SwRI IRD 2003--Pickup Ions Sources, Propagation, and Acceleration in the Heliosphere, 15-9352

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 km/s. Our sun is moving at about 26 km/s through a background of material called the interstellar medium. The interstellar medium is made up of material released from stars in our galaxy through stellar winds (like the solar wind), nova, and supernovae. 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 to 80–100 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, because they do not interact with the solar wind's magnetic fields. These neutrals therefore freely drift in toward the sun, prior to their ionization by solar UV radiation or charge-exchange with solar wind protons. Interstellar neutral hydrogen, for example, can penetrate to within three AU. Pickup ions are formed when interstellar neutral atoms become ionized, and are easily distinguished from the solar wind because 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 - Our research utilizes innovative theoretical techniques and several newly developed numerical simulations. At this point, we have developed and tested both 2.5D and 3D 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 2D, an octa-tree in 3D) 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. The 3D code utilizes parallel computing algorithms implemented with a set of Message Passing (MPI) functions.

Accomplishments - A recent discovery has added an important new source of pickup ions in the heliosphere, the "Outer Source" of pickup ions, which is needed to account for the presence of easily ionized material (e.g., Fe, C, Si) in Anomalous Cosmic Rays (pickup ions accelerated at the Termination Shock). The Outer Source, illustrated by blue lines and labels in Figure 2, is caused by small grains produced in collisions of Kuiper belt objects. The grains spiral inward toward the Sun (the slight inward drift is caused by the Poynting-Robertson effect). Sputtering and sublimation produces atoms that become ionized and picked up by the solar wind. These pickup ions then propagate with the solar wind to the termination shock where they become accelerated to form Anomalous Cosmic Rays. The outer source has a composition derived from the Kuiper belt grains. Because scattering due to gravitational interaction with planets removes most grains that drift inside of 10 AU, the source is generated between 10 and 50 AU.

Another recent discovery concerns the dominant sites of particle acceleration at the termination shock, which is a steady source of anomalous cosmic rays with energies 10–100 MeV/nucleon, among the most energetic particles generated in the heliosphere. So-called heliospheric "FALTS" (Favored Acceleration Locations at the Termination Shock; see Figure 3) are locations in the outer heliosphere where the magnetic field undergoes large-scale systematic departures from the standard spiral configuration thought for many years to be the only magnetic field configuration in the outer heliosphere. The FALTS field configurations lead to extremely efficient ion injection into particle acceleration at the termination shock, and therefore are the preferred sights where particles begin their acceleration to enormous energies by the shock.



Figure 2. An illustration of the physical processes involved in the production of Anomalous cosmic rays (ACRs). The yellow curves apply to the known interstellar source ACRs while the blue curves apply to the newly discovered outer source.

Figure 3. The three dimensional configurations of the magnetic fields in the distant interaction bands are indicated in the lower panels by black lines. The upper panel indicates the sense of footpoint motions and solar wind speeds measured by Ulysses as a function of latitude. Note that at mid latitudes there is a band where the solar wind speed steadily increases. It is in this band that magnetic field are subject to strong shearing by the solar wind, which has the effect of stretching out the magnetic fields and thereby form magnetic FALTS where particles become efficiently injected into ion acceleration at the Termination Shock. The left (right) lower panel applies for footpoint motions from fast (slow) wind into the slow (fast) solar wind. The streamline in slow wind is indicated by the red curve, and the streamline in fast wind is indicated by the blue curve.