ARLP019 Propagation de K7RA

For several weeks we expected today, May 2 to have active geomagnetic conditions. For instance, if you look at a forecast from April 23 at,, it shows an expected planetary A index for May 1-3 of 10, 20 and 15. The next day, April 24 (see this changed to 8, 20 and 15, and on April 25 it was 10, 15 and 15. For May 1 we see the actual planetary A index for that day was 9, and for the following two days, the predicted values are 10 and 12, which are much more moderate. So obviously as we moved closer to this date, the return of a solar wind stream seemed less likely, although earlier today the planetary K index rose as high as 4, indicating unsettled to active geomagnetic conditions.

The IMF (Interplanetary Magnetic Field) pointed south when the K index was high, which means our planet was vulnerable to radiation from solar flares and wind streams. Later the IMF pointed north, and the K index dropped to 0, which is very quiet. Nine planetary K index readings from 1500z May 1 through 1500z May 2 went from quiet to active to quiet again, 0, 1, 3, 3, 4, 4, 4, 3 and 0.

A solar wind stream flowing from a coronal hole is expected to reach earth on May 5. The expected planetary A index for May 2-11 is 10, 12, 12, 8, 10, 8, 5, 5, 5 and 5. That forecast is from the NOAA Space Weather Prediction Center and the U.S. Air Force. Those numbers are from the May 1 prediction. You can see the May 2 prediction some time after 2100z today by changing the last element of one of the URLs in the first paragraph from 042445DF.txt to 050245DF.txt.

That same forecast from May 1 is now predicting a 10.7 cm solar flux reading of 70 for the next 45 days. But on April 28 they were predicting solar flux to rise to 75 from May 20-28.

The Solar Department at the Ondrejov Observatory of the Prague Astronomical Institute (part of the Academy of Sciences of the Czech Republic) projects a 10.7 cm solar flux range of 66-73 and a sunspot number range of 0-25 for May 2-8.

For the same period Geophysical Institute Prague predicts geomagnetic activity of unsettled to active May 2, active on May 3, unsettled May 4-6, quiet to unsettled May 7, and quiet May 8.

F.K. Janda, OK1HH of the Czech Propagation Interest Group predicts for the next solar rotation (about four weeks), quiet conditions May 8, 11-12, 15-16, (17,) 18-19, (22, 24,) and 26, unsettled to active conditions May 2, 6-7, 9, 13 and 23, and active to disturbed May (3-5, 10, 14,) 20, (21, 25, and 27) The dates in parenthesis are days he expects a lower probability of solar activity enhancing propagation.

HF operators looking for good propagation like to see higher sunspot and solar flux numbers, because the MUF (Maximum Usable Frequency) is higher, and they want lower A and K index numbers (indicating quiet geomagnetic conditions) because absorption of HF signals is generally lower. The two don't always go together. High sunspot activity, which we haven't observed for several years, may often result in higher geomagnetic activity.

Now that we have all the sunspot data for April, we can take our monthly look at our 3-month moving average of daily sunspot numbers to give us a hint on where this cycle may be heading. The March 08 figures below are an average of daily sunspot data from February 1 through April 30.

May 06 39.7
Jun 06 28.9
Jul 06 23.3
Aug 06 23.5
Sep 06 21.2
Oct 06 24.1
Nov 06 23.1
Dec 06 27.3
Jan 07 22.7
Feb 07 18.5
Mar 07 11.2
Apr 07 12.2
May 07 15.8
Jun 07 18.7
Jul 07 15.4
Aug 07 10.2
Sep 07 5.4
Oct 07 3
Nov 07 6.9
Dec 07 8.1
Jan 08 8.5
Feb 08 8.4
Mar 08 8.4

We can see that a slight rise in sunspot activity from October levels has flattened. Although another digit isn't really significant, for readers who prefer to parse their parameters more precisely, the December through March averages may be expressed as 8.14, 8.52, 8.39 and 8.36.

An exciting email arrived earlier this week from Martin Ewing, AA6E of Branford, Connecticut regarding a free ionosphere visualization plug-in for Google Earth. I am just beginning to play with this. It allows users to view a multi-dimensional model of the ionosphere's electron density in real time, anywhere on earth. Follow the links from and for more information and to download the free software.

Donald Casillo, KD5UGY of Moore, Oklahoma suggested that at the end of our bulletin, we show the actual latitude of the magnetometer that the A index comes from. This is because higher latitudes usually have greater geomagnetic activity. But our planetary A index is actually derived from a number of magnetometers, and the mid-latitude index observatory is near Fredericksburg Virginia at 38.2 degrees north latitude. You can see a directory of USGS magnetometers at When you click on a link for a particular observatory on the right side of the page, you can see the coordinates to locate it precisely on a map or with one of the satellite imaging sites.

But look at the coordinates for FRD, the source of our mid-latitude A index. It says 51.8 degrees "co-latitude" and 282.63 degrees longitude. What does this mean? By looking at a number of these coordinates and comparing them with approximate locations on a mapping program, by observation I was able to parse out the conversion. At least for the northern latitude in the western hemisphere (I haven't verified this for other parts of the earth) just subtract their given co-latitude from 90 to get the kind of north latitude coordinate that you and I are familiar with, and the given longitude from 360 for west longitude. So this shows the Fredericksburg magnetometer at 38.2 degrees north latitude, and 77.37 west longitude, which indicates a spot about 8 miles southeast of Fredericksburg on the north side of Fort A.P. Hill.

Use that same method to find the locations for other magnetometers. For instance, the College station is where the high-latitude A and K indexes that we see at, come from. You can also see planetary and the Fredericksburg mid-latitude numbers on that page.

When you tune to WWV at 18 minutes after the hour, they give a mid-latitude A and K index from the Boulder magnetometer. Figure out the latitude of that station from that same observatory page mentioned above, and you can compare numbers with the Fredericksburg middle latitude numbers. You can also read this latest forecast at, This is updated every three hours.

Jon Kreski, AB9NN of Appleton, Wisconsin requests more information on how the numbers we report affect the bands he is interested in from his location. Because readers are in so many locations around the world, we haven't made enough general statements about propagation on different bands, because complicating this is the number of locations that these many stations in many places may want to communicate with. You can use W6ELProp, the free propagation prediction program from or one of many other programs that you can buy or try out to customize a forecast from your exact location at the time you want to operate on the band you are interested in. We review the use of this program from time to time (you can check back issues of the bulletin, as described below) and we will try to cover this in depth next week. You can also check the monthly propagation charts, as mentioned below for more general forecasts from several regions.

The past two bulletins offered a document from Dr. Kenneth Tapping of the Penticton Observatory, which provides us with our solar flux data. He was misquoted in several news items recently, giving the impression that we might be headed for decades of little or no sunspots. This document attempts to correct the record. You can still get this by sending a blank email to This email address is being protected from spambots. You need JavaScript enabled to view it., and you will get a PDF of Dr. Tapping's notes in return. The response has been tremendous, currently approaching 1,000 requests.

If you would like to make a comment or have a tip for our readers, email the author at, This email address is being protected from spambots. You need JavaScript enabled to view it..

For more information concerning radio propagation, see the ARRL Technical Information Service web page at, For a detailed explanation of the numbers used in this bulletin see, An archive of past propagation bulletins is at Monthly propagation charts between four USA regions and twelve overseas locations are at

Instructions for starting or ending email distribution of this bulletin are at

Sunspot numbers for April 24 through 30 were 11, 0, 0, 0, 0, 0, and 0 with a mean of 1.6. 10.7 cm flux was 70.4, 69.8, 69, 68.1, 68.5, 68.6, and 67 with a mean of 68.8. Estimated planetary A indices were 18, 8, 10, 10, 11, 8 and 9 with a mean of 10.6. Estimated mid-latitude A indices were 13, 7, 5, 7, 9, 4 and 4, with a mean of 7.

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