Charge State Ratios Research Articles

A model Ni-Al-Mo superalloy studied by ultraviolet pulsed-laser-assisted local-electrode atom-probe tomography.

This study investigates the effects of the charge-state ratio of evaporated ions on the accuracy of local-electrode atom-probe (LEAP) tomographic compositional and structural analyses, which employs a picosecond ultraviolet pulsed laser. Experimental results demonstrate that the charge-state ratio is a better indicator of the best atom-probe tomography (APT) experimental conditions compared with laser pulse energy. The thermal tails in the mass spectra decrease significantly, and the mass resolving power (m/Δm) increases by 87.5 and 185.7% at full-width half-maximum and full-width tenth-maximum, respectively, as the laser pulse energy is increased from 5 to 30 pJ/pulse. The measured composition of this alloy depends on the charge-state ratio of the evaporated ions, and the most accurate composition is obtained when Ni2+/Ni+ is in the range of 0.3-20. The γ(f.c.c.)/γ'(L12) interface is quantitatively more diffuse when determined from the measured concentration profiles for higher laser pulse energies. Conclusions of the APT compositional and structural analyses utilizing the same suitable charge-state ratio are more comparable than those collected with the same laser pulse energy.

SUPRATHERMAL ELECTRONS IN THE SOLAR CORONA: CAN NONLOCAL TRANSPORT EXPLAIN HELIOSPHERIC CHARGE STATES?

There have been several ideas proposed to explain how the Sun's corona is heated and how the solar wind is accelerated. Some models assume that open magnetic field lines are heated by Alfven waves driven by photospheric motions and dissipated after undergoing a turbulent cascade. Other models posit that much of the solar wind's mass and energy is injected via magnetic reconnection from closed coronal loops. The latter idea is motivated by observations of reconnecting jets and also by similarities of ion composition between closed loops and the slow wind. Wave/turbulence models have also succeeded in reproducing observed trends in ion composition signatures versus wind speed. However, the absolute values of the charge-state ratios predicted by those models tended to be too low in comparison with observations. This letter refines these predictions by taking better account of weak Coulomb collisions for coronal electrons, whose thermodynamic properties determine the ion charge states in the low corona. A perturbative description of nonlocal electron transport is applied to an existing set of wave/turbulence models. The resulting electron velocity distributions in the low corona exhibit mild suprathermal tails characterized by "kappa" exponents between 10 and 25. These suprathermal electrons are found to be sufficiently energetic to enhance the charge states of oxygen ions, while maintaining the same relative trend with wind speed that was found when the distribution was assumed to be Maxwellian. The updated wave/turbulence models are in excellent agreement with solar wind ion composition measurements.

COHERENT STRUCTURE IN SOLAR WIND C6 +/C4 +IONIC COMPOSITION DATA DURING THE QUIET-SUN CONDITIONS OF 2008

This analysis offers evidence of characteristic scale sizes in solar wind charge state data measured in-situ for thirteen quiet-sun Carrington rotations in 2008. Using a previously established novel methodology, we analyze the wavelet power spectrum of the charge state ratio C$^{6+}$/C$^{4+}$ measured in-situ by ACE/SWICS for 2-hour and 12-minute cadence. We construct a statistical significance level in the wavelet power spectrum to quantify the interference effects arising from filling missing data in the time series (Edmondson et al. 2013), allowing extraction of significant power from the measured data to a resolution of 24 mins. We analyze each wavelet power spectra for transient coherency, and global periodicities resulting from the superposition of repeating coherent structures. From the significant wavelet power spectra, we find evidence for a general upper-limit on individual transient coherency of $\sim$10 days. We find evidence for a set of global periodicities between 4-5 hours and 35-45 days. We find evidence for the distribution of individual transient coherency scales consisting of two distinct populations. Below the $\sim$2 day time scale the distribution is reasonably approximated by an inverse power law, whereas for scales $\gtrsim$2 days, the distribution levels off showing discrete peaks at common coherency scales. In addition, by organizing the transient coherency scale distributions by wind type, we find these larger, common coherency scales are more prevalent and well-defined in coronal hole wind. Finally, we discuss the implications of our results for current theories of solar wind generation and describe future work for determining the relationship between the coherent structures in our ionic composition data and the structure of the coronal magnetic field.

CHARGE STATE EVOLUTION IN THE SOLAR WIND. II. PLASMA CHARGE STATE COMPOSITION IN THE INNER CORONA AND ACCELERATING FAST SOLAR WIND

In the present work, we calculate the evolution of the charge state distribution within the fast solar wind. We use the temperature, density, and velocity profiles predicted by Cranmer et al. to calculate the ionization history of the most important heavy elements in the solar corona and solar wind: C, N, O, Ne, Mg, Si, S, and Fe. The evolution of each charge state is calculated from the source region in the lower chromosphere to the final freeze-in point. We show that the solar wind velocity causes the plasma to experience significant departures from equilibrium at very low heights, well inside the field of view (within 0.6 R sun from the solar limb) of nearly all the available remote-sensing instrumentation, significantly affecting observed spectral line intensities. We also study the evolution of charge state ratios with distance from the source region, and the temperature they indicate if ionization equilibrium is assumed. We find that virtually every charge state from every element freezes in at a different height, so that the definition of freeze-in height is ambiguous. We also find that calculated freeze-in temperatures indicated by charge state ratios from in situ measurements have little relation to the local coronal temperature of the wind source region, and stop evolving much earlier than their correspondent charge state ratio. We discuss the implication of our results on plasma diagnostics of coronal holes from spectroscopic measurements as well as on theoretical solar wind models relying on coronal temperatures.

IONIC COMPOSITION STRUCTURE OF CORONAL MASS EJECTIONS IN AXISYMMETRIC MAGNETOHYDRODYNAMIC MODELS

We present the ionic charge state composition structure derived from axisymmetric MHD simulations of coronal mass ejections (CMEs), initiated via the flux-cancellation and magnetic breakout mechanisms. The flux-cancellation CME simulation is run on the Magnetohydrodynamics-on-A-Sphere code developed at Predictive Sciences, Inc., and the magnetic breakout CME simulation is run on ARC7 developed at NASA GSFC. Both MHD codes include field-aligned thermal conduction, radiative losses, and coronal heating terms which make the energy equations sufficient to calculate reasonable temperatures associated with the steady-state solar wind and model the eruptive flare heating during CME formation and eruption. We systematically track a grid of Lagrangian plasma parcels through the simulation data and calculate the coronal density and temperature history of the plasma in and around the CME magnetic flux ropes. The simulation data are then used to integrate the continuity equations for the ionic charge states of several heavy ion species under the assumption that they act as passive tracers in the MHD flow. We construct two-dimensional spatial distributions of commonly measured ionic charge state ratios in carbon, oxygen, silicon, and iron that are typically elevated in interplanetary coronal mass ejection (ICME) plasma. We find that the slower CME eruption has relatively enhanced ionic charge states and the faster CME eruption shows basically no enhancement in charge states—which is the opposite trend to what is seen in the in situ ICME observations. The primary cause of the difference in the ionic charge states in the two simulations is not due to the different CME initiation mechanisms per se. Rather, the difference lies in their respective implementation of the coronal heating which governs the steady-state solar wind, density and temperature profiles, the duration of the connectivity of the CME to the eruptive flare current sheet, and the contribution of the flare-heated plasma associated with the reconnection jet outflow into the ejecta. Despite the limitations inherent in the first attempt at this novel procedure, the simulation results provide strong evidence in support of the conclusion that enhanced heavy ion charge states within CMEs are a direct consequence of flare heating in the low corona. We also discuss future improvements through combining numerical CME modeling with quantitative ionic charge state calculations.

Pulsed-laser atom probe studies of a precipitation hardened maraging TRIP steel

A precipitation hardened maraging TRIP steel was analyzed using a pulsed laser atom probe. The laser pulse energy was varied from 0.3 to 1.9 nJ to study its effect on the measured chemical compositions and spatial resolution. Compositional analyses using proximity histograms did not show any significant variations in the average matrix and precipitate compositions. The only remarkable change in the atom probe data was a decrease in the ++/+ charge state ratios of the elements. The values of the evaporation field used for the reconstructions exhibit a linear dependence on the laser pulse energy. The adjustment of the evaporation fields used in the reconstructions for different laser pulse energies was based on the correlation of the obtained cluster shapes to the TEM observations. No influence of laser pulse energy on chemical composition of the precipitates and on the chemical sharpness of their interfaces was detected.

Soft-x-ray fragmentation studies of molecular ions

Imaging of photofragments from molecular ions after irradiation by soft x-ray photons has been realized at the ion beam infrastructure TIFF set up at the FLASH facility. Photodissociation of the two-electron system HeH+ at 38.7 eV revealed the electronic excitations and the charge-state ratios for the products of this process, reflecting the non-adiabatic dissociation dynamics through multiple avoided crossings among the HeH+ Rydberg potential curves. Dissociative ionization of the protonated water molecules H3O+ and H5O+2 at 90 eV revealed the main fragmentation pathways after the production of valence vacancies in these ionic species, which include a strong three-body channel with a neutral fragment (OH + H+ + H+) in H3O+ photolysis and a significant two-body fragmentation channel (H3O++ H2O+) in H5O+2 photolysis. The measurements yield absolute cross sections and fragment angular distributions. Increased precision and sensitivity of the technique were realized in recent developments, creating a tool for exploring x-ray excited molecular states under highly controlled target conditions challenging detailed theoretical understanding.

Solar wind elemental abundances related to the Sun's open magnetic flux

Context. We present an investigation of the effects of solar magnetic field on the elemental abundance ratios of heavy ions in the solar wind.Aims. We compare the elemental abundance ratios Fe/O, Si/O, Mg/O, Ne/O, and C/O over the 23rd solar cycle, attention being given to the varying fraction of open magnetic flux on the visible side of the solar disc.Methods. We use ACE solar wind data, the current sheet source surface (CSSS) model of the corona, and SoHO/MDI data to relate solar wind compositional properties with the fraction of open magnetic flux on the Sun. We include both a quasi-stationary and intermittent solar wind.Results. For the six investigated solar wind speed ranges, the elemental composition shows a similar FIP-bias of the quasi-stationary solar wind composition with respect to the photosphere but with three plateaus separating low-, mid-, and high-FIP elements. The average enrichment factor for the low-FIP element group (Mg, Fe, Si) with respect to the photospheric values is between 1.67 and 2.47 when the fraction of open magnetic flux on the visible side of the solar disc is above 14%, and it ranges from 2.78 to 4.15 when this fraction is less than 7%. They are close to the enrichment of low-FIP elements with respect to the photospheric values in quasi-stationary fast- and slow- solar wind, which implies similar fractionation mechanisms for the stationary solar wind and the intermittent solar wind. In the low alpha ( is the fraction of open magnetic flux on the visible side of the solar disc), the enrichment of heavy elements for the fast solar wind is well correlated with the high fraction of CMEs. In addition, both the charge state ratios and the elemental abundance ratios exhibit a lower dependence on alpha in a slow solar wind than in a fast solar wind. The dependence of the charge state ratios on alpha is correlated with FIP, which implies that solar magnetic effects play a non-negligible role in the elemental fractionation of the solar wind. We suggest that lower hybrid waves play an important role in the possible linkage between solar magnetic field and elemental fractionation.

Large-angle xenon ion scattering in Xe-propelled electrostatic thrusters: differential cross sections

Elastic scattering between xenon ions and xenon atoms can produce ion currents at large angles with respect to the axis of electrostatic thrusters. Differential scattering cross sections are needed to properly predict off-axis currents that can cause significant material erosion due to sputtering. Guided-ion beam differential cross section measurements are presented for Xe+ + Xe and Xe2+ + Xe elastic scattering at laboratory ion energies between 5 and 40 eV per ion charge. For the singly charged system, the experimental absolute differential cross sections are in excellent agreement with classical elastic scattering calculations based on the most recent ab initio ion–atom interaction potentials. The measurements for the doubly charged system are used to derive an approximate effective Xe2+–Xe interaction potential. The potentials are used to calculate absolute differential cross sections for both ion charge states at a typical Hall thruster ion energy of 270 eV per unit charge. The differential cross sections for the doubly charged ions are approximately a factor of 3 smaller than those of the singly charged system at large scattering angles. The importance of doubly charged ions with respect to material erosion is discussed on the basis of known sputtering yields as a function of ion energy for molybdenum and boron nitride. It is concluded that at typical charge-state ratios, doubly charged ions only have an impact at elastic scattering angles where the scattered ion energy in the laboratory (thruster) frame of reference is low and the sputtering yields depend very strongly on ion kinetic energy.

Composition and magnetic structure of interplanetary coronal mass ejections at 1 AU

We study the magnetic structure and charge state ratio of interplanetary coronal mass ejections (ICMEs) observed by ACE and Wind spacecraft. Measurements of abundances and charge state ratio of heavy ions (e.g. O 7+/O 6+, C 6+/C 5+, and Mg 10+/O 6+) in the plasma as well as magnetic field structure are important tracers for physical conditions and processes in the source regions of ICMEs. We used ion composition (from ACE), plasma (from Wind) and magnetic field (from Wind and ACE) data from 1998 to 2002. Using the low proton temperature criterion, a common plasma signature of ICMEs, we identified 154 events which include magnetic clouds, non-cloud ejecta and complex ICMEs. The latter one refers to compound events resulting from the overtaking of successive ICMEs which can include both magnetic clouds and non-cloud ejecta. We find that there is a close relationship between the increase in the charge state ionization factor and the magnetic structure of ICMEs. Events with magnetic cloud topology show higher Q O 7 + / O 6 + and Q Mg 10 + / O 6 + charge state ratios than those with non-magnetic cloud structure. However, both magnetic cloud and non-cloud events show an increase in these ratios when compared with the ambient solar wind. In contrast, perhaps due to instrumental effects, the charge state ratio Q C 6 + / C 5 + for all events does not show a real enhancement when compared with the ambient solar wind. The difference in ionization states between non-cloud ejecta and magnetic clouds is more pronounced in fast solar wind than when events are embedded in slow wind.

Correlation of solar wind entropy and oxygen ion charge state ratio

Both proton specific entropy and solar wind composition have been recognized in the past as markers of boundaries between what was originally slow and fast solar wind during the declining phase of the solar cycle, when the solar wind alternates between the two regimes. During the rising phase, when boundaries between regimes are not apparent, ACE SWICS and SWEPAM data from 1998–1999 show that O7+/O6+ and proton specific entropy are well‐correlated over the full range of complicated time variations. The correlation holds in spite of the fact that unlike O7+/O6+, entropy is not a constant of the solar wind flow. At solar maximum however, particularly in 2000, the correlation between entropy and O7+/O6+ degrades. While the correlation inside known interplanetary coronal mass ejections (ICMEs) is much worse throughout 1998–2001, the correlation outside ICMEs also worsens at solar maximum, possibly owing to unidentified transient outflows. Outside ICMEs and shocks, entropy structures have decorrelation times of 5–40 hours and both ln(O7+/O6+) and entropy have Gaussian distributions, consistent with their correlation. We conclude that except at solar maximum, the processes affecting entropy in nontransient solar wind act at time scales much smaller than the scales found here and that entropy is a good a proxy for O7+/O6+.

Triple ionization of lithium by electron impact.

Ejection of the three electrons from lithium in a single electron collision has been observed for the first time. Triply charged lithium was observed in an ion time-of-flight spectrum following electron impact on a sample of ultracold, trapped lithium. The higher signal/background afforded by the trap environment made the observation of Li3+ possible. We measured the ratios of triple-to-double and double-to-single ionization at an impact energy of 1000 eV. The 3+/2+ ratio is approximately 0.001, a value 2 orders of magnitude lower than semiempirical predictions. We present a simple method that uses photoionization data combined with sum-rule analysis to predict the asymptotic charge-state ratios. The sum-rule predictions compare reasonably with experiment and shake calculations, but disagree sharply with the semiempirical estimates.

The Solar Wind and Its Magnetic Sources at Sunspot Maximum

We use in situ measurements from the Advanced Composition Explorer and magnetograph data from the National Solar Observatory to relate the properties of the solar wind during 1998-2002 to its source region magnetic fields. The great bulk of the solar maximum wind is characterized by low proton speeds (vp ~ 420 km s-1) and high oxygen charge state ratios (nO7+/nO6+ ~ 0.3). This slow wind originates from small, sheared open-field regions located near active regions and characterized by very large flux tube expansion factors (fexp 10) and high footpoint field strengths (B0 ~ 30 G). In contrast, the occasional high-speed streams emanate from weak-field regions (B0 ~ 5 G) with small expansion factors (fexp ~ 4) and show relatively low charge state ratios (nO7+/nO6+ ~ 0.1); their proton velocities (vp ~ 550 km s-1) are substantially reduced by interactions with the surrounding sea of low-speed wind. We attribute the high freeze-in temperatures of the slow wind to enhanced heating taking place in the low corona in the presence of the very strong, rapidly diverging source fields, which are found to be correlated with high mass and energy flux densities at the coronal base.

Interplanetary and solar surface properties of coronal holes observed during solar maximum

Data from the Solar Wind Ion Composition Spectrometer (SWICS) on board the Ulysses spacecraft and synoptic maps from Kitt Peak are used to analyze the relatively short‐lived coronal holes which exist during the maximum phase of the solar activity cycle 23. They are compared with the persistent polar coronal holes which prevail around solar minimum. A solar wind velocity increase coinciding with a shift of the ionic charge composition toward lower charge states serves as a robust criterion for identifying solar wind streams emanating from solar maximum holes. This allows an unambiguous association of every stream identified in interplanetary space with a coronal hole on the solar surface with consistent magnetic polarity. Solar wind streams emanating from the solar maximum holes generally show lower velocities of 400 to 600 km/s compared to the polar hole stream velocities of 700 to 800 km/s. However, the SWICS O7+/O6+ charge‐state ratios, which are a proxy for coronal temperatures, do not reveal a consistent difference. Though a number of solar maximum holes have a significantly, up to three times, higher temperature compared to the polar coronal holes, the majority of the investigated holes and specifically those with new cycle polarity have a coronal temperature within the range of polar hole temperatures. Likewise, the magnetic flux density in the solar maximum holes and in the polar coronal holes, as derived from the synoptic maps, is not strikingly different. Therefore any intrinsic difference between solar maximum holes and polar coronal holes is small. The striking discrepancy in their kinetic properties, namely the slower velocity of the solar wind streams emanating from solar maximum holes, may partly be attributed to deceleration of the solar wind during propagation to the spacecraft. The discrepancy may also be influenced by active regions in close proximity to the coronal holes, which presumably is more likely for smaller holes. There may, however, be a tendency for the faster wind streams to emanate from cooler holes.

Calibration of the Galileo microchannel plates with the Xe2+–Xe13+ and C2+–C6+ ions in the energy range from 0.5 to 150 keV/q (abstract)

The procedure of calibration of the detector assembly consisting of the two Galileo microchannel plates (MCPs) operated in a chevron configuration is described. The current gains and the analog particle gains of MCPs for Xe ions with charge states from q=3+ to q=13+ and C ions, with charge states from q=1+ to q=6+ and ion impact energies to charge state ratios from 0.5 to 150 keV/q have been measured. These results were compared to the earlier results obtained of calibration of this detector assembly with Xe ions with the charge states from q=7+ to q=43+ and ion impact energies to charge state ratios from 2 to 154 keV/q. We have shown the areas of ion parameters in which the secondary ion-electron emission coefficient of the investigated MCPs was dominated by kinetic or potential emission.

Differences in the O7+/O6+ ratio of magnetic cloud and non‐cloud coronal mass ejections

On its trajectory to Jupiter and over the poles of the Sun the Ulysses spacecraft has observed a considerable number of Coronal Mass Ejection (CME) transients in slow and in fast solar wind streams. The analysis of the magnetic field topology and the O7+/O6+ charge state ratio of 56 of these events has yielded strong evidence for a systematic connection between the two features. Coronal mass ejections with magnetic cloud structure have an increased O7+/O6+ ratio with respect to the ambient solar wind whereas non‐cloud CMEs do not show enhanced O7+/O6+ ratios. We discuss possible mechanisms based on the freezing‐in concept that can account for the observation.

Concurrent observations of solar wind oxygen by Geotail in the magnetosphere and wind in interplanetary space

We present an initial report on the first direct, concurrent measurements of solar wind oxygen (O) charge state variations inside and outside Earth's magnetosphere. On 10‐11 Jan 1997 a magnetic cloud, with a spatial scale of at least ∼100 RE engulfed the magnetosphere. At ∼1200 UT on 10 Jan 1997, the Wind/SWICS ion charge‐state spectrometer, at ∼90 RE upstream of Earth, detected an increase in the solar wind O charge state ratio Γo = (O+7 + O+8) / O+6 at ∼0.5‐30 keV/e which persisted until ∼2400 UT on 10 Jan 1997. After ∼1110 UT on 10 Jan 1997, Geotail was in and near the mid‐afternoon to dusk magnetosphere. The ∼9.4‐210 keV/e Γo value from the Geotail/STICS spectrometer showed a longer‐term, gradual increase over ∼8–10 hr. Then, after ∼0600 UT on 11 Jan 1997, Wind observed a Γo decrease at a high speed solar wind stream onset. About 6 hr later Γo decreased promptly at Geotail during a plasma sheet substorm injection. Both instances are long durations compared to nominal 3‐hr substorm time‐scales.

Beta-decay experiments of neutral atoms in a magneto-optical trap

Magneto optical traps (MOT) allow the cooling and storing of neutral atoms in a volume of a few cubic millimeters by use of laser beams and a magnetic field. Such devices offer new and exciting opportunities for precision measurements of radioactive isotopes. Here we present experiments performed with a double-MOT system coupled to the on-line separator TISOL at TRIUMF/Vancouver, Canada. For the first time, the Β-decay of free atoms stored in such a device could be observed. We report on coincidence measurements between beta-particles and the argon recoils in the decay of 37K and 38rm{m}}K. The charge state ratios of the recoil-ions were deduced by Time-Of-Flight separation in an acceleration field. The final goal of those investigations is a precision test of the Standard Model by measuring the –nu-correlation parameter a.

Electron energy distributions from multiple ionization in 20-120-keV H++Ar collisions.

Energy-analyzed secondary electrons from ${\mathrm{H}}^{+}$+Ar collisions at eight proton energies from 20 to 120 keV were detected in coincidence with recoil ions of charge states 1--3. The charge states were distinguished by time-of-flight measurements and the electron energies were analyzed by a newly designed electrostatic analyzer of the low-pass high-pass filter type which has a large (\ensuremath{\sim}1.45\ensuremath{\pi} steradian) solid angle acceptance. For each of nine electron energies from 10 to 160 eV, the ionizing events associated with each residual ion charge state were counted. The electron energy dependence of the charge-state ratios and the average charge determined from the experimental data were in satisfactory agreement with calculations made with a simple statistical model based on the assumption of independent interactions between the projectile and the ejected electrons. In this model the single-electron ionization probability is assumed to be given by P(b)=P(0)exp(-b/${\mathit{r}}_{0}$) where ${\mathit{r}}_{0}$ is a characteristic length and b the impact parameter. Values of ${\mathit{r}}_{0}$ amd P(0) were determined by fitting the experimental data with this model. The charge-state ratios averaged over ejected electron energies agree with the data of DuBois et al. [Phys. Rev. A 29, 70 (1984)]. \textcopyright{} 1996 The American Physical Society.

Mass spectrometric identification of cosmic-ray-produced neon in terrestrial rocks with multiple neon components

Studies of cosmic-ray-produced nuclides in terrestrial rocks are expected to provide important geomorphological and glaciological information, such as surface exposure ages, erosion rates and extent of glacial cover. In the case of the stable nuclide 21Ne, the cosmic-ray-produced component may represent but one of several components, and a component resolution based on three-isotope correlations is essential. We developed analytical techniques which permit accurate corrections for all interfering ions. We show (for our modified Nier source) that corrections for doubly charged species based on constant charge state ratios are not satisfactory. The improved techniques permit isotopic studies of < 10 6 atoms of 21Ne. The spallation ratio( 22Ne/ 21Ne) c from Si was calibrated using a quartz separate from an Allan Hills sandstone and yields a value of1.243 ± 0.022. This ratio is expected to be essentially constant for terrestrial quartz samples and should be useful in component resolutions. The stepwise release of Ne at increasing temperatures shows that cosmic-ray-produced Ne is released from quartz at rather low temperatures. This permits an estimate of the activation energy for Ne diffusion in quartz (90 ± 10 kJ/mol).