Solar Weather

How does Solar Weather affect the HF Activity Group?

Why is the HF Activity Group so concerned about X-Ray flux, Flares, and solar activity? Because ‘Radio”.

Solar Weather Information for HF Radio Operators

Ham radio enthusiasts and shortwave listeners often reference solar and geomagnetic conditions to predict HF radio propagation. This page provides key insights into understanding these conditions and their impact on radio communications and how it affects our group everyday.

Understanding Key Terms

Solar Flux: Measures the sun’s radio emissions, indicating activity levels. Higher numbers suggest improved HF conditions.

A and K Indices: Represent the Earth’s magnetic field disturbances, affecting radio wave propagation. The K index updates every 3 hours, while the A index provides a daily average.

Space Weather: Encompasses the conditions in space influenced by the sun and Earth’s magnetic field, affecting HF propagation.

Radio Blackouts and X-ray Levels: Solar X-ray emissions, monitored by GOES satellites, can cause radio blackouts by ionizing the Earth’s upper atmosphere, impacting HF radio waves’ travel.

X-ray Flare Classes

Flare ClassPeak Flux Range (Watts/sq. meter)Description
A< 1×10⁻⁸The weakest flares, not significantly affecting Earth’s atmosphere.
B1×10⁻⁷ to <1×10⁻⁶Slightly stronger than A, but still not noticeable in terms of HF propagation.
C1×10⁻⁶ to <1×10⁻⁵Small flares with minimal impact, causing slight disturbances.
M1×10⁻⁵ to <1×10⁻⁴Medium flares that can cause brief radio blackouts at the poles and minor radiation storms.
X≥1×10⁻⁴The strongest flares, capable of causing planet-wide radio blackouts and long-lasting radiation storms.

Monitoring solar flare classes, especially M and X, is crucial for planning and avoiding disruptions in HF radio communication.

Impact of M Class X-ray Flares on 18 MHz Communications

An M class solar flare represents a medium level of solar activity that can significantly influence HF radio propagation, especially on frequencies like 18 MHz. These flares emit intense X-ray radiation that increases the ionization levels in the Earth’s upper atmosphere, temporarily enhancing or disrupting the radio waves’ path.

Effect on Distance and Quality

During an M class flare, operators might experience:

  • Enhanced Propagation: Initially, the increased ionization can enhance signal strength, potentially extending the reach of transmissions beyond normal limits. This phase can last from minutes to hours, depending on the flare’s intensity.
  • Signal Fading: As the flare progresses, rapid fluctuations in ionization levels can cause signal fading and variations in signal strength, affecting the consistency and reliability of communications.
  • Increased Noise Levels: The ionospheric disturbance can introduce higher levels of background noise, making it harder to distinguish signals.
  • Temporary Blackouts: In some cases, especially with stronger M class flares, complete signal blackouts may occur, rendering communication on the affected frequency impossible for the duration of the flare’s impact.

Real-World Example – “M” Class Flare

Consider a scenario where an amateur radio operator attempts to establish contact from North America to Australia on 18 MHz during an M class solar flare. Initially, they might find that their signals reach further than expected, possibly even beyond Australia, into more distant regions. However, as the flare’s effects intensify, they may start to experience signal fading and increased noise, complicating the communication. If the flare is particularly strong, a temporary blackout could occur, during which no contacts could be made. After the flare subsides, conditions would gradually return to normal, requiring operators to adjust their strategies accordingly.

Effects of X Class X-ray Flares on 18 MHz and Higher Frequencies

X class flares are the most intense solar events and have a profound impact on HF radio communications, particularly at 18 MHz and higher frequencies. These frequencies are highly sensitive to ionospheric conditions, making them especially vulnerable during such powerful solar flares.

Impact on Communication

  • Severe Signal Degradation: The intense ionization from X class flares can severely degrade signal quality, to the point where reliable communication is nearly impossible. The ionosphere may absorb signals instead of reflecting them back to Earth, leading to significant loss in communication capability.
  • Widespread Communication Blackouts: X class flares can cause widespread communication blackouts across the daytime side of Earth. This affects not just amateur radio but also commercial and emergency communication frequencies, leading to a notable disruption in various communications.
  • Extended Duration of Effects: The effects of X class flares can last from several hours to a full day, depending on the intensity and duration of the flare. This can result in prolonged periods of radio silence, disrupting planned communications.
  • Increased Noise Levels: Background noise levels can spike dramatically during X class flares, further complicating communication attempts and making it challenging to receive clear signals.

Real-World Example – “X” Class Flare

Imagine an amateur radio operator trying to make long-distance contacts on 18 MHz during an X class solar flare. At first, they might notice an unusual extension in the reach of their signals, potentially making unexpected contacts far beyond the usual range. However, this is quickly followed by severe signal fading, drastically increased noise levels, and ultimately, a complete blackout of communication on 18 MHz and higher frequencies. Unlike the effects seen with M class flares, the blackout period during an X class flare can last for many hours, significantly disrupting all planned radio communications. Operators might find themselves needing to wait out the flare’s effects or switch to lower frequencies, less affected by the flare, to maintain some level of communication.

Understanding the severe impacts of X class flares is crucial for radio operators, emphasizing the need to monitor space weather forecasts closely and adjust communication strategies in anticipation of these disruptive solar events.

Know it all?

Not nearly, we never know what the Sun will throw at us, Literally. CME’s.

Coronal Mass Ejections (CMEs) are massive bursts of solar wind and magnetic fields rising above the solar corona or being released into space. When CMEs interact with the Earth’s magnetosphere, they can cause geomagnetic storms, potentially affecting HF radio communications at frequencies like 18 MHz. Effects include degraded signal quality, increased noise levels, and, in severe cases, complete blackout of HF bands. These impacts vary in duration and intensity based on the strength of the CME and its directness of impact on Earth.

During a CME, amateur radio operators can expect disruptions on the 18 MHz frequency, including signal fading, increased static, and potential blackout conditions. These effects are due to the CME’s interaction with Earth’s magnetic field, leading to changes in the ionospheric layers that facilitate HF propagation. Operators should monitor space weather alerts and may need to adjust operating frequencies or times to maintain communications.


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In March of 2024 an X1.1 CLASS FLARE erupted on the Sun causing radio blackouts around the globe for hours.