57 research outputs found
Nature of the metal-nonmetal transition in metal-ammonia solutions. I. Solvated electrons at low metal concentrations
Using a theory of polarizable fluids, we extend a variational treatment of an
excess electron to the many-electron case corresponding to finite metal
concentrations in metal-ammonia solutions (MAS). We evaluate dielectric,
optical, and thermodynamical properties of MAS at low metal concentrations. Our
semi-analytical calculations based on a mean-spherical approximation correlate
well with the experimental data on the concentration and the temperature
dependencies of the dielectric constant and the optical absorption spectrum.
The properties are found to be mainly determined by the induced dipolar
interactions between localized solvated electrons, which result in the two main
effects: the dispersion attractions between the electrons and a sharp increase
in the static dielectric constant of the solution. The first effect provides a
classical phase separation for the light alkali metal solutes (Li, Na, K) below
a critical temperature. The second effect leads to a dielectric instability,
i.e., polarization catastrophe, which is the onset of metallization. The locus
of the calculated critical concentrations is in a good agreement with the
experimental phase diagram of Na-NH3 solutions. The proposed mechanism of the
metal-nonmetal transition is quite general and may occur in systems involving
self-trapped quantum quasiparticles.Comment: 13 figures, 42 page
Neutral interstellar hydrogen in the inner heliosphere under the influence of wavelength-dependent solar radiation pressure
With the plethora of detailed results from heliospheric missions and at the
advent of the first mission dedicated IBEX, we have entered the era of
precision heliospheric studies. Interpretation of these data require precision
modeling, with second-order effects quantitatively taken into account. We study
the influence of the non-flat shape of the solar Ly-alpha line on the
distribution of neutral interstellar H in the inner heliosphere. Based on
available data, we (i) construct a model of evolution for the solar Ly-alpha
line profile with solar activity, (ii) modify an existing test-particle code
used to calculate the distribution of neutral interstellar H in the inner
heliosphere so that it takes the dependence of radiation pressure on radial
velocity into account, and (iii) compare the results of the old and new
version. Discrepancies between the classical and Doppler models appear between
~5 and ~3 AU and increase towards the Sun from a few percent to a factor of 1.5
at 1 AU. The classical model overestimates the density everywhere except for a
~60-degr cone around the downwind direction, where a density deficit appears.
The magnitude of the discrepancies appreciably depends on the phase of the
solar cycle, but only weakly on the parameters of the gas at the termination
shock. For in situ measurements of neutral atoms performed at ~1 AU, the
Doppler correction will need to be taken into account, because the
modifications include both the magnitude and direction of the local flux by a
few km/s and degrees, respectively, which, when unaccounted for, would
introduce an error of a few km/s and degrees in determination of the magnitude
and direction of the bulk velocity vector at the termination shock.Comment: 10 pages, 13 figures, accepted by A&
Interpretation of Pioneer 10 Lyman alpha based on heliospheric interface models: methodology and first results
The Very Local Interstellar Medium (VLISM) neutral hydrogen and proton
densities are still not precisely known even after three decades of deep space
research and the existence of the EUV and other diagnostic data obtained by
Pioneer 10/11, Voyager 1/2 and other spacecraft. The EUV data interpretation,
in particular, has suffered because of inadequate neutral hydrogen-plasma
models, difficulty of calculating the multiply scattered Lyman alpha glow and
calibration uncertainties. Recently, all these difficulties have been
significantly reduced. In the present work we have used the latest state of the
art supersonic VLISM neutral hydrogen-plasma and Monte Carlo radiative transfer
model, incorporating neutral density, temperature, and velocity variations,
actual solar line shape, realistic redistribution function, Doppler and
aberration effects. This work presents the methodology of the radiative
transfer code and the first results of the comparison of the model predictions
with the Pioneer 10 data. Monte Carlo radiative transfer calculations were
carried out for five neutral hydrogen- plasma models and compared with Pioneer
data. The first results are quite encouraging. We found that the VLISM
ionization ratio is between 0.2 and 0.5 and that the VLISM neutral hydrogen
density is less than 0.25 cm^{-3}. The present calculation suggests that the
Pioneer 10 photometer derived intensities (Rayleighs) need to be increased by a
factor of 2. If this model- derived calibration is used then the difference
between Pioneer 10 and Voyager 2 intensity values is reduced to about 2.2. The
model, neutral hydrogen density = 0.15 cm${-3} and proton density = 0.07
cm^{-3}, is found to best fit the Pioneer 10 data.Comment: accepted in JG
The Boundary Conditions of the Heliosphere: Photoionization Models Constrained by Interstellar and In Situ Data
The boundary conditions of the heliosphere are set by the ionization, density
and composition of inflowing interstellar matter. Constraining the properties
of the Local Interstellar Cloud (LIC) at the heliosphere requires radiative
transfer ionization models. We model the background interstellar radiation
field using observed stellar FUV and EUV emission and the diffuse soft X-ray
background. We also model the emission from the boundary between the LIC and
the hot Local Bubble (LB) plasma, assuming that the cloud is evaporating
because of thermal conduction. We create a grid of models covering a plausible
range of LIC and LB properties, and use the modeled radiation field as input to
radiative transfer/thermal equilibrium calculations using the Cloudy code. Data
from in situ observations of He^O, pickup ions and anomalous cosmic rays in the
heliosphere, and absorption line measurements towards epsilon CMa were used to
constrain the input parameters. A restricted range of assumed LIC HI column
densities and LB plasma temperatures produce models that match all the
observational constraints. The relative weakness of the constraints on N(HI)
and T_h contrast with the narrow limits predicted for the H^O and electron
density in the LIC at the Sun, n(H^0) = 0.19 - 0.20 cm^-3, and n(e) = 0.07 +/-
0.01 cm^-3. Derived abundances are mostly typical for low density gas, with
sub-solar Mg, Si and Fe, possibly subsolar O and N, and S about solar; however
C is supersolar. The interstellar gas at the Sun is warm, low density, and
partially ionized, with n(H) = 0.23 - 0.27 cm^-3, T = 6300 K, X(H^+) ~ 0.2, and
X(He^+) ~ 0.4. These results appear to be robust since acceptable models are
found for substantially different input radiation fields. Our results favor low
values for the reference solar abundances for the LIC composition.Comment: 14 pages, 4 figures, submitted to Astronomy & Astrophysics together
with papers from the International Space Sciences Institute workshop on
Interstellar Hydrogen in the Heliospher
Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations
By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements
near 5 AU close to the ecliptic right after the previous solar minimum, this
paper presents a determination of the neutral interstellar H density at the
solar wind termination shock and implications for the density and ionization
degree of hydrogen in the LIC. The density of neutral interstellar hydrogen at
the termination shock was determined from the local pickup ion production rate
as obtained close to the cut-off in the distribution function at aphelion of
Ulysses. As shown in an analytical treatment for the upwind axis and through
kinetic modeling of the pickup ion production rate at the observer location,
with variations in the ionization rate, radiation pressure, and the modeling of
the particle behavior, this analysis turns out to be very robust against
uncertainties in these parameters and the modeling. Analysis using current
heliospheric parameters yields the H density at the termination shock equal to
cm, including observational and modeling uncertainties.Comment: Re-edited version, density revised downward due to data
re-processing, accepted by A&
Herzfeld instability versus Mott transition in metal-ammonia solutions
Although most metal-insulator transitions in doped insulators are generally
viewed as Mott transitions, some systems seem to deviate from this scenario.
Alkali metal-ammonia solutions are a brilliant example of that. They reveal a
phase separation in the range of metal concentrations where a metal-insulator
transition occurs. Using a mean spherical approximation for quantum polarizable
fluids, we argue that the origin of the metal-insulator transition in such a
system is likely similar to that proposed by Herzfeld a long time ago, namely,
due to fluctuations of solvated electrons. We also show how the phase
separation may appear: the Herzfeld instability of the insulator occurs at a
concentration for which the metallic phase is also unstable. As a consequence,
the Mott transition cannot occur at low temperatures. The proposed scenario may
provide a new insight into the metal-insulator transition in condensed-matter
physics.Comment: 9 pages, 4 figure
An Unexplained 10 Degree - 40 Degree Shift in the Location of Some Diverse Neutral Atom Data at 1 AU
Four different data sets pertaining to the neutral atom environment at 1 AU
are presented and discussed. These data sets include neutral solar wind and
interstellar neutral atom data from IMAGE/LENA, energetic hydrogen atom data
from SOHO/HSTOF and plasma wave data from the magnetometer on ISEE-3.
Surprisingly, these data sets are centered between 262 degrees and 292 degrees
ecliptic longitude, about 10 degrees - 40 degrees from the upstream
interstellar neutral flow direction at 254 degrees resulting from the motion of
the Sun relative to the local interstellar cloud. Some possible explanations
for this offset, none of which is completely satisfactory, are discussed.Comment: 6 pages, 6 figures, 2 color peer-reviewed paper, in press, COSPAR/WS
Neutral H density at the termination shock: a consolidation of recent results
We discuss a consolidation of determinations of the density of neutral
interstellar H at the nose of the termination shock carried out with the use of
various data sets, techniques, and modeling approaches. In particular, we focus
on the determination of this density based on observations of H pickup ions on
Ulysses during its aphelion passage through the ecliptic plane. We discuss in
greater detail a novel method of determination of the density from these
measurements and review the results from its application to actual data. The H
density at TS derived from this analysis is equal to 0.087 \pm 0.022 cm-3, and
when all relevant determinations are taken into account, the consolidated
density is obtained at 0.09 \pm 0.022 cm-3. The density of H in CHISM based on
literature values of filtration factor is then calculated at 0.16 \pm 0.04
cm-3.Comment: Submitted to Space Science Review
Recommended from our members
Reflectivity of Venus’s Dayside Disk During the 2020 Observation Campaign: Outcomes and Future Perspectives
We performed a unique Venus observation campaign to measure the disk brightness of Venus over a broad range of wavelengths in 2020 August and September. The primary goal of the campaign was to investigate the absorption properties of the unknown absorber in the clouds. The secondary goal was to extract a disk mean SO2 gas abundance, whose absorption spectral feature is entangled with that of the unknown absorber at ultraviolet wavelengths. A total of three spacecraft and six ground-based telescopes participated in this campaign, covering the 52–1700 nm wavelength range. After careful evaluation of the observational data, we focused on the data sets acquired by four facilities. We accomplished our primary goal by analyzing the reflectivity spectrum of the Venus disk over the 283–800 nm wavelengths. Considerable absorption is present in the 350–450 nm range, for which we retrieved the corresponding optical depth of the unknown absorber. The result shows the consistent wavelength dependence of the relative optical depth with that at low latitudes, during the Venus flyby by MESSENGER in 2007, which was expected because the overall disk reflectivity is dominated by low latitudes. Last, we summarize the experience that we obtained during this first campaign, which should enable us to accomplish our second goal in future campaigns
Solar parameters for modeling interplanetary background
The goal of the Fully Online Datacenter of Ultraviolet Emissions (FONDUE)
Working Team of the International Space Science Institute in Bern, Switzerland,
was to establish a common calibration of various UV and EUV heliospheric
observations, both spectroscopic and photometric. Realization of this goal
required an up-to-date model of spatial distribution of neutral interstellar
hydrogen in the heliosphere, and to that end, a credible model of the radiation
pressure and ionization processes was needed. This chapter describes the solar
factors shaping the distribution of neutral interstellar H in the heliosphere.
Presented are the solar Lyman-alpha flux and the solar Lyman-alpha resonant
radiation pressure force acting on neutral H atoms in the heliosphere, solar
EUV radiation and the photoionization of heliospheric hydrogen, and their
evolution in time and the still hypothetical variation with heliolatitude.
Further, solar wind and its evolution with solar activity is presented in the
context of the charge exchange ionization of heliospheric hydrogen, and in the
context of dynamic pressure variations. Also the electron ionization and its
variation with time, heliolatitude, and solar distance is presented. After a
review of all of those topics, we present an interim model of solar wind and
the other solar factors based on up-to-date in situ and remote sensing
observations of solar wind. Results of this effort will further be utilised to
improve on the model of solar wind evolution, which will be an invaluable asset
in all heliospheric measurements, including, among others, the observations of
Energetic Neutral Atoms by the Interstellar Boundary Explorer (IBEX).Comment: Chapter 2 in the planned "Cross-Calibration of Past and Present Far
UV Spectra of Solar System Objects and the Heliosphere", ISSI Scientific
Report No 12, ed. R.M. Bonnet, E. Quemerais, M. Snow, Springe
- …