(C) ARRL Letter
This story was originally published by ARRL Letter and is unaltered.
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The ARRL Solar Report [1]
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Date: 2025-04
Solar activity is likely to be moderate (R1-R2, Minor-Moderate) with
a slight chance for X-class flares through April 5.
The geomagnetic field is expected to be quiet to minor storm levels
on April 4 and 5.
NOAA Geomagnetic Activity Probabilities for April 5 and 6 forecast a
15% chance, and then a 1% chance, of a Moderate storm for the
reporting period.
The forecast of solar and geomagnetic activity calls for a chance
for R1-R2 (Minor-Moderate) effects throughout the outlook period due
to multiple complex regions on the visible disk as well as on the
Sun's far side that are likely to return.
A slight chance exists for R3 (Strong) events over the next three
days primarily due to the flare potential from complex regions in
the Sun's eastern hemisphere.
Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - April 3, 2025, by F. K. Janda, OK1HH:
"A harbinger of a somewhat more dramatic development in the
Sun-Earth system was the helioseismological observation of a larger
active region on the far side of the Sun. This region appeared on
the northeastern limb of the solar disk as AR4046. It was followed
by the larger, more magnetically complex, growing and eruptively
active AR4048. Concurrent with the increase in solar activity, the
geomagnetic field remained quiet from March 29 to April 1, a
favorable combination for ionospheric evolution.
"The intensified solar wind from the coronal hole margins in the
western half of the solar disk was the cause of the geomagnetic
disturbances, which caused a significant decrease in the critical
frequencies of the ionospheric F2 layer from 2 April onwards,
together with an increase in the attenuation of radio waves, to
which the increased concentration of particles in the solar wind -
both electrons and especially protons - contributed.
"At the time of writing, the prediction of the onset of the
recurrent disturbance on the night of 4/5 April is valid. The cause
is the passage of the Earth through a region of enhanced solar wind
in the corotating interaction region (CIR) structure.
"In the next two weeks, the author of these lines will be in
hospital. He will send his next post after his return."
A greater electron flux at geosynchronous orbit is expected to be at
high levels on April 6 to 14 in response to multiple, recurrent,
Coronal Hole High-Speed Streams. The remainder of the outlook
period is expected to be at normal to moderate levels.
Geomagnetic field activity is expected to range from quiet to G2
(Moderate) geomagnetic storm conditions. G2 conditions are likely on
April 5 and 09, G1 (Minor) conditions are likely over April 8 and
10, active conditions are likely over April 11 and 13, unsettled
conditions are likely on April 6 and 7, 12, and April 14 and 15.
All increases in geomagnetic activity are anticipated in response to
multiple, recurrent Coronal Hole High-Speed Streams. The remainder
of the outlook period is likely to mostly quiet.
For more information concerning shortwave radio propagation, see
http://www.arrl.org/propagation and the ARRL Technical Information
Service web page at,
http://arrl.org/propagation-of-rf-signals. For
an explanation of numbers used in this bulletin, see
http://arrl.org/the-sun-the-earth-the-ionosphere . Information and
tutorials on propagation can be found at,
http://k9la.us/ .
Also, check this:
https://bit.ly/3Rc8Njt
"Understanding Solar Indices" from September 2002 QST.
The predicted Planetary A Index for April 5 to 10 is 35, 10, 12, 30,
35, and 25, with a mean of 24.5. The predicted Planetary K Index is
6, 3, 3, 5, 6, and 5, with a mean of 4.7. The predicted 10.7
centimeter flux is 180, 180, 185, 185, 180, and 175, with a mean of
180.8.
[END]
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