Partial Number Density Research Articles

Discovery of suprathermal Fe+ in Saturn's magnetosphere

AbstractMeasurements in Saturn's equatorial magnetosphere from mid‐2004 through 2013 made by Cassini's charge‐energy‐mass ion spectrometer indicate the presence of a rare, suprathermal (83–167 keV/e) ion species at Saturn with mass ~56 amu that is likely Fe+. The abundance of Fe+ is only ~10−4 relative to that of W+ (O+, OH+, H2O+, and H3O+), the water group ions which dominate Saturn's suprathermal and thermal ions along with H+ and H2+. The radial variation of the Fe+ partial number density (PND) is distinctly different from that of W+ and most ions that comprise Saturn's suprathermal ion populations which, unlike thermal energy plasma ions, typically have a prominent PND peak at ~8–9 Rs (1 Saturn radius, Rs = 60,268 km). In contrast, the Fe+ PND decreases more or less exponentially from ~4 to ~20 Rs, our study's inner and outer limits. Fe+ may originate from metal layers produced by meteoric ablation near Saturn's mesosphere‐ionosphere boundary and/or possibly impacted interplanetary dust particles or the Saturn system's dark material in the main rings.

Suprathermal magnetospheric minor ions heavier than water at Saturn: Discovery of 28M+ seasonal variations

AbstractWater group ions W+ (O+, OH+, H2O+, and H3O+), along with H+ and H2+, dominate Saturn's near‐equatorial magnetospheric suprathermal ion populations. The singly charged, minor heavy ions O2+ and 28M+ were also observed in the suprathermal energy range, but at much lower densities, having ≤10−2 the abundance of W+. From 2004 through 2013, Cassini's charge‐energy‐mass ion spectrometer has measured suprathermal 83–167 keV/e heavy ions at ~4–20 Rs (1 Saturn radius, Rs = 60,268 km). Christon et al. (2013) found apparent O2+/W+ transient and seasonal responses to variable insolation of Saturn's ring atmosphere prior to mid‐2012. A similar seasonal variation in 28M+/W+ (28M+ ~27–30 amu/e molecular minor ions) was suggested but inconclusive. Now with data from mid‐2012 through 2013, we find that both O2+ and 28M+ clearly exhibit seasonal recoveries from mid‐2012 onward. Prominent radial partial number density peaks at ~9 Rs identify W+, O2+, and 28M+ as clear ring current participants. It is presently unclear which part of Saturn's magnetosphere produces the seasonally varying 28M+ component. Dissimilar 28M+/W+ and O2+/W+ responses to a strong late 2011 solar UV burst suggest different seasonal ring‐based photolytic processes.

Inner magnetospheric heavy ion composition during high‐speed stream‐driven storms

AbstractIon composition data, taken by the Combined Release and Radiation Effects Satellite Magnetospheric Ion Composition Spectrometer instrument, are investigated across eight high‐speed solar wind‐stream‐driven storms (HSSs) during 1991. The HSSs are identified using solar wind data from OMNI alongside geomagnetic indices, and the behavior of ions in the energy range 31.2–426.0 keV is investigated. A case study of the single HSS event that occurred on 30 July 1991 is performed, and superposed epoch analyses of five events are conducted. The data show evidence of a local minimum (dropout) in the flux and partial number density of ionic species H+, He+, He++, and O+ close to the onset of magnetospheric convection. The flux and number density rapidly fall and then recover over a period of hours. The initial rapid recovery in number density is observed to consist primarily of lower‐energy ions. As the number density reaches its maximum, the ions show evidence of energization. Heavy ion‐to‐proton ratios are observed to decrease substantially during these HSS events.

Class of consistent fundamental-measure free energies for hard-sphere mixtures

In fundamental-measure theories the bulk excess free-energy density of a hard-sphere fluid mixture is assumed to depend on the partial number densities {ρ(i)} only through the four scaled-particle-theory variables {ξ(α)}, i.e., Φ({ρ(i)})→Φ({ξ(α)}). By imposing consistency conditions, it is proven here that such a dependence must necessarily have the form Φ({ξ(α)})=-ξ(0)ln(1-ξ(3))+Ψ(y)ξ(1)ξ(2)/(1-ξ(3)), where y≡ξ(2)(2)/12πξ(1)(1-ξ(3)) is a scaled variable and Ψ(y) is an arbitrary dimensionless scaling function which can be determined from the free-energy density of the one-component system. Extension to the inhomogeneous case is achieved by standard replacements of the variables {ξ(α)} by the fundamental-measure (scalar, vector, and tensor) weighted densities {n(α)(r)}. Comparison with computer simulations shows the superiority of this bulk free energy over the White Bear one.

Long term time variations of the suprathermal ions in Saturn's magnetosphere

[1] We use the Charge-Energy-Mass Spectrometer (CHEMS) on Cassini to study long-term time variations of the suprathermal (E/Q = 27–220 keV/e) ions in Saturn's equatorial ring current (Latitude = −10° to 10°, dipole L = 7–16) from the end of 2004 to the end of 2010. We identify five equatorial time periods, which vary in length from 124 to 282 days, and determine the average ion composition of the ring current for each period. The species examined are the water group W+ (O+, OH+, H2O+, and H3O+), H+, H2+, He++, He+, and O++. We find that the combined partial number density over this energy range varies less than a factor of two from a minimum of 1.3 × 10−3 cm−3 to a maximum of 2.3 × 10−3 cm−3, implying that long-term time variations in neutral source strength must be modest. The most abundant species (63% W+, 30% H+, 5% H2+) show no long-term trends. The H+/W+ and H2+/W+ ratios vary less than half as much as individual densities. The smaller variations in the ratios could be understood if all three species mostly originate from the Enceladus plumes either directly or via dissociation, but our results can accommodate differing neutral sources (currently thought to be Enceladus for W, Saturn's atmosphere for H, and Titan for H2) if the suprathermal ion density variations are largely caused by changes in the overall acceleration rate of the thermal plasma. Both He++, originating from the solar wind, and He+, interplanetary pickup ions of interstellar origin, show a substantial decrease in 2009–2010 near solar minimum.

The recombination of D+3 and D+5 ions with electrons in deuterium containing plasma

The recombination of D+3 and D+5 ions with electrons in a He–Ar–D2 flowing afterglow plasma is reported. Low temperature (T = 130–300 K) and high pressure of the He buffer gas (900–1200 Pa) was used to enhance the formation of D+5 ions in the afterglow plasma. The deuterium partial number density was varied over a large range ([D2] = 1 × 1012–3 × 1015 cm−3) to study its influence on plasma decay. At low [D2], D+3 ions dominate in the afterglow and the plasma decay is controlled by the recombination of D+3 ions (rate coefficient α3). At high [D2] and lower temperatures, D+5 ions are effectively formed and the plasma decay is controlled by the recombination of D+5 ions (α5). In the intermediate region, the rate of recombination is given by the partial densities of both ions, D+3 and D+5. These partial densities are influenced by conditions in the plasma and they are controlled by ion–molecule reactions (D+3 + D2 ↔ D+5). If the ion–molecule reactions are fast in comparison with the recombination processes the plasma decay can be characterized by the equilibrium constant KC and by recombination rate coefficients α3 and α5. By monitoring of the plasma decay, all three constants α3 (190 K) = (1.4 ± 0.5) × 10−7 cm3 s−1, α5 (190 K) = (3 ± 1) × 10−6 cm3 s−1 and KC (190 K) = (3.8 ± 2.0) × 10−16 cm3 are determined.

Recombination studies in a He-Ar-H2 plasma

The recombination of H+3 ions with electrons has been studied in afterglow plasma in three different experiments. In two experiments, using the Variable Temperature Stationary Afterglow (VT-AISA) and the Variable Temperature Flowing Afterglow (VT-FALP) techniques, a decay of the electron number density was measured by an electrostatic Langmuir probe to determine the recombination rate coefficient. In the third experiment a near infrared Cavity Ring-Down Absorption Spectrometer (CRDS) was used to monitor the decay of the H+3 (v = 0) ion density during the afterglow. Measurements were carried out in helium buffer gas with small admixtures of argon and hydrogen at total pressures ranging from 150 up to 1200 Pa and at buffer gas temperatures ranging from 100 up to 330 K. In the experiments the partial number density of hydrogen was varied from 5 × 1010 up to 1 × 1016 cm−3 and for this broad range of hydrogen number densities effective recombination rate coefficients were obtained, which varied over three orders of magnitude from 2 × 10−9 cm3s−1 at [H2] = 5 × 1010 cm−3 up to 3 × 10−6 cm3s−1 at [H2] = 1 × 1016 cm−3. Using our experimental results we discuss possible mechanisms of recombination in hydrogen plasma in a very broad range of several parameters: buffer gas pressure, temperature, electron number density, hydrogen number density and internal excitation of recombining ions.

A neutron scattering investigation of the structural order in RbBrRb solutions

Neutron scattering investigation of the RbBrRb system offers the advantage of giving directly the partial number density structure factor S NN . On the salt rich side not much deformation of the short range ionic correlations is observed except for a linear decrease with salt dilution. On the metal rich side the strong disorder introduced by the anions and already observed for KKCl is confirmed, as well as the detection of ionic short range correlations at 80% metal concentration. A tendency to segregation as noticeable from the scattering at low angles is observable over the whole concentration range. We also see that the strong shoulder K p appearing in all MCl salts at about twice the first maximum position of the S cc partial definitely belongs to S NN .