"We're looking for a storm to arrive from the Sun," Combs says. "We're looking at the solar wind—its density, its temperature, and all that goes along with that."
The driving force behind solar activity is the Sun's magnetic field. Like Earth, the Sun is a giant rotating magnet. Unlike Earth (or a simple magnet), however, the Sun is not a solid body: it is a highly conducting fluid, so its rotation, and the magnetic field that arises as a consequence, is complex and constantly on the move. Material near the solar poles rotates faster than material near the solar equator. Meanwhile, new material is constantly rising to the surface by convection, stirring the fluid further. As a solid body, Earth has a tidy magnetic field, with straight north-south field lines that surround the planet like a neatly wrapped ball of yarn. In comparison, the Sun is a knitter's nightmare: a huge sphere of rubbery ropes, of ever-shifting field lines that twist, kink, tangle and snap as the liquid body spins.
The Sun's magnetic field defines the shape and motion of its gaseous atmosphere. "Much of the structure we see on the Sun is the gas aligning along the magnetic lines of force," Combs says. Loops and prominences, two of the more charismatic solar features, are yawning arches of solar gas; they may last for months and can extend 30,000 miles above the Sun's surface. As the Sun progresses through many rotations, its magnetic field becomes increasingly distorted; this tumult, known as solar maximum, reaches a peak every 11 years or so. During this period, solar eruptions are more frequent and severe and the solar wind blows with added ferocity. "The sun has a cycle," Combs says. "It has a maximum period and a minimum period. Right now we're in that maximum period."
Using different photographic filters, the space-weather forecaster watches the Sun's magnetic field as it changes over a period of months, days, hours, even minutes. "We look at areas on the Sun that have the potential to produce activity," says Harrison Jones, a solar physicist with NASA's Goddard Space Flight Center. A magnetogram image, for example, reveals the varying intensity of the magnetic field in shades of gray. "Areas that are bright are areas where the magnetic field is pointing towards us and is strong. Areas that are black are areas where the magnetic field is strong, but pointing away from us. An area of strong magnetic field is an area that is likely to evolve and release high-energy particles." Forecasters also look for sunspots, slightly cooler regions of the Sun that appear dark in visible-light and ultraviolet images. Sunspots arise where the magnetic field is in flux, and are often sites of developing solar storms.
Solar storms occupy much of a forecaster's attention. They come in a variety of forms, each promising a unique brand of potential trouble. Solar flares are massive explosions on the Sun's surface. They often arise near sunspots and release a wide spectrum of energy-charged particles, X-rays and gamma rays-outward into space. Coronal holes are more subtle phenomena; to the forecaster, they appear as large, dark regions of the corona. The magnetic field lines around coronal holes are "open": instead of looping downward and trapping plasma against the Sun's surface, they point outward, spewing solar particles into space as quickly as 500 miles per second, adding extra punch to the solar wind. The biggest solar storms are coronal mass ejections. A CME is an enormous bubble of plasma expelled by the Sun; it contains billions of tons of fast-moving solar particles as well as the magnetic field that binds them. "A coronal mass ejection is actually ripping a part of that magnetic field away from the Sun and sending it out into space," says Terry Onsager, a scientist at the Space Environment Center.
Most solar storms are directed away from Earth. But when a storm points our way, the forecasters at the Space Environment Center begin to calculate what sort of space weather it may generate on Earth, and how soon to expect the turbulence.
"The Sun doesn't give a hoot about us," says Joe Kunches, the lead forecaster at the Space Environment Center. "We just happen to be in the way."