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.