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SEATTLE, WA, Jul 8, 2005--The Earth-facing disc of the sun went through a tremendous change over the past couple of weeks, ranging from a sunspot number of zero on June 26 to a reading of 192 on July 4. Along with this, geomagnetic conditions were very stable, an ideal combination for HF operators. The average daily sunspot number for the previous reporting week was only 19, but it jumped to more than 154 this week--a huge change. Average mid-latitude A index was lower by two points, with the planetary A index down by nearly five.
This is an example of how--even toward the solar cycle minimum--there can still be tremendous variation in activity from day to day. Those graphs you've seen of sunspot cycles are generally smoothed out using a moving average. In other words, if you draw your graph of daily sunspot numbers with every dot or section of line representing an average of the readings from the 90 days before and the 90 days after that date, the squiggly line tends to smooth out. This sort of representation helps us to visualize trends, and pay less attention to the day-to-day variations.
The July 4 sunspot number of 192 was the highest reading since November 26, 2003. Having the number go from zero to 192 is quite a jump, especially when we observe the last time prior to November 2003 that the sunspot number was zero. This was clear back on the other side of sunspot cycle 23 on January 7-9, 1998.
Someone wrote me about the big increase in sunspots, and mentioned a NASA article referenced in this bulletin a couple of months ago, entitled "Solar Myth." The message was about the fantastic conditions, how 15 meters has been open into the evening and that even lulls in the solar cycle can have spurts of activity. The article bears another reading.
On July 7, an explosion near sunspot 786--currently aimed squarely toward Earth--hurled a coronal mass ejection that's expected to cause a mild geomagnetic storm. Predicted planetary A index for July 8-11 is 25, 30, 25 and 15. Solar flux peaked on July 3 at 129.8 (highest solar flux reading since January 19, 2005) and for July 8-11 solar flux is predicted at 120, 120, 115 and 110. Flux values are expected to remain above 100 until mid month.
For more information concerning radio propagation and an explanation of the numbers used in this bulletin see the ARRL Technical Information Service Propagation page. An archive of past bulletins is available on this Web site.
Sunspot numbers for June 30 through July 6 were 96, 122, 168, 179, 192, 181 and 143, with a mean of 154.4. The 10.7 cm flux was 102.5, 114.6, 123.8, 129.8, 123.7, 126.8 and 123, with a mean of 120.6. Estimated planetary A indices were 8, 16, 13, 11, 7, 5 and 5 with a mean of 9.3. Estimated mid-latitude A indices were 4, 12, 12, 9, 5, 3 and 3, with a mean of 6.9.
Amateur solar observer Tad Cook, K7RA, Seattle, Washington, provides this weekly report on solar conditions and propagation. This report also is available via W1AW every Friday, and an abbreviated version appears in The ARRL Letter. Readers may contact the author via e-mail.
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