Forty meters is open. You’re making plenty of QSOs, even snagging a few DX stations. Suddenly, signals begin to fade or even disappear—and they don’t seem to come back. You look outside to see if your antenna is still standing. Fortunately, it’s still there. Your wattmeter indicates the transceiver on your desk is still operational. What happened?
You could be one of the many victims of propagation theft (PT). When we get solar storms and the A index climbs to about level 4-5, the band just falls apart, with lots of absorption and fading. Even DX is not possible. It seems this has happened a lot recently, and it’s not just on 40 meters.
On April 21, 2023 at 3:26 p.m. EDT, a coronal mass ejection (CME) erupted from the sun, releasing a burst of plasma that raced toward Earth at nearly two million miles per hour and generated a severe geomagnetic storm (level 4 out of 5 on NOAA’s space weather G-scale).
Here Comes the Sun
Have you ever sat around a campfire and been suddenly surprised when a flaming ember was ejected in your direction? The Sun can also do unexpected and dramatic things. Disturbances of propagation conditions here on Earth are caused by conditions on the Sun known as solar storms.
These solar events can significantly affect the ability to communicate on the medium and high frequency (300 kHz-30 MHz) bands. During these storms, some radio frequencies are absorbed and others are reflected, leading to rapidly fluctuating signals and unexpected propagation paths.
You may hear people complaining when propagation conditions are poor. Just like the weather, there’s not a lot we can do about solar disturbances—but we can continue to observe and determine when they’re coming our way.
Solar Flares
Solar flares are enormous explosions that occur on the surface of the Sun. They result in the release of colossal amounts of energy, including sustained high-energy bursts of radiation from VLF to X-ray frequencies and huge amounts of solar material. Most solar flares occur around the peak of the 11-year solar cycle. In addition to this, the larger solar flares also eject large amounts of material in the form of protons.
Flares erupt in just a few minutes with no apparent warning. The first indication of a huge flare is usually some visible brightness near a sunspot group, along with increases in UV and X-ray radiation and VHF radio noise. If the position of the Sun relative to the Earth is right, intense X-ray radiation takes eight minutes to travel the 93 million miles to our planet.
These flares generally only last for about an hour and the surface of the Sun returns to normal—though some post flare loops may remain for a time afterward. The flares affect radio propagation and radio communications on Earth, and the effects may be noticed for some time afterward.
Coronal Mass Ejections
You know what happens when you eat baked beans or indulge in fizzy drinks. Every so often, the sun also passes gas—but with the power of 20 million nuclear bombs. Scary.
During a CME, enormous bubbles of superheated gas, called plasma, are ejected from the sun. Over the course of several hours, a billion tons of material launch off the sun’s surface and accelerate to speeds of a million miles per hour (1.6 million kilometers per hour). The resulting shocks ripple through the solar system. Worst case, they can interrupt satellite operation, radio communication, and power grids on Earth. CMEs can happen several times a day when the sun is most active. During its quieter periods, CMEs occur only about once every five days.
Because CMEs leave the sun in all directions, most don’t come anywhere near Earth. But every so often, we’re in the crosshairs. When the plasma cloud hits our planet, a geomagnetic storm follows. The fastest of these CMEs can reach Earth in less than a day, with the slowest taking four or five days.
Geomagnetic Storms
Solar flares and CMEs are two significant triggers for geomagnetic storms. These storms result from variations in the solar wind which produce major changes in the currents, plasmas, and fields in Earth’s magnetosphere.
When the solar wind reaches Earth, it sends a flurry of charged particles into the magnetosphere and along Earth’s magnetic field lines, toward the poles. The combination of these particles with Earth’s atmosphere can produce glowing aurora displays above polar regions. Active auroras can sometimes interfere with communications, disrupting radio and radar signals.
The sudden increase in X-ray energy produced by a large solar flare can increase RF absorption in the Earth’s lowest ionospheric layers (D region), sometimes causing a condition known as a Sudden Ionospheric Disturbance (SID). A SID affects all HF communication on the sunlit side of the Earth, and signals in the 2-30 MHz range could disappear entirely. Even background noise may disappear in some extreme cases. These can last from several minutes to several hours.
Becoming Better Prepared
Solar disturbances are responsible for many of the major changes in the ionosphere. The effects of both CMEs and solar flares can cause changes to ionospheric radio propagation, often disrupting it for hours or sometimes days. NOAA’s Space Weather Prediction Center (SPWC) issues warnings and alerts, using several computer models. Whole Atmosphere Model-Ionosphere Plasmasphere Electrodynamics (WAM-IPE) provides guidance to forecasters when issuing alerts for terrestrial radio and Global Navigation Satellite System (GNSS) disturbances.
Obviously, we don’t want to be surprised by a powerful flare or Earth-directed CME. That’s why astronomers study the sun. With even just a few hours of warning before an impending CME strike, we could safely shut down our stations. Disruptions may then only last a few hours during the actual solar disturbances, rather than the days, weeks and possibly months needed to restore communications.
Propagation Check
Activity on the Sun has a major impact on ionospheric radio propagation. It affects a variety of forms of HF radio communications, including two-way radio communications, maritime mobile radio communications, radio broadcasting, and amateur radio communications. Knowing the Solar Flux Index and K Index will help you determine current conditions and know what to expect in the near future. There are dozens of sites on the web that provide this data.
For Best Conditions:
- Solar Flux should remain above 150 for a few days (scale from 0-300)
- K Index should be below 2. A 5 or more indicates a geomagnetic storm (scale from 0-9)
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