Pickup Ions Sources, Propagation, and Acceleration in the Heliosphere, 15-9352Printer Friendly Version
Inclusive Dates: 10/01/02 - 04/01/04
Background - The sun's upper atmosphere (the low corona) reaches enormous temperatures, several million degrees. These high temperatures cause complete ionization of the gas and the release of the solar wind - a supersonic flow of ionized atoms (a plasma) with speeds typically between 400 and 800 kilometers per second (km/s). Our sun is moving at approximately 26 km/s through a background of material called the interstellar medium. The solar wind carves out a cavity (see Figure 1) from the interstellar medium close to the sun. This immense cavity (the termination shock in Figure 1), extending from 80 to 100 astronomical units (AU, with 1 AU the distance from the sun to Earth), and its surrounding boundaries (the heliopause and bow shock) is called the heliosphere. The solar wind contains ions and electromagnetic fields swept out faster than sound (supersonically) to fill the heliosphere. Ions from the interstellar medium are deflected away beyond the heliopause and prevented from entering the inner heliosphere. Interstellar neutral atoms, however, travel freely through the solar wind, since they do not interact with the solar winds' magnetic fields. These neutrals, therefore, freely drift in toward the sun, prior to their ionization by solar ultraviolet radiation or charge-exchange with solar wind protons. Interstellar neutral hydrogen, for example, can penetrate to within 3 AU. Pickup ions are formed when interstellar neutral atoms become ionized, and are easily distinguished from the solar wind as a result of their high initial energies, their subsequent acceleration to form high-energy particles, and their single charge (solar wind ions are highly charged, and for many elements are almost fully stripped of their electrons). This project focuses on the properties of pickup ions; how they may be used to understand properties of the interstellar medium, and properties of the heliosphere.
Approach - The research utilizes innovative theoretical techniques and several newly developed numerical simulations. At this point, we have developed and tested both 2.5-D and 3-D magneothydrodynamic (MHD) codes for the description of solar wind evolution and its coupling to pickup ion evolution. The numerical method is based on a conservative shock-capturing scheme. Adaptive mesh refinement (AMR, based on a quadra-tree in 2-D, an octa-tree in 3-D) is implemented, allowing regions of interest and complexity to be computed with greater accuracy. The refinement can be performed dynamically during run time. The code is written in C++ using an object oriented approach.
Accomplishments - The internal research has been enormously successful, motivating not only new scientific results, but also allowing the development of a vital new MHD modeling capability that will provide considerable new opportunities and bridge new partnerships in years to come. We summarize here some of the new scientific opportunities explored in the internal research program.