The authors varied the ISM’s density and magnetic field, exploring how this changed the interaction between the ISM and the solar wind. An increase in the plasma wave frequency corresponds to an increase in the ambient plasma density. Bottom: Voyager 1 observations of plasma waves. Top: Simulation results for the plasma density observed by Voyager 1 along its trajectory. In order to combine the strengths of MHD and kinetic models, the authors also use adaptive mesh refinement - a technique in which the grid size is whittled down at key locations where small-scale physics can have a large effect - to resolve the important kinetic processes taking place at the heliopause while lowering the overall computational cost. Fully kinetic models, on the other hand, are too computationally expensive to be used for global time-dependent simulations. However, they struggle to capture processes that are better described kinetically, like charge exchange or plasma instabilities. MHD models have been used to understand many aspects of plasma flow in the heliosphere. In this study, Nikolai Pogorelov (University of Alabama in Huntsville) and collaborators use a hybrid magneto-hydrodynamical (MHD) and kinetic simulation to capture fully the physical processes happening in the outer heliosphere. Simulations of the plasma density in the meridional plane of the heliosphere due to the interaction of the solar wind with the ISM for the case of a relatively dense ISM with a weak magnetic field. In order to interpret the IBEX and Voyager observations, astronomers rely on complex models that must capture both global and local effects. IBEX makes global maps of the flux of neutral atoms, while Voyagers 1 and 2 record the plasma density and magnetic field parameters along their trajectories as they exit the solar system. Much of our understanding of the outer heliosphere and the local ISM comes from observations made by the International Boundary Explorer (IBEX) and the Voyager 1 and Voyager 2 spacecraft. The clash of plasmas generates a boundary region called the heliopause, the shape of which depends strongly on the properties of the solar wind and the local ISM. As our solar system travels through interstellar space, the magnetized solar wind flows outward and pushes back on the oncoming ISM, forming a bubble called the heliosphere. Schematic illustrating different boundaries of our solar system and the locations of the Voyager spacecraft.
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