Download
1 / 20
200 likes | 346 Views
The Spinning Terrella Experiment: Lab Analog for Earth's Magnetosphere. Robert Sheldon 1 , Eric Reynolds 2 1 National Space Science and Technology Center, 2 West Virginia University May 30, 2001. Why do we need global pix?.
E N D
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? • 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
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.
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 • Many space effects are topological, not microphysical • Global dynamics and imaging is easily accomplished
Terrella2 • 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
Parallel Dipoles @ High P -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
Plasma 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"
Anti-Parallel Merging -Left magnet grounded, right magnet w/ increasing bias -X-line plasma merging -Isotropization at min-B
Quadrupoles 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?
Aurora & Substorm -Left magnet biassed +10-60V -Aurora appears above magnet, and with ion colors -Substorm!
Conclusions (//) • 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-//) • 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?
