One of the handiest (and cheapest) methods to better understand the current state of shortwave radio propagation conditions is by monitoring the Geophysical Alert Broadcasts made at 18 minutes past each hour over the U. S. National Institute of Standards and Technology radio station WWV in Ft. Collins, Colorado and at 45 minutes past each hour via WWHV on the island of Kauai in Hawaii WWV broadcasts continuously on shortwave frequencies of 2.5, 5, 10, 15, and 20 MHz and WWVH broadcasts on 2.5, 5, 10 and 15 MHz. Their signals are audible throughout North America and the Pacific Region and often the rest of the world, depending upon radio propagation conditions.

These 45-second Geophysical Alert Broadcasts outline the current nature of the solar-terrestrial environment. They are produced by the National Oceanic and Atmospheric Administration's Space Environment Services Center (SESC). This center operates a worldwide network of sensors which continuously observe conditions between the earth and the sun. A listener familiar with the types of information presented can gain a surprising amount of insight into how the natural phenomena primarily responsible for long-distance HF radio communication are currently affecting it at the moment as well as in the near future.

Updated every three-hours beginning at 0000 UTC, the Geophysical Alert Broadcasts are concerned with two primary types of Earth-sun interaction: electromagnetic radiation and geomagnetic activity (which includes effects from solar sub-atomic particle emissions).The effects of each are summarized below:

Electromagnetic Radiation

The sun's electromagnetic spectrum is a continuum of radiation spanning not only infrared, visible, and ultraviolet wavelengths, but the radio portions, x-rays and beyond. Sensors on the Earth and in space continuously observe specific portions of the sun's energy spectrum to monitor their levels and give scientists indications of when significant events occur.

Solar emissions in this category are all electromagnetic in nature, that is, they move at the speed of light. Events detected on the sun in these wavelengths begin to affect the Earth's environment around 8 minutes after they occur.

Geomagnetic Activity

In addition to electromagnetic radiation, the sun constantly ejects matter in the form of atomic and subatomic particles. Consisting typically of electrons, protons, and helium nuclei, this tenuous gas is accelerated to speeds in excess of the sun's gravitational escape velocity and thus moves outward into the solar system. The collective term for the gas and the particles making them up is the Solar Wind. The sun's approximately 27-day rotation period results in the clouds being slung outward in an expanding spiral pattern which, at the earth-sun distance, overtakes the earth from behind as it moves along in it's orbit.

As the clouds encounter the earth, the geomagnetic field and the earth's atmosphere prevents the solar wind particles from striking the planet directly. Magnetic interactions between the clouds and the geomagnetic field cause the solar wind particles to flow around the field, forming a shell-like hollow with the earth at the center. The hollow, known as the earth's Magnetosphere, is actually distorted into a comet shape with the head of the comet always pointing directly into the solar wind and the tail directly away. In the absence of significant solar activity, the solar wind is uniform with a velocity of approximately 400 km/second. Under these conditions, the earth's magnetosphere maintains a fairly steady shape and orientation in space.

When disturbances occur on the sun, some clouds of solar particles can be blasted away at tremendous velocities. As these higher speed solar particle clouds encounter the earth's magnetosphere, they perturb it, changing the intensity and direction of the earth's magnetic field. This is analogous to a weather vane in gusty wind; sudden higher speed gusts can strike it and cause it to move around. Moreover, changes in solar wind density and velocity can cause the Earthìs surface and are referred to as a "sudden impulse" (SI).

Geomagnetic activity, including solar particle-caused variations in the geomagnetic field are carefully monitored by instruments both on the Earth and in space. High levels of geomagnetic activity act indirectly to degrade the ability of the ionosphere to propagate HF radio signals. So they are of interest to users of that portion of the radio frequency spectrum. Like the electromagnetic radiation portions of the sun's output, geomagnetic activity comprises another family of interactions observed and reported by groups such as IPS and SESC.