Probing the Structure of Our Solar System’s Edge

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The boundary between the solar wind and the interstellar medium (ISM) at the distant edge of our solar system has been probed remotely and directly by spacecraft, but questions about its properties persist. What can models tell us about the structure of this region?

The Heliopause: A Dynamic Boundary

heliosphere

Schematic illustrating different boundaries of our solar system and the locations of the Voyager spacecraft. [Walt Feimer/NASA GSFC’s Conceptual Image Lab]

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. 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.

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. 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. In order to interpret the IBEX and Voyager observations, astronomers rely on complex models that must capture both global and local effects.

heliosphere model

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. [Adapted from Pogorelov et al. 2017]

Modeling the Edge of the Solar System

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.

MHD models have been used to understand many aspects of plasma flow in the heliosphere. However, they struggle to capture processes that are better described kinetically, like charge exchange or plasma instabilities. Fully kinetic models, on the other hand, are too computationally expensive to be used for global time-dependent simulations.

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.

Physics of the Border

Voyager 1 observations

Top: Simulation results for the plasma density observed by Voyager 1 along its trajectory. Bottom: Voyager 1 observations of plasma waves. An increase in the plasma wave frequency corresponds to an increase in the ambient plasma density. Click for a closer look. [Adapted from Pogorelov et al. 2017]

The authors varied the ISM’s density and magnetic field, exploring how this changed the interaction between the ISM and the solar wind. Among their many results, the authors found:

  1. There exists a plasma density drop and magnetic field strength increase in the ISM, just beyond the heliopause. This narrow boundary region is similar to a plasma depletion layer formed upstream from the Earth’s magnetopause as the solar wind streams around it.
  2. The authors’ model for the plasma density along the trajectory of Voyager 1 is consistent with the actual plasma density inferred from Voyager 1’s measurements.
  3. The heliospheric magnetic field likely dissipates in the region between the termination shock — the point at which the solar wind speed drops below the speed of sound — and the heliopause.

While this work by Pogorelov and collaborators has brought to light new aspects of the boundary between the solar wind and the ISM, the challenge of linking data and models continues. Future simulations will help us further interpret observations by IBEX and the Voyager spacecraft and advance our understanding of how our solar system interacts with the surrounding ISM.

Citation

N. V. Pogorelov et al 2017 ApJ 845 9. doi:10.3847/1538-4357/aa7d4f

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