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ARRL Propagation Bulletin ARLP037 (2020)

ARLP037 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 37  ARLP037
From Tad Cook, K7RA
Seattle, WA  September 11, 2020
To all radio amateurs 

ARLP037 Propagation de K7RA

An extended lull in solar activity persists. The smoothed sunspot
minimum occurred last December, but the flurry of moderate sunspot
activity in August has not continued.

An event to look forward to is the Autumnal Equinox, which occurs at
1330 UTC on Tuesday, September 22. We should see a seasonal
improvement in HF propagation around that date because the northern
and southern hemispheres are bathed in roughly equal solar
radiation, enhancing north-south propagation.

Thursday, September 10 was the twentieth consecutive day with no
sunspots, but reported a small sunspot with a Solar
Cycle 25 magnetic signature may be forming in the Sun's southeastern
quadrant. As of Thursday night it was still too small to merit

Average daily solar flux barely budged from 69.6 to 69.7.
Geomagnetic indicators were very quiet, with average daily planetary
A index declining from 13.1 to 4.4.

Like last week's forecast, predicted solar flux is 70 on every day
over the next 45 days, from September 11 until October 25.

Predicted planetary A index is 5 on September 11-17, 8 on September
18-19, 5 on September 20-22, then 8, 10 and 15 on September 23-25,
then 10, 25, 15 and 10 on September 26-29, 5 on September 30 and 8
on October 1, 5 on October 2-14, 8 on October 15-16, 5 on October
17-19, then 8, 10, 15, 10. 25 and 15 on October 20-25.

Geomagnetic activity forecast for the period September 11 to October
6, 2020 from F.K. Janda, OK1HH.
"Geomagnetic field will be,
quiet on: September 11-13, 16-17, 20-21
quiet to unsettled on: September 14, 22-25, 29, October 1-2
quiet to active on: September 15, 18-19, 26-27, (October 5-6)
unsettled to active: (September 28-30, October 3-4)
active to disturbed: not expected
"Solar wind will intensify on: September (15, 19,) 22-23, (October

"Parenthesis means lower probability of activity enhancement."

Frank Donovan, W3LPL shared this on September 6:

"Exactly three years ago, on September 6th 2017, the Sun ejected the
strongest solar flare and Earth-directed coronal mass ejection (CME)
of Solar Cycle 24.  Solar region 2673 produced four X-class solar
flares including a massive X-class solar flare of magnitude 9.3 from
1153 to 1210Z September 6, 2017.

"Just over eight minutes later, powerful hard x-rays from the flare
increased D-layer ionization by several orders of magnitude,
completely blacking out HF skywave propagation on the daylight side
of the Earth for about an hour, an event called a Sudden Ionospheric
Disturbance (SID).

"Solar flares are commonly followed by CMEs.  Solar region 2673
ejected powerful Earth-directed CMEs on September 6th 2017, causing
visible aurora on September 7th and 8th and a severe geomagnetic
storm the planetary Kp index reached magnitude 8 on September 8th.


"The strongest solar flare measured in modern times was an X28 flare
on 28 October 2003 which blacked out HF skywave propagation on the
sunlit side of the Earth for several hours. Extremely powerful CMEs
on 28 and 29 October caused severe geomagnetic storms on 29, 30 and
31 October and the Kp index reached magnitude 9 during all three

Here is a 3-hour video from Dr. Tamitha Skov, WX6SWW, entitled,
"Coronal Holes and Sources of the Solar Wind - Part 2."

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 and 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

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

Sunspot numbers for September 3 through 9, 2020 were 0, 0, 0, 0, 0,
0, and 0, with a mean of 0. 10.7 cm flux was 70, 69.7, 69.2, 69.5,
70.2, 69.9, and 69.7, with a mean of 69.7. Estimated planetary A
indices were 4, 8, 6, 4, 4, 4, and 1, with a mean of 4.4. Middle
latitude A index was 3, 9, 7, 5, 5, 4, and 1 with a mean of 4.9.


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