Moving at the speed of light, electromagnetic radiation emissions from the Sun begin to effect the Earth's atmosphere about eight minutes after they occur. Infrared, visible, and ultraviolet radiation, as well as radio waves and x-rays are part of the Sun's electromagnetic spectrum that can be detected. In addition to this radiation, atomic and subatomic particles, typically electrons, protons, and helium nuclei matter are ejected by the Sun. This discharge is called the solar wind. As these gases and particles reach the Earth, the Earth's geomagnetic field and atmosphere cause the solar wind to flow around the Earth forming the Earth's magnetosphere. Sometimes high-speed solar wind impacts the magnetosphere, changing its intensity and direction. These changes from the solar wind are referred to as a "sudden impulse." It is these high levels of geomagnetic activity that indirectly act to degrade the ionosphere to propagate high frequency radio signals.
The Sun's ultraviolet and x-ray emissions are primarily responsible for causing the upper ionized layers of the Earth's atmosphere used in propagating radio-waves over long distances. Their intensity is called solar flux and it is proportional to sunspot activity, which changes during an approximate 11-year sunspot cycle. Sudden large x-ray emissions can produce sudden and extensive ionization in the lower regions of the atmosphere, thus absorbing radio waves and degrading radio communications from minutes to hours. These sudden ionospheric disturbances are known as "short-wave fadeouts." High solar x-ray emissions in the daytime cause common disruptions with disappearance of radio signals on lower frequencies that are opposite to the impact caused by geomagnetic storm disturbances. Geomagnetic storms can last for days with worldwide disruptions of the middle and higher altitude ionosphere regions which fade out higher frequency radio signals. When short wave propagation is affected in the F-layer it is call an ionospheric storm.
Ordinary weather is much easier to predict than "radio weather," but there are several indexes (A-index and K-index) used to report the geomagnetic activity along with the solar flux. The higher the A or K indexes that range from 0 to 400, or solar flux numbers that range from 67 to greater than 300, the greater solar activity and more severe the solar storm is categorized. "Geophysical Alert Broadcasts" made at 18 minutes past each hour over the U.S. National Institute of Standards and Technology radio station WWV and other foreign stations continuously report on and forecast the solar flux (electromagnetic radiation) and A and K Indexes (geomagnetic activity).
In general, the A-index should be under 14 and the solar flux low-moderate for long distance medium-wave radio propagation. If the A-index drops under 7 for a few consecutive days, excellent conditions exist.
Can you predict if radio communication is good today? Check out the radio weather forecast and see.
©Copyright 1998 Elizabeth
Beckett, Holly Bernitt, and Vishwa Chandra.