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ARRL Propagation Bulletin ARLP028 (2005)

ARLP028 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 28  ARLP028
From Tad Cook, K7RA
Seattle, WA  July 8, 2005
To all radio amateurs 

ARLP028 Propagation de K7RA

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.  Average daily sunspot number for the previous reporting
week was only 19, but it jumped to over 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 variation.

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, in 1998 on January 7-9.

Earlier this week someone sent an email about the big increase in
sunspots, and mentioned a NASA article referenced in this bulletin a
couple of months ago, titled ''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.  Unfortunately, the computer on which that email resides
has crashed, so around here this weekend will no doubt be spent
trying to recover.  But we still have the URL for the article, and
it bears another reading.  Propagation Bulletin 18 referenced the
article, which you can read at

Yesterday, July 7, an explosion near sunspot 786 (currently aimed
squarely toward Earth) hurled a coronal mass ejection, which is
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.

If you would like to comment or have a tip, email the author at,

For more information concerning radio propagation and an explanation
of the numbers used in this bulletin see the ARRL Technical
Information Service propagation page at, An archive of past
bulletins is found at,

Sunspot numbers for June 30 through July 6 were 96, 122, 168, 179,
192, 181 and 143 with a mean of 154.4. 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.


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