| SOLAR WIND AND GEOMAGNETIC
STORMS
SIMULATIONS
OF SPACE WEATHER
THE SPACE WEATHER
MODELING HORIZON  oon
after rockets and satellites began orbiting the Earth, scientists
realized the importance of studying the region between the Sun
and the Earth for interpreting data from satellites and predicting
solar storms. While the Earth's magnetic field creates a protective
shield, called the magnetosphere, plasma particles from the Sun
(electrons, protons, and the nuclei of helium atoms and heavier
atoms) can penetrate the magnetosphere. Magnetospheric disturbances
can cause vivid auroras, radio interference, power blackouts,
navigation problems for ships and airplanes, and spacecraft damage.
Researchers in the Space Plasma Simulations Group at UCLA have
advanced "space weather" forecasting by modeling the problems
using NPACI resources.
Beginning in
1980 with a grant from NASA, space physicist Maha Ashour-Abdalla
and colleagues at UCLA have conducted research into the physics
of energy and plasma transport through the Earth's magnetosphere.
"We have used supercomputers at SDSC for our modeling work since
1986," she said. Since July 2000, the group has been working with
NPACI's Strategic Applications Collaborations (SAC) staff to improve
their simulations. Using comparisons of NASA spacecraft observations
with supercomputer simulations of the magnetosphere and its response
to the solar wind, the group studies how the magnetosphere reacts
to changes in the solar wind. Top
| Contents | Next SOLAR WIND AND GEOMAGNETIC
STORMS The solar wind consists of charged particles
that stream out of the Sun at high velocities, pushing against
the Earth's magnetic field and stretching the magnetosphere into
a long tail on the night side, opposite the Sun. The interaction
of the solar wind and the magnetosphere leads to reconnection,
in which magnetic field lines intersect and merge. When the solar
wind transfers energy to the magnetotail, reconnection induces
an ejection of plasma back towards the Earth, depositing energy
in the upper atmosphere and creating auroral light. The details
of such geomagnetic "substorms" depend on many factors, including
the solar wind strength and the direction of the overall magnetic
field from the Sun. "The interactions of solar storms create complex
magnetospheric configurations," said physicist Jean Berchem of
the UCLA group. "We try to model these events to better understand
how the energy from the solar wind is transferred to the Earth's
environment. The implications of this transfer of energy can be
very dramatic, especially during solar maximum." In December 2000, the Earth was in the midst
of a solar maximum, a peak in the 11-year cycle over which solar
activity waxes and wanes. Occasionally, a coronal mass ejection
(CME) of high-energy particles from the Sun hits the magnetosphere.
Possible consequences of these solar flare-ups include particularly
intense precipitation of energetic particles in the auroral regions
creating vivid displays of light and colors at high latitudes
and overload conditions for power grids on the ground. High-energy
particles can also damage satellites and may even be hazardous
to astronauts. "CMEs are a very active research topic," Berchem
said. "During solar maximum, large CMEs occur very often, about
one or two a month. Coronal ejections have very different sizes
and dynamics. Sometimes, they have very little effect on the Earth's
magnetosphere, while other times times they can dramatically alter
the Earth's space environment, leading to strong geomagnetic storms." Solar disturbances have caused major problems
at the Earth over the last few decades. One of the strongest geomagnetic
storms occurred during the solar maximum in March 1989, when several
spacecraft had to have their orbits adjusted and a power blackout
occurred over eastern Canada. In 1997, a geomagnetic storm caused
AT&T to lose its Galaxy IV satellite, which knocked out pagers
throughout the United States. Last year, a space shuttle was sent
up to boost the International Space Station to a new orbit because
of the increased solar activity. "Magnetospheric research has
enabled us to recognize precursors to geomagnetic storms," said
David Schriver of the UCLA group. "We have about two days' notice
when we see a CME or its precursors, before the storm might possibly
reach the region of the Earth." Top
| Contents | Next SIMULATIONS OF
SPACE WEATHER
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Figure 2. Simulating an Aurora
These diagrams shows Earth's
polar regions as seen from above by satellite and simulations.
The solar wind comes in at 12 noon (to the top of each diagram),
and each concentric circle from the center indicates a lower
latitude by 10 degrees starting from 90 degrees at the center.
Satellite data (top) from the Polar spacecraft Visual Imaging
System (VIS) camera looking down on the north pole of the
Earth from space (courtesy of Louis Frank from the University
of Iowa) and an equivalent view from a global MHD simulation
(bottom) show the location of an aurora (red). The aurora
is seen near midnight on the Earth between about 60 and
70 degrees latitude in both panels. The MHD simulation used
upstream solar wind conditions on December 22, 1996, as
input.
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The Space Plasma Simulations Group use NPACI
supercomputers at SDSC to simulate the effects of changes in solar
wind conditions on the Earth's environment. "During a solar maximum,
many storms occur, changing the entire plasma system surrounding
the Earth," Berchem said. "Our research simulates these events
using the computer." "Simulation and modeling of plasmas near the
Earth has really benefited from the use of supercomputers," said
group leader Ashour-Abdalla. "The continual improvement of the
resources at SDSC has certainly changed the fortunes of this group.
Our ability to treat multicomponent plasmas realistically has
been greatly extended." The group tries to understand space weather
by modeling storms using bothlocal and global simulations. Global
magnetohydrodynamic (MHD) simulations display the big picture,
while local, particle-in-cell (PIC) simulations of individual
particles show more accurate, detailed interactions. Large-scale
kinetics (LSK) is an intermediate type of model in which particles
are used along with the global fluid field model to determine
the entry of particles from the solar wind into the magnetosphere,
and in particular penetration of these particles into the near-Earth
region of the magnetosphere. The results of these simulations
are then compared to actual observations. "To model large-scale interactions we use
data from spacecraft monitoring the solar wind far upstream from
Earth as input to our global MHD simulations" Berchem said (Figures
1-2). "Then, we assess the validity of our predictions by comparing
the results of the simulations with spacecraft observations from
the NASA International Solar Terrestrial Physics (ISTP) program,
as well as ground-based data obtained by magnetometer chains and
radar supported by the National Science Foundation. We have been
very successful in comparing simulated auroral patterns with auroral
observations using images from the Visible Image System on board
the NASA Polar spacecraft." Top
| Contents | Next THE SPACE WEATHER MODELING
HORIZON The UCLA group's codes were originally run
on the Cray T3E, but they soon found that the IBM SP Blue Horizon
enabled the kernels to run two or three times faster. "We are
trying to improve our global MHD simulations at each step," said
group researcher Vahe Peroomian. "Ultimately, when our simulation
code is able to run in real time, if an upstream spacecraft sees
energetic particles coming toward Earth, we might be able to predict
which satellites could be in danger. Because of our computer simulations,
we can model the consequences of a storm." The UCLA group specializes in the use of multiple
simulation methods to study space plasma physics. "In our research,
we have MHD (global simulations), and often we use MHD results
to run LSK particles," Peroomian said. "We also have the local,
PIC calculations. This allows us to study local effects and apply
our findings to space weather directly." "NPACI's Strategic Applications Collaborations
(SAC) program helps these researchers to do their science better
and faster," said Bob Sinkovits, SAC coordinator and SDSC computational
scientist. "As parallel machines become more complicated, it is
harder to get good performance without an insider's knowledge
of machine particulars. But our researchers should be focused
on science instead of computational issues, which is where our
staff comes in." The computational scientists in the SAC project
come from science and engineering backgrounds, creating a bridge
between the scientific and computer worlds. "The UCLA group's MHD code initially would
not run the MPI library on Blue Horizon", Sinkovits said. "So,
Dong Ju Choi and Dominic Holland worked on code modification such
that it could use MPI on Blue Horizon." Choi has also succeeded in porting the particle-in-cell
simulation code to the Sun Enterprise 10000 at SDSC. "Choi's work
has allowed us to speed up the code by a factor of 10," Schriver
said. "The collaboration is great because we are interested primarily
in the science. We know how the codes work, but in terms of optimization
and how to best use available computer resources, we can always
use help." Peroomian has also experienced greater code
success since moving his LSK code from an IBM SP at UCLA with
10 to 20 nodes to Blue Horizon, which can run the code on 30 to
80 nodes. "A 30-second job originally took 12 hours on the older
machines", he said. "Now, on Blue Horizon, even before Sinkovits'
speed-up, the same job would take four minutes. His work sped
up the code by a factor of three to four. It took me six months
to complete the first run of this code. Now a good run takes less
than eight hours." The group's short-term goal is to reproduce
observations by spacecraft and explain magnetospheric phenomena.
"Modeling past events lets us assess the accuracy of our models.
They help us to determine the physical processes our simulations
need to accurately model the interaction of the solar wind with
the Earth environment," Berchem said. "Ultimately, our goal is
to refine these models so they are accurate enough to forecast
weather in space." In 2000, the UCLA group ran 177 jobs on Blue
Horizon between May and November. They are performing jobs on
256 processors that require more than 100 hours of machine time
per year. "Computer time is precious, and it can be
difficult to find," Schriver said. "Originally, our codes ran
on vector machines. Advances in computational science have evolved
to the point where we can't go back to such machines. We would
like to see the SAC program continue. There is no other way to
get the kind of results we are getting now." -NB  Top
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Project
Leader
Maha
Ashour-Abdalla
UCLA Participants
Jean Berchem,
Mostafa El-Alaoui,
Vahe Peroomian,
Robert Richard,
David Schriver
UCLA SAC Team
Bob Sinkovits,
Dong-Ju Choi,
Dominic Holland
SDSC |
SDSC - UC San Diego, MC 0505 |
9500 Gilman Drive |
La Jolla, CA 92093-0505
