By James Mitchell Crow
Australian-based researchers have recreated inside their laboratory the atmospheric conditions that cause auroras. One day the results of their experiments could power a mission to Venus or Mars.
Christine Charles from the Australian National University told delegates at the Australian Institute of Physics conference in Melbourne that their work will help physicists to understand the formation of the auroras—the undulating bands of lights visible in the night sky over the north and south poles.
Charles has pioneered a technique to form plasmas of ionised gases within a magnetic field, like those that exist high above the earth in the aurora zone. Under such conditions, an electric field spontaneously forms, ejecting a jet of ions out of the plasma. It is this movement of ions and electrons in the ionosphere that causes the aurora to form.
Studying these phenomena in the lab can provide much more detailed information that can be obtained by sending probes into space, says Charles. It’s also much cheaper, she adds.
Charles is already investigating the plasma jets that form as a new way to power spacecraft. Subjecting the plasma to a magnetic field causes ions to shoot out in a single defined direction, generating thrust.
Oblique double layers; a comparison between terrestrial and auroral measurement
Christine Charles1, Rod Boswell 1, and Rhys Hawkins 2
1Space Plasma, Power and Propulsion laboratory
Research School of Physics and Engineering, The Australian National University, Canberra, ACT 0200, Australia
Satellites observations of electric field structures in the Earth’s aurora have been identified as belonging to a `U’ shaped potential structure that supports oblique electric double layers. This interpretation is verified by terrestrial laboratory measurements.
Humanity’s fascination with the northern lights has generated an ongoing interest in the physics of the Earth’s aurora. The basics underlying auroral physics also apply to remote planets as shown by recent imaging of Jupiter’s North polar aurora measured by the Hubble Space Telescope.
The Earth’s aurora mostly occur in a region surrounding each magnetic pole (North and South) which is connected (via magnetic field lines) to the conducting ionospheric plasma on the low altitude side and to the magnetosphere on the high altitude side. Large perpendicular electric fields (called electrostatic shocks) measured by the S3-3 satellite were the first signature of a converging potential structure, hence of parallel electric fields in the auroral acceleration region . These localised parallel electric fields have since been directly measured by the S3-3, POLAR and FAST satellites.
Statistical analysis of parallel and perpendicular electric field and accelerated charged particle flux data recorded during POLAR and FAST satellite paths across the auroral cavity strongly support the possibility of an electric double layer in the two auroral transition layers. The upward auroral current (inwards pointing electric fields, anti-earthward ion beam and earthward electron beam producing the visible auroral arcs) is generated in the low altitude transition layer and the downward auroral current (outwards pointing electric fields, earthward ion beam and anti-earthward electron beam of the non visible aurora) is generated in the high altitude transition layer.
II. LABORATORY MEASUREMENTS
The S3-3, POLAR and FAST satellites auroral observations of parallel and perpendicular electric field structures have been identified as belonging to a large `U’ shaped potential structure that supports oblique electric double layers. This interpretation is verified by terrestrial laboratory measurements of a self-consistently supported three dimensional oblique current-free double layer. Its width is a few tens of Debye lengths, its oblicity (with respect to the magnetic field) varies from 0 up to 30 degrees and its strength is a few times the electron temperature .
The laboratory study shows first evidence of the U-shaped potential structure and simultaneously measured accelerated ion beam. This result was achieved by using the Chi Kung expanding plasma apparatus consisting of a 15-cm-diam 31- cm long pyrex tube surrounded by a Helicon antenna and two axial solenoids and mounted on a 30-cm-long 32-cm-diam earthed aluminium diffusion chamber equipped with a rf compensated Langmuir probe (LP) and a Retarding Field Energy Analyser (RFEA).
The molecular gas CO2 was the working gas of choice so as to be closer to actual conditions existing in the auroral regions of planets and to generate the double layer outside the source tube: simultaneous mapping of the plasma potential and ion beam current was carried out using the RFEA; the plasma density and electron energy probability function was measured using the LP. The laboratory results are in consort with recent space observations.
REFERENCES F. S. Mozer et al, ‘Observations of paired electrostatic shocks in the Polar magnetosphere’, Phys. Rev. Lett. 38, 292 (1977).  C. Charles, R.W. Boswell, and R. Hawkins, ‘Oblique double layers: a comparison between Terrestrial and Auroral measurements ’, Phys. Rev. Lett. 103, 095001 (2009).
Christine Charles, firstname.lastname@example.org