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The Spinning Terrella Experiment: Lab Analog

for Earth's MagnetosphereRobert Sheldon1, Eric Reynolds2

1National Space Science and Technology Center, 2West Virginia University

May 30, 2001

UAHWhy do we need global pix?

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:

UAHRing Current in the Lab

1) N & e2) Saturated3) -400VDC4) 0.5Tesla5)10-200mT

UAHSpinning 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

UAHThe 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.

UAHThe 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'sphereMany space effects are topological, not microphysicalGlobal dynamics and imaging is easily accomplished

UAHTerrella2

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

UAHParallel Dipoles @ High P

-High latitude minimum, and Shabansky orbits-Bistable distributions- Quadrupolar regions of magnetosphere are important for trapping and feeding dipole.

UAHParallel 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

UAHPlasma Entry @ Cusps

-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"

UAHAnti-Parallel Merging

-Left magnet grounded, right magnet w/ increasing bias-X-line plasma merging-Isotropization at min-B

UAHQuadrupoles and Merging

-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?

UAHAurora & Substorm

-Left magnet biassed +10-60V-Aurora appears above magnet, and with ion colors-Substorm!

UAHConclusions (//)

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?

UAHConclusions (anti-//)

Merging X-line geometry is entry point for plasma, supporting reconnection theories

Isotropization of the population is observed at X-line 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?

UAHNegative Biassed Magnet

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