Global Hybrid Modeling of Coherent ~1 Hz Waves at Mercury

Mercury’s magnetosphere is a small, yet highly dynamic and complex plasma environment that remains comparitively mysterious, as in-situ measurements have been limited in both duration and spatial coverage. Coherent magnetic ~1Hz waves were first observed during the first MESSENGER flyby, and were subsequently detected repeatedly over numerous orbits throughout the mission. These waves occur predominantly on closed field lines close to the magnetic equator, with spectral power peaking on the post-midnight side and frequencies peaking close to, but below, the local proton gyrofrequency. Their occurrence is reported in roughly 10-20% of MESSENGER orbits, with enhanced occurrence under northward Interplanetary Magnetic Field (IMF) conditions, while wave activity appears to be suppressed during dipolarisation events and on recently-closed field lines. Together, these factors indicate a generation/growth mechanism that is highly sensitive to magnetic topology and local plasma conditions. The underlying physics of these waves remains uncertain, and effects on their distribution and occurrence rate are not well understood^[1].

Here, we analyse global hybrid simulations of Mercury’s magnetospheric solar wind interaction, performed using RHybrid (paRallel Hybrid), an open-source kinetic particle-in-cell model platform for the simulation of planetary plasma environments^{[2, 3, 4]}. The model combines kinetically propagated clouds of ions (macroparticles) with a charge-neutralising, massless fluid representation of electrons, enabling efficient resolution of large-scale magnetospheric processes while preserving ion dynamics and, for example, finite gyroradius, nonthermal, and anisotropic effects self-consistently coupled with the evolution of the magnetic field.

We employ RHybrid to investigate the occurrence, spatial distribution, and characteristics of ~1Hz wave activity in Mercury’s inner magnetosphere under varying upstream conditions. We identify ~1Hz wave events within the RHybrid simulation space through frequency analysis along simulated trajectories and regions, and contrast the simulated properties and appearance of the waves against representative MESSENGER observations. The 3-dimensional occurrence and power distribution of the simulated waves is analysed and compared with the distributions observed by MESSENGER, to assess how IMF orientation and magnetic topology affects ~1Hz wave activity. We categorise simulated occurrences according to the degree of field-line connectivity, and a possible association between the ~1Hz waves and non-linear Kelvin-Helmholtz instabilities is also tested.

These analyses provide a prediction of where and under which solar-wind conditions BepiColombo’s MPO and Mio are most likely to observe ~1Hz wave activity. The model-based interpretation, combined with the future joint observations of MPO and Mio and the preexisting MESSENGER observations, will likely improve constraints on ~1Hz wave generation and growth in Mercury’s magnetosphere. Future analysis will lead to a far more complete understanding of the formation and influence of these ~1Hz waves.

References

[1] – Boardsen, Scott A., et al. "Survey of coherent∼ 1 Hz waves in Mercury's inner magnetosphere from MESSENGER observations." Journal of Geophysical Research: Space Physics 117.A12 (2012), doi:10.1029/2012ja017822

[2] – https://github.com/fmihpc/rhybrid, https://planets.fmi.fi/

[3] – Jarvinen R., Alho M., Kallio E., Pulkkinen T.I., 2020, Ultra-low frequency waves in the ion foreshock of Mercury: A global hybrid modeling study, Mon. Not. R. Astron. Soc., 491, 3, 4147-4161, doi:10.1093/mnras/stz3257

[4] – Kallio E., Jarvinen R., Massetti S., Alberti T., Milillo A., Orsini S., De Angelis E., Laky G., Slavin J., Raines J.M., Pulkkinen T.I., 2022, Ultra-low frequency waves in the Hermean magnetosphere: On the role of the morphology of the magnetic field and the foreshock, Geophys. Res. Lett. 49, 24, doi:10.1029/2022GL101850