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ARRL Propagation Bulletin ARLP010 (2021)

ARLP010 Propagation de K7RA

QST de W1AW  
Propagation Forecast Bulletin 10  ARLP010
From Tad Cook, K7RA
Seattle, WA  March 5, 2021
To all radio amateurs 

ARLP010 Propagation de K7RA

We saw one day (Monday) with no sunspots during this week, so
average daily sunspot numbers declined slightly from 19.6 to 18.9.
Two new sunspot groups (2806 and 2807) appeared on the following

Average daily solar flux remained about the same, increasing
slightly during the reporting week (February 25 through March 3)
from 75.7 to 76.7.

Average daily planetary A index softened slightly from 16 to 14.7,
and the middle latitude average went from 12.4 to 10.4. Geomagnetic
indicators remained somewhat active due to persistent solar wind.
The most active day was Monday, when Alaska's High Latitude College
A index reached 34. reported a G2 class geomagnetic storm on Monday,
aided by a significant crack in Earth's magnetic field. Although
activity was otherwise moderate this week, the March 1 event was the
largest storm since a G3 event 94 weeks earlier, on May 14, 2019.

Predicted solar flux for the next 30 days is 80 on March 5, 78 on
March 6, 78 on March 7-9, 72 on March 10-11, then 71, 72, 70, 71, 72
and 71 on March 12-17, then 73, 76, 75, 76, 78 and 81 on March
18-23, then 80 on March 24 and 25, then 79, 78 and 73 on March
26-28, 74 on March 29-30, 73 on March 31 through April 1, and 74 on
April 2-3.

Predicted planetary A index is 10, 20 and 15 on March 5-7, 10 on
March 8-9, then 8, 5, 15, 10 and 5 on March 10-14, then 15, 8, 5 and
18 on March 15-18, 20 on March 19-20, then 18, 12, and 8 on March
21-23, 5 on March 24-27, then 20, 15 and 10 on March 28-30, 5 on
March 31 and April 1, then 12 on April 2, and 5 on April 3-7.
Geomagnetic activity forecast for the period March 5-30, 2021 from

"Geomagnetic field will be,
quiet on: March 9-10, 14, (26-27)
quiet to unsettled on: March 5, 16-17, 25
quiet to active on: March 7-8, 11, 13, 15, 20-24
unsettled to active: March 12, 29
active to disturbed: March 6, 18-19, 28, 30
"Solar wind will intensify on: March (5-9, 12-14, 16-22, 27,) 28-29.
- Parenthesis means lower probability of activity enhancement.
- Predictability of changes remains low."

W6MVT reports a surprising 10 meter opening to South America on the
same Tuesday when sunspots re-appeared.

"It never hurts to turn on 10 meters or check the spots on DXMaps or
your favorite spotter. I saw some action and was pleased I was at
the radio. On March 2 around 2130 UTC 10 meter SSB was alive with
South American stations and a good path to those of us in Southern
California. With 100w and a rotatable dipole only up 20 feet I was
able to log LU4DJB, PU2LUC, PY2EX, PY5QW, PU2SDX and PY4NY in rapid
succession, all with good reports both ways. Things faded out around
2200 UTC, but I was glad I caught it. W6MVT."

Larry, K8MU sent this article concerning a space plasma hurricane:

Don't miss Larry's page on, showing lines and arrows with
humorous text about his modest station, complete with steerable
ground plane and incoming QSL receptacle.

This is from an email exchange with Frank Donovan, W3LPL regarding
"Total Sunspot Area" which is shown daily along with SFI and SSN
(Sunspot Number) in this table:

(SFI is 10.7 cm solar flux, uHem (micro-Hem) is solar
micro-hemispheres, and EUV is extreme ultra-violet radiation.)

"Here are some additional insights regarding total sunspot area.

"SFI and total sunspot area are well correlated with each other and
with EUV flux at the wavelengths that ionize the F2 region.

"Daily sunspot number is not well correlated because tiny sunspots
greatly affect it but they have no effect on HF propagation. I
usually ignore daily sunspot numbers unless total micro-Hem exceeds

"Today is a classic case with daily SISLO sunspot number of 30 but
total sunspot area is under 100 micro-Hem and SFI is stuck at 75.

"Roughly 100 micro-Hem elevates the SFI into the mid 70s, but has
only a minor effect on HF propagation.

"200 micro-Hem roughly corresponds to SFI of 80 and usually improves
17 and 15 meter propagation. But the normal daily variability of F2
MUFs is not well correlated to SFIs of about 80 and often swamps out
the expected improvements from SFI of 80.

"HF MUFs increase more consistently when the SFI approaches 90. You
may recall active region 12786 area was as high as 1000 micro-Hem
last November and the SFI was above 100 for nine days. It greatly
improved 15 meter propagation during the CQWW CW DX Contest and
there was significant 10 meter DX propagation too. Daily sunspot
number varied wildly from 40 to 94 during this period mostly because
there were also three smaller active regions at during the period
when 12786 was by far the most significant contributor to SFI
greater than 100.

"The rough equivalencies are:

"uHem   SFI
 100   75    
 200   80
 400   90
 600   100
 800   110
1000   120
1200   130
1400   140
1500   150
1600   160
1800   170
2000   180"

This weekend is the ARRL International SSB DX Contest. See for details.

NASA video of solar flare from Science Times:

A recent video forecast from Dr. Tamitha Skov, WX6SWW:

If you would like to make a comment or have a tip for our readers,
please 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 February 25 through March 3, 2021 were 31, 16,
14, 13, 0, 28, and 30, with a mean of 18.9. 10.7 cm flux was 80.1,
80.1, 79.2, 77.7, 71, 74.7, and 74.2, with a mean of 76.7. Estimated
planetary A indices were 13, 11, 4, 6, 26, 20, and 23, with a mean
of 14.7. Middle latitude A index was 13, 8, 3, 4, 16, 14, and 15,
with a mean of 10.4.


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