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2. Background

A coronal mass ejection (or CME) is a giant cloud of solar plasma drenched with magnetic field lines that are blown away from the Sun during strong, long-duration solar flares and filament eruptions. A coronal mass ejection can escape from the Sun during eruptions on the Sun like solar flares and filament eruptions. However, not every event has a coronal mass ejection accompanied with it. Strong flares (M and X-class) are likely candidates to launch coronal mass ejections. C-class solar flares can also produce coronal mass ejections but only the long-duration and stronger C-class flares might do this.
Image of Coronal Mass Ejection
When the Sun isn't very active during solar minimum, coronal mass ejections are rare. There might only be one coronal mass ejection every week. When the Sun's activity increases towards solar maximum, coronal mass ejections become more common and we can see multiple coronal mass ejections every day.
In general, the physical danger is low and controllable. The biological hazard inherent in solar and geomagnetic storms comes from the exposure to radiation, which is mainly a concern for astronauts and people flying at high altitudes.
On the other hand, the disruptions that more severe storms can cause have the potential to bring about real damage. Milder storms may disrupt the satellites that handle GPS communications. But the more severe geomagnetic storms can spike the voltage in transmission lines which could damage grid transformers and potentially knock power out. A massive power outage in the province of Quebec in 1989 was blamed on a solar storm.
Dramatization of electrical grid effect
The solar wind distorts the Earth's magnetosphere. When the solar wind changes, this causes changes in the magnetosphere. Since antennas are large conductors in this moving magnetic field, currents are induced in the antenna which we see as noise.
When the sudden shock from a CME strikes the Earth, the violent vibration this causes in the magnetosphere causes a loud bang on HF, followed by eerie silence.
Image of Earth's magnetosphere
X-rays and extreme ultraviolet light from solar flares ionize the Earth's atmosphere, causing an enhancement of the lower part of the dayside (Sun-facing) ionosphere which blocks radio signals that normally are reflected off of the ionosphere. Reflection of radio waves off the ionosphere allows long distance radio communication without having clear line-of-sight between the transmitter and receiver. When the enhanced ionosphere absorbs the radio waves, no radio communication is possible. This creates conditions referred to as Radio Blackouts.
D-RAP image of blackout
Practically speaking, a Radio Blackout is the absence of a capability to communicate on High Frequency bands in the 5 to 35 MegaHertz spectral range, but lower frequency radio communications may also be significantly degraded during a Radio Blackout event. The violent magnetosphere motion is most noticeable at lower frequencies, so although the cause is different, lower frequencies tend to be also unusable.
Most very large CMEs actually deliver a double-whammy to our home planet. Initially, high energy, lighter particles leave the Sun at relativistic speeds. These particles arrive with just a few minutes of the ejection, slamming into the magnetosphere at incredible speed.
The bulk of the mass takes much longer to reach Earth. This time can vary from as few as eight to as much as seventy-two hours, depending on the violence of the ejection. Although traveling at much lower speed, the greater mass of this material causes a much more long-lasting effect on HF propagation.
Many DXers note that the initial hit of the higher velocity particle will cause a blackout for only a few hours, after which HF conditions will be enhanced for several hours. However, when the second wave hits, it is time to turn off the radio and melt solder. HF conditions will be blacked out for a long time, perhaps days, and will not return quickly.

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