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Logo Institut für Geophysik und Extraterrestische Physik der TU Braunschweig

BepiColombo

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A special feature of planet Mercury, the closest planet to the Sun, is its magnetic field. Usually, a planetary magnetic field is generated by electric currents in the planetary interior, the liquid iron core, in a so-called dynamo process. In the past, it was assumed that Mercury should not possess a liquid core since it was believed to be completely solidified long ago. In constrast to this, NASA’s Mariner 10 mission confirmed at two short flybys a small but global magnetic field. So there is probably still a liquid core and a dynamo process in the interior of Mercury. An important requirement for the dynamo is that at least a portion of the core is liquid and is constantly stirred by convection. Using ground based radar measurements of the slightly uneven rotation, this liquid layer was detected. Nowadays, it is assumed that a light alloying material like sulfur keeps the core from freezing until the present.

 

Vergleich des inneren Aufbaus der Erde mit Merkur. Der Merkur hat im Vergleich zur Erde einen relativ großen Eisenkern. Ein Teil des Merkurkerns muss flüssig sein und ermöglicht damit einen Dynamoprozess. Die Größe bzw. die Existenz eines inneren, festen Kerns ist noch völlig ungeklärt.
In comparison to Earth, Mercury has a relatively large iron core. The core must be least partially molten. The existence and the extent of the inner solid core is still not constrained.
Photographie vom Planeten Merkur im sichtbaren Licht.
Photography of Mercury in the visible light.

But again, the Hermean magnetic field caused a headache for scientists. The question is: Why is the magnetic (dipole) field so weak? If you transfer the knowledge gathered from the terrestrial dynamo And other solar system bodies to the Hermean case, the magnetic field should be at least 10 times stronger according to the scaling laws. A number of researchers have speculated on how this conflict with dynamo theory could be resolved. Models that rely on a special interior structure have been developed and implemented in full computer simulations. These simulations are an important tool to look inside inside a planet. Now, the goal is to map the magnetic field to a higher degree in order to compare the measurements with the simulations. This is one of the reasons why ESA has initiated the BepiColombo mission to Mercury in collaboration with the Japanese Aerospace Exploration Agency.

 

Fluxgate-Magnetometer
The IGEP uses such a magnetometer for the BepiColombo mission (tri-axial fluxgate magnetometer).

BepiColombo is going to be the first European Space mission to Mercury. The rocket launch is scheduled for 2015 from Korou in South America and only after 7 years of travel this mission will arrive. The mission consists of two sattelites:

  1. the Mercury Planetary Orbiter (MPO)
  2. the Mercury Magnetospheric Orbiter (MMO).

With these two, it is then possible to explore this enigmatic planet in detail. From an engineering point of view, the challenge is to provide instruments that are capable to cope with the temperature fluctuations in the vicinity to the Sun, because the distance to our star is only one third of that of the Earth – therefor the solar radiation is about 10 times stronger. One of the scientific instruments on board BepiColombo is a magnetometer from the TU Braunschweig, Institute for Geophysics and Extraterrestrial Physics (IGEP). This magnetometer was developed in collaboration with the Institute for Space Research, Graz, Austria and the Imperial College, London, UK.

Wie sehen die Konvektionsbewegungen im Planetenkern aus, die für das Magnetfeld verantwortlich sind? Dargestellt sind die Oberfläche des Planeten (grau) und die Konvektionszellen aus einem Computermodell (Isovolumen mit gleicher Z-Vortizität) an der Kern-Mantelgrenze (blau und rot).
How does convection look like inside the planetary core, that is responsible for the magnetic field? Here, the surface of the planet (grey) and the convective cells from a computer simulation (iso-volumes of equal z-vorticity) at the core-mantle boundary (blue and red) are shown.

At the IGEP, we do not only develop and build magnetometers but also model planetary magnetic fields. Because of the weak planetary magnetic field only a small magnetosphere is formed. This is also highly dynamic compared to the terrestrial one. The problem is that due to these highly varying magnetic fields it is challenging to separate spatial and temporal changes in the magnetosphere. Which part of the magnetic field has its origin in the Planetary core and which stems from the magnetosphere? This issue is followed by the researchers at the IGEP. The two satellites promise a great amount of data the separation problem can be tackled with. The NASA mission MESSENGER currently in orbit around Mercury consists only of a single space craft. Thus, the dataset will be as large as it is expected for the BepiColombo mission.

 

At the IGEP, also a special model explaining why Mercury possesses such a weak dipole field is developed. The magnetic field of the magnetosphere could also be the reason for the low magnetic field strength. Since the magnetic dipole field of the planet and the first degree magnetic field of the magnetosphere are inherently anti-parallel to each other – even after a polarity reversal – a negative feedback on the dynamo could arise. Researchers at the IGEP could demonstrate in a computer simulation that only a relatively weak external field from the magnetosphere is required to significantly alter the evolution of the dynamo. With the computer simulation, it was possible to derive a characteristic magnetic spectrum that can be compared to the BepiColombo data in the future.

 

Merkur Magnetosphäre
Schematic model of the feedback mechanism. Mercury is in the center. The crust is grey, the mantle dark red, the fluid outer core yellow and the solid inner core dark grey. The soloar wind enters from the lower left and interacts with the planetary dipole field (one field line is shown in green). As a result the magnetopause is created (blue paraboloid) on whiche the magnetopause currents flow (indicated with a white arrow). These current cause another magnetic field (red), that is anti-parallel directed to the internal field at the core-mantle boundary.

The BepiColombo-Team at the IGEP

  • Daniel Heyner (MPO-Mag Principal Investigator)
  • Hans-Ulrich Auster (Technical Manager)
  • Karl-Heinz Fornaçon (Sensor Development)
  • Daniel Heyner (Modelling of the Magnetosphere and the Dynamo, Experiment Tests)
  • Evelyn Liebert (Dataprocessing/-Modelling)
  • Christian Nabert (Dataprocessing/-Modelling)
  • Kai Okrafka (Experiment Tests, Data Archiving)
  • Ingo Richter (Calibration)
  • Bernd Stoll (Sensor Electronics)

Publications

  • Baumjohann, W., Matsuoka, A., Glassmeier, K.-H., Russell, C.-T., Nagai, T., Hoshino, M., Nakagawa, T., Balogh, A., Slavin, J.-A., Nakamura, R., Magnes, W., The magnetosphere of Mercury and its solar wind environment: Open issues and scientific questions, Advances in Space Research, 38, 604-609, doi:10.1016/j.asr.2005.05.117,  2006.
  • Baumjohann, W., Matsuoka, A. Magnes, W., Glassmeier, K.-H., Nakamura, R., Biernat, H., Delva, M., Schwingenschuh, K., Zhang, T., Auster, H.-U., Fornacon, K.-H., Richter, I., Balogh, A., Cargill, P., Carr, C., Dougherty, M., Horbury, T.-S., Lucek, E.-A., Tohyama, F., Takahashi, T., Tanaka, M., Nagai, T., Tsunakawa, H., Matsushima, M., Kawano, H., Yoshikawa, A., Shibuya, H., Nakagawa, T., Hoshino, M., Tanaka, Y., Kataoka, R., Anderson, B.-J., Russell, C.-T., Motschmann, U., Shinohara, M., Magnetic field investigation of Mercury's magnetosphere and the inner heliosphere by MMO/MGF, Planetary and Space Science, 58, 279-286, doi:10.1016/j.pss.2008.05.019,  2010.
  • Blomberg, L. G., J. A. Cumnock, K.-H. Glassmeier, and R. A. Treumann, Plasma Waves in the Hermean Magnetosphere, p. 393, 2008.
  • Glassmeier, K.-H., Currents in Mercury's Magnetosphere, in Washington DC American Geophysical Union Geophysical Monograph Series, Washington DC American Geophysical Union Geophysical Monograph Series, vol. 118, edited by S.-I. Ohtani, R. Fujii, M. Hesse, and R. L. Lysak, p. 371, 2000.
  • Glassmeier, K.-H., H.-U. Auster, and U. Motschmann, A feedback dynamo generating Mercurys magnetic field, Geophysical Research Letters, 34, L22201, doi:10.1029/2007GL031662,  2007a.
  • Glassmeier, K.-H., J. Grosser, U. Auster, D. Constantinescu, Y. Narita, and S. Stellmach, Electromagnetic Induction Effects and Dynamo Action in the Hermean System, Space Science Reviews, 132, 511-527, doi:10.1007/s11214-007-9244-9,  2007b.
  • Glassmeier, K.-H., Auster, H.-U,. Heyner, D., Okrafka, K., Carr, C., Berghofer, G., Anderson, B.-J., Balogh, A., Baumjohann, W., Cargill, P., Christensen, U., Delva, M., Dougherty, M., Fornacon, K.-H., Horbury, T.-S., Lucek, E.-A., Magnes, W., Mandea, M., Matsuoka, A., Matsushima, M., Motschmann, U., Nakamura, R., Narita, Y., O'Brien, H., Richter, I., Schwingenschuh, K., Shibuya, H., Slavin, J.-A Sotin, C., Stoll, B., Tsunakawa, H., Vennerstrom, S., Vogt, J., Zhang, T., The fluxgate magnetometer of the BepiColombo Mercury Planetary Orbiter, Planetary and Space Science, 58, 287-299, doi:10.1016/j.pss.2008.06.018,  2010.
  • Heyner, D., D. Schmitt, J. Wicht, K.-H. Glassmeier, H. Korth, and U. Motschmann, The initial temporal evolution of a feedback dynamo for Mercury, Geophysical and Astrophysical Fluid Dynamics, 104, 419-429, doi:10.1080/03091921003776839,  2010.
  • Heyner, D., D. Schmitt, K.-H. Glassmeier, and J. Wicht, Dynamo action in an ambient field, Astronomische Nachrichten, 332, 36, doi:10.1002/asna.201011466,  2011a.
  • Heyner, D., J. Wicht, N. Gomez-Perez, D. Schmitt, H.-U. Auster, and K.-H. Glassmeier, Evidence from Numerical Experiments for a Feedback Dynamo Generating Mercury's Magnetic Field, Science, 334, 1690, doi:10.1126/science.1207290,  2011b.
  • Heyner, D., K.-H. Glassmeier, and D. Schmitt, Stellar Wind Inuence on Planetary Dynamos, The Astrophysical Journal, 750, 133, doi:10.1088/0004-637X/750/2/133,  2012.
  • Müller, J., S. Simon, U. Motschmann, J. Schüle, K.-H. Glassmeier, and G. J. Pringle, A.I.K.E.F.: Adaptive hybrid model for space plasma simulations, Computer Physics Communications, 182, 946-966, doi: 10.1016/j.cpc.2010.12.033,  2011.
  • Olsen, N., K.-H. Glassmeier, and X. Jia, Separation of the Magnetic Field into External and Internal Parts, Space Science Reviews, 152, 135-157, doi:10.1007/s11214-009-9563-0,  2010.

Links

ESA BepiColombo

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