All The World's A Stage, And The Ions Merely Players...

October 28, 1998

Since the discovery of the magnetosphere in 1958, scientists have measured it with hundreds of instruments that have revealed a complex, ghostlike body enveloping the Earth. Because only a few instruments are operating at any time, it's like forecasting weather nationwide with rain gauges, wind scales, and barometers located just in Chicago, Miami, and Los Angeles.

To fill in the gaps, scientists build models, imitations of several million cubic kilometers of space crammed into a few thousand lines of computer code that play the roles of electrons, ions, neutral atoms, the Earth's magnetic field, and the solar wind. The models paint convincing images of activities in the magnetosphere, but they don't always match what the experimenters observe in space.

As part of the week-long workshop on the New Millennium Magnetosphere, being held in Guntersville, Ala., NASA brought together modelers and instrument scientists to discuss the best ways to use simulations so we can turn our observations into a global understanding of space around the Earth and to fill in the gaps as necessary.

"We wanted to do more than present the latest models," said Dr. Michael Liemohn of the University of Michigan at Ann Arbor. "We wanted to explain how these models can be used by the scientists who build the instruments and fly the satellites."

Liemohn and Dr. Charles Goodrich of the University of Maryland at College Park jointly chaired the modeling session on Monday evening and Tuesday. Four decades after its discovery, the magnetosphere remains one of the closest - and most challenging - places to explore in space.

Monday evening and Tuesday morning, some 15 scientists explained various models they have developed to explain various parts of the magnetosphere, from the effects of space storms on the Van Allen radiation belts to the Earth's atmosphere trickling upward from the polar regions into deep space. Modeling generally works on the inner or outer magnetosphere. Many of the inner magnetosphere models have Earth's atmosphere as one of its boundaries.

But the sheer size of the magnetosphere is a challenge even to the most powerful computers.

"You have an enormous volume to simulate," Goodrich said, "and with finite computers you have to make compromises."

In many simulations of the outer magnetosphere, scientists have to use "magneto-hydrodynamics" (MHD). The name means the mechanics of things flowing, like water, in a magnetic field. It's a complex mathematical process involving multiple equations for pressure, density, velocity, and other features.

"The problem is that every equation has a term that is solved in the next equation," Goodrich said. "The Earth's magnetosphere is an enormous system in which the structure on the large scale depends on processes happening on the smallest scales."

Even the most powerful modern computers do not allow modeling of the whole system with everything represented at the smallest scales. Some simulations may take the solar wind at one end and the open magnetotail at the other as boundaries. Another might take a slice through the Earth as a boundary. As a result, some models can depict highly detailed activities - but only in a tiny region of space. Others can describe all of space - but with only the coarsest of details. And joining the two types of models can be difficult.

To check the models, he said, "virtual satellites" are inserted to take measurements that are compared with actual measurements taken under similar conditions.

"If the answers match, then we know we're doing it right," he continued. But real world data are needed to know for sure.

Goodrich explained that several scientists are developing good physical models of reconnections, the events that happen deep in the magnetotail when magnetic field lines rejoin and snap back towards the Earth.

"Reconnection is the basic process that enables energy transfer from the solar wind to the inner magnetosphere," Goodrich said. "We don't know the onset process, but magnetic reconfiguration requires large-scale reconnections for energy release."

Spacecraft like Geotail, Polar, and Wind have provided real-world data that have let scientists set up realistic codes. But more data are needed from the tail region.

"It's a very under-sampled area," Liemohn said, because few spacecraft have spent much time there.

Future missions like Europe's Cluster 2 and NASA's proposed six-satellite Multiscale Magnetospheric Mission will provide detailed sampling that will give the model builders more of the details they need to guide their work. These and other missions will be discussed in Thursday's session.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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