A graph of sunspot counts from 1700 to 1993. Click on image for full size (98 Kb) Image courtesy NOAA/NGDC.

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.

A graph of sunspot counts from 1700 to 1993. Click on image for full size (98 Kb) Image courtesy NOAA/NGDC.

Scientists who study weather on Earth measure wind speed, rainfall amounts, and temperature. They use special terms, like dew point, relative humidity, and barometric pressure. Scientists who study space weather do the same sorts of things. They also use special terms and measure certain traits of the "weather" in space.

The first place to look when measuring space weather is the Sun. Sunspots are visible forms of active regions on the Sun. We have sunspot records over long time periods, so sunspot counts are an important metric for tracking activity levels on the Sun. Solar flares, gigantic explosions on the Sun, are classified using letters and numbers. X-class flares are the most powerful. Coronal Mass Ejections (CMEs) are a third type of solar phenomena 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 places on and near Earth. 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.

A graph of sunspot counts from 1700 to 1993. Click on image for full size (98 Kb) Image courtesy NOAA/NGDC.

As with weather on Earth, there are space weather reports and forecasts. In these reports and forecasts, scientists use metrics and indices.

Just what are these metrics and indices? They are numbers that have to do with the different players in the space weather scene during that time period. One example might be the number of sunspots on the Sun at that time. The higher the number of sunspots, the more active the Sun is likely to be. The more active the Sun, the more flares or coronal mass ejections we might see coming from the Sun. So, you can see that number of sunspots is a good metric for telling how active the Sun will be.

Other metrics and indices deal with the solar wind, the magnetic field out in space (IMF), the Earth's magnetic field and the Earth's atmosphere.