Transcript UAH The Spinning Terrella Experiment: Lab Analog for Earth's Magnetosphere Robert Sheldon1, Eric Reynolds2 1National Space Science and Technology Center, 2West Virginia University May 30, 2001

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The Spinning Terrella
Experiment: Lab Analog
for Earth's Magnetosphere
Robert Sheldon1, Eric Reynolds2
1National Space Science and Technology Center,
2West Virginia University
May 30, 2001
Why do we need global pix?
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• Chapman & Ferraro [1932] wanted the neutral plasma from the
sun to produce a "ring current" as it envoloped the Earth. This
current was needed to explain Dst. But they didn't know how.
• Alfven tried electric fields [1952?] and failed.
• Finally Fred Singer [1957] realized that dipoles trap charged
particles which carry a current via the grad-B drift.
– If they had taken
Birkeland's
apparatus [1910] and
applied a 400V DC
bias, this is what
they would have
seen:
Ring Current in the Lab
1) N & e
2) Saturated
3) -400VDC
4) 0.5Tesla
5)10-200mT
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Spinning Terrella Experiment
• Bell jar, oil roughing pump, HV
power supply, Nd-B ceramic magnet
• Needle valve used to control the
pressure from 10-400 mTorr
• Langmuir
• 2 ports
• Webcam
• ~$10k
The Distorted Dipole
-But the Earth is NOT a
simple dipole, it is distorted
by the Solar Wind.
- The SW acts as a
"superconductor" which
prevents B-normal across the
separatrix.
-This is equivalent to an
"image" dipole from
elementary electrostatics.
-Chapman 32, and Parker 61
simulated this distortion as
an image dipole "reflected"
by the magnetopause.
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The Double Dipole M'sphere
• Chapman & Ferraro 1932
• Parker 1961 used the
double dipole model to
explain geomagnetic
storms
– Laboratory plasmas can easily be
set up which are topologically
equivalent to Earth's M'sphere
• any space effects are topological, no
M
microphysical
•Global dynamics and imaging is easily
accomplished
2
Terrella
• Two Nd-B-Fe magnets, on
alumina standoffs.
• We vary the orientation of the
magnets, the aspect ratio of
the magnets, and the voltage
of the magnets.
• Glow discharges at 10-200
mTorr = m.f.p 0.3 - 6 mm
• The smallest dimension of
the glow is 1-10 mfp
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Parallel Dipoles @ High P
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-High latitude minimum, and
Shabansky orbits
-Bistable distributions
- Quadrupolar regions of
magnetosphere are important
for trapping and feeding
dipole.
Parallel Dipoles @ Low P
-Assymetries are caused by
2-dipoles.
-Possibly due to separatrix
of grad-B and ExB drifts.
-This leads to closed orbits
that do not encircle the
central magnet, "banana
orbits", which only occur
at narrow range of
Voltages at low pressure
-Region between the
dipoles has compressed
fields & appears brighter
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Plasma Entry @ Cusps
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-One magnet grounded, other
biassed
-Plasma generated by electrons
on one magnet, feed into other
trapping field due to diffusion
though "x-line"
-Like northward Bz, this
feeding happens at the cusps
-The cusps themselves hold the
plasma long enough to glow,
"Sheldon orbits"
Anti-Parallel Merging
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-Left magnet grounded, right
magnet w/ increasing bias
-X-line plasma merging
-Isotropization at min-B
Quadrupoles and Merging
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-Balanced voltages
-Appearance of trapped
plasma above and below
the null point.
- These orbits are
quadrupolar trapped
(drifting around a
minima)
-Is there a Earth analog?
Aurora & Substorm
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-Left magnet
biassed +10-60V
-Aurora appears
above magnet, and
with ion colors
-Substorm!
Conclusions (//)
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• We can learn a great deal from laboratory analogs of the
Earth's magnetosphere. Terrella experiments are not just
history!
• Separation of charge species observed in the lab, suggesting
space-charge and field-aligned potentials are possible at
Earth. (See poster)
• Quadrupole trapping appears in anti-merging geometries,
supporting Sheldon-Shabansky orbits
• Cusps appear to be entry location for plasma, supporting
"sash" theories
• "Banana" orbits appear important for narrow voltage
regimes--is this a feature in storm development?
Conclusions (anti-//)
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• Merging X-line geometry is entry point for
plasma, supporting reconnection theories
• Isotropization of the population is observed at Xline as well, with consequences for the footpoint.
• "Balanced" voltages at 2-dipole X-line generates
a unique quadrupolar trap. Is there any evidence
for this trap in space plasmas?
• Substorm like features can be generated in the
laboratory. Can we use these auroral dynamics to
probe parent population?
Negative Biassed Magnet
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