Manuscript received 1 Dec 2010. Guillaume Gronoff is from NASA Langley Research Center, and is sponsored by the NPP program. Cyril Simon Wedlund is from BIRA/IASB, Belgium. Publisher Identifier S XXXX-XXXXXXX-X

Auroral formation and plasma interaction between magnetized objects simulated with the Planeterrella

G. Gronoff and C. Simon Wedlund

Fig. 1. The classic Sun – Planet interaction. The big sphere (Sun) results in the creation of two auroral ovals on the planet (small sphere) (top panel). The simulation of a planet where one magnetic pole points towards the Sun results also in an auroral oval at the opposite pole (bottom panel).

Abstract - The Planeterrella is a space plasma simulator, based on Kristian Birkeland's historical experiment, the “Terrella”. This device makes it possible to simulate interactions between an electrode and a magnetized sphere in many different geometries but also to simulate interactions between two magnetized spheres. Such configurations allow the visualization of phenomena unknown to Birkeland, such as an emitting body (Io) immersed in a magnetosphere (Jupiter), or the aurora on the night side of a planet where one magnetic pole points towards the Sun (Uranus).

The Planeterrella is based on an improved design of Birkeland's original Terrella experiment from 1901 [1]. The experiment is placed in a plexiglas vacuum chamber where a primary vacuum is made (1–10Pa corresponding to the Earth’s atmospheric pressure at 80km altitude). Two metal spheres of different sizes equipped with 1-Tesla neodymium magnets and an electrode are positioned and lie on top of pedestals so that many different configurations can be tested. When an electric field (>300V) is applied between the objects

(electrode and/or spheres), a fluorescent glow appears similarly to the auroral phenomena of Earth and other planets.

With one electrode and two spheres with two possible polarities, many combinations are possible [2]. Some general patterns can be seen: – Emitting sphere: here, the small sphere ejects electrons outwards and accelerates them along the equator. It has similarities with a cyclotron. Since the big sphere has the same magnet, but not similarly positioned, it has a weaker magnetic field relative to its size, and is not dipolar. In turn, when the big sphere is a cathode, one can simulate a glow similar to that of the solar corona punctured by coronal holes where the local magnetic field is larger (Fig. 1, top panel). – Receiving sphere: in this configuration, a red glow is observed and wraps around the sphere, except at the position of the auroral ovals, which appear black in contrast. – Relative positions of the spheres: when objects are too closely positioned or when there is a strong anisotropy, parts of the previously described patterns can be significantly altered. “Jets” can appear in the form of a glow following the trajectory of the electrons between two objects.

The emissions observed in the Planeterrella correspond to the emission of an N2/O2 gas excited by electrons. As O2 emissions are not strong enough in the visible to be observed with the naked eye (the plexiglas dome filters the UV emissions for safety reasons), the main emissions come from the N2 and N2

+ species. The purple glow arises from the N2

+(B–X) first negative band transition, with main emissions at 391.4nm and 427.8nm (hence the purple tint). The reddish glow comes from the N2(B–A) first positive band, which emits in the 600-750nm region (red tint) [3].

As these excitations originate from electron impact on N2 molecules, simple interpretations of the observed colours of the emissions can be made. The purple N2

+ emissions are localized close to the emitting object where the Debye screening is efficient [2]. Because the electrons have to break the electrostatic gap they have a greater energy and therefore are able to ionize the ambient gas. While losing their energy, electrons can excite the N2(B–A) band giving the gas its red tint. On longer paths, electrons can lose a lot of their energy in collisions, hence no emission is visible until they reach the anode where they are re-accelerated by the electric field. In turn they can get enough energy to excite the N2(B–A) bands, producing there a red-coloured emission.

The trajectory of electrons is therefore determined by collisions, the electric field and the magnetic field. An interesting analogy can be made with Earth’s environment, with the emitting object being the equivalent of the reconnection point and the re-acceleration region the equivalent of the entry of electrons in the atmosphere.

While similarities exist between the Planeterrella and large-scale auroral phenomena, several limitations have to be considered on top of scaling effects. The absence of gradients of gravity and atmospheric pressure prevents a real magnetosphere-like trajectory of the electrons, and the appearance of complex sets of currents. Also, the experiment is static since no plasma flow exists and no protons are created from an ion source. Finally, the small size of the experiment prevents the observation of O(1S) green and O(1D) red transitions because of quenching.

Despite these limitations, the Planeterrella can in a first approximation reproduce electron trajectories in magnetospheres and give a visual representation of unusual configurations, such as a magnetized object (a sunspot) emitting towards a close planet (a hot Jupiter with a magnetic field or a magnetized accretion disk) or an emitting object immersed in the magnetosphere of a planet and creating auroral ovals (Io's plasma torus and Jupiter's magnetosphere). It is also possible to guess what a mission to Uranus will see when Uranus’ magnetic pole is pointing towards the sun, as simulated in Fig. 1 (bottom panel). On the ‘day side’, a bright spot is present exactly as was observed by Voyager 2 in 1986 [4]. Two auroral ovals/cusp are also seen, a bright one on the night side (opposed to the cathode) and a thinner one on the day side (facing the cathode) surrounding the emission bright spot. These observations should be confirmed by a “Uranus Pathfinder”-like mission, designed to observe this particular type of aurora, with a similar geometry.

We would like to encourage scientific teams and museums to make their own Planeterrella since it is not only a useful auroral simulator but also a very visual experiment which can be useful for outreach and education.

Acknowledgements: The authors are indebted to J.

Lilensten (LPG-IPAG, CNRS, France) for giving them the

opportunity to build and run the Planeterrella simulations as a team.

The Planeterrella was designed and brought to life by J. Lilensten

(CNRS). C.S.W wishes to thank M. S. Wedlund (LATMOS, France)

for her help in the photo session.

REFERENCES

[1] Birkeland, K., “The Norwegian aurora polaris expedition, 1902-1903”, H. Aschelhoug Pub., Oslo (1908) http://www.archive.org/details/norwegianaurorap01chririch

[2] Lilensten, J. et al., The Planeterrella, a pedagogic experiment in planetology and plasma physics, Acta Geophys., 57, vol. 1, 220-235 (2009)

[3] Gronoff, G., PhD Thesis, UJF/Grenoble (2009) [4] Herbert, F., Aurora and magnetic field of Uranus, J. Geophys.

Res., 114, A11206 (2009)

Website: http://planeterrella.obs.ujf-grenoble.fr/