NOAA / Space Weather Prediction Center
The Space Weather Scales of the National Oceanic and Atmospheric Administration were recently developed by NOAA's Space Weather Prediction Center (SWPC) to improve understanding of space weather Events among technical operators and the general public alike. The scales serve as a sort of Richter scale that correlates space weather events with their likely effects on technological systems. To many who see space weather as an obscure field, the NOAA Space Weather Scales will convey useful information and perhaps pique new scientific interest.
The SWPC has forecasted the space environment for more than 35 years, alerting users to existing conditions between the Sun and Earth. Demand for these services has grown as technology susceptible to space weather Damage has increased in use. Satellites, for example--which were once rare and only government-owned-- are now numerous, and many are commercially owned. Transmissions to and from those satellites now carry weather information and military surveillance, television and other communications signals, credit card and pager information, navigation data, and cell phone conversations.
As our dependence on satellites for communication and information become more sophisticated, the number of people that use this and other technologies vulnerable to space weather events has increased dramatically. Inevitably, an alert about a space weather event evokes the question, "What effect will that have?" among more and more people and organizations.
SWPC's mission is to describe the environment, not to report how systems are affected; and in any case, satellite-based communications engineers and others are in a better position to address particular potential problems or implement mitigation actions. Yet, customers and reporters who call SWPC deserve an answer about the effects of space weather that is meaningful for them. A more clear way to communicate with the general public and the expanding group of technical operators who may be new to and naive about space weather jargon was therefore needed.
The three categories of descriptive scales for space weather devised by NOAA, and which are most relevant to the possible effects on satellite-based and other systems, are:
- Geomagnetic storms: disturbances in the geomagnetic field caused by gusts in the solar wind that blows by Earth
- Solar radiation storms: elevated levels of radiation that occur when the numbers of energetic particles increase
- Radio blackouts: disturbances of the ionosphere caused by X-ray emissions from the Sun
This is because weather and storms are familiar terms that convey meaning to the public, describing disturbances in the space environment as "space weather" has aided in improving the public's understanding, and comparing events to storms has been equally effective.
It has been suggested that additional NOAA scales should be created to describe other types of events. This remains an open question. The Fujita scale for tornadoes, the National Weather Service categories of hurricanes, and the Richter scale for earthquakes are useful models for communicating the relative severity of weather and other disturbances to the public. Each scale is based on a physical measure and has proven to be effective in conveying, to the lay public and experts alike, the relative seriousness of an event. The NOAA Space Weather Scales break the continuum of possible event severity into categories, each of which is designated by a number and a single word descriptor.
Each scale provides lists of possible effects seen with each category of activity, the physical measure that determines the category of an event, and a climatological assessment that explains how often we can expect to see events of each magnitude during a solar cycle. For space weather, we use five levels or categories of severity, because effects can be usefully binned in five levels, and because the historic alerting system used by SWPC easily maps to those levels (for example, the geomagnetic index of disturbance, Kp, in the ranges of 5-9; X-ray flare of M5, X1, X5, X10, X20). A finer gradation would be less meaningful for users; a coarser one would have levels with enormous ranges of conditions.
The simplicity and the usefulness of the scales for some purposes leads to some unavoidable tradeoffs. Using only one physical measure for each scale leads to a description of events less precise than some users might like. Duration and timing, for instance, are not considered in a scale's assessment of an event; measures like integrated fluxes do not take into account the sometimes important aspects of spectral shape; and the cadence of the data or index may not be well-matched to the time-scale of the physical event, which complicates the climatology. (The average frequency was obtained from the National Geophysical Data Center's solar-terrestrial database covering the last several solar cycles). As an example, Kp values are derived and reported every 3 hours, although a single geomagnetic storm may last a day or more.
The requirements of providing real-time information to users, the reliance on established geophysical indices, and a desire for simplicity in interpreting the categories all argued against the use of more refined measures. The NOAA Space Weather Scales had to be based on repeatable, verifiable physical measures so that scientists and operators around the world could agree, independently, that a given event is of a certain category of severity.
Clearly stating the effects expected at each level was difficult, even more so at the category 5 level. Extreme effects are extrapolated from the rare known instances of effects at that event level. The obvious shortcomings inherent in the simplistic concept of the scales are outweighed by the usefulness of describing conditions and effects simply.
The categories of each scale are defined by the numeric value of a physical measure, but the intervals between the categories do not always scale uniformly with the physical measure's value. For geomagnetic storms, the categories differ by equal steps of Kp, which itself is a quasi-logarithmic index that is currently the best estimate of global geomagnetic activity available in near-real time. For solar radiation storms, each successive category is reached when the flux of greater than 10 MeV particles increases by a factor of 10, but the radio blackouts categories increase based on quantitative (and historically used) flare X-ray brightness levels, rather than "regular" intervals.
To ensure clear communication and avoid pitfalls in trying to communicate with a general audience, the vocabulary of the scales was tested in two focus groups drawn from the local community. Group members had little or no knowledge of space weather. They deliberated on the best descriptors of categories (Is "minor: better than "weak" for level 1? Is "strong" of lesser intensity than "severe"? etc.). The focus groups were also helpful in selecting the most meaningful names for the three events: geomagnetic storms, solar radiation storms, and radio blackouts. These names were selected from an array of choices suggested by SWPC staff.
The many internal versions of the scales have been refined to the version published here. We expect some evolution of the scales in response to comments from the user community and as better physical measures become available, although the basic structure of the classification scheme will remain largely unaltered.
Consequences of space weather disturbances are often hard to attribute, and then are reported only sporadically. As a result, many are confused and frustrated, not knowing why we care about space weather events. The NOAA Space Weather Scales tie physical measures to possible outcomes, at least in general terms. SWPC products and forecasts describing the environment or specific events, and cast in terms of the new NOAA Space Weather Scales, now carry with them easily accessible information about the importance and consequences of space weather.
As the NOAA Space Weather Scales work their way into SWPC products and forecasts, space weather services will become more useful and intelligible to the operators of affected systems. Variations in the space environment will also capture the interest of the general public.
For further information, browse the Space Weather Prediction Center Web Site =
Barbara B. Poppe, NOAA Space Weather Prediction Center, 325 Broadway, Boulder, CO 80303
Available online with permission from American
Geophysical Union and Eos - the
weekly newspaper of geophysics.
Eos, Transactions, American Geophysical Union, Vol. 81, No. 29, July 18, 2000, Pages 322, 328
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