The Genesis Solar-Wind Collector Materials

Genesis (NASA Discovery Mission #5) is a sample return mission. Collectors comprised of ultra-high purity materials will be exposed to the solar wind and then returned to Earth for laboratory analysis. There is a suite of fifteen types of ultra-pure materials distributed among several locations. Most of the materials are mounted on deployable panels (‘collector arrays’), with some as targets in the focal spot of an electrostatic mirror (the ‘concentrator’). Other materials are strategically placed on the spacecraft as additional targets of opportunity to maximize the area for solar-wind collection. Most of the collection area consists of hexagonal collectors in the arrays; approximately half are silicon, the rest are for solar-wind components not retained and/or not easily measured in silicon. There are a variety of materials both in collector arrays and elsewhere targeted for the analyses of specific solar-wind components. Engineering and science factors drove the selection process. Engineering required testing of physical properties such as the ability to withstand shaking on launch and thermal cycling during deployment. Science constraints included bulk purity, surface and interface cleanliness, retentiveness with respect to individual solar-wind components, and availability. A detailed report of material parameters planned as a resource for choosing materials for study will be published on a Genesis website, and will be updated as additional information is obtained. Some material is already linked to the Genesis plasma data website (genesis.lanl.gov). Genesis should provide a reservoir of materials for allocation to the scientific community throughout the 21st Century. This revised version was published online in August 2006 with corrections to the Cover Date.     

Numerical Simulation of the Ring Current: Review

Numerical simulation of the terrestrial ring current is reviewed. After mentioning ‘modules’ which are needed to be taken into consideration in a ring current simulation, we discuss growth and decay of the ring current. At least four different paradigms have been proposed to account for the ring current development in the past forty years, i.e., the convection paradigm, the substorm paradigm, the diffusion paradigm, and the ionosphere paradigm. As for the proton ring current, a simulation under the convection paradigm gives reasonable results which are in fair agreement with observations with respect to the Dst variation as well as the radial and longitudinal energy density variation of protons when the convection electric field depending on solar wind parameters is given. The proton energy density is observed to be enhanced (weakened) on the nightside, and be weakened (enhanced) near noon during a storm main phase (recovery phase). This characteristic is probably understood to mean that a large-scale and long-standing electric field dominates other electric fields during the storm main phase, e.g., a locally induced electric field (the substorm paradigm) and a highly fluctuated electric field (the diffusion paradigm). The declining of the ring current is shown to be triggered by the decrease in the convection electric field at the beginning of a storm recovery phase, but the decrease in the convection electric field hardly contributes the decay of the ring current. The charge exchange or other loss processes is needed for the substantial decay of it. An ultimate decay rate (several hours) is achieved when the strong diffusion takes place, or when the plasma sheet density drastically decreases while the charge exchange is estimated to provide rather slow decay (a half of day). Diagnosis tools for investigating the ring current, which are expected to bring us a new insight, are proposed in the latter section. This revised version was published online in June 2006 with corrections to the Cover Date.