Nature of the fine structure of the middle chromosphere

Fine dark H filaments fibrils form at the limb, apparently in most of the middle chromosphere corresponding to an altitude between 1500–2000 km and 4000 km. The space in between filaments is corona and the transition layer. The cool gas in fibrils is protected by the magnetic field against the conductive flux out of the hot corona. Therefore the fibrils stretch up to 4000 km where their temperature is about 18 000 K and the density about 5 × 10⁹ cm^–3. The gas in the fibrils is ionized by electronic collisions and by the external ultraviolet radiation. The second level of the hydrogen atoms in fibrils is populated by recombinations, electronic collisions and by Ly- quanta. The calculated optical thickness of the fibrils in H is about 1, it explains the absorption features on the spectroheliograms. The gas pressure in fibrils is lower than the coronal pressure, and the pressure equilibrium is achieved by a magnetic field of about 1.5–2 G. In the active regions the photospheric fields are stronger, therefore the fibrils in active regions are wider and show more contrast. The emission of the fibrils at the limb is explained by the scattering of the solar radiation. The temperature in arches reaching as high as 5000–6000 km, is stabilized near the top by the HeII emission. Thus the middle chromosphere is essentially a collection of magnetic arches.     

Precise wideband photometry of photospheric faculae with an emphasis on the disk center

The center-to-limb variation of the excess intensity in faculae was obtained for 266 active regions with an accuracy of 10^–3. For this observation full-disk images were obtained with a rotating one-dimensional diode array whose rotation axis was set at the disk center, at the wavelength of 5450 Å with a bandpass of 400 Å. From the center-to-limb variation of excess intensity of active regions the excess effective temperature was found to be 6.4 K on the average where the mean longitudinal magnetic field is 65 G as measured by 5233 Å line. In other words the ratio of the excess radiative flux to the total flux was 0.44% on the average for the present measurements of low spatial resolution of 20.The average excess intensity for 60 active regions near the disk center was found to be 4 × 10^–4 of the quiet Sun intensity. This very low excess brightness averaged over the whole active region, in contrast to the reported high excess brightness of facular points (diameter 0.2) of 0.4, leads to a hypothesis that the background in between facular points in the active region is darker than the true quiet photosphere by 1%. It is further surmised that the inferred darkness of intra facular points is due to partial compensation for excess total irradiance of facular points. This interpretation is also consistent with previous observations of the contrast of facular points near the limb.     

Time variation of the spectrum of Gyro-synchrotron emission from the Sun

The emission spectra and their time variations of gyro-synchrotron emission from an ensemble of energetic electrons are computed for some initial power-law distributions of the electron energies N()d= ^– with =2 or 4. The spectra and decay curves of the emission are compared with solar microwave bursts in order to separately estimate the magnetic field H and . From a limited number of observations, we have 3 and H 10³ gauss for the microwave impulsive bursts, and 2 and H (500–1000) gauss for the microwave type-IV bursts.     

An investigation of the structure of coronal active regions

The temperature and density structure of a typical coronal active region is deduced from X-ray observations of several active regions. Observations of the limb transits of three regions from OSO-5 indicate that the X-ray emission originates between 2 × 10⁴ km and 1.5 × 10⁵ km. An emission measure-temperature distribution is deduced from high resolution X-ray spectra obtained with a rocket observation of two similar regions. These observations are combined to give a model of a typical active region, the temperature varying from 2 to 6 × 10⁶ K with corresponding densities between 2 × 10⁹ and 10¹⁰ cms^–3.     

Estimates of the γ-ray flux from Sagittarius A

Analysis of the anisotropy of the -ray background observed by Clarket al. (1968) has lead to the suggestion that part of the anisotropy be due to a source at the Galactic Centre. The complex structure of radio and infrared emission of the region of Sgr A is considered. The -ray flux arising from Compton scattering and ₀ decay is calculated. For a value of the magnetic field H10^–4 Gauss, corresponding to equipartition, the -ray flux above 100 MeV at Earth from the Sgr A source is expected to lie between 10^–6 and 10^–5 gamma/cm² sec. Such a source should be detectable in the near future.     

Lyman continuum observations of solar flares

A study is made of Lyman continuum observations of solar flares, using data obtained by the Harvard College Observatory EUV spectroheliometer on the Apollo Telescope Mount. We find that there are two main types of flare regions: an overall mean flare coincident with the H flare region, and transient Lyman continuum kernels which can be identified with the H and X-ray kernels observed by other authors. It is found that the ground level hydrogen population in flares is closer to LTE than in the quiet Sun and active regions, and that the level of Lyman continuum formation is lowered in the atmosphere from a mass column density m 5/sx 10^–6 g cm^–2 in the quiet Sun to m 3/sx 10^–4 g cm^–2 in the mean flare, and to m 10^–3g cm^–2 in kernels. From these results we derive the amount of chromospheric material evaporated into the high temperature region, which is found to be - 10¹⁵g, in agreement with observations of X-ray emission measures. A comparison is made between kernel observations and the theoretical predictions made by model heating calculations, available in the literature; significant discrepancies are found between observation and current particle-heating models.     

Solar X-ray spectra observed from the ‘Intercosmos-4’ satellite and the ‘Vertical-2’ rocket

Results are given of the detailed analysis of fourteen Fe xxv-xxiii lines ( = 1.850–1.870 Å) in the spectra of a solar flare on 16 Nov. 1970. The spectra were obtained with a resolution of about 4 × 10^–4 Å, which revealed lines not previously observed and allowed the measurement of line profiles. The measured values of the wavelengths and emission fluxes are presented and compared with theoretical calculations. The analysis of the contour of the Fe xxv line ( = 1.850 Å) leads to the conclusion that there is unidirectional macroscopic gas motion in the flare region with the velocity (projection on the line of sight) ± 90 km s^–1.Measurements of the 8.42 Å Mg xii and 9.16 Å Mg xi lines in the absence of solar flares indicate prolonged existence of active regions on the solar disk with T _e = 4–6 × 10⁶K and emission measure ME 10⁴⁸ cm^–3. The profile of the Mg xii line indicates a macroscopic ion motion with a velocity up to 100 km s^–1.     

Coronal bright points at 6 cm wavelength

We report the results of the first observations of solar coronal bright points at 6 cm wavelength using the Very Large Array (VLA), with a spatial resolution of 1.2. The maximum brightness temperature of the sources observed is 3 × 10⁴ K with a mean value of 1 × 10⁴ K (above the quiet Sun value). The lifetime of most sources is between 5 and 20 min. The average diameter of the sources is about 5–15 arc. The sources are gaussian-like near the footpoint of miniature loops and they appear in groups. The observations indicate that significant fluctuations in the brightness temperature (sometimes quasi-periodic) and in the spatial extents of these sources can occur over periods of a few minutes.On leave from Beijing Observatory, Beijing, Peoples Republic of China.     

Models of clusters of point masses with large central redshifts

Conclusions In the Newtonian case we have obtained an isotropic self-consistent distribution of gravitationally interacting point masses which satisfies the transport equation without collisions, and the gravitational equation for an arbitrary powerfunction density distribution =r^–s, s<3.For =r^–2 the analogous self-consistent solution was obtained for the anisotropic distribution function both in Newtonian and GTR cases.The GTR solutions with =r^–2 have central redshifts which increase without limit in accordance with the law 1+zr^–1/ as we approach the center. In the isotropic case, they appear to be stable when the mean velocities are much less than the velocity of light u<0.2c, >21.The hydrodynamic GTR solution was found for a perfect gas at constant temperature (but variable T=T(g₀₀)^1/2) which also has z for r0.We should like to thank K. Thorne, L. Hazin, and M. Podurets for valuable discussions. K. Thorne was particularly helpful in supplying unpublished results on circular orbits obtained by American authors.Astrofizika, Vol. 5, No. 2, pp. 223–234, 1969     

X-ray luminous IRAS galaxies

A sample of 14708 extragalactic IRAS sources selected from the Point Source Catalog via statistical classification has been cross-correlated with the ROSAT All Sky Survey (Bolleret al., 1992). 244 galaxies emerge as being detected both by ROSAT and IRAS.The most interesting point is the discovery of a dozen of normal spirals whose X-ray luminosities reach nearly 10⁴³ erg s^–1 (0.1–2.4 keV), higher than previous detection limits of a few 10⁴¹ erg s^–1 (Stockeet al. 1991; Green, Anderson and Ward, 1992). We obtained optical spectra for nine of them, showing Seyfert spectra for three of the objects (thus previous mis-classifications), spectra close to LINERs for two further objects and normal IRAS galaxy spectra for the last four.In the case of normal galaxies, the source of energy is not clear yet and could be related to the high rate of star formation likely to occur in the central regions or also to a low level active nucleus. The study of these objects is of interest to the nature of LINERs and, more generally, to possible relations between AGN and starbursts (Sanderset al., 1988).     

Generation of γ-bursts by old magnetic neutron stars

A model of -bursts is considered that treats the flares of neutron stars as a result of convectiveoscillation instability associated with the stars having strong internal magnetic fields ( 10¹³ to 10¹⁴ G). In the context of this model only sufficiently old (10⁴ to 10⁷ yr), drastically cooled-down neutron stars may be sources of -bursts. The paper shows that major characteristics of a -burster in the Supernova N 49 remnant (energy release during burst up to 10⁴⁴ erg, age 10⁴ yr, burst-to-burst interval (I to 3)×10⁶s; rotation period P=8 s) may be explained under the assumption that the mass of the neutron star is about 0.14M _· while its mean magnetic field strength is 1.5×10¹⁴ G abd 10¹³ G within the star and on its surface, respectively. The observational tests of the model discussed conclude the paper.     

On the inability of magnetically constricted transition regions to account for the 105 to 106 K plasma in the quiet solar atmosphere

Although back conduction from the corona has been shown to be inadequate for powering EUV emission below T 2 × 10⁵ K, it is thought to be adequate in the temperature range 2 × 10⁵ K < T < 10⁶ K. No models to date, however, have included the large magnetic constriction which should occur in the legs of coronal loops where conductive transition regions, hitherto thought to contain the bulk of the plasma in this higher temperature range, are located. On the basis of fine scale magnetograms, Dowdy et al. (1986) have estimated that these magnetic flux tubes are constricted from end to end by an areal factor of approximately 100. Furthermore, on the basis of simple steady-state conductive models, Dowdy et al. (1985) have shown that the large constriction can inhibit the conductive flow of heat by an order of magnitude. We are thus led to re-examine static models of this region of the atmosphere which incorporate not only conduction and radiation but also the effects of large magnetic constrictions. We find that the structure of this plasma depends not only on the magnitude of the constriction but also on the tube's shape.Our results show that no model with a constriction of order 100 can simultaneously (a) produce the variation of differential emission measure with temperature derived from measured line intensities and (b) satisfy the observed constraint (Reeves, 1976) that EUV emission from below T 7 × 10⁵ K be confined to the supergranular network, covering no more than 0.45 of the solar surface. The failure of the models suggests that the bulk of the 10⁵–10⁶ K plasma in the quiet solar atmosphere is not in transition region structures, but is instead magnetically isolated from the corona and heated internally. Even though the transition region component of 10⁵–10⁶ K plasma in the legs of coronal loops should exist, it comprises only a small fraction of the total 10⁵–10⁶ K plasma and, hence, produces only a small fraction of the observed EUV emission from this temperature range.We also find that for any permitted tube shape, constriction factors of order 100 reduce the coronal conductive energy losses to the transition region to a value which is less than a third of the value for an unconstricted field, i.e., to less than 2 × 10⁵ erg cm ^–2 s ^–1. In particular, if the magnetic geometry of the upper transition region is extremely concave (i.e., horn-shaped geometry with most of the areal divergence near the hot end), then a constriction of order 100 results in a conductive loss of less than 1 × 10⁴ erg cm^–2 s^–1 and, hence, much less than the coronal radiative energy loss. For such geometries, the constriction in the magnetic field hence provides an effective thermal insulation of the corona from the cooler parts of the solar atmosphere.Presidential Young Investigator.     

Electrical conductivity of neutron star cores in the presence of a magnetic field

General theory of electrical conductivity of a multicomponent mixture of degenerate fermions in a magnetic fieldB, developed in the preceding article (this volume), is applied to a matter in neutron star interiors at densities ₀, where ₀ = 2.8×10¹⁴ g cm^–3 is the standard nuclear matter density. A model of free-particle mixture ofn, p, e is used, with account for appearance of ^–-hyperons at > _c , where _c 4₀. The electric resistivities along and acrossB, and , and the Hall resistivity _H are calculated and fitted by simple analytical formulae at _c and > _c for the cases of normal or superfluid neutrons provided other particles are normal. Charge transport alongB is produced by electrons, due to their Coulombic collisions with other charged particles; is independent ofB and almost independent of the neutron superfluidity. Charge transport acrossB at largeB may be essentially determined by other charged particles. If _c , one has = [1 + (B/B ₀)²] for the normal neutrons, and for the superfluid neutrons, while _H = B/B _e for both cases. HereB _e 10⁹ T ₈ ² G,B ₀10¹¹ T ₈ ² G, andT ₈ is temperature in units of 10⁸ K. Accordingly for the normal neutrons atBB ₀, the transverse resistivity suffers an enhancement, _{1/4 1}. When 5₀ andB varies from 0 toBB _p 10¹³ T ₈ ² G, increases by a factor of about 10³–10⁴ and _H changes sign. WhenBB _p , remains constant for the superfluid neutrons, and _H B ² for the normal neutrons, while _H B for any neutron state. Strong dependence of resistivity onB, T, and may affect evolution of magnetic fields in neutron star cores. In particular, the enhancement of at highB may noticeably speed up the Ohmic decay of those electric currents which are perpendicular toB.     

On the production of suprathermal grains

The physical conditions under which suprathermal grains can be produced when they are accelerated by radiation pressure against the drag of ambient gas are investigated. It is found that dust grains may attain a terminal velocityU (=10⁵ cm s^–1) in most regions and move out of the midplane of the source region about a distance |z|100 pc. Once clear of the main gas/dust layer the dust grains (a3×10^–6 cm) may then attain suprathermal energy (V _g 3×10⁸ cm s^–1) by the Fermi process.     

Neutrino reactions in strong magnetic field

We present the energy losses due to several neutrinos processes: (1) synchrotron neutrinos, (2) pair annihilation neutrinos, (3) plasmon neutrinos, and (4) photoneutrinos in the presence of a superstrong magnetic field. Numerical results are tabulated and illustrated for several values of densities and temperatures. In the low density regime (10⁷ g cm^–3) the presence of a magnetic field decreases the luminosity, whereas the opposite is true at higher densities. This last effect is however almost entirely due to the existence of a new process the synchrotron neutrinos that disappear whenH0. Even though the overall effect can only be quantitatively ascertain after a complete cooling computation is performed, one should however expect a much lower temperature for neutron star surface than the one computed in theH=0 case.     

A spectroscopic study of the microorganism model of interstellar grains

The microorganism model of interstellar grains is investigated by spectroscopy from the infrared (IR), visible to the ultraviolet (UV) wave regions.E. coli, yeast and spores ofBacillus subtilis exhibit absorption bands at =3.1 and 9.7 m; they also exhibit several absorptions at 68 m which are in agreement with the observed IS extinction curves.To obtain the extinction curves in the visible and UV regions, dry films of microorganism are prepared on a MgF₂ plate or synthesized quartz plate and their spectra measured. In the wavelength region 190400 nm, conventional spectrophotometers are adopted for the measurement. The extinction curve of the film ofE. coli is similar to the observed IS curve.For the wave-range 100<<400 nm, a vacuum UV spectrometer is adopted to avoid absorptions due to O₂ in the atmosphere. The extinction spectra by this method are in agreement with the result obtained by the conventional method where comparison is possible. The extinction curves ofE. coli and yeast are such that they incrase towards the short wavelength and exhibit a peak at -190 nm, which is different from the well-known IS peak at =220 nm. It remains to be seen whether interstellar low temperatures (1040 K) can shift the peak position in the extinction curve of biochemical materials.     

On hard X-ray spectra of accreting neutron stars

Formation of the spectra of X-ray pulsars and gamma bursters is investigated. Interpretation of a hard X-ray spectrum of pulsars containing cyclotron lines is feasible on the basis of an isothermal model of a polar spot heated due to accretion to a neutron star. It has been ascertained that in the regions responsible for the formation of continuum radiation and lines the mode polarization is determined by a magnetized vacuum rather than by a plasma. Bearing this in mind, the influence of the magnetic field of a star on the wide wings of the cyclotron line and on its depth is discussed. The part played by the accreting column in the case of strong accretion (10¹⁹ el cm^–3) needed for long sustaining of the high level of X-rays from a neutron star-pulsar is studied. There occur the gaps in spectrum at frequencies close to the electron gyro-frequency and its harmonics due to the screening of the hot spot by the opaque gyro-resonant layers located within the accreting column. These gaps ensure the formation of cyclotron lines in absorption irrespective of the presence of such lines in the X-ray spectrum of a polar hot spot.The spectra of gamma-bursters recorded by Venus 11 and Venus 12 are interpreted in terms of a two-layer model of a polar hot spot. The estimates are given of the distance to some of the bursters, of the emission measure from a high-temperature layer responsible for continuum radiation and of the dispersion measure of a colder layer forming cyclotron lines in absorption. It is noted that the action of an accreting column leads generally to the radiation depression at frequencies below cyclotron lines. By the observed depression for one of the bursters the electron density of near-star accreting plasma during the burst has been directly estimated (4×10^–14 el cm^–3). Possible appearance of false cyclotron lines associated with cyclotron scattering in accreting column has been revealed.The problem of measuring the magnetic fields of neutron stars taking account of the gravitational redshift and the quantum recoil effect in emission and in absorption is discussed. Possibilty for a more precise measurement of the magnetic fields of those bursters whose spectrum contains both a cyclotron and an annihilation lines is noted.     

Pulsar radio emission. I. Oblique rotator

The problem of pulsar radio emission for the case of a coaxial rotator was investigated in our preceding paper [G. S. Sahakian, Astrofizika,38, 143 (1995)]. In this paper it is solved for the realistic case in which the star's magnetic axis does not coincide with its rotational axis (an inclined rotator). It is shown that above the star's magnetic cap a special region, called a magnetic funnel, is formed in which vigorous processes of particle multiplication occur. The height of this region is h 8·10^60.2 ₃₀ ^1/3 R ₆ ^1.3 cm and its radius r(r/c)^0.5 depends little on the inclination angle a ( is the angular rotation rate, is the magnetic moment, R is the star's radius, and r is distance from the center of the star). It is shown that the pulsar radio emission is produced in the magnetic funnel. Here, in the course of active radiative processes, two main particle fluxes with a high ultrarelativistic energy are formed: an upward electron flux and a positron flux falling onto the star's magnetic cap. These main fluxes are accompanied by individual narrow strips of positron and electron fluxes with a relatively low energy, which are fairly powerful, coherent radio sources. Such secondary fluxes are formed immediately after the annihilation of photons of curvature radiation emitted by particles of the main fluxes. The pulsar's radio luminosity is estimated to be L7.4·10^233.52 ₃₀ ^8/3 (a), where (a) is a known function (1 for a<50°). Equating the theoretical and observed radio luminosities L and L₀, we obtain the formula ₃₀P^1.32R ₆ ^0.4 (2.1·10^–27L₀/)^3/8 for the magnetic moment of the pulsar's neutron star, where P is the pulsar's period. The magnetic moments of slow pulsars calculated from this formula turn out to be considerably larger than those of fast pulsars. This means that the masses of slow pulsars are larger, on the average, than those of fast pulsars. The magnetic funnel operates with interruptions, periodically undergoing a discharge, so that the production of pulsar radio emission operates with interruptions. The durations of the production of radio emission and of the interruptions between those processes are on the order of h/c2.7·10^–40.2 ₃₀ ^1/3 sec, i.e., pulsar radio emission has a microstructure. Consequently, a study of the microstructure of the profiles of observed radio pulses enables one to obtain additional information about the magnetic moments of the neutron stars.Translated from Astrofizika, Vol. 39, No. 2, pp. 313–335, April–June, 1996.     

Primordial magnetic fields and symmetry restoration

In the standard cosmological model symmetry breaking in grand unified theories will occur at times 10^–39 s after the initial singularity when the Universe has cooled to a temperature 10¹⁶ GeV. We investigate here whether it is possible for a uniform, large-scale, magnetic field present in the early universe to delay significantly the time at which symmetry breaking occurs. Given the present magnitude of the intergalactic B-field (10^–11–10^–9 G) it is found that no significant effects are introduced.     

Абсолютная система высот в краевой зоне луны по наблениям покрытий звэзд

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