Essay: Earth's Magnetosphere
Space weather begins on the Sun, but it isn't really space weather until it hits Earth. Fortunately, Earth's magnetosphere spares the planet's residents from the worst damage. Earth's rotation generates a large magnetic field around the planet, extending hundreds of miles into space. This field, known as the magnetosphere, deflects or safely absorbs much of the hazardous energy and particles the Sun sends in our direction. "We're in a cocoon provided by the Earth's magnetic field," says Terry Onsager, a solar scientist at NOAA's Space Environment Center. "It's a shield, a bubble that we live inside."
"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."
Technically, space weather is what happens when solar particles and energy interact with Earth's magnetosphere. In sizing up a space-weather event, then, scientists at NOAA's Space Environment Center in fact keep an eye on two environments simultaneously: the Sun's environment and the near-Earth environment. "The geomagnetic forecast and the solar forecasts are actually two different forecasts," says Larry Combs. "One is what we expect the Sun to do at different areas. The other is how we expect Earth's geomagnetic field to respond."
"There are three different things we look for," Combs continues. "The first are electromagnetic emissions, in the form of X-rays and ultraviolet rays. Those go right through the magnetosphere and enter our upper atmosphere and ionosphere." Solar flares are the primary sources of these high-energy events; their effect is felt on Earth within minutes. The X-rays, ultraviolet rays and gamma rays heat up Earth's upper atmosphere; this creates more drag, which slows orbiting spacecraft and satellites and disrupts ground communications and navigation devices that rely on satellites to work effectively. NASA controllers must constantly boost the orbit of the Hubble Space Telescope to counteract the effects of space weather. High-energy radio waves emitted by solar flares also can upset the ionosphere, a layer of charged gas 60 to several hundred miles above Earth's surface. We use the ionosphere to reflect radio signals back to Earth over long distances; sudden disruptions of the ionosphere can cause radio blackouts around the globe.
The second thing forecasters look out for is an influx of energetic particles, primarily protons and electrons, released by the Sun. "They travel more slowly," Combs says. "It may take anywhere from thirty minutes to several hours for them to reach the magnetosphere." Solar flares and coronal holes, which create extra-strong gusts of solar wind, are the main causes of particle events. A sudden influx of solar particles into Earth's upper atmosphere can damage the sensitive electronics of satellites, thereby disrupting communications and navigation on the ground. An excess of solar particles also raises radiation levels in the upper atmosphere to dangerous levels; astronauts are careful to schedule their spacewalks around times when solar activity is at a minimum.
The third thing forecasters watch for is fast-moving solar plasma, large bubbles of solar particles bound up in a magnetic field. Typically produced by coronal mass ejections, a slab of plasma may take three or four days to slam into Earth, but when it does, the results can be severe. Like a wet electric blanket, the plasma can cause Earth's magnetosphere to tremble, generating massive electrical currents that can black out power grids on Earth's surface. In 1989, a huge geomagnetic storm temporarily damaged the Hydro-Quebec Power company, plunging all of Quebec into darkness for several hours. These currents also heat Earth's atmosphere, slowing satellites in their orbits.
Society has grown increasingly vulnerable to space weather in recent decades, largely because it has put more at stake: Earth's upper atmosphere is busy with spacecraft, communication satellites and orbiting telescopes (including ones that study space weather) as never before. This vulnerability in turn has prompted a vigorous and fruitful effort to improve space-weather forecasting. Of course, no forecast can ever be perfect; weather, even the space kind, is fickle.
"I could hang up the phone, there could be a coronal mass ejection, and in two days, away we go," says Joe Kunches, the lead forecaster at the Space Environment Center. "So far, though, it looks like a normal day."