Einstein observations of cool stars

Observations of cool stars with the $\text{Einstein}$ Observatory (HEAO-2) have brought about a fundamental change in our knowledge and understanding of stellar coronae. The existence of X-ray emission from stars throughout the H-R diagram, the wide range of X-ray luminosity within a given spectral and luminosity class, and the strong correlation of X-ray luminosity with stellar age and rotation are among the more significant $\text{Einstein}$ results. These results are strong evidence for the influence of stellar dynamo action on the formation and heating of stellar coronae. A discussion of relevant consortium and guest observations will be given. The Hyades cluster, in particular, will serve as an example to demonstrate the usefulness of X-ray observations in the study of stellar activity and coronal evolution.     

Einstein observations of hot stars

Surveys with instruments on the $\text{Einstein Observatory}$ have shown that essentially all 0 and B main sequence stars are X-ray sources as are many, if not all, 0B supergiants and Wolf-Rayet stars. The X-ray luminosities are sufficient to explain broad lines from the superionization stages seen in the UV spectra of the stars. High energy resolution spectra from the Solid State Spectrometer are shown to place severe constraints on various models for the location of the X-ray sources in the outer atmospheres of the stars. Coronal and embedded shock models for the X-ray emission are discussed and each is found to have some problems in explaining the X-ray emission of 0B stars. X-ray line emission of Si XIII and S XV in ? Ori is discussed and interpreted as arising from magnetically confined loops.     

I.U.E. high resolution observations of hot stars emitting coronal x-rays

The far UV resonance lines of a sample of 21 early-type stars, which were observed in the soft X-Ray band with the $\text{Einstein}$ satellite, are examined using I.U.E. high resolution spectra to search for possible correlation between the X-Ray coronal emission and far UV spectral properties. In particular, those quantities that can give information on the structure of the outer envelope (such as wind terminal velocities, emission-absorption ratios) are measured and compared with the observed X-Ray flux.     

Solar flare proton release from coronal magnetic traps and strong Alfvén turbulence in the corona

Drift instabilities arising when accelerated protons are trapped by coronal magnetic fields of active regions are investigated theoretically. If β, the ratio of total (plasma + energetic particles) pressure and magnetic field pressure is larger than some value, $\text{β?0.1}\text{to}\text{0.3}$, the magnetic trap is destructed and protons are released into interplanetary space. If $\text{β < β}{}^1$, the trapped protons excite gradient instability due to magnetic drift resonance. This “universal” instability results in rapid development of strong Alfvén wave turbulence with small wavelengths transverse to the magnetic field. Particle diffusion due to the waves has a rather complicated character and appears to be weak as compared to quasilinear diffusion. The role of Alfvén waves may consist in additional heating of the corona in the regions of closed magnetic field lines.     

Magnetic flux ropes in the Venus ionosphere: In situ observations of force-free structures?

Force-free magnetic structures with cylindrical geometry appear under a variety of conditions in nature. Filamentary helical magnetic structures are observed to be associated with prominences and flares in the solar atmosphere, and can arise in superconductors and laboratory plasmas. Another example of cylindrical quasi-force-free configurations appears to exist in the Venus ionosphere. Magnetic flux ropes with diameters of ～20 – 30 km have been observed by the Pioneer Venus Orbiter to be a nearly ubiquitous feature of the dayside Venus ionosphere. Models of flux ropes suggest that many of these structures tend to be quasi-force-free, i.e., $\text{J}$×$\text{B}$～0, while others are correlated with pressure variations in the ambient thermal plasma, $\text{J}$×$\text{B}$=-?(nkT).     

Highly variable X-ray emitting objects in the Rho Oph dark cloud

The Rho Ophiuchi dark cloud region has been the subject of an extensive guest investigation using the $\text{Einstein}$ Observatory. The set of observations comprise 14 IPC fields and 3 HRI fields. The densest part of the cloud has been observed 6 times. Forty seven sources were detected at a level > 3.5 σ and twenty more above 2 σ. The majority of these sources have optical, IR, or even radio continuum counterparts; nine are identified with known T Tauri stars, while several others are identified with stars showing H α in emission. All show a high degree of time variability; flux variations reach factors of 5 in a few hours, or 25 in a day. Apparent luminosities are in the range 10(30) – 10(31)(1) erg.s^?1. The possibility that the X-ray variability is due to flares is examined. If this interpretation is correct, one source has been the seat of the largest stellar flare ever recorded in X rays [L_x = 10(32) erg.s^?1, E_x ?10(36) ergs^-].     

Magnetic flux expulsion as an acceleration mechanism for stellar winds

Observations of the Sun show that magnetic flux is emerging through the surface in small scales in rather copious amounts. In order to maintain a steady state field strength, this flux must either be locally dissipated or explelled or both. We believe that magnetic reconnection and subsequent flux explusion is the most effective manner in which to achieve this. If new flux emerges into an already preexisting coronal magnetic field, the ambient field must be pushed aside to allow room for the new flux. If the ambient field strength decreases outward with radial distance as is expected for all stars, it may pinch off the emerging flux through magnetic reconnection and expell it outward. The net force on an isolated diamagnetic plasmoid produced by this process is shown to assume a particularly simple form, depending only on the plasmoid's mass, its temperature, and the radial gradient of the logarithm of the undisturbed magnetic pressure. If a sufficient number of these magnetic elements are produced per unit time, this process translates to a net outward magnetic force on the coronal plasma which can be greater that the gas pressure force. Thus, a stellar wind can be produced by magnetic forces alone without the need for a high coronal gas pressure — a mechanism which could be effective in explaining why stars, such as the late-type giants, which possess cool coronae nevertheless exhibit vigorous coronal expansions.     

IUE observations of cool stars

A summary of IUE results concerning late-type stars is presented. Observations show that high-temperature outer atmospheres, as indicated by N V, C IV emission at T ≈ 10⁵K, are generally present only in high-gravity (log g ? 2) stars. Objects with high-temperature emission tend not to exhibit cool circumstellar shells, and vice versa, although there are several transition objects, the hybrid atmosphere stars, which combine C IV emission with cool winds. Ultraviolet emission from stellar transition regions correlates well with chromospheric and X-ray emission. Transition-region line ratios indicate that many stars have differential emission measure distributions similar to the Sun's. Ultraviolet observations also give indications of important dynamical effects in low-gravity stars. Density diagnostics indicate extended chromospheres for some red giants and supergiants. In addition, the large widths of lines of high temperature ions in several luminous stars indicate supersonic motions.     

Dust in magnetised plasmas: Basic theory and some applications

In this paper we review the theory of charged test particle motion in magnetic fields. This theory is then extended to charged $\text{dust}$ particles, for which gravity and charge fluctuations play an important role. It is shown that systematic drifts perpendicular to the magnetic field and stochastic transport effects may then have to be considered - none of which occur in the case of atomic particles (with the exception of charge exchange reactions). Some applications of charged dust particle transport theory to planetary rings are then briefly discussed.     

The observation of ocean-surface fluxes of heat,water and momentum for climate studies

The potential of satellite measurements to define the ocean surface fluxes of heat, water and momentum is reviewed. Only surface stress and possibly rainfall can be directly estimated, latent heat flux may be available through parametrization, sensible heat flux cannot be obtained. Each of the radiative flux components may be estimated including possibly the downward longwave flux. However it is emphasised that, even for those fluxes which can be obtained, improvements in absolute accuracy of the monthly mean, area averaged values are required. Sampling by a single polar orbiting satellite is likely to be at best, marginally adequate. In most cases a pair of satellites will be needed.Calibration and continued validation of the satellite data using improved $\text{in situ}$ data will be necessary, and a combination of measurement systems will have to be used if the accuracy requirements are to be approached. Provision of $\text{in situ}$ data systems should be considered as part of the planning for future satellite missions. Satellite data collection and location could result in a considerable improvement to the $\text{in situ}$ data set.     

Evidence for mass-loading of the Venus magnetosheath

The observed magnetic field configuration in the Venus magnetosheath contains information about the solar wind mass-loading processes occurring as a result of the extension of the neutral atmosphere into the magnetosheath. In this paper, magnetic field signatures of various mass-loading processes are discussed and experimental results from the Pioneer Venus Orbiter magnetometer experiment are examined for evidence of these signatures. The data suggest that the ?$\text{V}$X$\text{B}$ acceleration process, stochastic pickup of ionospheric ions, and $\text{J}$X$\text{B}$ force “scavenging” at the ionopause all occur at various times.     

On different sensitivities of microorganisms to lowered gravitation

The influence of lowered gravitation on biomass and CO₂ production in $\text{B.megaterium}$, a xerophyte, and $\text{Spirillum azotocolligens}$, an aqueous spirillum, in liquid nutrient medium on a horizontal clinostat at 0.1 $\text{g}$ has been studied. As controls we considered: 1) growth under stationary conditions of cultivation with test tubes oriented horizontally; 2) growth on a synchronously revolving centrifuge; and 3) growth on a swing with stirring. A horizontal clinostat at 0.1 g stimulates biomass production and CO₂ release in $\text{B.megaterium}$ as compared with the controls. $\text{Spirillum azotocolligens}$ growth is reduced as a result of clinostating. The best development and CO₂ production are observed under stationary conditions. The results do not support the assumption that microorganisms living in water are more resistant to lowered gravitation than those living in soil.     

部分开放多极磁场中的日冕磁绳灾变

采用球坐标下二维三分量理想MHD模型，研究部分开放多极背景磁场中日冕磁绳的灾变现象．背景磁场由含3个闭合双极场的冕流和带赤道中性电流片的开放场构成，磁绳位于中心双极场的下方，其特性由环向磁通和轴向磁通表征．对给定的环向磁通，存在轴向磁通的一个临界值；对给定的轴向磁通，也存在环向磁通的一个临界值．在该临界值以下，磁绳附着于太阳表面，系统处于平衡状态；该临界值一旦被超越，磁绳将脱离太阳表面向上喷发，说明部分开放多极背景磁场中的日冕磁绳系统存在灾变现象．本文算例表明，灾变点对应的磁能阈值超过对应部分开放场(中心双极场开放，两侧的双极场仍维持闭合)能量约15％，其超过部分可为日冕物质抛射一类太阳爆发提供能源．     

Photometric calibration and first results obtained with the UFT experiment on board the astron satellite

The UV telescope of 30 cm diameter equipped with a three channel scanner spectrometer aboard the highly eccentric orbit space station “ASTRON” is describedAbsolute calibration of the whole device is performed using spectra of standard stars, of which absolute values were obtained by TDI or IUE experiments. Several photometric controls have been made on the stability of the equipment after one year of operation which demonstrate reliability of some first results obtained by the UFT.The sensitivity curve of the spectrophotometric system has been observed with a reproducibility of 2σ = 10% or better.The repeated observation of one star (139 Tau) indicates that the overall performance exhibits the same quality after one year in orbit. The low resolution mode ($\text{Δ λ = 28}\text{A}\text{?}$) leads to a powerful new investigation of faint extended sources.     

Optical and electron-microscopic studies of the Funaria hygrometrica protonema after cultivation for 96 days in space

$\text{Funaria hygrometrica}$ protonema cells grown in the “IFS-2” (Inoculating fixing system) for 96 days on board the Salyut 6 — Soyuz 32 orbital scientific station were examined by light and electron-microscopy. Investigation of experimental and control cells of the moss protonema showed common features as well as distinctions in their structure. Protonema cells of $\text{Funaria hygrometrica}$ both differentiate and undergo photosynthesis during space flight. Changes in cell shape, decreased cell size, a reduction in the volume of starch granules, and altered chloroplast structure were observed.     

An analytical model of prominence dynamics

Solar prominences are magnetic structures incarcerating cool and dense gas in an otherwise hot solar corona. Prominences can be categorized as quiescent and active. Their origin and the presence of cool gas ($～{10}^4$?K) within the hot ($～{10}^6\text{K}$) solar corona remains poorly understood. The structure and dynamics of solar prominences was investigated in a large number of observational and theoretical (both analytical and numerical) studies. In this paper, an analytic model of quiescent solar prominence is developed and used to demonstrate that the prominence velocity increases exponentially, which means that some gas falls downward towards the solar surface, and that Alfvén waves are naturally present in the solar prominences. These theoretical predictions are consistent with the current observational data of solar quiescent prominences.