THE
PERFECT SOLAR STORM - SOHO SPACECRAFT BLASTED BY INTENSE SOLAR
STORM
(Click
on Images 1, 2 and 3 to see movies of the solar flare)
An
explosion on the Sun July 14, 2000 smacked Earth's magnetosphere
with one of the most intense blast of solar particles ever
detected by the Solar and Heliospheric Observatory (SOHO)
and the Advanced Composition Explorer Spacecraft (ACE). The
solar explosion, called a flare, was accompanied by an ejection
of hot, electrically charged gas directed toward the Earth.
NOAA's Space Environment Center predicted strong to severe
geomagnetic disturbances on Saturday and Sunday. The arrival
at Earth of this massive electrified gas cloud, called a Coronal
Mass Ejection (CME), can cause vivid auroral (northern and
southern lights) displays these days, depending on the orientation
of the magnetic field within the CME.
'PRESSURE-COOKER'
MODEL MAY HELP EXPLAIN--AND PREDICT-- GIANT SOLAR FLARES
(Lake
Tahoe, NV -- June 20, 2000) -- A three-dimensional numerical
model developed by scientists at the Naval Research Laboratory
(NRL) and 'field tested' by astronomers at the Smithsonian
Astrophysical Observatory (SAO) may help explain the nature
and origin of solar eruptions. These eruptions are known to
trigger stormy space weather, which can, in turn, damage communications
satellites, endanger astronauts in space, and disrupt transmissions
along electrical power lines on Earth.
A
theoretical reconstruction of the evolving magnetic fields
in a flaring region on the Sun, using NRL's "pressure-cooker
model" (as it has been informally nicknamed), matched
actual observations made by SAO scientists using NASA's Transition
Region and Coronal Explorer (TRACE) of the so-called "Bastille
Day" flare on July 14, 1998. According to the NRL-Smithsonian
team, this close agreement between theoretical predictions
and observations demonstrates the viability of the model for
understanding how energetic solar eruptions occur and perhaps
predicting them in advance.
The
key strength of the pressure-cooker model is its novel description
of the conditions that allow the release of energy, leading
to eruptive flares and coronal mass ejections (CMEs). Magnetic
energy emerges from the solar interior, appears low in the
solar atmosphere, and accumulates under a magnetic "lid."
When the lid has a weak spot -- in this case, the "null
point", where the magnetic field is equal to zero --
the accumulated low-ling energy can blow violently through
the lid of the "Pressure Cooker".
The
NRL "pressure cooker " differs from competing CME
models in that the magnetic field is assumed to have a more
complicated (quadrupolar) geometry, with a null point located
high in the solar atmosphere. Other models, which assume a
simpler (bipolar) geometry and rely on different mechanisms
for triggering eruptions, "have never been compared with
actual solar observations as precisely as we did in our study,"
notes Dr. Guillaume Aulanier. "This test," he continues,
"allowed us to confirm all of the conditions required
to initiate a CME according to our model."
This
is one of the first attempts by the NRL-SAO research collaboration
to reconstruct and interpret the complex magnetic field of
a real observed flaring region. TRACE's Bastille Day coverage
provided some of the finest detailed observations of an eruptive
flare ever obtained. In particular, the continuous telemetry
of the instrument provided more data on pre-flare activity
than has previously been available, which was essential for
validating the model and confirming the pressure cooker process.
The scientists intend to use additional TRACE data, SOHO/EIT
observations, and future data from the SOLAR-B and STEREO
missions to further test the pressure cooker model.
The
scientific paper describing this research will be published
in the Astrophysical Journal in September 2000.
Back
to Top
ACCURATE SPACE STORM WARNINGS NOW POSSIBLE
(Click
on right pic above to see CME visual clip taken by LASCO C-2
on June 6, 2000. Venus is visible to the right.)
The arrival from the Sun of billion-ton electrified-gas clouds
that cause severe space storms can now be predicted to within
a half-day, a great improvement over the best previous estimates
of two to five days.
Scientists
at the Catholic University of America, Washington, DC, and
NASA's Goddard Space Flight Center, Greenbelt, MD, have created
a model that reliably predicts how much time it takes for
these clouds, called Coronal Mass Ejections (CMEs), to traverse
the gulf between the Sun and the Earth, based on their initial
speed from the Sun and their interaction with the solar wind.
The new model uses recent observations from the European Space
Agency/NASA Solar and Heliospheric Observatory (SOHO) and
the NASA WIND spacecraft. The model has been validated and
made more accurate using historical observations from the
Helios-1 (Germany/NASA), the Pioneer Venus Orbiter (NASA),
and the Space Test Program P78-1 (United States Air Force)
spacecraft.
Earth-directed CMEs cause space storms by interacting with
the Earth's magnetic field, distorting its shape and accelerating
electrically charged particles (electrons and atomic nuclei)
trapped within. Severe solar weather is often heralded by
dramatic auroral displays (northern and southern lights),
but space storms are occasionally harmful, potentially disrupting
satellites, radio communications and power systems.
"The
new model more accurately predicts the arrival of Coronal
Mass Ejections, and will greatly benefit people who operate
systems affected by space storms," said lead author Dr.
Natchimuthuk Gopalswamy of Catholic University, a Senior Research
Associate at the National Academy of Sciences/National Research
Council. "The improved forecasts let operators of sensitive
systems take protective action at the proper time and minimize
the unproductive time when systems are placed in a safe mode
to weather the storm."
Gopalswamy and colleagues will present this research today
during a meeting of the Solar Physics Division of the American
Astronomical Society at Lake Tahoe, Stateline, NV.
Coronal Mass Ejections leave the Sun at various speeds, ranging
from 12 to 1,250 miles (about 20 to 2,000 kilometers) per
second. Only the CMEs directed at Earth are potentially harmful;
estimating when they will arrive is difficult because their
speed changes due to interaction with the solar wind, a stream
of electrically charged gas blowing constantly from the Sun
at about 250 miles (about 400 kilometers) per second.
Just as a motorboat heading downstream will slow to the speed
of the river's current if its motor is turned off, Coronal
Mass Ejections starting out from the Sun more quickly than
the solar wind eventually are slowed by the drag of this "stream."
If a boat pulls up anchor, it will gradually accelerate until
it is moving at the speed of the current. Similarly, CMEs
that start out more slowly than the solar wind are pulled
along until they match the solar wind's speed.
Using data from solar-observing spacecraft, Gopalswamy and
his team discovered how much the solar wind sped up or slowed
down various Coronal Mass Ejections according to their initial
speeds. If the initial speed of a CME is known, the
new model accurately accounts for the influence of the solar
wind on the CME speed, and the CME arrival time at Earth can
now be precisely estimated.
Back
to Top
|