Part III: Getting the Ionosphere Model Right
Moving towards Earth, Tom Moore leads efforts to add greater realism to simulations of our planet's magnetosphere and especially its ionosphere, the top atmosphere layer beginning 100 kilometers above the surface. The ionosphere has specific impacts on space exploration. It is the setting for magnetic and electrical disturbances that can interrupt space communications. Moreover, solar wind interactions with the upper atmosphere create dynamic changes that would make aero-braking reentry of a crew vehicle a challenging proposition–not only in Earth's atmosphere, but that of Mars as well. As NASA astronauts prepare to venture forth, it becomes more and more important to get the ionosphere's details right for space weather prediction.

"Current models don't allow the ionosphere to escape from Earth and circulate around, but in actuality it does," Moore said. It has been known for 30 years that the ionosphere "gushes out of the auroral zones," stealing as much as 300 to 400 tons per day from the atmosphere. Yet, modelers typically assume the ionosphere to be a thin, largely inactive layer. "Our job is to calculate how the ionosphere flows out and participates in the storminess going on around the planet," Moore said.

For accurate initial conditions, Moore's team drives simulations with observations of the upstream solar wind taken by NASA satellites. "We play games with what simple variations in the solar wind do to the system," he said. "Then, we use those results to interpret real events where you have a complicated time series of northward and southward turnings, rotations of the magnetic field, and gusts of the solar wind."

Describing how the ionosphere and magnetosphere react falls to single-particle calculations. Each virtual particle represents some larger number of real particles such as protons. Millions of these particles collectively act as a free fluid that can respond and expand. This fluid conducts electricity, and the currents distort and tear up the magnetic field.

These complex simulations have yielded several surprising findings. For one, it turns out that the escaping ionosphere fills the magnetosphere with plasma. This means that most of the plasma in the magnetosphere comes from the ionosphere rather than from the solar wind. "The old textbook picture that the solar wind comes in and lights up the aurora is completely wrong," Moore said. Some solar wind leaks in, but it is only about 1 percent of the plasma that hits the magnetosphere. The magnetospheric plasma usually contains comparable amounts of solar and terrestrial plasma, but at times the terrestrial plasma is much denser and especially more massive, because it contains oxygen.

Particularly when the ionosphere is buffeted by intense solar wind or southward-directed solar magnetic fields, hot plasmas from the ionosphere also inflate the magnetosphere. In what Moore calls "the self-inflating bicycle tube," ionospheric plasmas pump up the magnetosphere's peak pressure until it is 10 to 20 times the pressure that the solar wind exerts 0 on it. "You end up with much more pressure trying to get out than the pressure with which the solar wind is blowing on the system," Moore said. "It is amazing."

The most recent addition to Moore's simulations is the plasmasphere, a ring of plasma located just above the mid- to low-latitude ionosphere. As he pointed out, NASA's Imager for Magnetopause to Aurora Global Exploration (IMAGE) mission reminded scientists that it is a very dynamic part of the magnetosphere. Early simulation results indicate the plasmasphere to be a "huge source of action" during solar events (see Figure 6).