The Geophysical Alert Broadcasts consist of three primary sections to describe the Solar-terrestrial environment: The most current information, then a summary of activity for the past 24 hours, and finally a forecast for the next 24 hours. The actual wording of each section of the broadcast is explained below with a brief description of what is being reported. Similar wording is also used in other broadcasts, so the WWV example is relevant to other reports too.
Current Information ‘Solar-terrestrial indices for (UTC Date) follow: Solar flux (number) and (estimated) Boulder A index (number) . Repeat, solar flux (number) and (estimated) Boulder A index (number).
Current Information ‘Solar-terrestrial indices for (UTC Date) follow: Solar flux (number) and (estimated) Boulder A index (number) . Repeat, solar flux (number) and (estimated) Boulder A index (number).
In addition, the level of the sun’s ultraviolet and X-ray emissions is primarily responsible for causing ionization in the earth’s upper atmosphere. It is these emissions which produce the ionized ‘layers’ involved in propagating shortwave radio signals over long distances. The solar flux number reported in the broadcast is in solar flux units (s. f. u.) and is recorded daily at Ottawa at 1700 UTC to be forwarded to the SESC. Solar flux readings range from a theoretical minimum of approximately 67 to actually-observed numbers greater than 300. Low solar flux numbers dominate during the lower portions of the 11-year sunspot cycle, rising as the cycle proceeds with the average solar flux a fairly reliable indicator of the cycle’s long-term behavior. 1 s. f. u. = 10-22Watt/(meter2·Hz) = 104 jansky.
A Index
The A Index is an averaged quantitative measure of geomagnetic activity derived from a series of physical measurements. Magnetometers measure differences between the current orientation of the magnetosphere and compare it to what it would be under ‘quiet’ geomagnetic conditions.
But there is more to understanding the meaning of the Boulder A index reported in the Geophysical Alert Broadcasts. The Boulder A index in the announcement is the 24 hour A index derived from the eight 3-hour K indices recorded at Boulder. The first estimate of the Boulder A index is at 1800 UTC. This estimate is made using the six observed Boulder K indices available at that time (0000 to 1800 UTC) and the SESC forecaster’s best prediction for the remaining two K indices. To make those predictions, SESC forecasters examine present trends and other geomagnetic indicators. At 2100 UTC, the next observed Boulder K index is measured and the estimated A index is reevaluated and updated if necessary. At 0000 UTC, the eighth and last Boulder K index is measured and the actual Boulder A index is produced. For the 0000 UTC announcement and all subsequent announcements the word ‘estimated’ is dropped and the actual Boulder A index is used.
The underlying concept of the A index is to provide a longer-term picture of geomagnetic activity using measurements averaged either over some time frame or from a range of stations over the globe (or both). Numbers presented as A indices are the result of a several-step process: first, a magnetometer reading is taken to produce a K index for that station (see K INDEX below); the K index is adjusted for the station’s geographical location to produce an a index (no typographical error here, it is a small case ‘a’) for that 3-hour period; and finally a collection of a indices is averaged to produce an overall A index for the timeframe or region of interest. A and a indices range in value from 0 to 400 and are derived from K-indices based on the table of equivalents shown in the APPENDIX.
K Index
The K index is the result of a 3-hourly magnetometer measurement comparing the current geomagnetic field orientation and intensity to what it would have been under geomagnetically ‘quiet’ conditions. K index measurements are made at sites throughout the globe and each is carefully adjusted for the geomagnetic characteristics of its locality.
The scale used is quasi logarithmic, increasing as the geomagnetic field becomes more disturbed. K indices range in value from 0 to 9. In the Geophysical Alert Broadcasts, the K index used is usually derived from magnetometer measurements made at the Table Mountain Observatory located just north of Boulder, Colorado. Every 3 hours new K indices are determined and the broadcasts are updated.
- K = 0 Inactive
- K = 1 Very quiet
- K=2 Quiet
- K = 3 Unsettled
- K = 4 Active
- K = 5 Minor storm
- K = 6 Major storm
- K = 7 Severe storm
- K = 8 Very severe storm
- K = 9 Extremely severe storm
- A = 0-7 Quiet
- A = 8-15 Unsettled
- A = 16-29 Active
- A = 30-49 Minor storm
- A = 50-99 Major storm
- A = 100-400 Severe storm