Kinetic simulation of particle acceleration due to the Interaction of the Mercury magnetic field with the Solar wind

• Applicant: Professor Dr. Jörg Büchner
• Subject Area: Geophysics
• Term: from 2013 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 237376584

Due to the weak Hermean magnetic field and the close distance to the Sun the Solar wind interacts almost directly with almost all of Mercury's cross-section. This forms the most dynamic magnetosphere of all terrestrial planet and causes the generation of highly energetic particles. The Hermean magnetic field, plasma and particle environment was in situ investigated first by short Mariner 10 spacecraft flybys in 1974 and 1975. That already electrons with energies above 60 keV and protons with energies above 80 keV were discovered near Mercury, much above the particle energies in the upstream solar wind (1.5-10 keV). Since than the particle acceleration mechanism at Mercury is an unsolved miracle.Several mechanisms have been proposed to explain the observed high particle energies, e.g. reconnection in the Hermean magnetotail and acceleration at the Hermean bow shock wave, stochastic Ist and 2nd order Fermi acceleration. Due to the small spatial scales at Mercury compared to large Larmor radii and due to the short time scales of acceleration a proper description of the particle acceleration at Mercury requires a plasma kinetic approach. In this proposal we focus on the kinetic Simulation of particle acceleration due to reconnection in the Hermean magnetotail and shock acceleration.Previous investigations of Hermean particle acceleration were usually based on test particle calculations in prescribed magnetic and electric fields taken from a modified empirical models of the external Earth's magnetic field. In contrast we will start with Hermean magnetic fields models based on NASA's Messenger spacecraft observations just made after 2011 and currently under development, e.g. in the MPS Lindau dynamo group. For acceleration in the collisionless Hermean magnetotail plasma we will simulate the tail current sheets thinning down to the ion inertial length. In them the formation of the accelerating electric fields is a purely kinetic process, which we will treat accordingly. Due to the decoupling of electrons and ions the kinetic reconnection rate (electric field) is higher than for fluid (MHD) reconnection models.Already early models of Hermean bow shock particle acceleration models found that for the MHD Jump conditions the observed high energies cannot be explained. Current observations planetary bow shock waves have shown that they might become very thin, about a few electron inertial lengths (c/cope) and, therefore, a fraction of ion inertial length (c/o)pj) so that kinetic effects have to be considered to understand shock acceleration as well. While for the Earth's case ion acceleration was found to be also due to interactions in foreshock plasma "bubbles" their existence has still to be proven for the Mercury. Particle acceleration in them was so far treated only by hybrid simulations in which only the ions are treated kinetically and electrons äs a mass-less fluid. Since at Mercury the Alfven Mach number (MA~40) is much higher than for the Earth (MA<10) the conditions for shock and foreshock particle acceleration will be different from that at the other planetary shock waves.Hence we propose to treat the two assumed particle acceleration sites at Mercury, the Hermean magnetotail and its shock wave regions, fully kinetically. We plan to use first a 2D PIC code which we recently successfully applied to solar particle acceleration. In the course of the project we will apply also a massively parallelized 3D PIC Simulation code, developed in our group together with the Czech Academy of Sciences.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1488: Planetary Magnetism (PlanetMag)
• International Connection: Czech Republic
• Participating Persons: Dr. Miroslav Barta; Dr. Johannes Wicht
• Ehemaliger Antragsteller: Dr. Kuang Wu Lee