The last observable line in hydrogen emission spectrum

It is shown that outside continuous solar spectrum or diffused solar light in the Earth's atmosphere substantially reduces the contrast and quality of observable emission lines in the hydrogen spectrum. A formula is obtained which enables the electron concentration in hydrogen plasma to be determined by the last observable line with due account for outside (spurious) continuous spectrum.     

Thermal plasma enhancements in the topside ionosphere and their relationship to the auroral electrojets

Data from the Topside sounder aboard ISIS II are used to calculate the latitudinal distribution of thermal plasma at 1400 km when the satellite moves along a line of constant geographic longitude. The distribution of thermal plasma is related to the latitudinal regime of the auroral electrojets as inferred from data from a meridian line of magnetometers. It is found that thermal plasma at high altitude tends to be found above the poleward portion of the auroral electrojet. This finding is explained in terms of the spectrum of precipitating electrons across the auroral oval. It is found that the thermal plasma distribution in the post-noon sector is distinctly different from that in the premidnight sector despite the fact that both quadrants feature clear latitudinally confined eastward current flow. This difference can be used to define whether or not the polar cleft penetrates into the local time sector traversed by the satellite. On the nightside, the peak in thermal plasma poleward of the ionospheric trough can be used to identify the magnetic field lines which map to the boundary between the tail lobe and the plasma sheet.     

Incoherent Scatter Radar Studies of Daytime Plasma Lines

First results from wideband (electron phase energies of 5–51 eV), high-resolution (0.1 eV) spectral measurements of photoelectron–enhanced plasma lines made with the 430 MHz radar at Arecibo Observatory are presented. In the F region, photoelectrons produced by solar EUV line emissions (He II and Mg IX) give rise to plasma line spectral peaks/valleys. These and other structures occur within an enhancement zone extending from electron phase energies of 14–27 eV in both the bottomside and topside ionosphere. However, photoelectron–thermal electron Coulomb energy losses can lead to a broadened spectral structure with no resolved peaks in the topside ionosphere. The plasma line energy spectra obtained in the enhancement zone exhibit a unique relation in that phase energy is dependent on pitch angle; this relation does not exist in any other part of the energy spectrum. Moreover, large fluctuations in the difference frequency between the upshifted and downshifted plasma lines are evident in the 14–27 eV energy interval. At high phase energies near 51 eV the absolute intensities of photoelectron-excited Langmuir waves are much larger than those predicted by existing theory. The new measurements call for a revision/improvement of plasma line theory in several key areas.     

Interaction of hot plasma with radiation of magnetic white dwarfs

The influence of radiation on the electron velocity distribution in a hot nonrelativistic plasma localized near the surface of magnetic white dwarfs is investigated. The part played by the plasma in the formation of cyclotron features in the optical spectrum of these stars is studied. The region of parameters where the transverse temperature of plasma is defined by the brightness temperature of extraordinary radiation at the gyrofrequency is found. When escaping from the plasma in a homogeneous magnetic field, this component forms a cyclotron line in absorption. The ordinary radiation at the gyrofrequency and both modes at higher cyclotron harmonics are in emission or absorption depending on the magnetic field strength and hot plasma density. Possible interpretation of the observed spectral features of magnetic white dwarfs in terms of the developed theory is discussed.     

Multiple Compton scattering in strongly magnetized plasma

To provide the necessary data for solving the mystery of the γ-ray burst spectrum, we solved the equation of transport of γ photons in a strongly magnetized, purely scattering plasma. We mainly discussed the effect of multiple scattering on the annihilation line photons and γ-ray continuum. In our calculations we assumed the scattering layer to be a non-degenerate plasma with a temperature between (+7)K and (+8)K, and a strong magnetic field of 4.414(+12)G. We used the precise Compton cross-section for a strong field. The effect of the strong field as shown by the results of our calculation will help to gain a deeper understanding of the γ-ray burst spectrum.     

Analysis of the intensities and profiles of the spectral line Mg xii 8.42 Å in the solar X-ray spectrum

High resolution profiles of the Mg xii 8.42 Å line in the solar X-ray spectrum were recorded from the Intercosmos 7 satellite. The Mg xii line intensity provides a sensitive indicator of the hot plasma content (T ? 3 × 10⁶ K) in coronal condensations and X-ray flare volumes. The ratio of the line intensity to the intensity of the adjacent continuum has been used to compute approximate thermal models of the emitting regions. For all the investigated coronal condensations the temperature distribution of plasma has been found to be a function monotonically decreasing with temperature. But for some X-ray bursts there occurred a distinct excess of the hot plasma of temperature between 6–10 × 10⁶K. FWHM values of the Mg xii line profiles have been used to estimate ion temperature in the emitting regions.     

Influence of relativistic effects and vacuum polarization on the transfer of gyroresonance radiation and the stability of the atmospheres of compact stars

We consider the transfer of radiation and calculate the force of its pressure in the electron gyroresonance line in the atmospheres of magnetic degenerate stars. We specify the atmospheric parameters for which an outflow of plasma is possible under radiation pressure in the cyclotron line. We show that the permittivity tensor of a mildly relativistic plasma in a strong magnetic field found by applying relativistic corrections to the cyclotron resonance condition and by taking into account the vacuum polarization and recoil effects during photon scattering should be used to obtain proper results. We have determined the real and imaginary parts of the refractive indices and the polarization coefficients for normal electromagnetic waves when scattering dominates over absorption. Relativistic effects, which change greatly the dispersion and resonant absorption of waves propagating almost perpendicular to the magnetic field, and vacuum polarization have been found to change qualitatively the gyroresonance radiation spectrum and pressure for a wide range of parameters of stellar magnetospheres.     

Soft X-ray spectroscopy of Compton-thick Seyfert 2 galaxies with BeppoSAX

We present an X-ray spectroscopic study of the bright Compton-thick Seyfert 2 galaxies NGC 1068 and the Circinus Galaxy, performed with BeppoSAX . Matt et al. interpreted the spectrum above 4 keV as the superposition of Compton reflection and warm plasma scattering of the nuclear radiation. When this continuum is extrapolated downwards to 0.1 keV, further components arise. The NGC 1068 spectrum is rich in emission lines, mainly owing to K _α transitions of He-like elements from oxygen to iron, plus a K _α fluorescent line from neutral iron. If the ionized lines originate in the warm scatterer, its thermal and ionization structure must be complex. From the continuum and line properties, we estimate a column density, N _warm, of the warm scatterer less than a few×10²¹ cm⁻². In the Circinus Galaxy, the absence of highly ionized iron is consistent with a scattering medium with U _X≲5 and N _warm∼ a few×10²² cm⁻². In both cases the neutral iron line is most naturally explained as fluorescence in the medium responsible for the Compton reflection continuum. In NGC 1068 an optically thin plasma emission with kT ≃500 eV and strongly sub-solar metallicity is required, while such a component is only marginal in the Circinus Galaxy. We tentatively identify this component as emission of diffuse hot gas in the nuclear starbursts. Possible causes for the metal depletion are discussed.     

Unsteady State of the Turbulent Current Sheet of a Flare

The stability of the turbulent current sheet of a flare is analyzed. It is argued that the equilibrium state of the current sheet is extremely unstable relative to certain processes: dissipative tearing instabilities, MHD instabilities of line pinches, overheating of the turbulent plasma, and the threshold dependence of conductivity on the current value. The final state of a flare current sheet must be an extremely inhomogeneous layer containing numerous clusters of bad, turbulent, low-conductivity domains and good, normal ones. The propagation of current in this medium is a percolation process with certain fundamental properties: a threshold regime for current dissipation, which explains the threshold character of the flare phenomenon itself, a universal power-law spectrum of the statistical dependence on flare parameters, and a universal power-law energetic spectrum for the accelerated high-energy particles.     

Si VII/Mg VII and Si IX/Mg IX line diagnostics for solar plasma

New calculations for Si vii/Mg vii and Si ix/Mg ix theoretical line ratios as a tool for plasma density diagnostics and for the estimation of relative element abundance in the solar atmosphere are presented, making use of the EUV spectrum of an active region obtained by the Solar EUV Rocket Telescope and Spectrograph (SERTS). Theoretical line ratios, hitherto not observed, are discussed for diagnosing solar plasma from the observations obtainable with the Coronal Diagnostic Spectrometer (CDS) instrument on board the SOHO mission.In the honour of Dr P. K. Raju, Indian Institute of Astrophysics, Bangalore, on the completion of his 60 years.     

On the influence of resonant absorption on the iron emission‐line profiles from accreting black holes

The fluorescent iron K α emission-line profile provides an excellent probe of the innermost regions of active galactic nuclei. Fe  xxv and Fe  xxvi in diffuse plasma above the accretion disc can affect the X-ray spectrum by iron K α resonant absorption. This in turn can influence the interpretation of the data and the estimation of the accretion disc and black hole parameters. We embark on a fully relativistic computation of this effect and calculate the iron line profile in the framework of a specific model in which rotating, highly ionized and resonantly absorbing plasma occurs close to the black hole. This can explain the features seen in the iron K α line profile recently obtained by Nandra et al. for the type 1 Seyfert galaxy NGC 3516. We show that the redshift of this feature can be mainly gravitational in origin and accounted for without the need to invoke fast accretion of matter on to the black hole. New X-ray satellites such as XMM , ASTRO-E and Chandra provide excellent opportunities to test the model against high-quality observational data.     

The effect of asymmetry on toroidal hydromagnetic waves in a dipole field

The meridional and azimuthal electric wave fields are considered as the characteristic toroidal and poloidal components. Neglecting the exchange of energy between these fields leads to a toroidal mode wave equation which retains the principal longitudinal or asymmetric contribution. The asymmetric spectrum appears as a logical extension of the results for the symmetric field line oscillations. The model for this study consists of a dipole field magnetized plasma, whose density is commensurate with conditions in the plasmapause. Eigenperiods are calculated for a broad range of asymmetric modes. Because of the similarity in the latitudinal variation between the symmetric and asymmetric periods, it is imperative to revise current idealized magnetospheric models and incorporate such similarity in future models.     

Evidence of decay of flux ratio of Fe to Fe-Ni line features with electron temperature in solar flares

We report observational evidence of the decay of the flux ratio of Fe to Fe-Ni line features as a function of plasma electron temperature in solar flares in comparison to that theoretically predicted by Phillips (2004). We present the study of spectral analysis of 14 flares observed by the Solar X-ray Spectrometer (SOXS) — Low Energy Detector (SLD) payload. The SLD payload employs the state-of-the-art solid state detectors, viz., Si PIN and Cadmium-Zinc-Telluride (CZT) devices. The sub-keV energy resolution of Si PIN detector allows us to study the Fe-line and Fe-Ni line features appearing at 6.7 and 8 keV, respectively, in greater detail. In order to best-fit the whole spectrum at one time in the desired energy range between 4 and 25 keV we considered Gaussian-line, the multi-thermal power-law and broken power-law functions. We found that the flux ratio of Fe to Fe-Ni line features decays with flare electron temperature by the asymptotic form of polynomial of inverse third order. The relative flux ratio is ∼30 at temperature 12 MK which drops to half, ∼15 at 20 MK, and at further higher temperatures it decreases smoothly reaching to ∼8 at ∼50 MK. The flux ratio, however, at a given flare plasma temperature, and its decrease with temperature is significantly lower than that predicted theoretically. We propose that the difference may be due to the consideration of higher densities of Fe and Fe-Ni lines in the theoretical model of Phillips (2004). We suggest revising the Fe and Fe-Ni line densities in the corona. The decay of flux ratio explains the variation of equivalent width and peak energy of these line features with temperature.     

Measurements of the ambient photoelectron spectrum from atmosphere explorer: II. AE-E measurements from 300 to 1000 km during solar minimum conditions

The ambient photoelectron spectrum above 300 km has been measured for a sample of 500 AE-E orbits during the period 13 December 1975 to 24 February 1976 corresponding to solar minimum conditions. The 24 h average and maximum ΣK_p were 19 and 35, respectively. The photoelectron flux above 300 km was found to have an intensity and energy spectrum characteristic of the 250–300 km production region only when there was a low plasma density at the satellite altitude. Data taken at local times up to 3 h after sunrise were of this type and the escaping flux was observed to extend to altitudes above 900 km with very little modification, as predicted by several theoretical calculations. The flux at high altitudes was found to be extremely variable throughout the rest of the day, probably as a result of attenuation and energy loss to thermal plasma along the path of the escaping photoelectrons. This attenuation was most pronounced where the photoelectrons passed through regions of high plasma density associated with the equatorial anomaly. At altitudes of 600 km, the photoelectron fluxes ranged from severely attenuated to essentially unaltered—depending on the specific conditions, Photoelectron fluxes from conjugate regions were often less attenuated than those observed arriving from the high density regions immediately below. Comparison of the observed attenuations, photoelectron line broadening, and energy loss due to coulomb scattering from the thermal plasma with rough calculations based on stopping power and transmission coefficients of thermal plasma for fast electrons yielded order of magnitude agreement—satisfactory in view of the large number of assumptions necessary for the calculations. Overall, the impression of the high altitude photoelectron flux which emerges from this work is that the fluxes are extremely variable as a consequence of interactions with the thermal plasma whose density is in turn affected by electrodynamic and neutral wind processes in the underlying F region.     

Scintillations of cosmic radio sources in the decametre waveband

The effect of fluctuations of the interplanetary plasma and the ionosphere upon the scintillation spectra of radio sources at decametre waves is considered with due regard for the finite antenna aperture, fluctuation anisotropy, and the direction of their drift in space. It has been shown that scintillation due to interplanetary plasma (IPP), can be reliably separated from the ionospheric scintillation background at decametre wavelengths.For elongations between 90° to 150°, the IPP scintillation power spectrum observed in the 12.6–25 MHz waveband is of a power law form with the index 3.1±0.6, which is in close agreement with the values known for smaller elongations. The solar wind velocity projection orthogonal to the line of sight is estimated for elongations about 110° and has been found to be 300±80 km s^–1. As in the case of smaller elongations, the velocity dispersion is significant.At night, wideband spectra of ionospheric scintillations are observed in the decametre band, with the breaking point at approximately 0.01 Hz in the 12 m band, and narrow-band spectra whose cut-off frequency is below 0.01 Hz. The power spectrum of ionospheric scintillations is of a power-law form with the index 3.4±0.5. In some cases steeper spectra are observed.     

Wind signatures in the X-ray emission-line profiles of the late-O supergiant ζ Orionis

X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a simple line-profile model. We show that there is distinct evidence for blueshifts and profile asymmetry, as well as broadening in the X-ray emission lines of ζ Ori. These are the observational hallmarks of a wind-shock X-ray source, and the results for ζ Ori are very similar to those for the earlier O star, ζ Pup, which we have previously shown to be well fit by the same wind-shock line-profile model. The more subtle effects on the line-profile morphologies in ζ Ori, as compared to ζ Pup, are consistent with the somewhat lower density wind in this later O supergiant. In both stars, the wind optical depths required to explain the mildly asymmetric X-ray line profiles imply reductions in the effective opacity of nearly an order of magnitude, which may be explained by some combination of mass-loss rate reduction and large-scale clumping, with its associated porosity-based effects on radiation transfer. In the context of the recent reanalysis of the helium-like line intensity ratios in both ζ Ori and ζ Pup, and also in light of recent work questioning the published mass-loss rates in OB stars, these new results indicate that the X-ray emission from ζ Ori can be understood within the framework of the standard wind-shock scenario for hot stars.     

Analytical Approach to Calculate Quasi-thermal Noise Spectrum in Irregular Plasma

Basic properties of quasi-thermal noise spectrum in irregular plasma have been investigated using an analytical point of view. A simple formula for the plasma frequency splitting effect has been obtained for ionospheric conditions. A passive electric antenna, immersed in a stable plasma, detects the fluctuations of the electric potential due to thermal motion of the ambient particles. Properties of this quasi-thermal noise spectrum in homogeneous plasma are relatively well known and are effectively used for diagnostics of space plasma (Aksenov et al., 1978; Trakhtengerts and Chugunov, 1978; Kellog, 1981; Meyer-Vernet and Perche, 1989). Especially, in the Earth's ionosphere or solar wind plasma, random irregularities of electron density are always present. These irregularities may substantially change properties of these media through electromagnetic radiation and may also modify quasi-thermal noise spectrum, which depends on the effective dielectric permittivity tensor. This tensor is defined as the dielectric permittivity tensor of some imaginary `effective' regular medium in which the field of point source is the same as the mean field in the medium with random irregularities (Ryzhov, Tamoikin and Tatarskii, 1965; Ryzhov 1968). Since the correlation function of electric field fluctuations in random medium may be expressed through the effective dielectric constant tensor (Ryzhov, 1968), it may be used for direct calculation of quasi-thermal noise spectrum. In Zabotin et al. (2000), the Born approximation was used to calculate numerically the effective dielectric permittivity tensor and the modified noise spectrum while we analytically estimate herein the modified noise spectrum. This revised version was published online in July 2006 with corrections to the Cover Date.     

康普顿化的计算-推广的Kompaneets方程与Monte Carlo方法比较

在X射线天文学以及辐射物理学中,当硬X射线穿过" 冷"的等离子体时所发生的Compton软化是一种重要的辐射转移过程.简要介绍推广的Kompaneets方程,该方程在hv〈mec2及kTe〈mec2较宽松的条件下广泛成立,因此不仅能处理Compton硬化过程,而且也适用于Compton软化过程,后者在目前快速发展的X射线和γ射线天文学中十分重要.基于此方程,我们对天体物理中4种常见辐射谱(Gauss型发射谱线、黑体辐射谱、幂律辐射谱和热轫致辐射谱)在Compton软化情况下的谱演化进行了数值求解,并同Monte Carlo模拟结果做比较,证实了推广的Kompaneets方程的正确性和有效性.最后指出此方程在X射线天文学和γ射线天文学中的重要意义和潜在应用.     

Diurnal variations in the spectrum of lower-ionosphere plasma irregularities

We examined the variations caused in the spectrum of lower-ionosphere plasma irregularities by diurnal changes of ionospheric parameters. We demonstrate that at a height of 95 km, where the irregularities arise due to the turbulence of the neutral atmosphere, the expected level of plasma irregularity fluctuations at a constant turbulent mixing intensity is smaller in the daytime than at night. The one-dimensional spectrum which can be measured in experiments also changes, namely, the spectrum slope is greater at night.     

An evaluation of the possibility of studying flare plasma turbulence using the satellites of Hei line forbidden components

Using the Baranger-Mozer method, we explore the possibility of diagnosing the flare plasma of forbidden Hei lines, that permits the determination of the plasma oscillation frequency and noise level. Examination of the Hei lines observed in solar flare has led us to conclude that:

1. the appearance of satellites of forbidden components in the flares spectrum, due to turbulent electric fields, is the most probable for Hei 3819.606 Å lines;
2. the Baranger-Mozer method is more sensitive to the high-frequency component of turbulent fields than to the low-frequency ones;
3. the upper limit of the turbulent oscillation level in flares is evaluated.

In the spectrum of the solar flare of 26 September, 1963 we detected satellites of the forbidden component of the 3820 Å line and used its relative intensity to derive the level of low-frequency oscillations (～1.5 kVcm^-1).