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A graph of sunspot counts from 1700 to 1993.
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Image courtesy NOAA/NGDC.

Metrics & Indices that Describe Space Weather

Meteorologists measure quantities like wind speed, rainfall amounts, and temperature. They use specialized terminology, like dew point, relative humidity, and barometric pressure. Scientists who study space weather also use special terms and measure key aspects of the "weather" in space.

The first arena for defining important space weather metrics is the Sun. Sunspots are visible manifestations of active regions on the Sun for which we have records over long time periods, so sunspot counts are an important metric for tracking activity levels on our neighborhood star. Solar flares, gigantic explosions on the Sun, are classified in severity using an alphanumeric scheme, with X-class flares being the most powerful. Coronal Mass Ejections (CMEs) are a third type of solar phenomena with important space weather ramifications for which scientists have developed classification schemes.

A second realm for which measures of space weather are needed is interplanetary space. The solar wind is characterized by its speed, particle density, pressure, and temperature. The interplanetary magnetic field (IMF) is described in terms of the magnitude of the magnetic force and the directions of the polarity of that field.

The third and region in which we need quantities that describe space weather phenomena is Earth and near-Earth space (geospace). Some measures describe the strength and orientation of Earth's magnetic field at various locales on the surface of Earth and in the vicinity of our planet. Other metrics relate to characteristics of Earth's atmosphere, especially the various layers of the ionosphere. A third set of metrics describe the flow of electrical currents in the upper atmosphere and the magnetosphere. Finally, indices that describe features of auroras round out the set of metrics needed to track space weather phenomena in geospace.

Last modified September 11, 2008 by Randy Russell.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA