QST de W1AW
Propagation Forecast Bulletin 38 ARLP038
From Tad Cook, K7VVV
Seattle, WA September 14, 2001
To all radio amateurs
SB PROP ARL ARLP038
ARLP038 Propagation de K7VVV
Average daily sunspot numbers were up over 86 points this week over last, and average solar flux rose 39 points. The daily sunspot number peaked on Sunday at 291, the highest value since April 1. Solar flux peaked on Tuesday at 249.7, also the highest since April 1. Solar flux is expected to level off over the next few days, to 235 on Friday, 225 on Saturday and Sunday, and around 215 on Monday. A large complex of sunspots has just crossed the sun's central meridian. Holographic images of the sun's far side show another large sunspot, probably region 9591 which produced a large solar flare at the end of August when it was on the sun's earth side.
There have been several coronal mass ejections since September 11, but they were not earth-directed. Geomagnetic conditions have been quiet this week, but on Thursday we are experiencing a rise in activity. On Friday the predicted planetary A index is 20, and conditions are expected to be unsettled after that.
K9LA recently wrote a better version of the explanation of the terms and numbers that runs in this bulletin from time to time. He has generously offered to share it, and it runs below. You can contact firstname.lastname@example.org with questions.
The Sun emits electromagnetic radiation and matter as a consequence of the nuclear fusion process. Electromagnetic radiation at wavelengths of 100-1000 Angstroms (ultraviolet) ionizes the F region, radiation at 10-100 Angstroms (soft X-rays) ionizes the E region, and radiation at 1-10 Angstroms (hard X-rays) ionizes the D region. Solar matter (which includes charged particles - electrons and protons) is ejected from the sun on a regular basis, and this comprises the solar wind. On a ''quiet'' solar day the speed of this solar wind heading toward Earth averages about 400 km per second.
The Sun's solar wind significantly impacts the Earth's magnetic field. Instead of being a simple bar magnet, the Earth's magnetic field is compressed by the solar wind on the side facing the sun and is stretched out on the side away from the sun (the magnetotail, which extends tens of Earth radii downwind). While the Sun's electromagnetic radiation can impact the entire ionosphere that is in daylight, charged particles ejected by the Sun are guided into the ionosphere along magnetic field lines and thus can only impact high latitudes where the magnetic field lines go into the Earth.
The Earth's magnetic field plays an important part in propagation. When electromagnetic radiation from the Sun strips an electron off a neutral constituent in the atmosphere, the resulting electron can spiral along a magnetic field line. Thus the ionosphere is critically dependent on the state of the Earth's magnetic field. Variations in the Earth's magnetic field are measured by magnetometers. There are two measurements readily available - the daily A index and the 3-hour K index. The A index uses a linear scale and goes from 0 (quiet) to 400 (severe storm). The K index uses a quasi-logarithmic scale (which essentially is a compressed version of the A index) and goes from 0 to 9 (with 0 being quiet and 9 being severe storm). Generally an A index at or below 15 or a K index at or below 3 is best for propagation.
Sunspots are areas on the Sun associated with ultraviolet radiation. Thus they are tied to ionization of the F region. The daily sunspot number, when plotted over a month time frame, is very spiky. Averaging the daily sunspot numbers over a month results in the monthly average sunspot number, but it also is rather spiky when plotted. Thus a more averaged, or smoothed, measurement is needed to measure solar cycles. This is the smoothed sunspot number (SSN). SSN is calculated using five and a half months of data before and after t he desired month, plus the data for the desired month. Because of this amount of smoothing, the official SSN is about a half year behind the current month.
Sunspots come and go in an approximate 11-year cycle. The rise to peak (4 to 5 years) is usually faster than the descent to minimum (6 to 7 years). At and near the peak of a solar cycle, the increased number of sunspots causes more ultraviolet radiation to impinge on the ionosphere. This results in significantly more F region ionization, and allows the ionosphere to refract higher frequencies (15 meters, 12 meters, 10 meters, and even 6 meters) back to Earth for DX contacts. At and near the minimum between solar cycles, the number of sunspots is so low that higher frequencies go through the ionosphere into space. Commensurate with solar minimum, though, is less absorption and a more stable ionosphere, resulting in the best propagation on the lower frequencies (160 meters and 80 meters). Thus high SSNs are best for high frequency propagation, and low SSNs are best for low frequency propagation.
Most of the disturbances to propagation come from solar flares and coronal mass ejections (CMEs). The flares that affect propagation are called X-ray flares due to their wavelength being in the 1-8 Angstrom range. X-ray flares are classified as C (the smallest), M (medium size) and X (the biggest). Class C flares usually have a minimal impact to propagation. Class M and X flares can have a progressively adverse impact to propagation. The electromagnetic radiation from these flares can cause the loss of all propagation on the sunlit side of the Earth due to increased D region absorption. In addition, big class X flares can emit very energetic protons that are guided into the polar cap by the Earth's magnetic field. This results in a polar cap absorption event (PCA), with high D region absorption on paths passing through the polar areas of the Earth.
A CME is an explosive ejection of large amounts of solar matter, and can cause the average solar wind speed to take a dramatic jump upward - kind of like a shock wave heading toward Earth. When the shock wave hits the Earth's magnetic field, it can cause large variations in and distortions to the Earth's magnetic field. This is seen as an increase in the A and K indices. Distortions to the magnetic field can cause those electrons spiraling around magnetic field lines to be lost into the magnetotail. With electrons gone, maximum usable frequencies (MUFs) decrease, and return only after the magnetic field returns to normal and the process of ionization replenishes lost electrons.
Solar flares and CMEs are related, but they can happen together or separately. Scientists are still trying to understand the relationship between solar flares and CMEs. One thing is certain, though - the electromagnetic radiation from a big flare, traveling at the speed of light, can cause short-term radio blackouts on the sunlit side of the Earth within about 10 minutes of the eruption. The energetic particles ejected during a flare and the shock wave from a CME can take up to a couple days to arrive in the vicinity of Earth to cause their disruptions to propagation.
Each day the National Oceanographic and Atmospheric Administration (NOAA) and the US Air Force jointly put out a Report on Solar and Geophysical Activity (RSGA). These reports are archived at http://www.sec.noaa.gov/ftpdir/forecasts/RSGA . Each daily report consists of six parts.
Part IA gives an analysis of solar activity, including flares and CMEs. Part IB gives a forecast of solar activity. Part IIA gives a summary of geophysical activity. Part IIB gives a forecast of geophysical activity. Part III gives probabilities of flare and CME events. These five parts can be summarized as follows: normal propagation (no disturbances) generally occurs when no X-ray flares higher than class C are reported or forecasted, along with solar wind speeds due to coronal mass ejections near the average of 400 km/sec.
Part IV gives observed and predicted 10.7 cm solar flux. A comment about the daily solar flux - it has little to do with what the ionosphere is doing on that day. This will be explained later.
Part V gives observed and predicted A indices. Part VI gives geomagnetic activity probabilities. These two parts can be summarized as follows: good propagation generally occurs when the forecast for the daily A index is at or below 15 (this corresponds to a K index of 3 and below).
WWV, at 18 minutes past the hour every hour, puts out a shortened version of this report. It gives the previous day's 10.7 cm solar flux, the previous day's A index, and the current 3-hour K index. Current solar activity and geomagnetic field activity are also given, along with forecasts for both of them. As in the RSGA report, normal propagation (no disturbances) is expected when solar activity is low and the geomagnetic field is quiet. A comment is appropriate here - both the RSGA report and WWV give a status of solar activity. This is not a status on the 11-year sunspot cycle, but rather a status on solar disturbances. For example, if the solar activity is reported as low, that doesn't mean we're at the bottom of the solar cycle - it simply means the sun has not produced any major flares or CMEs.
In order to predict propagation, much effort was put into finding a correlation between sunspots and the state of the ionosphere. The best correlation turned out to be between SSN and monthly median ionospheric parameters. This is the correlation that our propagation prediction programs are based on, which means the outputs (usually MUF and signal strength) are values with probabilities tied to them. They are not absolutes - understanding this is a key to the proper use of propagation predictions.
Sunspots are a subjective measurement - they are counted visually. It would be nice to have a more objective measurement, one that actually measures the Sun's output. 10.7 cm solar has become this measurement. But it is only a general measurement of the activity of the Sun, since a wavelength of 10.7 cm is way too low in energy to cause any ionization. Thus 10.7 cm solar flux has nothing to do with the formation of the ionosphere. The best correlation between solar flux and sunspots is smoothed 10.7 cm solar flux and smoothed sunspot number - the correlation between daily values, or even monthly average values, is not very acceptable.
Since our propagation prediction programs were set up based on a correlation between SSN and monthly median ionospheric parameters, the use of SSN or the equivalent smoothed 10.7 cm solar flux gives the best results. Using the daily 10.7 cm solar flux, or even the daily sunspot number, can introduce a sizable error into the propagation prediction outputs due to the fact that the ionosphere does not react to the small daily variations of the Sun. Even averaging 10.7 cm solar flux over a week's time frame can contribute to erroneous predictions. For best results, use SSN or smoothed solar flux, and understand the concept of monthly median values. If there was a good correlation between what the ionosphere is doing today and today's solar flux or sunspot number, then we'd have a daily propagation model as opposed to a monthly median model.
Sunspot numbers for September 6 through 12 were 204, 288, 281, 291, 217, 180 and 228 with a mean of 241.3. 10.7 cm flux was 222.2, 226.1, 249.5, 236.2, 244.5, 249.7 and 235.1, with a mean of 237.6, and estimated planetary A indices were 8, 6, 7, 7, 5, 9 and 13 with a mean of 7.9.