Field-Aligned Current Systems at Mercury

• Applicant: Dr. Daniel Heyner
• Subject Area: Geophysics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 397023160

• Final Report Year: 2022

Final Report Abstract

Mercury has a weak dipole field and is subject to a strong solar wind flow. This flow consists of charged particles that are deflected by the Hermean magnetic field. At the same time, this deflection changes in turn the magnetic field. A tiny and very dynamic magnetosphere is formed. Because of the stress that the solar wind flow imposes on the magnetosphere special kind of currents are driven that follow along the magnetic field lines along which the plasma particles can move easily. This type of current is called field-aligned current. The planetary magnetic field carries these field-aligned currents towards the planet. At Earth, these currents also exist and hit the ionosphere which allows these currents flow from the morning to the evening side. However, at Mercury, there is no such ionosphere and the question arises how the field-aligned currents are transported from one hemisphere to the other. It was commonly thought that the currents must actually flow inside the planetary body. Using computer simulations, we could show that a good part of the currents may actually flow over the polar region if the sodium ion content in the vicinity of the planet is high enough. These sodium particles are created by various generation mechanisms such as bombardment of small meteoroids or just energetic photons from the sun. Another thing that came as surprise at Mercury is the persistence of the field-aligned currents. At Earth, these currents only appear when the magnetic field that permeates the solar wind turns southward. This kind of behavior is strangely not observed at Mercury. But some doubts about the analysis remain. The in-situ MESSENGER probe could not observe both the solar wind and the magnetosphere at the same time. Thus, it was blind to the solar wind conditions when it entered the magnetosphere on each orbit. We could show that the dual satellite BepiColombo mission currently underway to Mercury will have better orbits to study these strange field-aligned currents in more detail. For the orbital mission phase, we also successfully tested new robust methods to determine both the currents as well as the internal field at the same time. We concluded the project with a study of extreme solar wind events that further compress the magnetosphere. During these, the current systems overlap and thus it will be hard to disentangle the individual currents observationally.

Publications

• (2018). Coronal mass ejection hits Mercury: A.I.K.E.F. hybrid-code results compared to MESSENGER data, Planetary and Space Science, 153, 89
  W. Exner and D. Heyner and L. Liuzzo and U. Motschmann and D. Shiota and K. Kusano and T. Shibayama
  
• (2020). Influence of Mercury’s exosphere on the structure of the magnetosphere, Journal of Geophysical Research: Space Physics, 125
  W. Exner and S. Simon and D. Heyner and U. Motschmann
  
• (2021). Cross-comparison of global simulation models applied to Mercury’s dayside magnetosphere, Planetary and Space Science, 198, 105176
  Aizawa, S., Griton, L. S., Fatemi, S., Exner, W., Deca, J., Pantellini, F., Yagi, M., Heyner, D., Génot, V., André, N., Amaya, J., Murakami, G., Beigbeder, L., Gangloff, M., Bouchemit, M., Budnik, E., Usui, H.
  
• (2021). The BepiColombo Planetary Magnetometer MPO-MAG: What Can We Learn from the Hermean Magnetic Field?, Space Science Reviews, 217(4), 52
  Heyner, D., Auster, H.-U., Fornaçon, K.-H., Carr, C., Richter, I., Mieth, J. Z. D., Kolhey, P., Exner, W., Motschmann, U., Baumjohann, W., Matsuoka, A., Magnes, W., Berghofer, G., Fischer, D., Plaschke, F., Nakamura, R., Narita, Y., Delva, M., Volwerk, M., Balogh, A., Dougherty, M., Horbury, T., Langlais, B., Mandea, M., Masters, A., Oliveira, J. S., Sánchez-Cano, B., Slavin, J. A., Vennerstrøm, S., Vogt, J., Wicht, J., Glassmeier, K.-H.
  
• (2021). The Mie representation for Mercury’s magnetospheric currents. Earth, Planets and Space, 73(1), 204
  Toepfer, S., Narita, Y., Exner, W., Heyner, D., Kolhey, P., Glassmeier, K. H., Motschmann, U.
  
• Modelling Mercury’s Magnetosphere Under Different Solar Wind Conditions, Doktorarbeit, Technische Universität Braunschweig, 2021
  W. Exner