Stormy Weather in Space
The Challenges of Exploring Vast Substorms
Simulating Substorms in the Computer
|Figure 1. Northern Lights from Space
View from space of the Earth’s Northern Lights, which are produced by the impact on the upper atmosphere of fast-moving charged particles from magnetic storms and substorms in the Earth’s magnetotail. Image of northern auroral oval obtained on March 25, 1996 is superposed on image of Earth’s surface. Auroral image, in the ultraviolet spectrum, is from the Visible Imaging System (VIS) on the Polar spacecraft. Underlying image of Earth’s surface is a subset of the Face of the Earth™ © 1996, ARC Science simulations.
he shimmering curtains of the Northern Lights or aurora borealis have long fascinated all who see them, including the scientists who study this "space weather" in the Earth’s protective magnetosphere. But these Northern Lights also have a dark side–they are the product of magnetic substorms that can damage satellites, communications networks, and even power grids. Developing a theory that explains the rapid onset of substorms and the trigger that unleashes them has eluded physicists for decades, both because of the expense of obtaining satellite data and the great computing resources required for complex models. Now, Philip Pritchett of UCLA and colleagues are gleaning new insights using NPACI supercomputers to simulate substorms more accurately than previously possible, as they continue the hunt for the elusive triggering mechanism.
Watching the eerie display of the aurora borealis overhead can awaken both feelings of awe and questions about this mysterious light show in the heavens. Where do these lights come from? What governs their comings and goings?
To answer these questions, research physicist Philip Pritchett of the Department of Physics and Astronomy at UCLA and colleagues Ferdinand Coroniti and Viktor Decyk are trying to penetrate the mysteries of magnetic substorms. "Magnetic substorms are one of the oldest problems in physics that is still unsolved–people have been working on this for 35 or 40 years–and now simulations on powerful supercomputers are playing a crucial role in letting us understand them," said Pritchett.
In addition to giving scientists a better understanding of the basic mechanisms of substorms, the knowledge the researchers are gaining should lead to practical benefits such as better predictions of the space weather around the Earth, including the magnetic substorms that can be so disruptive. And beyond substorms, the mechanisms Pritchett is studying play a role in other important areas such as the eruptions on the Sun known as solar flares and the Tokamak fusion energy device that may one day generate low-cost electricity.
Stormy Weather in Space
The Sun gives charged particles (protons, electrons, and heavier ionized atoms) that blow through interplanetary space toward the Earth. When this "solar wind" reaches the Earth, the charged particles interact in complex ways with the Earth’s magnetic field, forming the protective magnetosphere with its long magnetotail extending millions of miles or hundreds of Earth radii downstream. Just as the dynamics of the atmosphere give rise to the Earth’s weather, so the dynamics of the solar wind as it buffets and distorts the Earth’s magnetic field give rise to space weather–complete with magnetic storms, which have a timescale of days, and substorms which can have a timescale of hours.
During the period leading up to a magnetic substorm, the solar wind acts to stretch the Earth’s magnetic field lines like rubber bands, until eventually they break and reattach in a process known as reconnection. During reconnection, the magnetic field lines release their stored energy, accelerating the charged particles around them in a sudden explosion that scatters the electrons and ions great distances at high velocities.
Magnetic substorms occur on the night side of the Earth facing away from the Sun, and the explosion does not scatter the electrons and ions uniformly, but preferentially in two directions. Some are projected outward along the Earth’s magnetotail, while another "beam" is projected toward the Earth, and guided down the Earth’s magnetic field as it dips toward the poles. "The impact of these fast-moving charged particles hitting the Earth’s upper atmosphere excites atoms there so that they emit the light we see in the aurorae, which are mostly visible at high latitudes, although on rare occasions they can be seen as far south as Los Angeles," said Pritchett. He explains that an aurora acts as a kind of signature or mirror, providing a telltale visual display that reveals what is happening in the substorm farther above the Earth (Figure 1).
"Although many things about magnetic substorms have been understood for years, there are some fundamental questions that have turned out to be surprisingly difficult to answer," said Pritchett. One question involves the great suddenness with which substorms appear--far faster than earlier models have been able to account for. A second basic question is what triggers substorms. Is it an external event, such as some change in the solar wind that tilts the interplanetary magnetic field orientation, or is the trigger a local event in the magnetotail itself? And although the disruptions of substorms eventually extend from near the Earth to far down the magnetotail, where is the substorm triggered--relatively near the Earth at five or 10 Earth radii, or farther out in the magnetotail at perhaps 25 Earth radii? (See Figure 2.) Through their supercomputer simulations the researchers are finding new answers to these basic questions, which continue to intrigue space physicists.
|Figure 2. Possible Substorm Trigger Zones
The solar wind interacts with Earth’s magnetic field to create a long downstream magnetotail (black arrows indicate direction of magnetic field lines), where magnetic storms and substorms produce the Northern Lights (Figure 1). These simulations are investigating substorm dynamics, including two proposed mechanisms for substorm triggering. Top shows trigger far down magnetotail at around 25 Earth radii with inward high-speed flow, while bottom shows alternate theory with trigger of outward-moving rarefaction wave occurring relatively near the Earth at five or 10 Earth radii.
The Challenges of Exploring Vast Substorms
Magnetic substorms unfold throughout enormous volumes of space extending far above the Earth, presenting researchers with major difficulties in both the experimental and computational approaches used to explore them. "Until now we’ve never been able to have more than one satellite observing in the same general region at a time. Like a blind man exploring an elephant, the single measurement point of one satellite gives only a partial view of substorms. In a similar way, although we get a great deal of information from our simulations, in one respect we’re still limited in that we can’t yet include the entire substorm zone," said Pritchett.
Complementing more general studies with global magnetohydrodynamics models, the researchers’ model concentrates on the detailed local physics that leads to the triggering and onset of substorms, capturing the effects of individual particle dynamics in what are known as particle in cell simulations. But this sets a minimum size for each computational cell of several tens of km, so that the entire substorm zone would contain too many cells for even today’s largest supercomputers. Thus, these simulations encompass one part of the substorm zone at a time.
Along with an experiment that the researchers hope will one day be done with a dozen or more satellites observing simultaneously, being able to run larger simulations will help researchers answer further questions about substorm origins and dynamics.
"Our simulations are part of computational physics, a third branch that lies between the traditional theoretical and experimental branches of physics. The way I approached the substorm problem is to perform numerical experiments in which we follow the motion of a large number of charged particles in 3-D simulations in self-consistent electric and magnetic fields described by Maxwell’s equations," said Pritchett. The model can follow as many as 100 million particles, reproducing the interactions the particles experience in the earth’s magnetosphere and substorms.
"One of the most important things we’ve discovered is that instead of treating all charged particles the same, in line with previous assumptions, when we include certain kinetic effects of the plasmas and allow separate behavior for electrons and ions, the release of energy becomes much more rapid in the reconnection process," said Pritchett. While earlier models predicted rates of solar flare eruption, for example, that were far too slow, on the order of 10,000 years, Pritchett’s improved simulations produce a rate on the order of 30 minutes, consistent with what is actually observed in nature. "The simulations show that the proposed mechanism of reconnection can account for the rapid onset of magnetic explosions that form substorms and solar flares," said Pritchett.
The researchers have also tested predictions from each of the two competing theories for the trigger mechanism of substorms, finding some behavior consistent with the theory that predicts a near-Earth substorm origin. However, further simulations with larger grids will be necessary to fully answer the question of where substorms are triggered.
Simulating Substorms in the Computer
In their simulations, Pritchett and colleagues use a 3-D grid with 128 grid points per axis. Because the code uses a domain decomposition algorithm that is 1-D, it has been limited to 128 processors. To scale up the code for larger simulations, the researchers are adopting a 2-D domain decomposition, which will enable the simulations to run on 10,000 or even 20,000 processors.
"Data sets include the full particle and field information for up to 100 million particles, which is then used in post-processing to create statistical averages and calculate things like current densities," said Pritchett.
Looking ahead, the researchers have nearly finished migrating their code from the Cray T3E to Blue Horizon, which will allow a larger model with from 256 to 512 grid points per axis running on the same number of processors. This will encompass a greater part of the substorm domain, corresponding to a distance of about 10 Earth radii, allowing the researchers to more realistically model substorms and better test the two models for substorm origins.
"This is an interesting time to be studying substorms," said Pritchett, "because with better simulations we’re finally able to answer some very old questions." –PT
From the Sun: Auroras, Magnetic Storms, Solar Flares, Cosmic Rays,
Edited by S.T. Suess and B.T. Tsurutani, American Geophysical Union,
Washington, DC, 1998.