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ARRL Propagation Bulletin ARLP016 (2018)

ARLP016 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 16  ARLP016
From Tad Cook, K7RA
Seattle, WA  April 20, 2018
To all radio amateurs 

ARLP016 Propagation de K7RA

At 0631 UTC on April 20 the Australian Space Forecast Centre issued
a Geomagnetic Disturbance Warning.

"Earth is currently under the influence of a high speed stream from
a negative polarity equatorial coronal hole. Geomagnetic activity at
Quiet to Active levels is expected and at times may reach up to
Minor Storm levels if there are notable southward Bz periods.


20 Apr: Quiet to Active
21 Apr: Quiet to Active"

This link to Space Weather Live explains the reference to "southward
Bz periods" above:

Average daily sunspot number over the recent reporting week (April
12-18) rose from 0 to 5.4. Of course, any non-zero sunspot number
lower than 11 is imaginary, because of the arcane method used for
calculating sunspot numbers.

Every sunspot group counts for 10 points, and each sunspot within
that group counts as one point. So one sunspot in one group yields a
sunspot number of 11. Three sunspots in two groups yields a daily
sunspot number of 23. The last time we saw a sunspot number greater
than 22 was February 9-12 when the numbers were 23, 35, 24 and 26.

So an average daily sunspot number lower than 11 must include at
least one zero-sunspot day. In the past reporting week, only three
days had any sunspots.

Average daily solar flux rose from 67.7 to 69.9. Average daily
planetary A index declined from 9.4 to 6.1, while average
mid-latitude A index went from 8.1 to 5.6.

Predicted solar flux for the next 45 days is 72 on April 20-26, 71
on April 27, 69 on April 28 through May 5, 70 on May 6-12, 72 on May
13-19, 69 on May 20 through June 1, and 70 on June 2-3.

Predicted planetary A index is 15, 12 and 8 on April 20-22, 5 on
April 23-25, 8 on April 26, 5 on April 27 through May 5, 10 on May
6, 15 on May 7-8, 10 on May 9-10, 5 on May 11-15, 12 and 10 on May
16-17, 5 on May 18 through June 1, then 10 and 12 on June 2-3.

F.K. Janda, OK1HH of the Czech Propagation Interest Group brings us
his weekly geomagnetic activity forecast for the period April 20 to
May 15, 2018.

"Geomagnetic field will be:
Quiet on April 24-25, 27-28, May 13-14 
Mostly quiet on April 22-23, May 15
Quiet to unsettled on April 26, 29-30, May 1, 3-5, 9, 12
Quiet to active on April (21,) May 2, 7-8, 10-11 
Active to disturbed on April (20,) May 6 

"Solar wind will intensify on April (24)-25, May (2-3,) 5-6, (7-11) 

- Parenthesis means lower probability of activity enhancement.
- With regard to ongoing changes, current forecasts are less
  reliable again."

From SolarHam, which presents a weekly Monday observation:

Dr. Skov reports from a scientific conference in Switzerland this

"On the Hunt for Stealthy Solar Storms

"Dear Tad,

"I'm writing you while sitting in the ISSI meeting room in Bern,
Switzerland listening to twelve fascinating scientists argue about
solar observations. The discussion is quite lively as we learn how
we can work together to detect and forecast an invisible kind of
solar storm. These storms are called 'stealth CMEs' and they used to
be rare events. But with our quiet Sun, all that is changing. There
is a very good chance that these near invisible events will continue
to cause surprise storms at Earth over the next decade, disrupting
amateur radio, GPS, and causing issues for power grids with little
or no warning of their impact. Solving this significant problem is
exactly why we have assembled here in Bern.

"This week I highlight this amazing team of solar and space
physicists, who are helping shape the future of Space Weather
prediction. The forecast is shot in my hotel room so it has more of
a 'live' feel than my usual studio productions. Hopefully, it not
only gives you a look into the new activity our Sun brings this
week, but also gives you a glimpse into the important part of my
world that is Space Weather research.

"Cheers, Tamitha"

A look at the STEREO image on Friday morning shows an active region
just beyond the horizon:

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 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 April 12 through 18, 2018 were 13, 14, 11, 0, 0,
0, and 0, with a mean of 5.4. 10.7 cm flux was 70, 69.7, 69.5, 70.6,
69.2, 69.3, and 70.8, with a mean of 69.9. Estimated planetary A
indices were 9, 9, 6, 6, 3, 4, and 6, with a mean of 6.1. Estimated
mid-latitude A indices were 8, 8, 6, 5, 3, 3, and 6, with a mean of


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