Review of optical observations of solar flares

The recent observations of solar flares, made with a Lyot filter and a spectrograph in H, HeD3, higher Balmer lines, metallic lines, and continuum, are discussed. It is important to study the energy supply of non thermal particle/ conduction/ irradiation into the lower atmosphere from the optical observations with high temporal and spatial resolutions. Simultaneous observations from ground-based observatories and instruments on board satellites are necessary for understanding flare plasma of low and high energy.     

The Zeeman broadening of high n solar recombination lines

The Zeeman broadening of high n lines is derived. While in areas of the quiet Sun with field strengths of 1 G the upper boundary of the observable frequency region of recombination lines is defined by electron impact broadening, in active regions with field strengths 25 G the Zeeman broadening will shift this boundary to higher frequencies. The frequency region most favourable for observations is derived and the corresponding Doppler, electron impact and Zeeman broadening are discussed. The strengths of the recombination lines obtained in earlier calculations is reduced when one considers besides the Doppler broadening the electron impact and Zeeman broadening also. The frequency region favourable for observations is compared with the atmospheric transparency of the microwave region; it is found that observations require at least high altitude stations or space-based platforms. Details of using the Zeeman broadening of high n recombination lines for mapping (coronal) magnetic fields are given.     

Relativistic blue‐ and red‐shifted absorption lines in AGNs

Current, accumulating evidence for (mildly) relativistic blue‐ and red‐shifted absorption lines in AGNs is reviewed. XMM‐Newton and Chandra sensitive X‐ray observations are starting to probe not only the kinematics (velocity) but also the dynamics (accelerations) of highly ionized gas flowing in‐and‐out from, likely, a few gravitational radii from the black hole. It is thus emphasized that X‐ray absorption‐line spectroscopy provides new potential to map the accretion flows near black holes, to probe the launching regions of relativistic jets/outflows, and to quantify the cosmological feedback of AGNs. Prospects to tackle these issues with future high energy missions are briefly addressed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Simultaneous Stokes-V diagnostic of a Sunspot using Mg b and Fe I lines

Simultaneous observations of Stokes profiles in photospheric Fei (630.15 nm and 630.25 nm) and chromospheric Mgi b ₁ and b ₂ (518.4 nm and 517.3 nm) lines over a sunspot are presented. Observations were carried out using the Advanced Stokes Polarimeter of HAO/NSO, VTT, SacPeak, U.S.A. The Stokes-V amplitude asymmetries for these lines are analyzed. The values of amplitude asymmetry in Mgb lines are negative in disk-center-side penumbra while they are positive in limb-side penumbra. This trend is similar in nature to photospheric Fei line observations. Further, the spatial distribution of Stokes-V asymmetry is analyzed using Net Circular Polarization (NCP) maps. The chromospheric and photospheric NCP maps are different in many aspects. These observations with longitudinal magnetic field, estimated using weak field approximation, are discussed in this paper.     

Evidence for thin-target X-ray emission in a small solar flare on 26 February 1972

We compare solar X-ray observations from the UCSD experiment aboard OSO-7 with high resolution energetic electron observations from the UCAL experiment on IMP-6 for a small solar flare on 26 February 1972. A proportional counter and NaI scintillator covered the X-ray energy range 5–300 keV, while a semiconductor detector telescope covered electrons from 18 to 400 keV. A series of four non-thermal X-ray spikes were observed from 1805 to 1814 UT with average spectrum dJ/d (hv) (hv)^–4.0 over the 14–64 keV range. The energetic electrons were observed at 1 AU beginning 1840 UT with a spectrum dJ/dE E ^–3.1. If the electrons which produce the X-ray emission and those observed at 1 AU are assumed to originate in a common source, then these observations are consistent with thin target X-ray production at the Sun and inconsistent with thick target production. Under a model consistent with the observed soft X-ray emission, we obtain quantitative estimates of the total energy, total number, escape efficiency, and energy lost in collisions for the energetic electrons.     

Theoretical models of planetary nebulae

Theoretical models are attempted for eight moderate excitation planetary nebulae which have been intensively observed with the image-tube scanner at Lick and in other investigations. The models are uniform density structures, often truncated, with central star energy distributions and chemical compositions so adjusted as to give an optimum fit with the observations. Although a satisfactory representation can be obtained for most of the stronger, usually observed lines, those arising from the 3p ³ configuration, specifically the nebular transitions of [Sii], [Cliii], and [Ariv], often give difficulties. A comparison with results of model predictions with those from simple, traditional procedures generally shows a good agreement, except in those instances where different judgments are made in interpreting emissions of ions of 3p ⁿ configurations.     

Solar limb effect and meridional flows

Results of observations of the solar limb effect in the non-magnetic lines Fei 512.4, 543.4, and 709.0 nm are presented. The detailed form of the limb-effect curves depends on the line strength. The weakest line, 709.0, shows a relative blueshift with respect to heliocentric angle from disc center to = 0.8 ( = cos(heliocentric angle)). With increasing line strength the relative blueshift disappears and the redshift increases monotonically from the disc center to the limb. The strongest line, 543.4, shows a nearly linear redshift with decreasing , indicating little influence from horizontal motions.Small, but systematic differences are found for all three lines between the line-shift measurements along the north-south and east-west diameters. The latitudinal variation of the limb effect is largest for the lines formed lowest in the atmosphere. The observations of the strongest line suggest the existence of a poleward meridional flow of 20–25 m s^–1. Based on the measurements of the weaker lines a latitude variation in the granulation is suggested. One explanation is that the size of the granulation is about 5% larger at high latitudes as compared to the equatorial region.     

Detection of small scale structure in metal lines at the extreme solar limb

We describe a tangential limb spectrum at 5870 Å which geometrically probes the high photosphere through the low chromosphere. Velocity and brightness structures with sizes ranging from 500 to 1500 km are present in the stronger emission lines. Such structure is consistent between the Fe i and Ba ii lines, and emission knots in these lines coincide with continuum bright streaks. But no correlation is evident between structure in the He i D₃ line, emission in the Na i D₂ line, and emission in the Fe i and Ba ii lines as a group. Two classes of near-horizontal velocity structure are seen in the height range from 0 to 500 km above the limb: υ ? 1 km s^?1 for the weaker metals and υ ～ 7–10 km s^?1 for the Na i line. Differences in line opacity are suggested as the cause of the low correlation between the fine structure in the various lines.

     

Periodicity in spectral variations of IC 4997?

The emission lines in the spectra of some planetary nebula experience variations with the time. These variations (due to evolution effects and by their nature) differ from the spectral changes which we usually observe in stars. It appears that the long-scale and systematic observations on the intensity behaviour of different emission lines of nebula may be used as an independent indicator to receive a principally new and quite unpredicted category of information related to the physics of nebulae and their nuclei. Particularly, the analysis of long standing observations carried out in relation to the forbidden lines 4363 [Oiii] andN ₁+N ₂ [Oiii] of double-ionized oxygen lead to a conception about the possible existence ofrelativistic electrons of moderate energy in planetary nebulae and the generation oftransition radiation as a result of electrodynamic interaction of these electrons with the dust particles in nebula (Gurzadyan, 1991). Just on the basis of the same observational material, the long-scale periodicity in the activity or variations of a star-like planetary nebula IC 4997 is discovered about which is the present note.     

Analysis of the chromospheric hydrogen spectrum at the 1962 eclipse

Slitless spectrograms of the chromosphere obtained during the eclipse of 4–5 February 1962 have been analyzed to obtain the decrements of the level populations of hydrogen, the self-absorption in the Balmer lines, and parameters useful in construction of models of the low chromosphere.The decrement of the high energy levels of hydrogen inferred under the optically thin assumption does not vary significantly with height, and it appears to be unnecessary to seek large deviations from local thermodynamic equilibrium in the high levels. The observed Balmer-to-Paschen line intensity ratios have been used to infer self-absorption and opacities in the Balmer lines. The resulting population of the second energy level is about an order of magnitude smaller than that found by Athay and Thomas from the 1952 data.The chromospheric continuum was generally underexposed; the absence of observed continuum in the visible region of the spectrum made it impossible to derive a unique model from the 1962 data alone. However, the high Balmer line data and new theoretical solutions of the statistical equilibrium equations for hydrogen combined with corrected 1952 observations at 4700 A are compatible with a model having approximately the same temperature and neutral hydrogen structure as the 1952 model by Pottasch and Thomas but half the electron density: T _e = 6200K, N ₁ = 7.4 × 10¹³ cm^-3, N _e = 2.3 × 10¹¹ cm^-3 at 500 km and T _e = 7200K, N ₁ = 2.6 × 10¹² cm^-3, N _e = 1.7 × 10¹¹ cm^-3 at 1000 km.Based in part on a Ph.D. thesis submitted to the Department of Astro-Geophysics, University of Colorado.Now at the Department of Astronomy, Indiana University.     

Local burst model of CMB temperature fluctuations: scattering in primordial hydrogen lines

The propagation of an instantaneous burst of isotropic radiation from the time of its onset at some redshift z ₀ to the time of its detection at the present epoch (at z = 0) is considered within the framework of a flat Universe. Thomson scattering by free electrons and scattering in primordial hydrogen lines (Hα, Hβ, Pα, and Pβ) are believed to be the sources of opacity, with the single-scattering albedo in the lines being calculated by taking into account the deexcitation of the upper levels of the transitions being considered under the action of background blackbody radiation. The profiles of these lines in the burst spectrum at the present epoch have been constructed for various z0 at various distances from the burst center. To a first approximation, these profiles do not depend on the burst radiation spectrum and intensity. It is shown that the lines are purely absorption ones at a sufficiently large distance, but an emission component can appear with decreasing distance, which strengthens as the distance decreases, while the absorption component weakens. The absorption depth in the combined profile can reach 2 ×10^?4 for the Hα and Hβ lines and 7 × 10^?6 for the Pα and Pβ lines. In this case, the relative amplitude of the temperature fluctuations lies within the range 10^?7?10^?9. The calculations have been performed for bursts with different characteristic initial sizes. At the same z ₀, the hydrogen line profiles essentially coincide for sizes smaller than some value, and the contrast of the lines decreases with increasing burst size for greater ones.

     

High resolution observations of small-scale magnetic elements and interpretation

This contribution deals with the properties of small-scale magnetic elements in plages. Spectro-polarimetric observations, obtained with the highest possible spatial resolution with the German solar telescopes at the Observatorio del Teide on Tenerife, were analysed. We conclude from the spread of line parameters measured in the Stokes I and V profiles of Fe I and Fe II lines that a wide range of magnetic properties is realised in the solar atmosphere. The flow velocities in small-scale magnetic flux tubes, deduced from the zero-crossing of the V profiles at high spatial resolution, show a fluctuation of v _Doppler = 580 m s^-1. This is substantially smaller than the turbulent broadening velocities of v _Doppler = 2 – 3 km s^–1 commonly derived by fitting V profiles from flux tube models to low spatial resolution data, e.g. from a Fourier Transform Spectrometer. Attempts to explain the high resolution I and V profiles by models of hydrostatic flux tubes are discussed. It appears impossible to accomplish agreement between the modeled and observed radiation of lines with strong and weak magnetic sensitivity at the same time. We suggest a scenario in which small-scale magnetic elements possess substructure and are dynamic, with gas flows and magnetic field strengths varying in space and time.     

The complex X-ray spectrum of the isolated neutron star RBS1223

We present a first analysis of a deep X-ray spectrum of the isolated neutron star RBS1223 obtained with XMM-Newton. Spectral data from four new monitoring observations in 2005/2006 were combined with archival observations obtained in 2003 and 2004 to form a spin-phase averaged spectrum containing 290 000 EPIC-pn photons. This spectrum shows higher complexity than its predecessors, and can be parameterised with two Gaussian absorption lines superimposed on a blackbody. The line centers, E ₂≃2E ₁, could be regarded as supporting the cyclotron interpretation of the absorption features in a field B∼4×10¹³ G. The flux ratio of those lines does not support this interpretation. Hence, either feature might be of truly atomic origin.      

On helium-like 1s2l−1snl′ transitions in solar flare spectra

Expected wavelengths and intensities are computed for 1s2l–1snl transitions in helium-like ions of the abundant elements from oxygen to iron, under coronal conditions. Probable observations of some of these lines, in the spectra of solar flares, are discussed and attention is called to a possible reversal of singlet and triplet intensities as compared to laboratory observations.     

No compelling evidence of distributed production of CO in Comet C/1995 O1 (Hale-Bopp) from millimeter interferometric data and a re-analysis of near-IR lines

Based on long-slit infrared spectroscopic observations, it has been suggested that half of the carbon monoxide present in the atmosphere of Comet C/1995 O1 (Hale-Bopp) close to perihelion was released by a distributed source in the coma, whose nature (dust or gas) remains unidentified. We re-assess the origin of CO in Hale-Bopp’s coma from millimeter interferometric data and a re-analysis of the IR lines.Simultaneous observations of the CO J(1–0) (115 GHz) and J(2–1) (230 GHz) lines were undertaken with the IRAM Plateau de Bure interferometer in single-dish and interferometric modes. The diversity of angular resolutions (from 1700 to 42,000 km diameter at the comet) is suitable to study the radial distribution of CO and detect the extended source observed in the infrared. We used excitation and radiative transfer models to simulate the single-dish and interferometric data. Various CO density distributions were considered, including 3D time-dependent hydrodynamical simulations which reproduce temporal variations caused by the presence of a CO rotating jet. The CO J(1–0) and J(2–1) observations can be consistently explained by a nuclear production of CO. Composite 50:50 nuclear/extended productions with characteristic scale lengths of CO parent L_p > 1500 km are rejected.Based on similar radiation transfer calculations, we show that the CO v = 1–0 ro-vibrational lines observed in Comet Hale-Bopp at heliocentric distances less than 1.5 AU are severely optically thick. The broad extent of the CO brightness distribution in the infrared is mainly due to optical depth effects entering in the emitted radiation. Additional factors can be found in the complex structure of the CO coma, and non-ideal slit positioning caused by the anisotropy of dust IR emission.We conclude that both CO millimeter and infrared lines do not provide compelling evidence for a distributed source of CO in Hale-Bopp’s atmosphere.     

Modelling OH in regions of star formation

There is a large, diverse and rapidly growing body of OH observations from regions associated with star formation, the physical conditions of which are of great interest. To interpret these observations we need to calculate the populations of the OH energy levels using as accurate a model as is feasible. We have developed a large velocity gradient (LVG) model using the lowest 48 hyperfine states of OH and including the phenomenon of far-infrared line overlap using a new theory which treats all thermal and velocity overlaps. If we use a theory of maser propagation (Field and Gray, 1988) we are then able to see how competing maser lines develop.The generic behaviour which we find is in good agreement with the extensive maser observations of Gaume and Mutel 1987 and our calculations allow us to associate general physical conditions with the four ground state maser frequencies. In addition we are able to present preliminary results of the interpretation of the OH absorption observations of Guilloteauet al. (1984) and Walmsleyet al. (1986) for the compactHII region DR21.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain     

On the variability timescales and magnetic field in OJ 287

The continuum spectrum of OJ 287, like most other BL Lac objects, is featureless- no emission or absorption lines are observed. However, OJ 287 shows variations at different timescales in flux and polarization at various wavelength bands. Using the available variability data one can estimate the sizes of the emission regions in the source from light travel time arguments. We assume the emission mechanism to be synchrotron radiation by high energy electrons with single power law energy distribution. Theoretical synchrotron spectrum in the frequency range 10¹¹–10¹⁷ H _z is compared with the observed spectral shape, obtained from new multifrequency quasi-simultaneous observations, to estimate the lower and upper cut off frequencies. These frequencies are used to obtain theoretical values of the variability timescales and magnetic field in the emission region. We obtain a value of 0.93 G for the magnetic field and 5.184×10⁴ sec for the cooling time from the quiescent continuum spectrum. The shock-in-jet model explains the spectrum where shocks accelerate the particles and amplify the magnetic field in the jet. This timescale is compared with the one obtained from observed short timescale variability (20 minutes) with proper beaming correction. The short timescale variations (200 minutes in the source frame), possibly caused by an additional, flaring, component of the source, are also used to calculate compressed magnetic field. The observed and theoretically estimated variability timescales and the shape of the spectrum suggest that there are more than one emission components in OJ 287.     

Spectroscopy and photometry of the protoplanetary nebula candidate stHα62 = IRAS 07171+1823

We present the results of spectroscopic and photometric observations for the B star StHα62 with an IR excess, a post-AGB candidate identified with the IR source IRAS 07171+1823. High-resolution spectroscopy has allowed the λ4330–7340 Å spectrum of the star to be identified: it contains absorption lines of an early B star and emission lines of a gaseous shell. The residual line intensities have been measured. The heliocentric radial velocities measured from absorption lines of the star and emission lines of the shell are 〈V _r 〉 = +45 ± 1 and +52 ± 1 km s^?1, respectively. The line-of-sight velocities of gas-dust clouds determined from the interstellar Na I lines are 12 and 33 km s^?1. The He I λ5876 Å line exhibits a P Cyg profile, which is indicative of an ongoing mass loss by the star. The expansion velocity of the outer shell estimated from forbidden lines is 12–13 km s^?1. Quantitative classification gives the spectral type B0.51 for the star. The parameters of the gaseous shell have been determined: N _e = 3.1 × 10³ cm^?3 and T _e ～ 21 000 K. Over 4 years of its observations, the star showed rapid irregular light variations with the amplitudes ΔV = We present the results of spectroscopic and photometric observations for the B star StHα62 with an IR excess, a post-AGB candidate identified with the IR source IRAS 07171+1823. High-resolution spectroscopy has allowed the λ4330–7340 ? spectrum of the star to be identified: it contains absorption lines of an early B star and emission lines of a gaseous shell. The residual line intensities have been measured. The heliocentric radial velocities measured from absorption lines of the star and emission lines of the shell are 〈V _r 〉 = +45 ± 1 and +52 ± 1 km s⁻¹, respectively. The line-of-sight velocities of gas-dust clouds determined from the interstellar Na I lines are 12 and 33 km s⁻¹. The He I λ5876 ? line exhibits a P Cyg profile, which is indicative of an ongoing mass loss by the star. The expansion velocity of the outer shell estimated from forbidden lines is 12–13 km s⁻¹. Quantitative classification gives the spectral type B0.51 for the star. The parameters of the gaseous shell have been determined: N _e = 3.1 × 10³ cm⁻³ and T _e ∼ 21 000 K. Over 4 years of its observations, the star showed rapid irregular light variations with the amplitudes ΔV = , ΔB = , and ΔU = and no color-magnitude correlation. We estimate the total extinction for the star from our photometric observations as A _v = . Near-IR observations have revealed dust radiation with a temperature of ∼1300 K. We estimate the distance to StHα62 to be r = 5.2 ± 1.2 kpc by assuming that the star is a low-mass (M = 0.55 ± 0.05 M _⊙) protoplanetary nebula. Original Russian Text ? V.P. Arkhipova, V.G. Klochkova, E.L. Chentsov, V.F. Esipov, N.P. Ikonnikova, G.V. Komissarova, 2006, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2006, Vol. 32, No. 10, pp. 737–747.     

Radiative and conductive cooling in a solar flare

We investigate the radiative and conductive cooling in the solar flare observed by RHESSI on 2005 September 13. The radiative and conductive loss energies are estimated from the observations after the flare onset. Consistent with previous findings, the cooling is increased with time, especially the radiation becomes remarkable on the later phase of flare. According our method, about half of thermal energy is traced by RHESSI soft X-rays, while the other half is lost by the radiative (∼38%) and conductive (∼9%) cooling at end of the hard X-rays in this event. The nonthermal energy input of P _nth (inferred from RHESSI hard X-ray spectrum) is not well correlated with the derivative of thermal energy of \fracdE_thdt\frac{\mathrm{d}E_{\mathrm{th}}}{\mathrm{d}t} (required to radiate the RHESSI soft X-ray flux and spectrum) alone. However, after consideration the radiation and conduction, a high correlation is obtained between the derivative of total thermal energy ( \fracdE_th+E_rad+E_conddt\frac{\mathrm{d}E_{\mathrm{th}}+E_{\mathrm{rad}}+E_{\mathrm{cond}}}{\mathrm{d}t}) and nonthermal energy input (P _nth) from the flare start to end, indicating the relative importance of conductive and direct radiative losses during the solar flare development. Ignoring the uncertainties to estimate the energy from the observations, we find that about ∼12% fraction of the known energy is transferred into the thermal energy for the 2005 September 13 flare.     

Spectro-Polarimetric Observations of Solar Magnetic Fields and the SOHO/MDI Calibration Issue

Comparisons of solar magnetic-field measurements made in different spectral lines are very important, especially in those lines in which observations have a long history or (and) specific diagnostic significance. The spectral lines Fe i 523.3 nm and Fe i 525.0 nm belong to this class. Therefore, this study is devoted to a comprehensive analysis using new high-precision Stokes-meter full-disk observations. The disk-averaged magnetic-field strength ratio R=B(523.3)/B(525.0) equals 1.97±0.02. The center-to-limb variation (CLV) is R=1.74−2.43μ+3.43μ ², where μ is the cosine of the center-to-limb angle. For the disk center, we find R=2.74, and for near-limb areas with μ=0.3, R equals 1.32. There is only a small dependence of R on the spatial resolution. Our results are rather close to those published three decades ago, but differ significantly from recent magnetographic observations. An application of our results to the important SOHO/MDI magnetic data calibration issue is discussed. We conclude that the revision of the SOHO/MDI data, based only on the comparison of magnetic-field measurements in the line pair Fe i 523.3 nm and Fe i 525.0 nm (increasing by a factor of 1.7 or 1.6 on average according to recent publications) is not obvious and new investigations are urgently needed.