Analysis of ion charge states in solar wind and CMEs

We discuss needs in dielectronic recombination data motivated by recent work directed at a quantitative understanding of ion charge states of various elements observed in situ in the solar wind and CMEs. The competing processes of ionization and recombination lead to departures from collision ionization equilibrium. The use of this as a diagnostic of acceleration and heating processes of the solar wind and CMEs is sensitive to the accuracy of the atomic rates in a way that steady state ionization equilibrium plasmas are not. The most pressing need is dielectronic recombination rates for ions Fe8+-12+. These are among the dominant species observed in various regions of the solar wind and CMEs, and in remotely sensed EUV spectra.     

Relaxational dissipation in atmospheric waves—I. Basic formulation

Relaxation of molecular thermal energy between translational and internal degrees of freedom, which occurs in the course of molecular collisions, acts to dissipate dynamical energy of the atmosphere and so produces attenuation of atmospheric waves. This paper establishes a basic formalism by means of which the effects may be incorporated readily into dynamical studies. It does so with the aid of a frequency-dependent complex thermal capacitance, c_ω, for which general formulae are developed. These formulae are extended to include the effects of radiative relaxation as well, and the imaginary part of c_ω is recast in a generalized form of Newtonian cooling coefficient; the relative importance of collisional relaxation and radiative relaxation in the dissipation of dynamical energy may be assessed readily with the aid of either parameter.The ‘volume viscosity’ that is sometimes attributed to the atmosphere is shown to have its origins in the same relaxation processes and to be nothing other than an alternative to C_ω, for expressing their effects, an alternative that is inferior and even illegitimate in some circumstances and therefore one that should be abandoned in favour of c_ω.Though the paper is largely tutorial in nature, it establishes generalizations and limitations that appear to be absent from other work. Its formal results are applied to waves in the Earth's upper atmosphere in a companion paper.     

Stochastic models of hot planetary and satellite coronas: Atomic oxygen in Europa’s corona

This paper analyzes the formation, kinetics, and transport of hot oxygen atoms in the atmosphere of the Jovian satellite Europa. Atmospheric sources of suprathermal oxygen atoms are assumed to be represented by the processes of dissociation of molecular oxygen, which is the main component of the atmosphere, by solar UV radiation and electron fluxes from the inner magnetosphere of Jupiter, as well as by the reaction of dissociative recombination of the main ionospheric ion O ₂ ⁺ which thermal electrons. It is shown that dissociation in Europa’s near-surface atmosphere is balanced by the processes of the loss of atomic oxygen due to the effective escape of suprathermal oxygen atoms into the inner magnetosphere of Jupiter along the orbit of Europa and due to ionization by magnetospheric electrons and catalytic recombination of oxygen atoms on the icy surface of the satellite. It thus follows that atomic oxygen is only a small admixture to the main atmospheric component—molecular oxygen—in the near-surface part of the atmosphere. However, the outer exospheric layers of Europa’s atmosphere are populated mostly by suprathermal oxygen atoms. The near-surface molecular envelope of Europa is therefore surrounded by a tenuous extended corona of hot atomic oxygen.     

Numerical simulations for MHD coronal seismology

Magnetohydrodynamic（MHD） processes are important for the transfer of energy over large scales in plasmas and so are essential to understanding most forms of dynamical activity in the solar atmosphere. The introduction of transverse structuring into models for the corona modifies the behavior of MHD waves through processes such as dispersion and mode coupling. Exploiting our understanding of MHD waves with the diagnostic tool of coronal seismology relies upon the development of sufficiently detailed models to account for all the features in observations. The development of realistic models appropriate for highly structured and dynamical plasmas is often beyond the domain of simple mathematical analysis and so numerical methods are employed. This paper reviews recent numerical results for seismology of the solar corona using MHD.     

Radiation condensation instability in a plasma with ionization and recombination

In this work, we consider radiation (thermal) instability in a weakly ionized plasma with continuous ionization and recombination. The situation can be visualized in the case of envelopes of planetary nebulae, which are envelopes of ionized plasmas surrounding red giant stars. Various observations report continuous photoionization of these plasmas by the highly energetic streams of photons emanating from the parent star. Recently, it has been shown that thermal instability can be a probable candidate in such plasmas for the existence of small scale structures (viz., striations) whose kinematic age is much smaller than that of the parent nebula. We therefore report a systematic study of these plasmas with photoionization and determine the instability domain. We have shown that the continuous ionization and recombination may lead to modification of the underlying instability, which may limit the size of the small structures that are believed to form from these instabilities, and may thus provide an explanation of the physical processes responsible for the existence of these structures. We further show that in many cases the system bifurcates to an ovserstable (growing wave) state from a condensation instability (monotonic) and vice versa.     

Stellar adiabatic mass loss model and applications

Roche-lobe overflow and common envelope evolution are very important in binary evolution, which is believed to be the main evolutionary channel to hot subdwarf stars. The details of these processes are difficult to model, but adiabatic expansion provides an excellent approximation to the structure of a donor star undergoing dynamical time scale mass transfer. We can use this model to study the responses of stars of various masses and evolutionary stages as potential donor stars, with the urgent goal of obtaining more accurate stability criteria for dynamical mass transfer in binary population synthesis studies. As examples, we describe here several models with the initial masses equal to 1 M _⊙ and 10 M _⊙, and identify potential limitations to the use of our results for giant-branch stars.     

Advances in numerical modeling of astrophysical and space plasmas

Plasma science is rich in distinguishable scales ranging from the atomic to the galactic to the meta-galactic, i.e., themesoscale. Thus plasma science has an important contribution to make in understanding the connection between microscopic and macroscopic phenomena. Plasma is a system composed of a large number of particles which interact primarily, but not exclusively, through the electromagnetic field. The problem of understanding the linkages and couplings in multi-scale processes is a frontier problem of modern science involving fields as diverse as plasma phenomena in the laboratory to galactic dynamics.Unlike the first three states of matter, plasma, often called the fourth state of matter, involves the mesoscale and its interdisciplinary founding have drawn upon various subfields of physics including engineering, astronomy, and chemistry. Basic plasma research is now posed to provide, with major developments in instrumentation and large-scale computational resources, fundamental insights into the properties of matter on scales ranging from the atomic to the galactic. In all cases, these are treated as mesoscale systems. Thus, basic plasma research, when applied to the study of astrophysical and space plasmas, recognizes that the behavior of the near-earth plasma environment may depend to some extent on the behavior of the stellar plasma, that may in turn be governed by galactic plasmas. However, unlike laboratory plasmas, astrophysical plasmas will forever be inaccessible to in situ observation. The inability to test concepts and theories of large-scale plasmas leaves only virtual testing as a means to understand the universe. Advances in in computer technology and the capability of performing physics first principles, fully three-dimensional, particle-in-cell simulations, are making virtual testing a viable alternative to verify our predictions about the far universe.The first part of this paper explores the dynamical and fluid properties of the plasma state, plasma kinetics, and the radiation emitted from plasmas. The second part of this paper outlines the formulation for the particle-in-cell simulation of astrophysical plasmas and advances in simulational techniques and algorithms, as-well-as the advances that may be expected as the computational resource grows to petaflop speed/memory capabilities.Dedicated to the memories of Hannes Alfvén and Oscar Buneman; Founders of the Subject.     

On the Lifetime of Hot Coronal Plasmas Arising from Nanoflares

The cooling of plasmas in closed coronal loops by thermal conduction is important when considering their detectability at X-ray and EUV wavelengths. A non-local formalism of thermal conduction originating in laboratory plasmas is used and it is shown that while the effect is unlikely to be important for loops that are in a steady state, it does play a significant role in loops that are impulsively heated (e.g., by nanoflares). Such loops are “under-dense”, and so hot electrons have a relatively long mean free path. Analytic and numerical models are presented, and it is shown that conduction cooling times are lengthened quite considerably. A comparison of various cooling times with ionisation times is also presented, and it is noted that this conductive physics may enhance the chances of observing hot nanoflare-heated plasma.     

Comments on the observability of coronal variations

We discuss the observable variability of spectral lines in the soft X-ray and XUV region. Rapid variability of coronal emission, both in flaring and non-flaring structures has been reported and is particularly prominent when high spatial resolution is available. Examination of the ionization and recombination time-scales for the formation and removal of ions with prominent solar emission lines shows that, even though ionization equilibrium generally prevails, the observable variability time-scales are often limited by these atomic processes, independent of the physical process which is causing the change in the solar atmosphere. Rapid heating can lead to an initial freezing-in of abundances of some ions; observations of at least one low- and one high-excitation line from such an ion would permit studies of the time evolution of the emission measure and temperature. In a very limited number of cases, rapid cooling leads to freezing-in of the abundance of an ion and observations of a low-excitation line of this ion will not yield accurate information about the thermal evolution. Thus, future observations of Mgx 609 Å should be augmented by simultaneous observation at another wavelength, such as 63 Å. In addition, with the ability to produce images in isolated spectral lines it becomes possible to select those for which rapid variability is observable, such asOvii, rather than lines which were selected on the basis of previous hardware constraints, such asOvii.     

On hierarchical cosmology

Progress in laboratory studies of plasmas and in the methods of transferring the results to cosmic conditions, together within situ measurements in the magnetospheres, are now causing a ‘paradigm transition’ in cosmic plasma physics. This involves an introduction ofinhomogeneous models with double layers, filaments, ‘cell walls’, etc. Independently, it has been discovered that the mass distribution in the universe is highly inhomogeneous; indeed,hierarchical. According to de Vaucouleurs, the escape velocity of cosmic structures is 10²–10³ times below the Laplace-Schwarzschild limit, leaving avoid region which is identified as a key problem in cosmology. It is shown that a plasma instability in the dispersed medium of the structures may produce this void and, hence, explain the hierarchical structure. The energy which is necessary may derive either from gravitation or from annihilation caused by a breakdown of cell walls. The latter alternative is discussed in detail. It leads to a ‘Fireworks Model’ of the evolution of the metagalaxy. It is questioned whether the homogeneous four-dimensional big bang model can survive in an universe which is inhomogeneous and three-dimensional.     

Time dependent studies of the aurora—I. Ion density and composition

Ion densities and composition are investigated in a time varying model aurora. There is a time lag between turning on the source of ionization and the resulting increase in ion densities that depends on the species and the height level in the ionsophere, so that altitude profiles of auroral electron densities evolve with time. Characteristic buildup times for the ionization are a few seconds at the altitude of maximum energy deposition, increasing to tens of seconds above and below this level. A wide range of composition ratios, n(NO⁺/n(O₂⁺ and n(NO⁺/n(O⁺), can be expected, depending on the time an observation is made during buildup or decay of ionization. The concentrations of atomic nitrogen and nitric oxide increase as a result of auroral ionization, but the associated characteristic times are long compared to the average duration of ‘auroral event’. Thus, intermittent auroral bombardment could result in a gradual buildup of these minor neutral constituents in the auroral atmosphere. Variations in the electron density during pulsating, fluctuating or coruscating aurora lag the source function variations by a few seconds in a typical aurora.     

Dynamical equation for the degree of ionization of the cosmic substratum before the formation of the galaxy

We discuss the variable degree of ionizationx of a hydrogen-helium plasma in the early Universe for a quasi-static expansion in thermal equilibrium. The final equation for the degree of ionization can be reduced to a polynomial of fourth order inx with known coefficients depending on the temperature. Restricting to a pure hydrogen-plasma applied to the recombination era, where the main ionization effects are due to photo-electric and collisional processes, we study the dynamical evolution of the degree of ionization for nonstatic and nonequilibrium situations. The calculation can be reduced to a pure quadrature. In the linear case, we also calculate the rate of ionization.     

Orientation of galaxies and a magnetic ‘urfield’

Brown's results (1964, 1968) concerning the distribution of orientation angles of spiral galaxies in different areas of the sky are discussed and a graphical statistical test is applied. The deviations from randomness are found to be significant. It seems difficult to ascribe them to selection effects. It is shown that the observed distributions can be explained, if the angular momenta in greater aggregations of galaxies are distributed at random on congruent precession cones with parallel axes. This hypothesis may apply if the following cosmological conditions hold:

1. the matter in the universe was reheated after the recombination at an epoch, when most of the angular momentum was already transferred to the protogalaxies by tidal interaction, and the angular momenta of the protogalaxies in greater aggregations were predominantly parallel at the epoch of reheating;
2. the ‘magnetic’ model of the universe is valid and the ‘urfield’ was uniform at least at the epoch of reheating. Under these assumptions, the ‘frozen-in’ magnetic field will give rise to forces, which — apart from slowing down the rotation of the protogalaxies — will cause precession of their angular momenta around the direction of the ‘urfield’.

For a rigid body approximation the equations of motion are derived and solved numerically. Approximate analytic solutions are also given. The precession period is in the range of 10⁴ to 10⁸ yr for plausible values of the parameters of the problem. The observed distributions in the four regions of the sky investigated are — via the precession hypothesis — compatible with a direction of the ‘urfield’ indicated by the work of Sofueet al. (1969) and Reinhardt and Thiel (1970) ofl ^II≈280°,b ^II≈+30° tol ^II≈100°,b ^II≈?30°.     

X-ray Time Lags in TeV Blazars

We use Monte Carlo/Fokker?CPlanck simulations to study the X-ray time lags. Our results show that soft lags will be observed as long as the decay of the flare is dominated by radiative cooling, even when acceleration and cooling time scales are similar. Hard lags can be produced in the presence of a competitive achromatic particle energy loss mechanism if the acceleration process operates on a time scale such that particles are slowly moved towards higher energy while the flare evolves. In this type of scenario, the ??-ray/X-ray quadratic relation is also reproduced.     

The structure of the polar ionosphere during exceptionally quiet periods

Knowledge of the structure of the polar ionosphere during exceptionally quiet periods is basic for studying complicated ionospheric behaviors during disturbances. On the basis of data from an airborne ionosonde as well as a meridian chain of ground-basedionosondes, the circumpolar structure of the E,-and F-regions is elucidated. There are two circumpolar zones of E-region ionization with differing characteristics. The first is an auroral E,-layer and/or retarded type sporadic E-band that has previously (Whalen et al., 1971) been found to be identical with the continuous aurora. The second is a zone of non-retarded type spora die E located poleward of the former band. In general, discrete auroras are co-located with the latter. The main trough, a prominent feature of the night sector F-region, is most pronounced in the early morning. The main trough is bounded on the poleward side by a well defined ‘wall’ of F-region ionization. The night sector poleward trough wall is located approximately three degrees of latitude equatorward of the auroral oval. A ‘plateau’ of F-region ionization extends from the poleward trough wall to the auroral oval.     

Physical adsorption of hydrogen on interstellar graphite grain surfaces

We review existingsingle-particle theories concerning parameters of importance which determine the kinetics of hydrogen molecule formation and ejection from cold (T _gr?20K) graphite grain surfaces. The nature of thesingle-particle quantum states of low mass gas atoms and molecules in a periodic surface lattice potential is considered. Contributions to the physical adsorption potential due to dynamic polarizability effects arising from thelong-range collective valence-electron charge-density oscillations (plasmons) of the substrate are discussed.Short-range electron correlation effects at the surface may lead to the formation of a ‘quasimolecular state’ of adsorbed H₂ with a bond length ～3.5 Å and a reduced bond energy ～0.075 eV. It is proposed, that one consequence of this dynamical screening of the adsorbed molecules is that they are ejected normal to the grain surface with velocities ?20 km s^?1 and not necessarily in a high vibrational state. Similar dynamical effects could be important in determining activation processes and long-range ordering in monolayer films of adsorbed H₂. The astrophysical consequences of thesemany-body effects are discussed in the light of recent experimental and observational results.     

Ionization freeze-out and hydrogen excitation in the SN IIP atmosphere

We study the nonstationary recombination of hydrogen in the atmosphere of SN 1987A by taking into account ion-molecular processes. The hydrogen excitation due to nonstationary recombination is shown to be enough to explain the observed hydrogen lines in a time interval until day 30 in the absence of additional excitation mechanisms. Thus, the problem of a deficit in the hydrogen excitation that has recently been found in modeling the hydrogen spectrum of SN 1987A at an early photospheric stage by assuming statistical ionization equilibrium is resolved. The mass of the radioactive ⁵⁶Ni with a spherically symmetric distribution in the outer layers is shown to be close to 10^?6 M _⊙. Our model predicts the appearance of a blue peak in the Hα profile between days 20 and 30. This peak bears a close similarity to the observed peak known as the Bochum event. The presence of this peak in the model is attributable to nonstationary recombination and to a substantial contribution of hydrogen neutralization involving H^? and H₂.     

Analysis of the non-Gaussian spectra of interplanetary scintillations

The non-Gaussian intensity fluctuation spectra observed by Cohenet al. (1967) are analysed. Computations of the length scales derived from the phase autocorrelation functions using Buckley's method (1971, I) indicate that for a rms phase deviation of 4 radians or more the diffracting medium behaves as one with its phase structure having ‘inner’ and ‘outer’ scales of turbulent blobs or eddies which are present in a turbulent medium.