Simulation of Charge-Equilibration and Acceleration of Solar Energetic Ions

Recent measurements of the mean ionic charge states of solar energetic iron and silicon by SAMPEX and ACE during the large solar events of 1992 November 1 and 1997 November 6 show a mean ionic charge that increases with energy. This feature has implications for the use of the observed charge state as a probe of the coronal electron temperature and density, as well as for models of ion acceleration and transport in the coronal plasma. In this paper, we show results of a nonequilibrium model for the mean ionic charge that includes shock-induced acceleration in addition to charge-changing processes. The model is able to reproduce the general features observed without, however, specifying uniquely the acceleration time and the plasma electron density. Based on our simulations for iron and silicon for the 1992 and 1997 events, and assuming a characteristic shock-acceleration time of ~10 sec, our model suggests an equilibration-acceleration site at heights ~1 solar radius above the solar surface, a density ~10^9 cm^(–3), and an electron temperature ~1–1.33 MK. For ions with kinetic energy ≳ 30 MeV/nucleon we estimate the amount of coronal material the ions traverse to be ~100 µg/cm^2

Hydrogen-Impact Ionization Cross Sections in the Bates-Griffing Formalism

We describe a simple and effective procedure to estimate the hydrogen-impact ionization cross sections over an energy range relevant to studies of ACR heliospheric transport. The procedure is valid in the first Born approximation using known or estimated electron-impact cross sections. The original Bates-Griffing relation between the two sets of cross sections is reexpressed and a correction factor due to multiple transitions is introduced. Sample cross sections calculations for He, C, 0 and Ne collisions with hydrogen are presented

The Transition from Singly to Multiply-Charged Anomalous Cosmic Rays: Simulation and Interpretation of SAMPEX Observations

Multiply-charged anomalous cosmic rays (ACRs) can arise when singly-charged ACR ions are stripped of one or more of their electrons during their acceleration via, e.g., the process of diffusive shock-drift acceleration at the solar-wind termination shock. Recent measurements of the charge states of ACR neon, oxygen, and nitrogen by SAMPEX at 1 AU have shown that above ≈ 25 MeV/nucleon these ions are multiply charged. In addition, SAMPEX observations have also established that the transition from mostly singly-charged to mostly multiply-charged ACRs (defined as the 50% point) occurs at a total kinetic energy of ≈ 350 MeV. Preliminary simulations for ACR oxygen based on a theory of multiply-charged ACRs were able to show a transition energy at ≈ 300 MeV. However, the simulated intensity distribution among the various charge states was inconsistent with observations. This paper reexamines the predictions of the theory in light of new SAMPEX ACR observations and recently developed and refined estimates of hydrogen-impact ionization cross sections. Based on simulations for multi-species ACR ions, we find that the transition energy is only weakly dependent on characteristic transport parameters, and that the new ionization rates distribute the intensity among the charge states in a manner consistent with observations. The calculated transition energy is in excellent agreement with the measured value

Kuang's Semi-Classical Formalism for Calculating Electron Capture Cross Sections and Sample Application for ENA Modeling

Accurate estimates of electron-capture cross sections at energies relevant to ENA modeling (approx. few MeV per nucleon) and for multi-electron ions must rely on detailed, but computationally expensive, quantummechanical description of the collision process. Kuang's semi-classical approach is an elegant and efficient way to arrive at these estimates. Motivated by ENA modeling efforts, we shall briefly present this approach along with sample applications and report on current progress

Kuang's Semi-Classical Formalism for Calculating Electron Capture Cross Sections: A Space- Physics Application

Accurate estimates of electroncapture cross sections at energies relevant to the modeling of the transport, acceleration, and interaction of energetic neutral atoms (ENA) in space (approximately few MeV per nucleon) and especially for multi-electron ions must rely on detailed, but computationally expensive, quantum-mechanical description of the collision process. Kuang's semi-classical approach is an elegant and efficient way to arrive at these estimates. Motivated by ENA modeling efforts for apace applications, we shall briefly present this approach along with sample applications and report on current progress

Helium vs. Proton Induced Displacement Damage in Electronic Materials

In this project, the specific effects of displacement damage due to the passage of protons and helium nuclei on some typical electronic materials will be evaluated and contrasted. As the electronic material absorbs the energetic proton and helium momentum, degradation of performance occurs, eventually leading to overall failure. Helium nuclei traveling at the same speed as protons are expected to impart more to the material displacement damage; due to the larger mass, and thus momentum, of helium nuclei compared to protons. Damage due to displacement of atoms in their crystalline structure can change the physical properties and hence performance of the electronic materials

Kinetic and Potential Sputtering of Lunar Regolith: The Contribution of the Heavy Highly Charged (Minority) Solar Wind Ions

Solar wind sputtering of the lunar surface helps determine the composition of the lunar exosphere and contributes to surface weathering. To date, only the effects of the two dominant solar wind constituents, H+ and He+, have been considered. The heavier, less abundant solar wind constituents have much larger sputtering yields because they have greater mass (kinetic sputtering) and they are highly charged (potential sputtering) Their contribution to total sputtering can therefore be orders of magnitude larger than their relative abundances would sugges

Simulation of the Charge State and Energy Spectrum of Solar Energetic Iron

In a nonequilibrium model that includes shock-induced acceleration, we simulate the charge state and energy spectra of solar energetic iron. In this model the mean charge state exhibits an energy dependence seen in recent large solar events by SAMPEX and ACE. The simulated energy spectrum is a power-law and charge distributions are roughly Gaussians. The density distributions are smooth in charge-momentum space, suggesting that the accelerated ion retains no memory of its initial charge as well as momentum conditions

Cosmic-Ray Sources and Source Composition

Present data on cosmic-ray elemental and isotopic relative abundances are shown to be unable to distinguish between various models of cosmic-ray sources and their composition. For example, the model of freshly nucleosynthesized material from supernova explosions as the cosmic-ray source is unable to account for some measured, key cosmic-ray elemental abundances. This and two other models are evaluated here in light of recent isotopic and elemental measurements. It is shown that model-dependent preferential injection, acceleration, and reacceleration do not allow a clear distinction of one model against the others. Future measurements of critical elements and isotopes are suggested, which should afford us the ability to do that. We base our suggestions on measurements and a quantitative comparison between the predictions of the standard leaky-box model for the Galactic propagation of cosmic rays and one in which reacceleration is taken into account

The Exploration Atmospheres Working Group's Report on Space Radiation Shielding Materials

This part of Exploration Atmospheres Working Group analyses focuses on the potential use of nonmetallic composites as the interior walls and structural elements exposed to the atmosphere of the spacecraft or habitat. The primary drive to consider nonmetallic, polymer-based composites as an alternative to aluminum structure is due to their superior radiation shielding properties. But as is shown in this analysis, these composites can also be made to combine superior mechanical properties with superior shielding properties. In addition, these composites can be made safe; i.e., with regard to flammability and toxicity, as well as "smart"; i.e., embedded with sensors for the continuous monitoring of material health and conditions. The analysis main conclusions are that (1) smart polymer-based composites are an enabling technology for safe and reliable exploration missions, and (2) an adaptive, synergetic systems approach is required to meet the missions requirements from structure, properties, and processes to crew health and protection for exploration missions