Evolution du spectre de HR Del de 1973 À 1979

Résumé De 1973 à 1979, on observe une diminution de l'intensité des raies d'émission et du continuum de HR Del au cours du temps.En 1978, on note la disparition des raies de haute excitation.La répartition énergétique spectrale dans le continu reste sensiblement la même dans l'intervalle 4000–5000 Å. Le fort excès ultraviolet trouvé de 3600 à 3250 Å est confirmé par les observations dans l'ultraviolet lointain effectuées à l'aide du satellite IUE.Nos observations montrent un important changement dans la structure des raies 4959 et 5007 Å de [Oiii] entre 1973 et 1976, les composantes polaires devenant plus faibles que les composantes équatoriales.

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From 1973 to 1979, the intensity of emission lines and continuum of HR Del has steadily diminished.High excitation lines have disappeared in 1978.The spectral energy repartition in the continuum remains the same in the spectral range 4000–5000 Å. The strong ultraviolet excess found in the spectral range 3600–3250 Å is confirmed by IUE satellite observations in the ultraviolet.Our observations show an important change in the structure of the [Oiii] lines 4959 and 5007 Å between 1973 and 1976, the polar components becoming weaker than the equatorial ones.

     

The EUV continuum emission (1400–1960 Å) in a solar flare observed from Skylab

The total radiative output in the EUV continuum (1400–1960 Å) from the 5 September 1973 flare has been obtained from the EUV spectra of the flare observed with the NRL slit spectrograph (SO82B) on Skylab. The radiative energy in the EUV continuum is of the order of 10²⁹ ergs, which is more than a factor of 2 greater than those radiated in soft X-rays (8–20 Å) and in H for the flare. Thus, the EUV continuum emission is an important radiative energy loss, and should be included in the consideration of the energy balance of the flare.Ball Corporation.Now at the Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway.     

Calculated solar X-radiation from 1 to 60 Å

The fluxes of about 230 spectral lines in the range 1–60 Å from coronal ions of C, N, O, Ne, Na, Mg, Al, Si, S, Ar, K, Ca, Ti, Cr, Mn, Fe, and Ni are computed for a range of electron temperature from 10⁵ to 10⁹ K. The relative ion abundances are derived from Jordan's ionization equilibrium calculations. The continuum emission is derived from computations of Landini and Monsignori Fossi with a correction for the free-free emission.     

The hydrogen emission spectrum in three white-light flares

We present spectral data for three white-light flares (WLFs) showing Balmer continuum at wavelengths 3700 Å. These flares also have a weaker continuum extending toward longer wavelengths, from which, in one flare where this continuum is sufficiently bright, we are able to identify a Paschen jump near 8500 Å. The presence of the latter suggests that the Paschen continuum may be a substantial contributor to the WLF continuum at visible wavelengths. We note the possibility, therefore, that the entire continuum of this particular flare may be dominated by H _fb emission.In all three flares the head of the Balmer continuum, as well as the head of the Paschen continuum in the flare where it was identified, is advanced toward longer wavelengths as a result of the blending of the hydrogen emission lines of the respective series. The principal quantum number of the last resolvable line of the Balmer or Paschen series is approximately 16. The electron density, as measured from the halfwidths of the high Balmer lines in two of the flares, is approximately 5 × 10¹³ cm^–3. Due to possible misplacements of the spectrograph slit, however, the electron density in the brightest kernels of the WLFs may not have been obtained.Operated by the Association of Universities for Research in Astronomy, Inc. under contract AST 78-17292 with the National Science Foundation.     

Hydrogen and helium excitation by EUV radiation for the production of white-light flares

White-light flares are defined as those flares that produce significant enhancement of emission in the visible light continuum. The source of energy for this emission has not yet been identified with several possibilities being suggested: heating of the lower chromosphere by some mechanical or magnetic means, or by soft X-ray or extreme ultraviolet radiation from coronal loops being absorbed in the lower chromosphere and re-emitted in the visible.Using non-LTE radiative transfer calculations for hydrogen and helium in a simple model atmosphere we show that EUV ( < 912 Å) radiation cannot be the main energy source for white-light flares. Estimates of the observed energy emitted in the visible and the EUV indicate that there may be enough energy in the EUV to account for the white light flare with this mechanism. Using enhancements in the wavelength region below 912 Å of up to 7 × 10⁹ ergs cm^–2 s^–1 ster^–1 (5 × 10⁵ times the estimated q radiation field) to represent flare EUV emission from above we investigated the non-LTE level populations for hydrogen and helium and the lower atmospheric heating resulting from this radiation. The basic result is that the opacities in the Lyman continuum and the helium I and II continua are so much larger than even the enhanced opacity in the visible hydrogen continuum that the EUV radiation is absorbed before it can have a significant effect in the visible light continuum. However, the EUV radiation can cause a significant enhancement of H emission.Operated by the Association of Universities for Research in Astronomy Inc. for the National Aeronautics and Space Administration.     

Relation between VLF phase deviations and solar X-ray fluxes during solar flares

Sudden phase anomalies (SPA's) observed in the phase of GBR 16 kHz VLF signals during the years 1977 to 1983 have been analysed in the light of their associated solar X-ray fluxes in the 0.5–4 Å and 1–8 Å bands. An attempt has been made to investigate the solar zenith angle () dependence of the integrated solar X-ray flux for producing SPA's. It is deduced from the observations for < 81° that the phase deviation increases linearly as a whole with increasing solar X-ray fluxes in these two bands. The threshold X-ray flux needed to produce a detectable SPA effect has been estimated to be 1.6 × 10^–4 ergcm^–2 s^–1 and 1.8 × 10^–3 ergcm^–2 s^–1 in the 0.5–4 Å and 1–8 Å bands, respectively. For both bands the average cross section for all atmospheric constituents at a height of 70 km is almost equal to the absorption cross section for the 3 Å X-ray emission.     

Soft X-ray and microwave observations of hot regions in solar flares

Hot regions in solar flares produce X-radiation and microwaves by thermal processes. Recent X-ray data make it possible to specify the temperature and emission measure of the soft X-ray source, by using, for instance, a combination of the 1–8 Å (peak response at about 2 keV) and the 0.5–3 Å (peak response at about 5 keV) broad-band photometers. The temperatures and emission measures thus derived satisfactorily explain the radio fluxes, within systematic errors of about a factor of 3. Comparison of 15 events with differing parameters shows that a hot solar flare region has an approximately isothermal temperature distribution. The time evolution of the correlation in a single event shows that the hot material originates in the chromosphere, rather than the corona. The density must lie between 10¹⁰ and 2 × 10¹¹ cm^–3. For an Importance 1 flare, this implies a stored energy of roughly 2 x 10³⁰-10²⁹ ergs. A refinement of the data will enable us to choose between conductive and radiative cooling models.     

The UV continuum 1450–2100 Å and the problem of the solar temperature minimum

This study is based on a set of ten solar rocket spectra well exposed for photometry photographed on July 27, 1966 by Purcell, Snider, and Tousey.The photometry of the far UV continuum illustrates the transition of the solar temperature minimum at 1700 Å in the solar spectrum - (a) the continuum intensity decreases by 30–50% between 1700 Å and the¹ D limit of silicon at 1682 Å, and (b) the equivalent brightness temperature shows minimum values throughout the spectral range 1540–1682 Å, which average just under 4700 ± 100K.The minimum UV brightness temperature is 300K higher than the far infrared measurement of the solar minimum temperature, and possible reasons for this are discussed.Brightness temperatures measured in prominent CO band heads and in the aluminum 1937 Å auto-ionization line also are given.     

Analysis of a white-light flare spectrum

The continuum emission of an X₁ flare on 26 March, 1970 observed close to the solar limb (N 05 E 64) was analyzed by a photometric determination of the contrast I()/I ₀() in the wavelength range 3558–5920 Å. Two possible mechanisms for the emission were investigated, namely hydrogen Paschen and H^– continua. We show the unlikeness of the Paschen possibility and derive strong constraints on the temperature structure and energy deposition mechanism imposed by the H^– continuum process.We conclude that the emission must have originated in deep atmospheric layers. The relevance of this result with respect to flare energy release and transport processes is also discussed.     

Coronal survey in X-rays of O vii and Ne ix

We report some results of a rocket experiment flown on 29 April, 1971. A survey of the solar corona was carried out with a pair of collimated Bragg spectrometers to study the resonance, intersystem and forbidden line emission from the helium-like ions O vii (22 Å) and Ne ix (13 Å). In the direction of dispersion the collimator provided a field of view of 1.7. Also, the continuum radiation near 3 Å was monitored by a collimated proportional counter within a view angle of 4.2. The observed X-ray emission came from the general corona, seven plage regions, and one dynamic feature- the late stage of a small flare. From the intensity of the O vii and Ne ix resonance lines the electron temperature and emission measure of the individual emitting regions are derived on the basis of two models, one (a) in which the region is assumed to be isothermal and another (b) in which the emission measure decreases exponentially with increasing temperature. The latter model, which is the most adequate of the two, yields for the electron temperature of the time-varying feature 2–3 × 10⁶ K, for the other active regions 1.5–2.5 × 10⁶ K, and for the general corona 1.3–1.7 × 10⁶ K. The Ne ix emitting regions are about 1.5 times as hot as the O vii regions. The emission measure ranges from 0.4–2.3 × 10⁴⁸ cm^–3 for all active regions and is about 2 × 10⁴⁹ cm^–3 for one hemisphere of the general corona above 10⁶ K. From an analysis of the ratio, R, of the forbidden and intersystem lines of O vii we conclude that none of the regions producing these lines at the time of the rocket flight had electron densities exceeding about 3 × 10⁹ cm^–3. Our data demonstrate a dependence of R upon temperature in agreement with the theory of Blumenthal et al. (1971). The wavelengths for the intersystem, the 1s ²2s ² S ^e–1s2p2s ² P ⁰ satellite, and the forbidden transition show in the case of Ne ix improved agreement with predictions. The observed strength of the satellite lines for both O vii and Ne ix agrees with the predictions of Gabriel's (1972) theory, which attributes their formation to dielectronic recombination.We are saddened to report the death of A. J. Meyerott on 13 November, 1971.     

Photoelectric measurements of the green coronal line during the eclipse of November 12, 1966

A line to continuum ratio of the green line equal to 2.1 Å was measured above the West limb at the height p = 1.75 solar radii during the eclipse. The lower limit to the abundance of iron relative to hydrogen obtained from these measurements is 4 × 10^–5.     

Discrete subresolution structures in the solar transition zone

During operations on the Spacelab-2 Shuttle mission, the NRL High Resolution Telescope and Spectrograph (HRTS) recorded spectra of a variety of solar features in the 1200–1700 Å wavelength region which contains spectral lines and continua well suited for investigating the temperature minimum, the chromosphere and transition zone. These data show that, at the highest spatial resolution, the transition zone spectra are broken up from a continuous intensity distribution along the slit into discrete emission elements. The average dimensions of these discrete transition zone structures is 2400 km along the slit, but an analysis of their emission measures and densities shows that the dimensions of the actual emitting volume is conciderably less. If these structures are modelled as an ensemble of subresolution filaments, we find that these filaments have typical radii of from 3 to 30 km and that the cross-sectional fill factor is in the range from 10^–5 to 10^–2. The transport of mass and energy through these transition zone structures is reduced by this same factor of 10^–5 to 10^–2 which has significant consequences for our understanding of the dynamics of the solar atmosphere. Because the HRTS transition zone line profiles are not broadened by resolved large-spatial-scale solar velocity fields, the line widths of the Civ lines have been analyzed. The average line width is 0.195 Å (FWHM) and requires an average nonthermal velocity of 16 km s^–1 (most-probable) or 19 km s^–1 (root-mean-square) which is lower than previously observed values.     

A study of non-thermal radio emission features using fine spectral BAO and high-sensitivity RATAN observations of a solar active region

Based on spectral observations of active region NOAA 8545 on 19 May 1999, we describe the processes responsible for non-thermal long-lasting radio emission and for narrow-band non-drifting bursts observed at the same time. Non-thermal long-lasting radio emission consisted of two components: short-duration (1–2 s) microbursts with fluxes about 0.001 s.f.u. and continuum emission with growing spectrum in the range of 1000–2000 MHz. Energetic electrons continuously existed in the active region for more than 2.2 hours. The nature and parameters of microbursts were discussed by Bogod, Mercier, and Yasnov (2001). Here we consider the continuum source nature. It is shown that the model, taking into account the cyclotron loss-cone instability of hot electrons and the generation of plasma waves at the upper hybrid frequency, may explain the observed continuum source parameters. For the narrow-band non-drifting bursts we consider two models: the first taking into account an excitation of weak shock waves across the magnetic field and the second with an excitation of the upper hybrid waves under the double plasma resonance. Continuum source parameters are close to the last model. Our estimations for the magnetic field strength are as follows: H=120–126 G, which is valid for the region where the electron density of background plasmas n=(1.4–1.9) ×10⁹ cm^–3; H=180–190 G for the region where n=(3.0–4.3) ×10⁹ cm^–3; H=290 G for the region where n _e=2.5×10¹⁰ cm^–3; and H=350 G for the region where n _e=3.5×10¹⁰ cm^–3. The speed of the fast electrons is about 0.10–0.14 c.     

Interpretation of the solar continuum from 1680 to 600 Å. Model of the transition region photosphere-chromosphere and of the chromosphere

An interpretation is given of the observations of the continuous solar radiation in the spectral range 600–1700 Å. The model allows for deviations from LTE of H, C, Si and S, and is in hydrostatic equilibrium. The predicted intensity from 1680 to 1520 Å has virtually no dependence on the electron temperature variation in the optical depth range 10^–3–4 × 10^–5, at 5000 Å; the brightness temperature is compatible with a low electronic temperature minimum near the optical depth 10^–4. The model of the low chromosphere is characterized by a steep temperature gradient. The model satisfies observations at millimeter wavelengths.     

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.     

Extreme ultraviolet flashes of solar flares observed via sudden frequency deviations: Experimental results

Properties of solar-flare EUV flashes measured via a type of ionospheric event, called a sudden frequency deviation (SFD), are presented. SFD's are sensitive to bursts of radiation in the 1–1030 Å wavelength range. He ii 303.8 Å, O v 629.7 Å, HL 972.5 Å and C iii 977.0 Å have essentially the same impulsive time dependence as the 1–1030 Å flash responsible for SFD's. Soft X-rays (2–20 Å) and certain EUV lines have a much slower time dependence than the 1–1030 Å flash. Most SFD's have some fine structure, but marked quasi-periodicity in EUV flashes is quite rare. EUV flashes are closely associated with hard X-ray bursts, white-light emission, microwave radio bursts and small bright impulsive kernels in the H flare. The intensity of EUV flashes depends on the central meridian distance of the H flare location; the intensity decreases at the limb. The total energy radiated in the 10–1030 Å flash for the largest events observed is about 10³¹ ergs.     

Extreme ultraviolet spectrum of the solar flare of 2114 UT March 27, 1967

Scanning spectrometer measurements in the range 1310–270 Å, observed from the satellite OSO 3, are reported for the solar flare of 2114 UT March 27, 1967. This flare was a long lasting sequence of bursts with EUV spectra consisting of enhanced lines and recombination continua normally emitted from the chromosphere and chromosphere-corona transition region, with unusually small increases in lines normally emited from the corona. An EUV flare spectrum is presented and suggested as one example for interpreting broadband observations of EUV bursts. Any broadband continuum other than known recombination continua contributed less than 6 % of the meassured line and hydrogen recombination continua in the range 270–1310 Å. The ratio of photon flux of Ciii 1176 Å to that of Ciii 977 Å was 0.86, which suggests an ambient density in the region of emission greater than 10¹² cm^-3 at temperatures near 60000 K.     

A high resolution study in time,position, intensity,and frequency of the radio event of January 14, 1971

A high resolution study in time, frequency, position, and intensity was made at 169 MHz and neighbouring frequencies of the solar radio event of 1971, January 14, 11^h 20^m–30^m UT. The event consisted of two closely resembling groups of type III bursts and type II like details.Before, during, and after the outburst a stationary type IV continuum was seen, with small amplitude pulsating structure. The size of the pulsating structure (which was located inside the continuum) was considerably smaller than the continuum size, and in agreement with an explanation by fluctuating magnetic inhomogeneities inside the continuum source.The continuum moved outward after each outburst at a high speed (2–4000 km s^–1). After the second event the continuum source returned inward slowly ( 450 km s^–1). The outward motion is discussed. It can be explained by a combination of the impact of a fast magnetohydrodynamic shock and the injection of highly energetic particles during the event, the required number being also necessary to account for the observed radio flux. The slow returning motion can be related to mhd restoring of the magnetic field configuration.     

On the nature of interstellar grains

Data on interstellar extinction are interpreted to imply an identification of interstellar grains with naturally freeze-dried bacteria and algae. The total mass of such bacterial and algal cells in the galaxy is enormous, 10⁴⁰ g. The identification is based on Mie scattering calculations for an experimentally determined size distribution of bacteria. Agreement between our model calculations and astronomical data is remarkably precise over the wavelength intervals µ^–1 < ;^–2 < 1.94µ^–1 and 2.5µ^–1 < ;^–1 < 3.0 ;^–1. Over the more restricted waveband 4000–5000 Å an excess interstellar absorption is found which is in uncannily close agreement with the absorption properties of phytoplankton pigments. The strongest of the diffuse interstellar bands are provisionally assingned to carotenoid-chlorophyll pigment complexes such as exist in algae and pigmented bacteria. The 2200 Å interstellar absorption feature could be due to degraded cellulose strands which form spherical graphitic particles, but could equally well be due to protein-lipid-nucleic acid complexes in bacteria and viruses. Interstellar extinction at wavelengths <1800 Å could be due to scattering by virus particles.     

Intensity measurements of chromospheric fine structures in Lyman-α

The intensity of the sun was measured in the Lyman- emission line with 2.5 arc-seconds of resolution. The experiment was flown in an Aerobee-150 rocket on April 28, 1966. It contained a Cassegrain telescope with a pinhole aperture placed at the focus followed by a gas-gain ionization chamber whose spectral response was 1050 Å to 1350 Å.An isophote map 1.5 by 3 arc-minutes in size made from a composite of 90 linear scans shows an enhanced region and adjacent to it a prominent dark lane 20 arc-seconds wide. The measured intensity ratio of these two regions is nine. Bright features between 6 and 20 arc-seconds in size showed typical peak intensities of 20% greater than the surrounding chromosphere. The smallest features observed were 2.5 arc-seconds in size. A direct measurement of the absolute intensity at 1216 Å gave a value of 5.9 × 10⁴ erg cm^–2 s^–1 sterad^–1 in the quiet chromosphere.Based on observations made by the author at the E. O. Hulburt Center for Space Research (supported jointly by the Office of Naval Research and the National Science Foundation) at the Naval Research Laboratory.