Secure Site Login

ARRL Propagation Bulletin ARLP036 (2018)

ARLP036 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 36  ARLP036
From Tad Cook, K7RA
Seattle, WA  September 7, 2018
To all radio amateurs 

ARLP036 Propagation de K7RA

Sunspots disappeared again, since August 28. Average daily sunspot
number dropped from 17.7 (during the prior week) to 0, (naturally).

Average daily solar flux declined from 70.6 to 67.8. Geomagnetic
indicators quieted, with average daily planetary A index changing
from 19.9 to 6.3, and mid-latitude A index going from 13.4 to 5.9.

Predicted solar flux is 68 on September 7-14, 75 on September 15-17,
72 on September 18-22, 70 on September 23, 68 on September 24
through October 1, 70 on October 2-6, 72 on October 7, 70 on October
8-9, 75 on October 10-14, 72 on October 15-19, 70 on October 20 and
68 on October 21.

Predicted  planetary A index is 12, 10, 5 and 5 on September 7-10,
20, 15 and 12 on September 11-13, 12 on September 13, 10 on
September 14-15, then 15 and 10 on September 16-17, 5 on September
18-21, then 12 and 8 on September 22-23, 5 on September 24-29, 8 on
September 30, 5 on October 1-3, then 8, 12, and 8, on October 4-6,
then 5, 18 and 15 on October 7-9, 12 on October 10-11, then 10, 15
and 10 on October 12-14, 5 on October 15-18, then 12, 8 and 5 on
October 19-21.

When might sunspots return? In recent periods such as this when the
Sun has been blank for days or weeks, I've referenced predicted
solar flux values and assumed that relatively higher flux values may
indicate when we may see the return of sunspots. But this has often
led to disappointment.

Looking at the latest forecast (from it would seem that
September 15-17 (when predicted solar flux is 75) and October 10-14
(the same) are likely times to see sunspots again, or at least more
likely than days with lower solar flux predictions. We'll see.

In each case when an expected sunspot return did not appear, the
solar flux forecast changed in advance of the predicted enhanced

"OK1HH Geomagnetic activity forecast for the period September 07
till October 03, 2018

"Geomagnetic field will be:
Quiet on September 9, 17
Quiet to unsettled on September 10, 18-20, 25-28
Quiet to active on September 8, 13-15, 24, October 2
Unsettled to active on September 7, 12, 16, 21, 29-30, October 1
Active to disturbed n September 11, 22-23

"Solar wind will intensify on September (10-11,) 14-17, (21,) 22-24,
(25), October 1

- Parenthesis means lower probability of activity enhancement.
- Reliability of predictions remains low, of course.

"F. K. Janda, OK1HH (from Czech Propagation Interested Group
compiling this geomagnetic activity weekly forecasts since 1978)."

Frank Donovan, W3LPL of Glenwood, Maryland wrote on August 31:

"The Solar-Terrestrial Centre of Excellence (STCE) reports that last
weekend's reverse polarity sunspot group 2720 belongs to current
Solar Cycle 24.

"Because of its reversed polarity, some websites claimed sunspot
group 2720 was possibly one of the first groups of new Solar Cycle
25. This is simply not true, in view of its very low eight degree

"The next Solar Cycle 25 sunspot groups should have both reversed
magnetic polarity and much higher heliographic latitude, typically
20 to 40 degrees from the equator.

"Only two tiny short-lived numbered sunspot groups are currently
assigned to new Solar Cycle 25, sunspot group 2620 in December 2016
and 2694 in January 2018.  While both tiny sunspots were assigned to
Solar Cycle 25, there is some uncertainty about which sunspot cycle
they actually belong to. A few additional sunspot groups belong to
Solar Cycle 25, but they were so tiny and very short-lived that they
did not get an assigned sunspot number.

"During each solar cycle, about 3% of all active regions have
reversed polarity but do not belong to the previous or next solar
cycle. This percentage varies somewhat from one solar cycle to the
next. With 2000 to 3000 sunspot groups per solar cycle, this means
that every solar cycle has a few dozen reverse polarity sunspots
that belong to the ongoing sunspot cycle despite their reverse

"See the full STCE story at:


"This STCE news item provides more details on these numbered and
unnumbered Solar Cycle 25 regions and how solar magnetograms are
used to detect opposite polarity sunspots:

" "

Thank you, Frank. Readers may want to check the W3LPL page on for impressive photos of his antennas, including this one:

Here is the latest from Dr. Tamitha Skov, dated September 4:

Ran across this, from the end of August:

If you would like to make a comment or have a tip for our readers,
email the author at, .

For more information concerning radio propagation, see the ARRL
Technical Information Service web page at, For an explanation of
numbers used in this bulletin, see

An archive of past propagation bulletins is at More good
information and tutorials on propagation are at

Monthly propagation charts between four USA regions and twelve
overseas locations are at

Instructions for starting or ending email distribution of ARRL
bulletins are at

Sunspot numbers for August 30 through September 5, 2018 were 0, 0,
0, 0, 0, 0, and 0, with a mean of 0. 10.7 cm flux was 68.3, 67.5,
68.3, 67.7, 68.1, 67.5, and 67.5, with a mean of 67.8. Estimated
planetary A indices were 4, 5, 5, 5, 5, 9, and 11, with a mean of
6.3. Estimated mid-latitude A indices were 5, 4, 5, 5, 4, 9, and 9,
with a mean of 5.9.