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"Terrella Cubica" : a Planeterrella at Aalto University, Finland

At Aalto University, Helsinki, Finland, the Space Plasma Physics team led by Prof. Esa Kallio has toyed with the idea of rebuilding a Planeterrella from scratch using in-house expertise and building materials. The design of the vacuum chamber would have to be cubic in shape as a reminder of Birkeland’s original experiment, optimising the field of view for photography without deformations from the curved glass and a more usable space. The question remained : can we really do it all at the University, using only the on-site available personnel and equipments ?

The "Terrella Cubica" project was born. During the Spring 2014, Aalto University PhD student Tomi Kärkkäinen created the first CAD modelling designs (Fig. 1) and started to work on the manufacture of the most technically challenging part of the project : a single aluminium structure, which must be strong enough not to collapse under the huge pressure difference between the inside and the outside of the box.

Fig. 1 : Design phase of the Terrella cubica. From early designs (left) to final designs (middle and right). (c) Tomi Kärkkäinen

By September 2014, the aluminium structure was built at the University foundry under Tomi’s supervision (Fig. 2, left). On each face, two grooves were machined for fitting two rubber seals on which the glass windows would rest, ensuring the design to be as air-tight as possible. After calculations on the strengths and thickness needed, the 12-mm-thick glass windows were ordered externally and fitted to our experiment’s specifications.

The small pieces (spheres, pedestals) as well as the base plate were created from off-the-shelf raw
materials by CNC (Computer Numerical Control) and manual milling. Neodymium magnets of 1 Tesla intensity are used in the current setup.

The first version of the mounted Planeterrella as of March 2015 is shown in Fig. 2 (right).

Fig. 2 : Manufacturing and mounting phase. Terrella cubica aluminium frame after machining (left). First version of the mounted experiment at Aalto University (right). (c) Cyril Simon Wedlund

Nearly one year after we started this adventure on our spare time, the first light occurred in April 2015, with promising results (Fig. 3). Most of the original Planeterrella configurations can be reproduced.

The aim of the experiment is to support the current space physics cursus of the Aalto University Department of Electrical Engineering, by demonstrating basic plasma phenomena, such as auroral ovals or ring currents, or the role of the Lorentz force and the Debye shielding, usually only witnessed by instruments onboard space missions.

Fig. 3 : Testing phase : ’tilted’ ring current (left), auroral ovals (middle) and multidipolar ’Sun’ configuration (right). (c) Esa Kallio, Cyril Simon Wedlund & Markku Alho

A summary of the project is available on the team’s website ( http://space.aalto.fi/outreach.html ).

Contacts :
Prof. Esa Kallio, esa.kallio_AT_aalto.fi
M.A. Tomi Kärkkäinen, tomi.j.karkkainen_AT_aalto.fi
Dr. Cyril Simon Wedlund, cyril.simon.wedlund_AT_aalto.fi
Website : http://space.aalto.fi/

Aalto University,
School of Electrical Engineering (ELEC), Department of Radio Science and Engineering (RAD),
P.O. Box 13000
FI-00076 Aalto, Finland

Institut de Planétologie et d'Astrophysique de Grenoble (IPAG)