Andreas Ekenbäck and Mats Holmström
Swedish Institute of Space Physics (IRF)
P.O. Box 812
SE-981 28 Kiruna, Sweden
emails: andreas@irf.se and matsh@irf.se
The solar wind is a plasma consisting mostly of protons and electrons that are ejected from the Sun's corona, and streams radially outward through the solar system at speeds of several hundred kilometers per second. Embedded into the solar wind is also the interplanetary magnetic field. When this highly supersonic solar wind meets with solar system objects, such as planets, comets and asteroids, an interaction region is formed. The type of the interaction depends on the object. Around planets with a strong internal magnetic field, such as earth, a cavity is formed, a magnetosphere, shielding the upper atmosphere from direct interaction with the solar wind. This shielding is missing at planets without a strong intrinsic field, e.g., Mars. Then currents in the planet's ionosphere forms an obstacle that diverts the solar wind flow. Objects without any significant atmosphere, such as the Moon, are basically just physical obstacles to the flow, and a wake is formed behind the object. Comets have a very small nucleus compared to the size of the surrounding cloud of ions and neutrals and their interaction with the solar wind is governed by the production of photo-ions from neutrals.
In space physics, the numerical modeling of the interaction between the solar wind and solar system objects is an important tool in understanding the physics of the interaction region. The results of simulations is also important as inputs to further modeling, e.g., to predict loss of planetary atmospheres due to the interaction.
Although self consistent particle and kinetic models best capture the actual physics, the computational cost limits the number of particles and the refinement of grids. Since the computational cost of using a fluid model is less, more refined grids can be used. Thus, the modeling of the interaction between the solar wind and solar system objects is often done by using magnetohydrodynamic (MHD) fluid models.
In this work we investigate the possibility of using the open source FLASH code (University of Chicago) as a tool for modeling the interaction between the solar wind and solar system objects. The FLASH code is a parallel MHD solver on a Cartesian adaptive grid. By using this generic MHD code, we can benefit from all the testing, development and performance tuning that has been done. What we add is the specific initial conditions, boundary conditions and source terms for the problem at hand.
We present results from the simulation of the solar wind interaction with a comet and discuss performance, accuracy and implementation details. We also discuss how the FLASH code can be adapted and extended to the solar wind interaction with the other types of solar system objects.