UAH 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

UAH 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:

UAH Ring Current in the Lab 1) N & e 2) Saturated 3) -400VDC 4) 0.5Tesla 5)10-200mT

UAH Spinning Terrella Experiment •Bell jar, oil roughing pump, HV power supply, Nd-B ceramic magnet •Needle valve used to control the pressure from mTorr •Langmuir •2 ports •Webcam •~$10k

UAH 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.

UAH 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

UAH Terrella 2 •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 mTorr = m.f.p mm •The smallest dimension of the glow is 1-10 mfp

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

UAH Parallel 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

UAH Plasma 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"

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

UAH 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?

UAH Aurora & Substorm -Left magnet biassed V -Aurora appears above magnet, and with ion colors -Substorm!

UAH 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?

UAH 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?

UAH Negative Biassed Magnet

UAH