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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

08/15/2025

   

 

  Solar activity remained at moderate levels earlier this week. There
was an approximately 20-degree filament eruption on June 4 and a
possibly related Coronal Mass Ejection (CME). The CME is unlikely to
have an Earth-directed component, but analysis is in progress.

The geomagnetic field is expected to be quiet to unsettled on June 7
as High Speed Stream (HSS) activity continues. G1 (Minor) storm
levels are likely with the anticipated onset of a CME - that left
the Sun on June 3 - around mid-to-late on June 7. On June 8,
unsettled to active levels are expected.

Unsettled to active conditions are likely June 10 to 12 due to
recurrent negative polarity Coronal Hole High Speed Stream (CH HSS)
influences. Unsettled to G1 (Minor) conditions are likely on June 13
to 22 due to positive polarity CH HSS influences. Unsettled to G1
(Minor) conditions are likely on June 23 to 28 due to negative
polarity CH HSS influences.

Solar activity is expected to be low with a chance for M-class
flares (R1-R2/Minor-Moderate) and a slight chance for an X-class
flare (R3-Strong) on June 7.

Solar wind speed parameters increased from approximately 550 km/s to
nearly 810 km/s before decreasing to around 760 km/s. This could
either be transient influence or a transition back into the Coronal
Hole High Speed Streams.

Quiet to active levels are expected to prevail on June 7.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere - June 5, 2025, from F. K. Janda, OK1HH:

"When assessing solar activity based on the sunspot, or more
modernly based on the intensity of solar radio noise, we can
tentatively conclude that the maximum of the 25th cycle occurred
last spring and summer, or early autumn (the highest smoothed
sunspot number was in October 2024: R12 = 160.8, and then declined).
Even so, it was much higher than most astronomers had predicted.

"But that's not the end of the story. In May of this year in
particular, there was a surprising increase in the number and energy
of particles in the solar wind, especially during larger solar
flares. Particle ionization also affects the Earth's ionosphere,
although not as nicely as we would like given the state of the
Earth's ionosphere. In short, shortwave propagation conditions were
rarely good during May and especially early June 2025. They were
mostly unstable, disrupted, with irregular daily cycles and frequent
occurrences of increased attenuation.

"An exceptional phenomenon is the so-called Forbush effect, also
known as the 'Forbush decrease' in the intensity of galactic cosmic
rays after the arrival of a CME in the vicinity of Earth. The
largest decrease in cosmic ray intensity in more than 20 years, by
as much as 25%, was recorded on June 1, 2025 (the last time this
happened was on October 30, 2003). Particles ejected by the Sun will
remain in our vicinity and reduce the intensity of cosmic rays of
galactic origin for another week or two.

"A decline is generally expected in the further development of solar
activity. Only optimists admit that there will be one more increase
this year, probably in the northern half of the solar disk."

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 June 7 to 13 is 10, 8, 5, 15,
12, 10, and 35, with a mean of 13.6.  Predicted Planetary K Index is
3, 3, 2, 4, 4, 3, and 6, with a mean of 3.6.  Predicted 10.7
centimeter flux is 155, 155, 155, 155, 150, 150, and 155, with a
mean of 153.6. 

The ARRL Solar Report

08/8/2025

   

 

  

Solar activity was at low levels on 28 July to 02 August and moderate levels on 03 August.  The largest flare of the period was an M2.9/2b event observed from Region 4168 on 03 August.  The region also produced numerous C-class events.  Numerous C-class activity was observed from Regions 4153, 4155, and 4167.  No Earth-directed CME activity was observed.

No proton events were observed at geosynchronous orbit.

Geomagnetic field activity was at quiet to unsettled levels the entire highlight period.  A single active period was observed late on 03 August.  A majority of the unsettled periods was due to weak negative polarity CH HSS influence.  Solar wind speeds were at mostly 400 km/s from 28 July through midday on 30 July and 475-500 km/s from midday 30 July through 03 August with a peak velocity observed at 600 km/s late on 03 August.

Solar activity is expected to be at mostly low levels with a chance for M-class flares for the outlook period.

Geomagnetic field activity is expected to be at active to G1 storm levels on 11 to 15 August, 18 to 22 August and 25 to 30 August due to recurrent CH HSS effects.  Quiet to unsettled levels are expected on the remaining days of the outlook period.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere for August 7, 2025 by F. K. Janda:

Overall solar activity has not changed much in recent days, but it has been higher than on the same days of the previous solar cycle. However, eruption activity increased in the northwestern part of the solar disk.  The M4.4 eruption on May 5, with a maximum at 1553 UTC, was of greatest significance for further development, as it was accompanied by a CME, part of which is heading toward Earth.  It is expected to arrive on August 8, at the same time as a co-rotating interactive region (CIR), should cause an intensification of the solar wind blowing from a large coronal hole in the southern hemisphere of the Sun.

The combination of these two phenomena will cause an increase in geomagnetic activity on August 8.  Thanks to the intensified solar wind, the geomagnetic field will remain active on May 9.  On the first day, there will likely be a positive phase of development, accompanied by an increase in the highest usable frequencies of the F2 ionospheric region, while on the second day, the development will transition to a negative phase, with a decrease in usable frequencies and an overall deterioration in shortwave propagation conditions due to increased attenuation.

After that, rather turbulent developments can be expected until approximately August 12, followed by a calming down from August 13. Overall solar activity should remain only slightly elevated.

The latest report from Dr. Tamitha Skov can be found on YouTube at: https://www.youtube.com/watch?v=SItdIe1X7RU

The Predicted Planetary A Index for August 9 to 15 is 5, 5, 25, 18, 20, 12, and 8, with a mean of 13.2.  Predicted Planetary K Index is 2, 2, 5, 5, 5, 4, and 3, with a mean of 3.7.  10.7 centimeter flux is 135, 135, 130, 130, 130, 135, and 135, with a mean of 132.9.

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

08/1/2025

   

Spaceweather.com reports solar activity was at low levels with only minor C-class flares observed.  Region 4154 produced a C1.4 flare on 31 July.  Region 4153 contributed the majority of the flares, including a C1.3 flare, a C1.5/Sf flare, and a C1.6 flare on 31 July.  Both of these regions remained mostly unchanged during the period.  Region 4166 exhibited growth during the period yet remained mostly inactive.  Newly numbered Region 4167 exhibited growth during the period and produced a C1.9/Sf flare.  The remaining regions were stable or in decay.

Solar activity is expected to be low with a chance for M-class flares, Minor-Moderate, through 03 August.

Solar wind parameters are expected to continue a waning trend on 01 August as the negative polarity CH HSS moves from its geoeffective location.  An additional disturbance cannot be ruled out for 02 August due to the 30 July CME event mentioned in the previous discussion.  Aside from the possible CME influence, conditions are expected to remain at mostly background levels through 03 August.

Quiet conditions, with isolated unsettled periods, are expected to prevail on 01 August as CH HSS effects gradually wane.  Quiet to unsettled levels are expected on 02 to 03 August, with a slight chance for an isolated active period on 02 August, due to the possible July CME passage.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere for July 31, 2025 by F. K. Janda, OK1HH:

We are most likely still in the period of the maximum of 11-year solar activity cycle No. 25.  Its peak was preliminarily recorded in the fall of 2024, but this year's course was very unusual, while its response in the ionosphere was unexpected, especially in May and June.  Solar activity should continue to decline slowly this year, with a more rapid decline expected starting in 2026.  Therefore, there is still hope for favorable shortwave propagation conditions this fall, especially in the shorter part of the range (say, at frequencies above 20 MHz).

Last week, only relatively small sunspot groups were observed on the Sun, with no major flares.  This was accompanied by smaller fluctuations in the speed of the solar wind and, therefore, a calmer geomagnetic field.  Although the summer season in the northern hemisphere of the Earth is not favorable for long-distance shortwave propagation, it was still an improvement over previous months.

There should be fewer sunspot groups on the far side of the Sun. But it seems that those that will soon appear will be larger than those we are currently observing.

Solar ultraviolet and X-ray radiation, together with the solar wind, are the main causes of changes in the Earth's ionosphere, but they are not the only ones.  The ionosphere also reacts to processes taking place below it.  These include gravitational waves (formed at the interface between air masses of different densities, for example in meteorological fronts) and infrasound caused by earthquakes and typhoons.  Scientific research into these phenomena is still in its infancy, and although records of changes in the ionosphere following the earthquake on the east coast of Kamchatka (M8.8, Tue 29 July 2025, 23:24:51 UTC) are available, for DX signal hunters this is still more of a curiosity.

The latest solar report from Dr. Tamitha Skov, WX6SWW, can be found on YouTube at:

https://www.youtube.com/watch?v=-XM4kZ-vvRk

The Predicted Planetary A Index for August 2 to 8 is 5, 5, 12, 8, 5, 20, and 15, with a mean of 10.  Predicted Planetary K Index is 2, 2, 4, 3, 2, 5, and 4, with a mean of 3.1.  10.7 centimeter flux is 140, 150, 155, 155, 155, 155, and 160, with a mean of 152.9.

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

06/13/2025

   

 

  

Solar activity has been at low levels with mostly weak C-class flares observed from Regions 4172 and 4180.  The largest flare of the period was a C6.8/Sf at 15/1037 UTC from Region 4172.  There are 12 regions on the disk with most being simple in magnetic complexity.  Region 4172 remained the largest region on the disk and continued to decay with its magnetic field decreasing in complexity. All remaining regions were quiet and stable.

No Earth-directed CMEs were observed in available coronagraph imagery. Solar activity is likely to continue at low levels through 17 August.  Despite their decreasing activity, Region 4172 will help maintain a slight chance (35%) for isolated M-class flares (R1-2/Minor-Moderate radio blackouts) in the forecast through 15 August.  Chances decrease to 30% as the region moves beyond the western limb.

Waning influences from the CH HSS will steadily decrease the flux of 2 MeV electrons, but concentrations at geostationary orbit will remain high most likely through 17 August.  Last rotation, electrons remained significantly above the 1,000 pfu threshold for seven day with diurnal maximas above threshold for 11 days.

Solar activity is expected to be at predominately low levels with M-class flare (R1-R2/Minor-Moderate events) activity likely over 11 August to 06 September.

No proton events are expected at geosynchronous orbit, barring significant flare activity.

Geomagnetic field activity is expected to reach active levels on 11 August due to waning positive polarity CH HSS influence.  Periods of minor storming are likely on 18-20 August, with periods of active conditions likely on 22 August, due to negative polarity CH HSS influence.  Active conditions are likely again on 28 August in response to anticipated negative polarity CH HSS influence.  Periods of moderate storming are likely on 05 September, with active levels likely on 04 and 06 September, due to positive polarity CH HSS influence.

Weekly Commentary on the Sun, the Magnetosphere, and the Earth's Ionosphere for August 14, 2025 by F. K. Janda, OK1HH:

The overall level of solar activity did not change significantly in August.  Medium-size flares were observed almost daily in one or two sunspot groups, while the parameters of the solar wind were most influenced by a coronal hole in the southwestern part of the solar disk.  This clear situation made it possible to correctly predict the positive phase of the geomagnetic disturbance on August 8 (G1), which manifested itself in improved conditions for ionospheric  propagation of short waves.  Particles from the solar flare on August 5 and the intensification of the solar wind from the coronal hole contributed to the disturbance.  This was followed by a negative phase of the disturbance (G2), during which propagation conditions deteriorated significantly.  This was followed by only slightly turbulent development.

The development after August 15 should be calmer, as there are currently no major active areas near the emerging coronal hole in the northeast of the solar disk.

The Predicted Planetary A Index for August 16 to 22 is 5, 5, 15, 25, 15, 8, and 12, with a mean of 12.1.  Predicted Planetary K Index is 2, 2, 5, 5, 5, 3, and 4, with a mean of 3.7.  10.7 centimeter flux is 145, 150, 145, 145, 145, 140, and 145, with a mean of 145.

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

05/30/2025

   

 

 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.

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 


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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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