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Sunspot Cycle and Its Influence

Propagation Page

Propagation Page

 The Phenomenon of Solar Flares and Shortwave Propagation 


Solar flares, intense bursts of radiation from the Sun can significantly impact shortwave radio broadcasts on Earth. These flares occur when magnetic energy built up in the solar atmosphere is suddenly released. The energy from a solar flare can disrupt the Earth’s ionosphere, a layer of the atmosphere crucial for shortwave propagation. When solar flares happen, they can cause sudden ionospheric disturbances (SID), leading to degraded or completely blocked shortwave radio signals, a phenomenon often referred to as “solar flare and radio disturbances.” 


 

Solar Flares and Radio Disturbances

The relationship between solar flares and radio disturbances is complex. Shortwave radio waves travel long distances by reflecting off the ionosphere. During a solar flare, the ionosphere’s density and composition change rapidly, causing shortwave signals to be absorbed rather than reflected. This can lead to shortwave radio blackouts, significantly weakening or losing transmission. Such occurrences are often termed “solar flares and radio blackouts.”


 

Solar activity, particularly solar flares, can significantly impact shortwave radio propagation, likely contributing to the issues you’re experiencing with broadcast reception. The National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction Center provides detailed and current information on space weather conditions that affect radio communications. The NOAA website provides various resources, including forecasts, reports, and models that track and predict solar activity and its impact on different aspects of space weather, including HF radio communications.

Solar flares emit X-rays that can penetrate the Earth’s ionosphere, particularly the D-layer, causing it to become more ionized. This increased ionization can reflect or absorb radio waves at different frequencies, leading to HF (High Frequency) radio communications disruptions. This is particularly problematic for frequencies in the 1 to 30 MHz range, commonly used for shortwave broadcasting. The impact of these solar flares is most intense on the Earth’s dayside, where the sun is directly overhead, and can cause radio blackouts.

Moreover, other space weather phenomena like Radiation Storms caused by solar protons can also disrupt HF radio communication. These protons, guided by Earth’s magnetic field, collide with the upper atmosphere near the poles, enhancing the D-Layer and blocking HF radio communication at high latitudes.


Sunspot Cycle and Its Influence

The sunspot cycle, approximately 11 years, significantly influences shortwave radio propagation. Sunspots, dark spots on the Sun’s surface, are indicators of solar magnetic activity, which can lead to solar flares. During periods of high sunspot numbers (SSN), the Sun is more active, increasing the likelihood of solar flares. High SSN usually means better shortwave propagation conditions due to a more reflective ionosphere, except during solar flares. Understanding the “sunspot cycle” is essential for predicting shortwave radio propagation conditions.


 

SSN (Sun Spot Number) and Shortwave Propagation

SSN, or Sun Spot Number, is a simple count of the number of sunspots and groups of sunspots visible on the Sun’s surface. A higher SSN indicates a more active Sun, which can enhance or disrupt shortwave propagation. Increased solar radiation can boost the ionosphere’s reflectivity during a high SSN period, improving shortwave signal reach. However, the increased solar activity also raises the risk of solar flares, which can cause shortwave radio blackouts. Therefore, the relationship between SSN and shortwave propagation is a delicate balance.


Source: https://www.nexus.org/solar-flares-impact-on-shortwave-radio-broadcasts/


The ARRL Solar Report

05/30/2025

   

 

 Solar activity reached moderate levels early this past week with
several flares. An X1-class flare erupted mid-week, but activity has
slowed down with the majority of the low-level C-class flares. A
coronal mass ejection (CME) was observed on May 27 with a flare from
Region AR4100.

Modelling determined the CME to be well behind Earth's orbit. No
other potentially Earth-directed CMEs were detected in available
coronagraph imagery. M-class flare activity, minor - moderate, is
likely, with a slight chance for X-class flare events, strong or
greater, through May 30.

The forecast of Solar and Geomagnetic Activity to June 21, 2025:

Solar activity is likely to reach moderate levels
(R1-R2/Minor-Moderate), with a chance for R3 (Strong), over the next
three days as Region AR4098, the most productive region on the
visible disk, makes its way to the west limb of the Sun. A chance
for M-class X-ray activity (R1-R2) will persist throughout the
outlook period due to multiple regions on the visible as well as
multiple active regions scheduled to return from the far side of the
Sun.

No proton events are expected at geosynchronous orbit.

The Geomagnetic field activity is expected to be at a mostly
elevated level due to anticipated influence from multiple, recurrent
coronal holes. G1 (minor) geomagnetic storms are likely on June
13-14; active conditions are likely over June 2, June 5, June 10 and
11, and June 15 to 17.

Unsettled conditions are likely over June 3 and 4, June 6 and 7, and
June 18 to 21.  Quiet conditions are only expected on June 8 and 9.

On May 30, Spaceweather.com reports on a Super-Fast Solar Wind, and
a 24-hour Geomagnetic Storm.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - May 29, 2025, from F. K, Janda, OK1HH:

"On May 29, geomagnetic activity reached the level of a strong storm
(Kp 7) worldwide, which came as no surprise to those who had been
monitoring the recurring disturbances during the last four solar
rotations, and especially the developments on the Sun over the last
five days or so.

"Solar flares were observed, only occasionally with coronal mass
ejections (CMEs). Although the large coronal hole in the southwest
of the solar disk disappeared, other coronal holes appeared across
the solar disk in the meantime. The largest of these extends from
the southwest to the northeast and has been crossing the central
meridian since May 26. It has negative polarity and is associated
with the arrival of a high-speed stream (up to about 730 km/s). Even
in the coming days, after the current disturbance subsides, mild
active storm conditions (Kp 4) will continue.

"Overall solar activity will increase slightly in the coming days,
and geomagnetically active days will alternate irregularly with
calmer ones. Ionospheric conditions for shortwave propagation will
improve, but at best only to average levels."

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 May 31 to June 6 is 8, 10, 12,
8, 10, 12, and 10, with a mean of 10.  The Predicted Planetary K
Index is 3, 3, 4, 3, 3, 4, and 3, with a mean of 3.3.  Predicted
10.7 centimeter flux is 125, 120, 115, 110, 110, 115, and 120, with
a mean of 116.4. 

The ARRL Solar Report

05/23/2025

   

 

Solar activity ranged from low to high.  Region 4087 produced the strongest event of the period, an impulsive R3 flare on May 14 at 0825 UTC near the NE limb.  The region also produced R2 flares at 0325 and 1119 UTC.  Finally, three R1 events were produced by the region on May 14 and 15.  Region 4086 also produced an R3 event, with a flare observed on May 13 at 1538 UTC.

No proton events were observed at geosynchronous orbit.  However, a weak enhancement, which peaked below the S1 threshold, was observed on May 13 following the R3 event from Region 4086.

Geomagnetic field activity was at quiet levels on May 12 and 13. Late on May 16, enhancements in solar wind parameters, associated with the onset of a positive polarity CH HSS and possible influence from the southern periphery of the CME that left the Sun on May 12, increased activity to active levels.

Forecast of Solar and Geomagnetic Activity May 19 - June 14 2025:

Solar activity is expected to be at low levels, with a chance for R1-R2 (minor-moderate) events, over the outlook period.

No proton events are expected at geosynchronous orbit.

The remainder of the outlook period is expected to be at normal to moderate levels.

Geomagnetic field activity is likely to be elevated above quiet levels for most of the next 27 days due to multiple, recurrent, coronal hole features.  G2 (moderate) conditions are likely on May 29 and June 13; G1 (minor) conditions likely on May 19, May 28, and June 14; active conditions are likely on May 30 - June 01, and June 10; unsettled levels are likely on May 20-23, May 27, June 02-07, and June 11-12.  Quiet conditions are expected for the few remaining days of the outlook period.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere - May 22, 2025, from F. K. Janda, OK1HH.

Compared to previous months, there was a significant decrease in solar activity in May.  For example, on May 2, there were only two groups of spots on the entire solar disk observed.  After that, spot activity in the northern half of the solar disk increased slightly, but solar flux dropped significantly during the second third of the month.  Additionally, a large coronal hole appeared in the southern hemisphere of the sun.  Although this coronal hole was observed during previous rotations in March and April, it is much larger this time.

Consistent with this observation, after geomagnetic activity quieted down at the end of April, we experienced several days of strong geomagnetic disturbances in the first third of May.  Starting in the second third of May, the solar flux dropped significantly, reaching values not seen since October 2024.

The combination of low solar activity and high geomagnetic activity resulted in a significant deterioration of ionospheric shortwave propagation conditions, manifested by a drop in the critical frequencies of the F2 layer and an increase in attenuation and scattering.  This occurred especially on May 7-12 and May 14-18, and is likely to continue in the coming days.

The latest attempts to forecast further developments suggest that conditions should improve by the end of May.  The summer ionosphere of the northern hemisphere of Earth is characterized by lower maximal frequencies and higher lows, not counting the sporadic layer E surprises.

The Predicted Planetary A Index for May 24 to 30 is 6, 5, 5, 8, 25, 30, and 20, with a mean of 14.1.  The Predicted Planetary K Index is 2, 2, 2, 3, 5, 6, and 4, with a mean of 3.4.  10.7 centimeter flux is 120, 120, 120, 130, 135, 135, and 140, with a mean of 128.5.  

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:  

"Understanding Solar Indices" from September 2002 QST:

https://bit.ly/3Rc8Njt

The ARRL Solar Report

05/16/2025

   

 After weeks of calm, solar activity is suddenly high again, with two
strong solar flares erupting from opposite sides of the Sun. A
Coronal Mass Ejection (CME) was associated with a strong solar flare
on May 12, but modeling shows the ejection passing behind Earth. The
geomagnetic field has been at unsettled to active conditions.

A CME associated with a filament eruption in the northern hemisphere
is expected to pass above Earth on May 17, glancing influences will
likely enhance the solar wind field during this time. The
co-rotating interacting region (CIR) associated with the large,
positive polarity Coronal Hole (CH) in the southern hemisphere is
expected to become geoeffective on May 18 which will further enhance
the field.

There remains a 65% chance that M-Class (R1-R2, Minor-Moderate)
level flare activity will occur through May 18 with a 30% chance for
X-Class (R3-Strong) levels during the same time due to the complex
magnetic field within Region AR4087.

The solar wind reflected influences from the Coronal Hole High-Speed
Stream (CH HSS) originating from the negative polarity coronal hole
in the southwest part of the sun.

The forecast of Solar and Geomagnetic Activity for May 17 to June 2,
2025:

Solar activity is expected to be predominantly low through the
outlook period, with varying chances for M-class flare activity.

No proton events are expected at geosynchronous orbit.

The greater than 2 MeV electron flux at geosynchronous orbit is
expected to be at moderate levels until May 28, and again on June 6
and 7.  High levels are expected from May 29 to June 5 as CH HSS
influences increase during this time.

Geomagnetic field activity is anticipated to reach minor storm
levels on May 28 to June 1 under negative polarity CH HSS
influences.

Active levels are likely June 2 and 6.  Mostly unsettled levels are
likely on May 18 to 21, as well as June 2 and 5.  Quiet levels are
expected on May 22 to 26.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - May 15, 2025, from F. K. Janda, OK1HH:

"The level of solar activity in the first half of May mostly did not
resemble the current peak of the 25th 11-year cycle. The total
number of sunspot groups across the disk ranged from two (May 2) to
six (May 11), with no more than one larger group - first the
relatively quiet AR4079 and then the eruptively very active AR4086
(whose growth could already be tracked prior to disk rise on the
http://jsoc.stanford.edu/ - 'Helioseismic Far-Side Imaging'
website).

"Ionospheric shortwave propagation conditions were mostly poor to
below average. The main culprit was not the slightly lower level of
solar radiation, but was mainly the solar wind. The Earth's
ionosphere was under the influence of elevated free electron
concentrations on most days, while was later bombarded by protons
following the increase in flare activity in AR4086 with subsequent
CMEs.

"The consequence was not so much lower MUF values as increased
attenuation and scattering in the ionosphere. This was all the more
advantageous for stations with higher powers and, in particular,
with antenna systems with low radiation angles.

"Further increases in solar activity in the northern half of the
solar disk are still expected in the coming months but cannot be
predicted more accurately. In modern times, however, we can monitor
it closely - and prepare for it all the better and in good time."

On May 16, Spaceweather.com reports that an Aurora warning is issued
for the planet Mars!

The latest solar report from Dr. Tamitha Skov, WX6SWW, can be found
on YouTube at: https://youtu.be/5hRRuaP-bT0?si=bQr5hSz0vc8WPepn .

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 May 17 to 23 is 5, 8, 10, 8, 8,
6, and 6, with a mean of 7.3.  The Predicted Planetary K Index is 2,
3, 4, 3, 3, 2, and 2, with a mean of 2.7.  10.7 centimeter flux is
115, 115, 120, 125, 125, 125, and 130, with a mean of 122.1.

The ARRL Solar Report

05/9/2025

   

 The Solar Geophysical Activity Report and Forecast from the
USAF/NOAA indicates that solar activity has been at low levels for
the past 24 hours.

There are currently four numbered sunspot regions on the solar disk.
Solar activity is expected to be low with a chance for M-class
flares on May 10 and 11.

A simultaneous filament eruption produced a narrow Coronal Mass
Ejection (CME) signature that was first observed on May 6, and an
additional slower filament eruption was also observed. Analysis of
the events is ongoing and there was no significant growth or decay
observed in the spotted regions on the visible Sun.

The geomagnetic field is likely to experience periods of active
conditions on May 10 and 11 due to continued Coronal Hole High-Speed
Stream influences (CH HSS).

Geomagnetic field activity is likely to reach G1 (Minor) storm
levels on May 18, and May 29 to 31 due to negative polarity coronal
hole influences. Periods of active conditions are likely on May 16
and 17, and on May 19 to 21 in response to CH HSS influences.

On Spaceweather.com for May 9, there is a video of a "Solar Tornado"
occurring on the Sun's surface.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere, May 8, 2025, from F. K. Janda, OK1HH:

"Although we were expecting more activity in the relatively large
sunspot group AR4079 that crossed the central meridian on May 4, it
did not happen. Indeed, its magnetic configuration did not promise
it. Instead, a sufficiently fast and proton - and especially
free-electron-rich solar wind blew from the edges of the coronal
holes, which for most of the past days since the beginning of May
caused not only increased geomagnetic activity, but also increased
attenuation and scattering of radio waves in the ionosphere.

"Only a slight improvement in the situation can be predicted. In a
few days, the only one major active region that we know about thanks
to helioseismology should appear on the eastern limb of the solar
disk. Again, it is likely to be the only one in the disk, while the
small number of remaining ones will more or less not contribute to
the overall level of solar activity.

"It remains the case that solar activity is likely to shift from the
southern hemisphere to the northern hemisphere this year, but this
will probably not yet happen in May."

The latest video report from Dr. Tamitha Skov, WX6SWW, can be found
on YouTube at: https://www.youtube.com/watch?v=XZ7C6ja3ZmE .

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 May 10 to 16 is 12, 10, 5, 5, 5,
5, and 12, with a mean of 7.7.  The Predicted Planetary K Index for
the same period is 4, 3, 2, 2, 2, 2, and 4, with a mean of 2.7.
Predicted 10.7 centimeter flux is 165, 165, 160, 155, 155, 155, and
155, with a mean of 158.6.

The ARRL Solar Report

05/2/2025

   

  On May 2, Spaceweather.com reported: "Astronomers are monitoring a
very large sunspot now turning toward Earth. Sunspot AR4079
stretches more than 140,000 km from end to end and has two dark
cores each large enough to swallow Earth. Moreover, it is surrounded
by a ring of Ellerman Bombs.

"Ellerman bombs are a sign of magnetic complexity in a sunspot.
Opposite polarities bump together, reconnect, and--boom! A
full-fledged flare may not be far behind."

Solar activity increased to moderate levels this past week. The
largest flare was on April 30 from an area where there was a major
M-class flare producer on its previous rotation last week.

No Earth-directed Coronal Mass Ejections (CMEs) were observed. Solar
activity is expected to be low with a chance for M-class flares
(R1-R2, minor-moderate), and a slight chance for X-class flares
(R3-strong) through May 2.

Radio blackouts reaching the R1 levels were observed over the past
24 hours. The largest was on April 30.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - May 1, 2025, by F. K. Janda, OK1HH:

"In the past few weeks I have allowed myself to take a break because
I have been under the care of excellent doctors and caring nurses in
a top Prague hospital. Fortunately, I had a receiver available and
could and did use it occasionally, but the computer was at home, QRB
53 km away. Now I'm back and continuing my usual activities:
observing the events between the Sun and the Earth, analyzing the
context and trying to predict the future developments (yes, I know
that more accurate predictions are not possible, while I can well
justify why - but why not take science a bit as a sport too,
right...?).

"In the last few days of April, AR4079 rose on the northeastern limb
of the solar disk, while soon increased in area to over 1000
millionths. Already during the early months of this year, the Sun
had subtly hinted that the centre of gravity of activity might shift
from its southern to its northern hemisphere, but only now can we
see this trend more clearly. So, it's possible that we're in for
another upward swing within the current 11-year cycle, with active
regions mostly north of the equator. This could hold the promise of
improved ionospheric shortwave propagation conditions this Autumn.

"During this April, we experienced a really large number of
geomagnetically disturbed days and mostly low MUF values. The calm
and improvement occurred only in the last decade of April, when we
witnessed all six geomagnetically quiet days of the whole April.

"Going forward, although we do not expect a major increase in solar
activity for the time being, it will not be important during the
Northern Hemisphere ionospheric Summer. Solar activity will more or
less remain at the current level, the geomagnetic field will be
calmer compared to the past weeks - and the situation in the
ionosphere will be more favourable."

Solar wind parameters remained slightly enhanced due to waning
positive polarity of the Coronal Hole High-Speed Stream (CH HSS)
influences. A return to a mostly ambient-like state is expected for
May 1, but by May 2, an enhancement in solar wind parameters is
likely with the arrival of a recurrent, negative polarity CH HSS.

Unsettled to active levels will likely persist into May 4 as an
additional negative polarity CH HSS moves into place.

Geomagnetic field activity is likely to reach G1 (Minor) storm
levels on May 5 and 6 due to negative polarity CH HSS influences,
and again on May 18 due to positive polarity CH HSS influences.

Periods of active conditions are likely on May 07 to 10, and on May
16 and 17 in response to CH HSS influences. Quiet and quiet to
unsettled levels are expected to prevail throughout the remainder of
the period.

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 May 3 to 9 is 10, 8, 18, 18, 15,
15, and 15, with a mean of 14.1.  Predicted Planetary K Index is 3,
3, 5, 5, 4, 4, and 4, with a mean of 4.  Predicted 10.7 centimeter
flux is 140, 140, 150, 160, 160, 165, and 165, with a mean of 154.3.

The ARRL Solar Report

04/25/2025

   

 Solar activity has been at low levels with only minor C-class
flaring through April 24, 2025, and is expected to be moderate with
a chance of M-class flares (R1-R2, Minor-Moderate) through April 26.
No Earth-directed Coronal Mass Ejections (CMEs) have been observed.
A southwest CME was observed on April 23 and is considered to be
far-side due to the lack of any on-disk plasma motion/flare
activity.

Solar wind parameters continued to be slightly enhanced due to the
Coronal Hole High-Speed Streams (CH HSS) associated with the
positive polarity coronal holes in the southern hemisphere.  Solar
wind parameters are expected to continue to be influenced by
positive polarity, with CH HSS conditions through April 26.  Nominal
conditions are expected thereafter.

On April 25, Spaceweather.com reported the Earth was struck by an
interplanetary shockwave on April 24 around 0700 UTC. An
interplanetary shock wave is an abrupt change in the solar wind -
probably caused by the Coronal Mass Ejection.

Overall, solar activity remained at low levels.  Region AR4064
remained the largest sunspot group on the disk.  The regions
intermediate spots grew in penumbral area over the past 24 hours.

The geomagnetic field is expected to remain at mostly quiet to
unsettled levels, with active conditions likely on April 27 due to
the influence from multiple positive polarity CH HSS.

There are currently 11 numbered sunspot regions on the solar disk.

Solar activity is expected to be low with a chance for M-class
flares on April 26 and 27.

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/ .
 

The ARRL Solar Report

04/18/2025

   

Spaceweather.com reports a Cannibal Coronal Mass Ejection on April 15 sparked geomagnetic storms.  On April 16, the storm became severe (G4) with Northern Lights sighted as far south as France.  The storm is subsiding now to a category G1/G2, which could still produce high-latitude auroras.

Although Regions AR4062 and AR4064 have been relatively quiet they are more structurally complex than anticipated.  They will maintain a 60 chance for R1-R2 (Minor-Moderate) flare activity through April 19.  Chances for X-class (Strong) flare activity remain around 10.

Unsettled to active levels are expected on April 18 followed by quiet to unsettled conditions on April 19.

Solar activity is expected to be at moderate to high levels on April 27 to May 10 with the return of Region AR4055.  Low to moderate levels are expected for the remainder of the outlook period. Unsettled to active levels are expected on April 18, April 22 and 23, and on May 3 and 4.

G2 (Moderate) storm levels are expected again on May 2 due to recurrent Coronal Hole High-Speed Stream influences.  G1 (Minor) storm levels are expected on May 1, and May 5 and 6, all due to recurrent Coronal Hole High-Speed Stream activity.

The latest report from Dr. Tamitha Skov, WX6SWW, can be found on YouTube at: https://www.youtube.com/watch?v=zpyPrcMKvTY .

The Predicted Planetary A Index for April 19 to 25 is 10, 8, 8, 15, 15, 15, and 10, with a mean of 11.6.  The Predicted Planetary K Index is 3, 3, 3, 4, 4, 4, and 3, with a mean of 3.4.  10.7 centimeter flux is 145, 145, 150, 155, 155, 160, and 165, with a mean of 153.6.

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/ .
 

The ARRL Solar Report

04/11/2025

  Due to most of the regions on the solar disk being fairly simple in
their magnetic complexity, solar activity is expected to remain at
low levels with a chance for M-class flares (R1 to R2, or Minor to
Moderate).

Solar wind parameters are expected to trend further towards nominal
levels as the Coronal Hole High-Speed Stream effects continue to
wane.

Additional enhancements from another negative polarity Coronal Hole
is likely on April 12, combined with potential effects from the
glancing blow of a Coronal Mass Ejection that left the Sun on April
8.  Elevated conditions are likely to continue through April 13 as
the influences persist of the Coronal Hole.

Solar activity has been at moderate levels for the past 24 hours.
There are currently 8 numbered sunspot regions on the disk.  Solar
activity is expected to be low with a chance for M-class flares on
April 12.

The long range forecast of Solar and Geomagnetic Activity for April
12 to May 3:

The declining trend in solar flux and activity is expected to bottom
out around April 15, after which a slowly increasing period is
expected. The anticipated return on April 22 of the active
longitudes that gave rise to Region AR4046 (responsible for X-flare
activity) should bring solar activity to moderate and occasionally
high levels through the end of the forecast period.

No proton events are expected at geosynchronous orbit until the
expected increase in flare activity beginning on April 22. Then
there will be an increasing chance for an isolated proton event as
the more potent regions approach the west limb by the end of the
forecast period.

Flux will subside to moderate levels after April 12 as the effects
from the fast stream wane. April 19 is expected to bring a return to
high levels, again in response to another recurrent fast stream. The
elevated conditions are expected to remain through April 28 before
returning to moderate levels.

Geomagnetic field activity is expected to be primarily quiet to
unsettled, with an isolated active period, until the return of a
recurrent geoeffective coronal hole between April 19 to 21. Active
conditions are expected to prevail through April 24 before the fast
solar wind stream wanes.

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/ .

 

The ARRL Solar Report

04/04/2025

 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/ . 

The ARRL Solar Report

03/28/2025

 On Wednesday, March 26, around 2130 UTC a Major Storm occurred.  At
one point, the Planetary A Index was 49, and the Planetary K Index
was at 6.

Solar wind parameters are expected to remain enhanced due to
continued coronal hole high-speed stream influences with waning
effects likely by March 29.

The Solar Radiation Storm Forecast for March 29 and 30 has a 1
percent chance of a S1 or greater storm.

No S1 (Minor) or greater solar radiation storms are expected.  No
significant active region activity favorable for radiation storm
production is forecast.

The Radio Blackout forecast calls for a chance for isolated R1-R2
(Minor-Moderate) radio blackouts due to M-class flare activity will
persist through March 30 primarily due to the potential exhibited by
AR4043.

From Space Weather Prediction Center: "Since February 25, 2025, the
Space Weather Prediction Center (SWPC) has presented new coronagraph images and data from the GOES-19 Compact Coronagraph-1 (CCOR-1). The images and data are updated every 15 minutes.

"Imagery from the Compact Coronagraph (CCOR) instruments is used by
the SWPC Forecast Office to characterize activity in the outermost
part of the Sun's atmosphere known as the corona. This includes
monitoring data for transient events like coronal mass ejections
(CMEs), as well as monitoring the impacts the corona has on the
steady stream of plasma, referred to as the solar wind, emanating
from the Sun. Ultimately, information derived from CCOR images will
be used as inputs to the WSA-Enlil model to forecast the impacts of
CMEs and the solar wind on Earth.

"Note: Until such time as GOES-19 becomes operational, currently
planned for April 4th, the animations and data are to be considered
'preliminary and non-operational.' In particular, CCOR-1 data will
not update between 3/21 and 4/1 due to the spacecraft drifting to
its operational location."

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - March 27, 2025, by F. K. Janda, OK1HH:

"Most forecasts, including those from NOAA, have been consistent
over the past week that a strong G3-class geomagnetic storm would
develop on Sunday, March 23. In fact, a CME was expected to directly
impact the Earth. Although the source of the CME was only the
M1-class solar flare from AR4028, even weaker CMEs can produce
strong geomagnetic disturbances, especially on days around the
equinoxes. In the end, however, paradoxically, 23 March was the
relatively quietest day.

"However, a large coronal hole caught our attention this week, while
we expected that once it reached the central meridian region, the
solar wind stream would head directly towards Earth. This is what
happened and in the following days, especially on March 26, a G2
class geomagnetic storm developed. The solar wind speed increased
from 400 km/s to 700 km/s. On March 27, the increase continued to
over 800 km/s (1.8 million mph).

"Shortwave propagation conditions were particularly degraded along
paths through the higher latitudes of the Northern Hemisphere.
Outside of these, there was also a more pronounced decrease in MUF
on routes that crossed South America and the South Atlantic where
geomagnetic anomalies are located.

"While late March and early April are periods with traditionally
better-than-average shortwave propagation conditions, this time our
expectations will only be partially met. Solar activity is lower
than would be consistent with the current phase of the solar cycle -
and disturbances are relatively common."

A partial Solar Eclipse occurs on March 29, 2025. Details can be
found at,
https://science.nasa.gov/eclipses/future-eclipses/mar-29-2025-eclipse/
.

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/ .
 

The ARRL Solar Report

03/21/2025

Solar activity reached moderate levels due to an isolated M1.5/1n
flare on March 19 at 2040 UTC from Region AR4031.  Regions AR4028,
AR4034, and AR4035 exhibited slight growth.

Region AR4026 re-emerged in the Southwest quadrant. Regions in the
Northwest quadrant, including AR4020, AR4022, AR4025, and AR4031 all
appeared to be in a decay phase.

No Earth-directed CMEs were detected in available coronagraph
imagery.

No S1 (Minor) or greater solar radiation storms are expected through
March 23.

No significant active region activity favorable for radiation storm
production is forecast.

C-class flares are expected to continue to March 22, with a chance
for isolated M-class (R1-Minor) flares.

A chance for R1-R2 (Minor to Moderate) radio blackouts due to
M-class flares will persist through March 23.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - March 20, 2025, by F. K. Janda, OK1HH:

"While the current solar activity is a bit lower than we would like
and then would be consistent with the current phase of the solar
cycle, it may be a prelude to another peak within the current
11-year cycle maximum.

"This hypothesis is supported by the starting shift of the sunspot
activity from the southern hemisphere of the Sun to the northern
hemisphere (see also the M-class flares in AR4031 and the following
filament flare, i.e., in the northwest quadrant of the solar disc).

"At the same time, the presently forecasted period is likely to
begin with a transient decrease in geomagnetic activity. But this
may be interrupted as early as March 23 if an enhanced solar wind
blowing from Solar Coronal Hole 24 hits Earth.

"If this happens in the daytime, a so-called positive phase of the
disturbance could follow with an increase in MUF and a general
improvement in ionospheric shortwave propagation conditions."

Spaceweather.com has a link to an article that discusses new
evidence that cosmic rays spark lightning.

Weak disturbances in the solar wind are anticipated through March 20
in response to persistent transient/High-Speed Stream (HSS) effects
as well as possible flanking influences of a nearby CME passage that
departed the Sun on March 17. Solar wind parameters are expected to
slowly diminish on March 21.  Another enhancement is expected late
on March 22 due to the arrival of a CIR preceding a negative
polarity Coronal Hole HSS.

A CIR - or Corotating Interaction Region - is a recurring plasma
structure in the heliosphere formed when fast solar wind streams
interact with slower solar wind ahead of them.

Quiet to unsettled levels are expected on March 21. By late on March
22, unsettled to active levels are likely due to the aforementioned
CIR arrival.

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/ . 

Propagation

You and the ionosphere . . . a reader participation post By Jock Elliott, KB2GOM

 

Here’s a shocker for you: we live at the bottom of the sky. Above us there are multiple layers of the atmosphere, pressing down on us at 14.7 pounds per square inch.

 

Of particular relevance to us as shortwave listeners and hams, there is a special layer of the atmosphere, not shown on the chart above called the ionosphere. The ionosphere starts around 30 miles above us and extends up to about 600 miles and includes parts of the layers above.


The Sun’s upper atmosphere, the corona, is very hot and produces a constant stream of Ultra-Violet and X-rays, some of which reach our atmosphere.  When the high energy UV and X-rays strike the atmosphere, electrons are knocked loose from their parent atoms and molecules, creating a layer of electrons.

Now, here’s the cool part: this layer – the ionosphere – is important because radio waves bounce off of it.


The sun, however, is not constant in its action on the ionosphere. The amount of UV and x-ray energy (photon flux) produced by the sun varies at by nearly a factor of ten as the sun goes through an 11 year cycle. The density of the ionosphere changes accordingly, and so does the ability of the ionosphere to bounce radio waves. When the sun is at peak activity, and the ionosphere is “hot,” SWLs and hams are likely to experience excellent long-range propagation. When the sun is quieter, long-range propagation diminishes.

Every 11-year solar cycle is unique, but early indications are that we may on the verge a cycle that favors long-range propagation: https://swling.com/blog/2022/03/termination-event-may-indicate-solar-cycle-strength/

The results can be spectacular. Decades ago, during a particularly hot solar cycle, I once spoke from my station near Albany, NY, to a station in the state of Georgia on a mere 4 watts. On another occasion, I conversed with a ham in Christchurch, New Zealand – a distance of over 9,000 miles – with 100 watts single sideband transmit power. During that same period, I would routinely listen to shortwave stations halfway around the world.

6 Meters!! The magic band. Should be some real magic happening at the end of 2024.

 

Ain't this solar maximum great?   
Solar Cycle 25 is now much, much stronger than anyone anticipated,
and it's slowly growing stronger through at least this weekend.
Today's estimated international sunspot number is 281.

It's increasingly likely that we'll have widespread coast-to-coast and
worldwide 6 meter F2 propagation during about half of the days
between late October and at least early February.  Widespread F2
openings are likely to bring 6 meter CW and SSB to life like we haven't
experienced in more than 20 years.

The first sign of enhanced 6 meter F2 will be increasingly frequent TEP
from Europe and North America to South America and the South Atlantic islands. TEP may begin very sporadically by late August and become increasingly frequent later in September and especially during October.

Coast-to-coast F2 propagation and propagation crossing the Atlantic
to Europe and Africa may begin sporadically during September and
October and become frequent and long lasting by early November.

Effective 6 meter antennas can be very small.  3 element Yagis are small, lightweight and very effective. 20 foot antenna height is adequate but sloping terrain or higher antennas perform much better. Heights higher than 50 feet are unnecessary and in many cases perform poorly.
Are you ready for this once in a lifetime experience?


73
Frank
W3LPL

Ionospheric Propagation of Radio Waves Gives Ham Radio Operators "Seven League Boots"!

 

Thanks to ionospheric propagation of radio waves, ham radio operators can rely on HF ionospheric radio signal propagation to communicate with fellow hams located way beyond the horizon.

The ionized layers of the ionosphere make HF radio wave propagation possible much beyond line of sight distances. These layers can be viewed as our "Seven League Boots" which, by leaps and rebounds, give our ham ra

Show More

 

I'll explain, in a moment, how the 'F' layer is the most useful ionized layer for DX.

Best of all, solar sunspot cycles improve HF propagation because they revitalize our ionosphere. The good news is, solar cycle 25 has begun! Ham radio operators, all over the world, are looking forward to its increasing activity.

 The simplified drawing above illustrates how radio wave 'C' is refracted, by the ionized layer 'F', back toward the earth's surface, rebounds off the earth's surface a great distance away from its origin, goes upwards again as 'C1' to be refracted again by the 'F' layer and bounce off the earth further on  as 'C2' and so on.

The radio signals 'A' and 'B', arriving at the ionized 'F' layer at too 


Show More

Improve Your Communication with Eyebank Net’s Reliable information

 The HF signals will gradually lose energy after each refraction by the 'F' layer and after each rebound off the earth's surface... until it is no longer discernible. But, by that time, it will have traveled thousands of miles and been heard by countless radio amateurs and shortwave listeners!

That's the magic of HF ionospheric radio signal propagation. 

How Do Ionized Layers Form to Enable Ionospheric Propagation

 Ionization of the upper reaches of earth's atmosphere occurs when ultraviolet radiation from the sun collides with hydrogen and helium molecules that are few and far between up there. These collisions detach electrons from the gaseous molecules.

As a result, positive hydrogen and helium ions are generated and negatively charged free electrons are liberated from their nucleus. These regroup into ionized layers above the earth. 

 However, ionized layers only form when the sun is "active", which it is for about 9-10 years, every eleven years or so. It's commonly called the 11-year sunspot cycle.

We can see the progression of the last few sunspot cycles in the graph shown earlier. You can obtain more information on the 11-year cycle of sunspots here. 

 

The Ionized Layers and Their Respective Role in HF Radio Wave Propagation


 

Ionized Layer 'D'

During the day, the ionized layer 'D' mostly hinders ionospheric propagation of radio waves.
It is the ionized layer closest to the earth's surface. It is located between 60 km and 100 km (37-62 miles) above the earth.

In the daytime, it forms under the sun's intense UV radiation and constitutes a barrier preventing amateur radio signals in the 40-meter, 80-meter and 160-meter bands from getting far and from being heard in the intense atmospheric noise.
Meanwhile, signals 10 MHz and above can get through to reach the ionized layers above and make their way beyond the horizon.


The 'D' layer dissipates at sunset. 

Signals in the 160-meter to 40-meter bands then become free to reach the 'F' layer and reach DX amateur radio stations like the other higher-frequency signals.


Ionized Layer 'E'

The 'E' layer lies between 90 km and 150 km (56-93 miles) above the earth but its most useful portion is located between 95 km and 120 km (59-75 miles) of altitude.

During daytime hours, in theory, layer 'E' could refract 5-20 MHz signals and help them along their way.

However, in reality, the 'D' layer (below) absorbs much of the energy of signals at these frequencies. Only signals in the 7-14 MHz range - transmitted near vertically - will be able to punch through the 'D' layer with enough remaining energy to reach the 'E' layer and be refracted along to reach as far as 1200 km (750 miles) at times.

That's where NVIS antennas come in handy.

The periods just before dawn and right after dusk  are best to make use of the 'E' layer. At night, the 'E' layer disappears almost completely, while still remaining somewhat useful to the propagation of signals in the 160-meter band.


The "Sporadic E" Layer

Sometimes, dense ionized clouds will form suddenly in the 'E' layer and disappear just as suddenly, minutes, rarely hours later.

Sporadic 'E' propagation (Es) is useful at frequencies above 28 MHz, in the VHF range, rarely below. We cover their usefulness in extending the reach of VHF signals beyond the horizon on another page of this website.

Both 'E' and 'Es' propagation contribute to 50 MHz activity.


Ionized Layer 'F1'

During daytime hours, in summer, this layer will often be useful to the propagation of HF radio signals of the 30-meter and 20-meter bands. Its role in the propagation of HF signals is rather negligible.


Ionized Layer 'F2'

The 'F2' layer forms during daytime hours between 200 km and 400 km (125-250 miles) above the earth. It is higher in altitude in the summer than it is in the winter.

It is usually around all year round.

At night, layers 'F1' and 'F2' merge into one 'F' layer, a little lower than the daytime 'F2' was located.

The 'F2' ionized layer is present during the major part of a solar cycle.

However, it will sometimes disappear completely for days on end during a deep solar cycle minimum!

The 'F2' layer will reach its highest density at the peak of a solar sunspot cycle.

It will then refract toward earth radio signals ranging from 7 MHz to 30 MHz and enable them to reach distances as far as 4000 km from their origin, rebound off the earth to rise again to the 'F2' layer... and repeatedly do so… sometimes to travel right around the earth and come back from behind their point of origin!

During the better nine years or so of a solar cycle, QRP operators (5 watts of radiated power or less), using simple dipoles, can make DX contacts as far and as often as the QRO operators (using up to 200 to 300 times more power) using a multi-element directional antenna!

During such wonderful periods, every ham radio operator has an equal chance under the sun to make DX contacts.

 

Ionospheric Propagation of Radio Waves is a Complex Topic


 The information I have presented to you in this article is a very brief summary of what could be said about HF ionospheric radio signal propagation. I have really only scratched the surface!

Countless scientific publications have covered many aspects of the subject since the discovery of the ionosphere's existence and, later, its role in the propagation of HF radio signals.

Research is ongoing, involving and scientists and ham radio operators alike.

For more on our sun's behaviour, visit the Solar and Heliospheric Observatory 


by VE2DPE

https://www.hamradiosecrets.com/ionospheric-propagation-of-radio-waves.html

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