Phase diagrams describing solid-gas equilibria in the system Fe-Mg-Si-O-C-H,and its bearing on redox states of chondrites

Abstract— Phase diagrams describing solid-gas equilibria in the system Fe-Mg-Si-O-C-H under H-rich conditions (～700–2000 K and 10^?2–10^?20 atm of Ph ₂), including solar nebula conditions, were constructed based on thermochemical calculations. Boundaries of vaporous phases, which are the first phases to condense from a gas, can be obtained without calculating condensation temperatures of individual gas compositions because the numbers of major gaseous species are the same as those of components in the concerned systems. Fractionations by condensation and/or evaporation can be discussed easily in such phase diagrams. A thermal divide, which is a barrier that vapors cannot cross by a single cooling process, was recognized in the phase diagrams. This is present on the Fe-MgO-SiO₂-CO-H plane at high temperatures (≥500–700 K) and plays an important role in fractionations. Oxidizing states of ordinary chondrites and carbonaceous chondrites before aqueous alteration are located at the O-rich side of the thermal divide. Such oxidizing states can be formed from the solar gas by fractionation in the primordial solar nebula because the solar composition is located on the O-rich side. On the other hand, the reducing states of enstatite chondrites, located at the O-poor side, cannot be formed as long as the thermal divide is present. The reducing states can be obtained by CO to CH₄ molecular reaction at low temperatures (≤500–700 K), where the high-temperature thermal divide is absent. Addition of H₂O-rich and CH₄-rich ice can explain establishment of the redox states of ordinary and enstatite chondrites, respectively.     

A solar flare in the Fe I 5324 line on 24 June, 1993

A solar flare with both H and Fe i 5324 emissions was observed in AR 7529 (S13, E65) on 24 June, 1993 at the Bejing Astronomical Observatory. Our calculations show that the Fe i 5324 emission region of the flare was located in the low photosphere at a height of about 180 km above ₅₀₀₀ = 1, which is lower than many previous studies of white-light flares. To study a Fe i 5324 flare, which represents a kind of extreme case in solar flares, would be useful for clarifying some arguments in the researches of white-light flares as well as for understanding the mechanism of solar flares.The synthetic analyses from vairous features of the flare lead to the following possible exciting mechanism of the Fe i 5324 flare: owing to the flow of energetic electrons from the corona and probably also the thermal conduction downward into the lower atmosphere, a condensation with a temperature higher than that below it was formed near the transition region. Then the low photosphere was heated through backwarming. The Fe i 5324 flare occurred as an indicator of the excitation in the low photosphere.     

The formation of solar quiescent prominences by condensation

We model the formation of solar quiescent prominences by solving numerically the non-linear, time-dependent, magnetohydrodynamic equations governing the condensation of the corona. A two-dimensional geometry is used. Gravitational and magnetic fields are included, but thermal conduction is neglected. The coronal fluid is assumed to cool by radiation and to be heated by the dissipation of mechanical energy carried by shock waves. A small, isobaric perturbation of the initial thermal and mechanical equilibrium is introduced and the fluid is allowed to relax. Because the corona with the given energy sources is thermally unstable, cooling and condensation result.When magnetic and gravitational fields are absent, condensation occurs isotropically with a strongly time-dependent growth rate, and achieves a density 18 times the initial density in 3.5 × 10⁴ s. The rapidity of condensation is limited by hydrodynamical considerations, in contrast to the treatment of Raju (1968). When both magnetic and gravitational fields are included, the rate of condensation is inhibited and denser material falls.We conclude that: (1) condensation of coronal material due to thermal instability is possible if thermal conduction is inhibited; (2) hydrodynamical processes determine, in large part, the rate of condensation; (3) condensation can occur on a time scale compatible with the observed times of formation of quiescent prominences.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Dynamic and Thermal Disappearance of Prominences and Their Geoeffectiveness

This paper is a qualitative study of 42 events of solar filament/prominence sudden disappearances (“disparitions brusques”; henceforth DBs) around two solar minima, 1985 – 1986 and 1994. The studied events were classified as 17 thermal and 25 dynamic disappearances. Associated events, i.e. coronal mass ejections (CMEs), type II bursts, evolution of nearby coronal holes, as well as solar wind speed, and geomagnetic disturbances are discussed. We have found that about 50% of the thermal DBs with adjacent (within 15° from the DB) coronal holes were associated with CMEs within a selected time window. All the studied thermal disappearances with adjacent coronal holes or accompanied by dynamic disappearances were associated with weak and medium geomagnetic storms. Also, nearly 64% of dynamic DBs were associated with CMEs. Ten (40%) dynamic disappearances were associated with intense geomagnetic storms, even when no CMEs was reported, six (24%) dynamic disappearances corresponded to extreme storms, and five (20%) corresponded to medium geomagnetic storms. The extreme geomagnetic storms appeared to be related to combined events, involving dynamic disappearances with adjacent coronal holes or including thermal disappearances. Furthermore, the geomagnetic activity (Dst index) increased if the source was close to the central meridian (±30°). The highest interplanetary magnetic field (B), longest duration, lowest southward direction B _z component, and lowest Dst were highly correlated for all studied events. The Sun – Earth transit time computed from the starting time of the sudden disappearance and the time its effect was measured at Earth was about 4.3 days and was mainly well correlated with the solar wind speed measured in situ (daily value).     

The influence of chromospheric condensation on Hα line profiles

The influence of the chromospheric condensation on H line profiles for the thermal model of a solar flare has been empirically studied in this paper. The so-called thermal model here means that there is no temperature increase relative to the quiet-Sun chromosphere but with a chromospheric condensation in the lower part of its transition region, which case is assumed to represent the early stage of the impulsive phase. The main results include: when the temperature within the condensation region is assumed to be equal to that in front of it, the influence is to create an additional absorption profile overlapping on the original one; by increasing the condensation strength, the H line profile changes from a little line-center increase to broadened red asymmetry, then to the reversed red asymmetry, and finally to two independent absorption profiles; the thickness of the condensation determines the absorption of the additional profile; descending the transition region has no obvious effect on the basic characteristic of the H line profile except a little increase in the line center. Assuming that the temperature within the condensation is higher than that ahead of the condensation, the calculated H line profiles may be strong enough to be comparable with the observations. This means that if the condensation in the purely thermal model can reach a higher temperature, we may also use only thermal origin to explain a chromospheric flare.Alexander von Humboldt Research Fellow, on leave from Purple Mountain Observatory, Nanjing, China.     

The physics of thermal instability in two dimensions

Previous studies of a thermal (radiative) instability in a sheared magnetic field have shown that, under solar coronal conditions, cool condensations can form in a small neighborhood about the shear layer. Such results have served to model the formation of solar filaments (or prominences) observed to occur above photospheric magnetic polarity-inversion lines. A surprising conclusion of these studies is that the width of the condensation does not depend on the thermal conductivity (). By examining the mass-flow patterns of two-dimensional condensations in the absence of thermal conduction, we demonstrate that local plasma dynamics and the constraints imposed by boundary conditions are together sufficient to explain the size of the condensation width. In addition we present the results of a series of numerical calculations which illustrate the characteristic mode structure of sheared-field condensations.     

Iron/silicate fractionation and the origin of Mercury

Compared with the other terrestrial planets, Mercury has anomalously low mass and high iron content. Equilibrium condensation and inhomogeneous accretional models are not compatible with these properties, unless the solar nebula's thermal structure and history meet stringent conditions. Also, such models predict a composition which does not allow a presently molten core. It appears that most of the solid matter which originally condensed in Mercury's zone has been removed. The planet's composition may be explained if the removal process was only slightly more effective for silicates than for iron. It is proposed that planetesimal orbits in the inner solar nebula decayed because of gas drag. This process is a natural consequence of the non-Keplerian rotation of a centrally condensed nebula. A simple quantitative model shows good agreement with the observed mass distribution of the terrestrial planets. The rate of orbital decay is slower for larger and/or denser bodies, because of their smaller area-to-mass ratios. With plausible assumptions as to planetesimal sizes and compositions, this process can produce fractionation of the sense required to produce an iron-rich planet. Cosmogonical implications are discussed.     

Thermophysical analysis of infrared observations of asteroids

Abstract— Visual photometry, which measures reflected solar radiation, can be combined with infrared radiometry, which measures absorbed and re‐radiated solar energy, to determine key properties of small solar system objects. This method can be applied via thermophysical model concepts not only for albedo and diameter determination, but also for studies of thermal parameters like thermal inertia, surface roughness or emissivity. Hence, a detailed analysis of the asteroid surface is possible and topics like surface mineralogy, the density of the regolith or the presence of a rocky surface, lightcurve influences due to shape or albedo, porosity of the surface material, etc. can be addressed. The “radiometric technique” based on a recently developed thermophysical model is presented. The model was extensively tested against observations from the infrared space observatory, including spectroscopic and photometric measurements at infrared wavelengths between 2 and 200 μm of more than 40 asteroids. The possible model applications are discussed in terms of the different levels of knowledge for individual asteroids. The effects of the thermal parameters are illustrated and methods are presented as to how to separate different aspects. Possibilities and limitations are evaluated for the possible transfer of this model to near‐Earth asteroids. In the long run, this kind of study of near‐Earth asteroids may provide answers to questions about their surface properties which are crucial to develop mitigation scenarios.     

Gravity Anomaly During the Mohe Total Solar Eclipse and New Constraint on Gravitational Shielding Parameter

By using a high-precision LaCoste-Romberg gravimeter, continuous and precise measurements were carried out during the March 9, 1997 total solar eclipse in Mohe region in Northeast China. The gravity variations were digitally recorded during the total solar eclipse so as to investigate the possible anomaly of the Sun and the Moon's gravitational fields on the Earth. After the careful processing and analysis of the observed data, no significant anomaly during the very solar eclipse was found. However, there are two ‘gravity anomaly valleys’ with near symmetrical decrease of about 6 ∼ 7 μg at the first contact and the last contact. This is the anomaly phenomenon observed and reported for the first time in the literature. This paper is intended to explain the observed anomaly by conducting the tilt experiment due to the thermal stress and temperature change in the solar eclipse. A new constraint limit on gravitational shielding is thus obtained. Some analysis and discussions are presented although further studies and research are highly needed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Precondensed matter: Key to the early solar system

Chemical and isotopic anomalies in meteorites may be understandable in terms of the chemical fractionation routinely expected in the interstellar medium (ISM). Dust of distinct composition is idealized as being of three types: (1) thermal supernova condensates (SUNOCONS), (2) thermal condensation during other stellar mass-loss processes (STARDUST), and (3) nonthermal sticking processes in cold nebulae (NEBCONS). Great depletions in ISM of Ca Al Ti are due to SUNOCONS, although STARDUST is about twice as abundant. An abundance table of interstellar SUNOCONS is presented. Parent bodies in the solar system are accumulated directly from the ISM. No hot solar condensation sequence is assumed. Only relatively volatile elements within NEBOCONS are vaporized in the warm solar accretion disk. Variations in the relative amounts of these components during accumulation processes plus subsequent solid chemistry may have produced such chemical anomalies as the meteoritic fractionation patterns and the Ca Al-rich inclusions. Isotopic anomalies result from four processes that selectively site specific isotopes: (1) extinct radioactivities, (2) distinct supernova shells, (3) gas-dust separation, and (4) gas-dust age difference. Planetary accumulation will have been fingerprinted by the chemical state of the ISM if this picture is correct.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

The condensation with partial isolation (CWPI) model of condensation in the solar nebula

Abstract— We have developed a nebular condensation model and a computational routine that potentially can account for the unequilibrated mineral assemblages in chondritic meteorites. The model assumes that as condensation proceeds, a specified fraction (called the isolation degree, ξ) of the existing condensate is steadily withdrawn from reactive contact with the residual gas, presumably as a result of the growth and aggregation of condensed mineral grains. The isolated condensates may remain in the condensing system as coarse inert objects; whereas, the mineral grains that are still in reactive contact with residual nebular gases are in the form of fine dust. This paper describes the condensation with partial isolation (CWPI) model of condensation and uses it to study condensation in a nebula of solar composition at a total pressure of 10^?5 bar. The systematic isolation of condensates from residual nebular gases has profound effects on the condensation sequence. At ξ values <0.2%, the condensation sequence is essentially independent of the isolation degree and identical to the classic condensation sequence. At ξ values >2.5%, the condensation sequence is also independent of the isolation degree and closely resembles the “inhomogeneous accretion model” or “chemical disequilibrium model” of condensation. In the intermediate range of ξ values, the character of the condensation sequence is very sensitive to the degree of chemical fractionation caused by condensate isolation. The mineralogy of chondritic meteorites is not consistent with condensation sequences having ξ > 2.5; this is an upper limit on the ξ values that is characteristic of condensation in the solar nebula. The mineralogy and chemistry of carbonaceous and enstatite chondrites can be explained by accretion of isolated condensates formed at ξ values of ≤0.1% and 0.7–1.5%, respectively, providing that segregation of the inert coarse objects and fine reactive dust occurred in the nebula. Segregation of these two categories of condensate may have been responsible for the observed volatility-based chemical fractionations among chondritic meteorites.     

Titan's atmospheric engine: an overview

Heating occurs in Titan's stratosphere from the absorption of incident solar radiation by methane and aerosols. About 10% of the incident sunlight reaches Titan's surface and causes heating there. Thermal radiation redistributes heat within the atmosphere and cools to space. The resulting vertical temperature profile is stable against convection and a state of radiative equilibrium is established. Equating theoretical and observed temperature profiles enables an empirical determination of the vertical distribution of thermal opacity. A uniformly mixed aerosol is responsible for most of the opacity in the stratosphere, whereas collision-induced absorption of gases is the main contributor in the troposphere. Occasional clouds are observed in the troposphere in spite of the large degrees of methane supersaturation found there. Photochemistry converts CH₄ and N₂ into more complex hydrocarbons and nitriles in the stratosphere and above. Thin ice clouds of trace organics are formed in the winter and early spring polar regions of the lower stratosphere. Precipitating ice particles serve as condensation sites for supersaturated methane vapor in the troposphere below, resulting in lowered methane degrees of supersaturation in the polar regions. Latitudinal variations of stratospheric temperature are seasonal, and lag instantaneous response to solar irradiation by about one season for two reasons: (1) an actual instantaneous thermal response to a latitudinal distribution of absorbing gases, themselves out of phase with the sun by about one season, and (2) a sluggish dynamical response of the stratosphere to the latitudinal transport of angular momentum, induced by radiative heating and cooling. Mean vertical abundances of stratospheric organics and aerosols are determined primarily by atmospheric chemistry and condensation, whereas latitudinal distributions are more influenced by meridional circulations. In addition to preferential scavenging by precipitating ice particles from above, the polar depletion of supersaturated methane results from periodic scavenging by short-lived tropospheric clouds, coupled with the steady poleward march of the continuously drying atmosphere due to meridional transport.     

Formation of the first oxidized iron in the solar system

For fayalite formation times of several thousand years, and systems enriched in water by a factor of ten relative to solar composition, 1 μm radius olivine grains could reach 2 mole% fayalite and 0.1 μm grains 5 mole% by nebular condensation, well short of the values appropriate for precursors of most chondrules and the values found in the matrices of unequilibrated ordinary chondrites. Even 10 μm olivine crystals could reach 30 mole% fayalite above 1100 K in solar gas if condensation of metallic nickel‐iron were delayed sufficiently by supersaturation. Consideration of the surface tensions of several phases with equilibrium condensation temperatures above that of metallic iron shows that, even if they were supersaturated, they would still nucleate homogeneously above the equilibrium condensation temperature of metallic iron. This phenomenon would have provided nuclei for heterogeneous nucleation of metallic nickel‐iron, thus preventing the latter from supersaturating significantly and preventing olivine from becoming fayalitic. Unless a way is found to make nebular regions far more oxidizing than in existing models, it is unlikely that chondrule precursors or the matrix olivine grains of unequilibrated ordinary chondrites obtained their fayalite contents by condensation processes. Perhaps stabilization of FeO occurred after condensation of water ice and accretion of icy planetesimals, during heating of the planetesimals and/or in hot, dense, water‐rich vapor plumes generated by impacts on them. This would imply that FeO is a relatively young feature of nebular materials.     

Stochastic scattering and other contributions to the Sun's wake in the local hydrogen cloud

Gas streaming through the solar system experiences both destructive and scattering processes, the latter primarily in collisional interactions with the solar wind protons. The scattering interactions can be important in filling the downstream wake. They may effectively increase the velocity dispersion and also cause discrete orbit changes.The downstream intensity moment is here evaluated analytically for particles suffering a single, discrete collision, and compared with the moment from a thermal velocity dispersion (both in the absence of a central force field). The elastic scattering collisions of protons in H-gas lead to a contribution to theL backscatter from the wake equivalent to an initial thermal velocity of about 1 km s^–1, giving an intensity for cool gas of the order of 10R. This exceeds the contribution due to focussing in the solar gravitational field if the radiation pressure is not less than 0.8 of the gravitational attraction.     

The formation of solar prominences by magnetic reconnection and condensation

A model for solar quiescent prominences nested in a Figure 8 magnetic field topology is developed. This topology is argued to be the natural consequence of the distention of bipolar regions upward into the corona. If this distention is slow enough so that hydrostatic equilibrium holds approximately along the field lines, the transverse gas pressure forces fall exponentially with height whereas the inward Lorentz forces fall as a power law. At a low height in the corona, the pressure forces cannot balance the Lorentz forces provided the field lines remain tied to the photosphere and an inward collapse with subsequent reconnection at the point of closest approach should occur. Because of initial shear in the magnetic field, the reconnection would produce isolated helices above the point of reconnection since field lines would not interact with themselves but with their neighbors. This resulting topology produces a field above the elevated neutral line which is opposite in polarity to that of the photospheric field as in the current sheet models of Kuperus and Tandberg-Hanssen (1967). Raadu and Kuperus (1973), Kuperus and Raadu (1974), and Raadu (1979) and in agreement with recent observations of Leroy (1982), and Leroy et al. (1983).Assuming the isolated helices formed by reconnection are insulated from coronal thermal conduction and heating, the radiative cooling process and condensation is considered for the temperature range of 10⁴-6000 K. This condensation results in a steady downflow to the bottom of the helices as the temperature scale-height falls, thus forming a dense, cool, prominence at the bottom of the helical configuration resting on the elevated neutral line with the remainder of the helix being essentially evacuated of material. We identify this neutral line at the bottom of the prominence with the sharp lower edge often seen when viewing quiescent prominences side-on and the evacuated helix with the coronal cavity observed around prominences when seen during total eclipses.Downflow speeds associated with the condensation process are calculated for prominence temperatures and yield velocities in the range of the observed downflows of about 1 km s^–1.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Systematically Asymmetric Heliospheric Magnetic Field: Evidence for a Quadrupole Mode and Non-Axisymmetry with Polarity Flip-Flops

Recent studies of the heliospheric magnetic field (HMF) have detected interesting, systematic hemispherical and longitudinal asymmetries which have a profound significance for the understanding of solar magnetic fields. The in situ HMF measurements since the 1960s show that the heliospheric current sheet (HCS) is systematically shifted (coned) southward during solar minimum times, leading to the concept of a bashful ballerina. While temporary shifts can be considerably larger, the average HCS shift (coning) angle is a few degrees, less than the 7.2^∘ tilt of the solar rotation axis. Recent solar observations during the last two solar cycles verify these results and show that the magnetic areas in the northern solar hemisphere are larger and their intensity weaker than in the south during long intervals in the late declining to minimum phase. The multipole expansion reveals a strong quadrupole term which is oppositely directed to the dipole term. These results imply that the Sun has a symmetric quadrupole S0 dynamo mode that oscillates in phase with the dominant dipole A0 mode. Moreover, the heliospheric magnetic field has a strong tendency to produce solar tilts that are roughly opposite in longitudinal phase. This implies is a systematic longitudinal asymmetry and leads to a “flip-flop” type behaviour in the dominant HMF sector whose period is about 3.2 years. This agrees very well with the similar flip-flop period found recently in sunspots, as well as with the observed ratio of three between the activity cycle period and the flip-flop period of sun-like stars. Accordingly, these results require that the solar dynamo includes three modes, A0, S0 and a non-axisymmetric mode. Obviously, these results have a great impact on solar modelling.     

Non-equilibrium Condensation ina Primordial Solar Nebula: Formation of Refractory Metal Nuggets

We present a theoretical investigation of non-equilibrium condensation of refractory metalsinthe primordial solar nebula, in relation to the origin of “Fremdlinges” included inCAIs. Todescribe the nucleation process of grains from vapor, weadopted asemi-phenomenological modelmodified fromthe classical nucleation theoryby the introductionof the second virial coefficient of vapor. This modelachieves excellent agreement with nucleation rate experiment. However,the second virial coefficients are unknown for a vapor of refractory metals. To overcome this, weexpress the nucleation rate by theuse of the chemical potential of dimersinsteadof the second virial coefficient.On the basis of this new nucleation theory,we have performed numerical simulations ofnon-equilibrium condensation of refractory metals andfind thattheircondensation temperatures, T_c, decrease considerably in comparison withequilibrium condensation. Even if the characteristiccooling time scale is aslarge as 1×10⁵ years, the decrease in T_c isfrom 200 to 400 Kfor rare elements such as W, Re,and Os. This remarkablenon-equilibrium behaviormainly stemsfromthelow totalpressure in the primordial solar nebula. From our new modelwealso obtainthe typical size ofgrainsformed in condensation. We findthatthe cooling time should be ?1×10⁵ years for sub-micron-sized or largerrefractory metal nuggetsto form.     

Recurrency and the origin of the vertical component of the interplanetary magnetic field

The autocorrelation functions of the solar wind velocity and of the IMF components as well as of the geomagnetic activity indices are studied for two periods: August–December, 1965 and January–May, 1974. The vertical component of the IMF is shown to exhibit a rather definite recurrency relatively independent of the recurrency of the solar wind velocity.The daily mean values of the Z-component of the IMF are shown to correlate ( = -0.5) with the intensity of the meridional component of the large scale solar magnetic field with time delay of about 5 days with respect to the latter. This result is interpreted as an evidence for the Z- component of the IMF to be carried away by the solar wind from the Sun.     

Solar wind heating beyond 1 AU

The effect of an interplanetary atomic hydrogen gas on solar wind proton, electron and α-particle temperatures beyond 1 AU is considered. It is shown that the proton temperature (and probably also the α-particle temperature) reaches a minimum between 2 AU and 4 AU, depending on values chosen for solar wind and interstellar gas parameters. Heating of the electron gas depends primarily on the thermal coupling of the protons and electrons. For strong coupling (whenT _p ≳T _e ), the electron temperature reaches a minimum between 4 AU and 8 AU, but for weak coupling (Coulomb collisions only), the electron temperature continues to decrease throughout the inner solar system. A spacecraft travelling to Jupiter should be able to observe the heating effect of the solar wind-interplanetary hydrogen interaction, and from such observations it may be possible of infer some properties of the interstellar neutral gas. Currently a National Research Council Resident Research Associate.     

Long-term behavior of latitudinal cosmic-ray gradients

We have used neutron monitor data covering a wide range of energy over a period of 22 years (1966–1987), as well as sea-level multidirectional meson telescope data from Nagoya to examine the latitude effect of solar diurnal vectors and its dependence on the polarity of interplanetary magnetic field (IMF). By sorting the daily cosmic-ray data according to whether the IMF is toward (T) or away (A) from the Sun, the annual mean solar diurnal variations (amplitude and phase) for the T and A days were determined separately. Results showed a northward-pointing latitudinal gradient from neutron monitors of the most northerly latitudes, and a predominant southward gradient at high southerly latitudes. The resultant latitudinal cosmic-ray gradients are the sum of two gradients: a north-south symmetry gradient (occurring in minimum and maximum solar activity years), and a north-south asymmetry gradient (occurring during different phases of solar activity cycles). The difference vector (T - A) between the solar diurnal vector for two groups was calculated, which represents a good indicator for the resultant perpendicular gradient relative to the Earth. This difference vector shows a considerable change in phase for detectors located in the northern hemisphere of the Earth. On the other hand, there exists much less change in phase for detectors located in the southern hemisphere.