Geomagnetic Effects on Radio Propagation
The Sun emits electromagnetic radiation that spans a continuum of wavelengths from radio, through microwave, infrared, visible, ultraviolet x-ray and beyond. Ultraviolet radiation interacts with the upper atmosphere to form an ionized layer known as the ionosphere. Radio waves interact with the ionosphere in a variety of ways depending on their frequencies. For frequencies below about 30 MHz, the ionosphere can act as a reflector, and this property permits very long distance radio communications around the world. At higher frequencies, above 30 MHz, radio signals usually pass through the ionosphere.
The ionosphere sometimes becomes disturbed as a reaction to some types of solar activity and, as a result, radio wave propagation may be degraded or disrupted. Solar flares emit electromagnetic radiation, such as x-ray emissions which can cause increases in ionization in the lower ionosphere, with consequent phase shifts in low frequency radio signals and increased absorption (fading) in HF and VHF radio signals. The wide spectrum of radio noise emitted from a flare may interfere with a wanted radio signal. These effects may be experienced at all latitudes. At frequencies above 30 MHz, unexpected reflections of the radio waves by the ionosphere may cause radio interference. Ionospheric irregularities may produce fluctuating signals (a phenomenon known as scintillation) and may distort the paths of radio waves. During geomagnetic storms and the associated ionospheric disturbances, scintillation activity may affect certain applications of navigational aids such as the Global Positioning System (GPS). See Space Weather Effects on GPS.
Solar flares may be accompanied by streams of very energetic particles that travel at near the speed of light. These particles (mainly protons and electrons) enter the upper atmosphere in the regions near the magnetic poles. As a result, the lower levels of the polar ionosphere become very ionized, with severe absorption of HF and VHF radio signals. Such an event is known as a polar cap absorption (PCA) event and may last from days to weeks, depending on the strength of the stream of solar particles and the location of the source region on the Sun. HF radio communication in polar regions is often impossible during PCA events.
Large clouds of plasma (ionized gases), known as Coronal Mass Ejections (CME), can be emitted from the Sun, and may reach Earth, causing disturbances in the geomagnetic field and in the ionosphere. Coronal holes, regions of the solar corona with diminished x-ray emissions, also emit streams of charged particles that can result in disturbances of the ionosphere. Ionospheric disturbances are especially significant at auroral latitudes, such as over much of Canada, and during magnetic storms and substorms at these latitudes, HF radio communication may be unreliable.
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