September 4, 1998

ZCZC AP36

QST de W1AW

Propagation Forecast Bulletin 36 ARLP036

From Tad Cook, K7VVV

Seattle, WA September 4, 1998

To all radio amateurs

SB PROP ARL ARLP036

ARLP036 Propagation de K7VVV

We just experienced a new high in solar flux for the current cycle. On Monday, the last day of August, the flux was 178.5, which is the highest solar flux since it was 179.6 on February 10, 1993 during the downside of the last solar cycle.

Now that August has passed, it is time to look at the average solar flux for the month and compare it with previous months. Average solar flux for August was 136, which is a nice increase over May through July, which were 106.7, 108.5 and 114.1. The average solar flux for the past week was 157.6, so the trend continues. Compare this with the same week last year from ARLP036 in 1997, which had an average flux of 92.7, and the increase is almost 65 points. The average solar flux for the previous 90 days increased from 117 to 121 this week, and the solar flux was above these values on every day. All of these observations point to a strong upward trend in solar activity.

Solar flares produced some tremendous geomagnetic disturbances, with the planetary A index hitting 112 on August 27 and the K index rising as high as 8.

Here is the forecast for the next few days and beyond. Solar flux for Friday through Sunday is predicted at 160, 160 and 155. For the same period the predicted planetary A index is 12, 12 and 10. Solar flux is expected to go below 150 after September 8, below 140 two days later, then bottom out around 125 from September 12 through 14. It is expected to rise again above 140 after September 17, to 150 two days later, 160 by September 24, and above 170 after September 26. Geomagnetic conditions could be unsettled around September 5, and stormy around September 18 and 19. All of this is based on activity during the previous solar rotation.

VE3BSJ asked a question that deserves coverage again, and that concerns the significance of the A and K indices. He wanted to know if HF operators should hope for high or low A and K numbers. My answer is, that unless you are looking for VHF auroral propagation, low geomagnetic indices are good. The K index is updated every three hours and can be heard on the WWV at 18 minutes after each hour. A one point change in the K index is quite significant, and generally conditions are better if the K is below three and are worse if it is higher. When the K index hit 8 last week, that was extremely high.

The A index represents the K indices for the previous 24 hours, and a few points on the A index scale are not very significant. If the you had 24 hours of K at 1, the A would be 4. Likewise, K of 2 corresponds to A of 7, K of 3 corresponds to A of 15, K of 4 means A of 27, K of 5 means A of 48, K of 6 means A of 80, K of 7 means A of 132, K of 8 means A of 207, and K of 9 means A of 400. Of course the K index rarely stays the same for 24 hours, so the resulting A index is somewhere in between these values. For example, on Wednesday the planetary K index was 2, 1, 1, 2, 2, 3, 3, and 3, and the resulting A index for the day was 8. Last Friday the Boulder K index was 5, 4, 3, 2, 5, 4, 3 and 3, and the Boulder A index was 26.

KH6BZF reminds us that as we head toward the Fall equinox in a little more than two weeks, good openings toward Europe before noon local time should be common, as well as to the southern hemisphere later in the day, with South America first, and the Pacific later. Ten and twelve meters should be getting better as the days get shorter and the solar flux rises.

WM7D has a nice graph on the web showing the increase in solar activity at http://www.wm7d.net/hamradio/flux.html.

In last week's bulletin the packet and email addresses for the author were slightly garbled with the omission of the sign in some versions. The author can be contacted via email at tad@ssc.com, or via packet at K7VVV @ N7FSP.WA.USA.NOAM.

Sunspot Numbers for August 27 through September 2 were 153, 167, 128, 136, 133, 129 and 116 with a mean of 137.4. 10.7 cm flux was 135, 139.2, 146.5, 163.3, 178.5, 177 and 163.4, with a mean of 157.6, and estimated planetary A indices were 112, 25, 18, 15, 22, 18, and 8, with a mean of 31.1.