Case Studies
Berkeley, CA - October 2002
Understanding the dynamics of the solar atmosphere helps explain eruptions
called coronal mass ejections (CME). Predicting the cause of these eruptions
is an important scientific challenge.
Plasma
Plasma
is a distinct state of matter containing electrically charged particles
that affect its electrical properties and behavior. In addition to being
important in many aspects of our daily lives, plasmas may constitute more
than 99 percent of the visible universe. Plasma is often called the fourth
state of matter, the other three being solid, liquid and gas.
Solar Atmosphere
Both the Sun's atmosphere and its surface consist of plasma. The density
of plasma in the solar atmosphere — called corona — is orders of magnitude
less than at the surface — called photosphere. Because of this, the behavior
of plasma in these two regions is very different.
At the surface — photosphere — forces associated with plasma's fluid pressure
are comparable to the magnetic forces imposed on plasma from the Sun's
magnetic field. In contrast, the more rarefied (or less dense) plasma
of the corona is primarily controlled by the magnetic field. As a result,
coronal plasma tends to align itself along the magnetic field lines. This
forms large scale arches and arcades which constrain the plasma like huge
magnetic pipes. These massive, super-heated, electrified gas configurations
are called coronal loops.
Coronal Loops
The Sun is covered in millions of coronal loops at any given time. The
most massive arches are more than 300,000 miles high and would span the
Earth 30 times.
The footpoints of coronal loops anchor at the solar surface. Dense fluid
motion on the surface slowly moves the footpoints around. This slow motion
can hardly be detected over long periods of time. Thus, the corona appears
relatively quiet.
The loops can persist in the corona for days, or even weeks, then suddenly
erupt over the course of several hours. Huge bubbles of gas threaded with
magnetic field lines — called coronal mass ejections (CME) — are released
during some of the explosions.
The Sun undergoes an eleven year cycle of activity. As the cycle approaches
its maximum, the frequency of CMEs increase from approximately once a
week to several each day.
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| Figure 1: Image of the low-lying
sigmoidal lines in the solar atmosphere. |
Figure 2: Image captured using
a sun-observing satellite called TRACE (Transition Region and Coronal
Explorer). This image reveals thread-like loop structures. |
Coronal Mass Ejections
Massive CME (also called Space Storms) sends billions of tons of plasma
into space. Magnetic flux and ejected mass cause major disturbances in
the interplanetary medium. Earth-directed CMEs send huge bursts of charged
particles that disrupt radio communications, cause surges in power grids
and damage satellites.
The exact causes and mechanism of CME are not well understood and debated
in the scientific community. Since some of the explosions are directed
towards our planet and carry massive amounts of magnetised plasma, understanding
these events and developing predictive capabilities is a very important
scientific challenge. While the importance of the predictive capabilities
for solar eruptive activity is clear, much remains to be understood about
coronal plasma dynamics.
Figure 3: Top view of the twisting
magnetic fields lines formed by the rotational motion at the photosphere.
You can see the low-lying sigmoidal lines (orange) that look similar to
figure 1.
Mayya Tokman
Mayya Tokman, Ph.D. is a visiting Assistant Professor of Mathematics at
the University of California, Berkeley. She models the dynamics of coronal
loops and their evolution using three-dimensional magneto-hydrodynamic
numerical simulations. In particular, she studies the response of coronal
loops to the slow rotational motion of their footpoints anchored in a
denser plasma at the surface of the Sun.
Figure 4: Side view
of twisting magnetic fields lines formed by the rotational motion at the
photosphere. The formation of geometrically different regions as a result
of such dynamics are visible. You can see the low-lying sigmoidal lines
(orange) and the overlaying arcades (yellow and red). The green, purple,
red and yellow streamlines are magnetic field lines anchored to the solar
surface (bottom contour plot). Click
here to download an animated gif.
The results of her research shows formation of sigmoids
(orange lines in figures 3 and 4) and overlying arcades (yellow and red
lines in figures 3 and 4) which closely resemble those seen in observations
(figures 1,2, 5 and 6). Her simulation results indicate that these structures
form as a result of the slow rotation of the coronal loop footpoints.
Further enhancements to the model will help understand the physics and
evolution of coronal loops in more detail.
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| Figure 5: This TRACE image is
one of the sharpest pictures ever taken of magnetic arches. |
Figure 6: Image of a coronal
mass ejection (CME). You can see the similarity between Figure 4 and
6. |
Credits
- Figure 1: Sigmoid Image. Sterling, et al.
2000.
- Figure 2: Image of coronal mass ejection
(CME) observed by the Solar Maximum Mission. Image enhanced and reprinted
from Illing & Hundhausen, 1983.
- Figure 5 and 6: Images
provided by the TRACE
solar observatories project.
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