from all the reporting stations.
The SOLAR FLUX tells you the general radiation output of the sun. But, don't expect it to make a sudden jump to open up 15 or 10M. That takes years ... or an M or X-class solar flare. The A-Index tells you what our geomagnetic field did YESTERDAY. It tells you almost nothing about what the bands sound like TODAY. For that, find out what the current K-Index is. The lower the number, the better. Above about 6 or 7, conditions on the LOWER bands will be very rough.
For QRPers, a good time to operate is right after a solar flare. Once the solar flare is over, our E and F layers are highly ionized for good reflectivity and higher MUF, and will stay that way for the duration of your local sunlight. This can cause several hours of unexpected openings on the higher bands and fairly quiet signals on 20M. Conditions will be normal the following day or two ... until the shockwave from the flare arrives, triggering a geomagnetic storm. This can last from a few hours to over a day. Once the geomagnetic storm is over (when the K-Index falls back to 1-3), our geomagnetic field tends to get very quiet for a day or two (unless triggered by another CME, though unlikely where we are right now in the solar cycle). Therefore, another good time for QRPers to operate is following a geomagnetic storm when conditions can often be fairly quiet on 30 and 40M, and sometimes even 80M. Especially at night. This is really all you need to know to understand the effects of HF propagation due to solar flux and the K- and A-Indices. The only other real variable is throughout the year, our sun "shines" on the earth at different latitudes (higher in the summer, lower in the winter for northern hemisphere). This changes the paths signals bounce off the E and F layers ... whether the "skip" is more east-west from your location, or other directions. This changes throughout the day and throughout the year. This is what the propagation programs primarily exploit to determine good times to work South America or the Middle East from your QTH. Let's be honest. For a QRPer, if your propagation program says conditions are good to the Pacific and you end up in a QSO with Israel ... who's going to be disappointed over that! Heck, from New Mexico, I get excited when I work Alaska or Nova Scotia :-)
72, Paul NA5N

> Bob Henning writes:
> Hi Ron, I have been doing some research on this very subject, Is this what
> they call a "Ten-flare" ??

Bob, and others,
In my post yesterday, I pointed out that 2880MHz, or 10-cm., was the frequency used to measure the solar flux since that frequency is seldom effected by solar flares. However, once in awhile a particularly large flare or one in the right geometry will cause the daily solar flux measurements to suddenly increase. When a solar flare contaminates the 10-cm. solar flux measurements, it is called a "ten flare" (for "10-cm. emission flare"). NOAA tries to "smooth out" the reading for that day, but reports it as a "ten flare" so you know the value was likely contaminated by the solar flare. As reported numerous places, the solar flux measurements are made at the Penticton Observatory in Canada. Actually, the proper and legal name of the facility is the Dominion Radio Astrophysical Observatory, or DRAO. They have been been the official radio observers of the sun and recording the daily solar flux since the 1950s. They have a proud and outstanding record of virtually no downtime or missed measurements over the past 50 years.
72, Paul NA5N