The behaviour of the infrared lines of neutral oxygen and the balmer lines of hydrogen in the spectra of early-type stars with emission lines

The equivalent widths of the oxygen lines at 7774 and 8446 and of H (and some H) have been measured for 22 early-type, emission-line stars. A strong correlation between H and 8446 intensities has been found, although there is no such correlation between H and 7774. This confirms the probability that Bowen's mechanism is operative (the neutral oxygen 3³ D state is overpopulated because the excitation energy of Ly- nearly coincides with that of theOi 1025 line). The possibility of using 8446 and H equivalent widths for a comparison of oxygen to hydrogen abundances in these stars is discussed.     

Solar flare nomenclature

The evolution of solar flare nomenclature is reviewed in the context of the paradigm shift, in progress, from flares to coronal mass ejections (CMEs) in solar-terrestrial physics. Emphasis is placed on: the distinction between eruptive (Class II) flares and confined (Class I) flares; and the underlying similarity of eruptive flares inside (two-ribbon flares) and outside (flare-like brightenings accompanying disappearing filaments) of active regions. A list of research questions/problems raised, or brought into focus, by the new paradigm is suggested; in general, these questions bear on the interrelationships and associations of the two classes (or phases) of flares. Terms such as eruptive flare and eruption (defined to encompass both the CME and its associated eruptive flare) may be useful as nominal links between opposing viewpoints in the flares vs CMEs controversy.     

A classification of magnetic field configurations associated with solar flares

On the assumption that solar flares are due to instabilities which occur in current sheets in the Sun's atmosphere, one may classify magnetic-field configurations associated with flares into two types. One is characterized by closed current sheets, magnetic-field lines adjacent to these sheets beginning and ending at the Sun's surface. The other is characterized by open current sheets, magnetic-field lines adjacent to these sheets beginning at the Sun's surface but extending out into interplanetary space. Flares associated with open current sheets can produce Type III radio bursts and high-energy-particle events, but flares associated with closed current sheets cannot. The flare of July 6, 1966 apparently consisted of one flare of each type.     

Космическое черенковское излучение

, . N/E, E _r, (E _r)=1. , (, ) . .     

Hα and hard X-ray development in two-ribbon flares

Morphological features of two-ribbon flares have been studied, using simultaneous ISEE-3 hard X-ray records and high-resolution Big Bear H movies for more than 20 events. Long-lasting and complex hard X-ray bursts are almost invariably found associated with flares of the two-ribbon type. We find at least three events, namely March 31, 1979, April 10, 1980, and July 1, 1980, where the occurrence of individual spikes in hard X-ray radiation coincides with suddenly enhanced H emission covering the sunspot penumbra. There definitely exist important ( 1B) two-ribbon H flares without significant hard X-ray emission.     

Kα line emission during solar X-ray bursts

K X-ray line emission from S, Ar, Ca and Fe is calculated for conditions likely to exist in solar flares. We consider both the non-thermal and thermal phases of flares as indicated by X-ray observations. Impulsive non-thermal events seen at the onset of a flare at photon energies > 20 keV generally give rise to small K line fluxes (<250 photons cm^-2 s^-1) on the basis of data presented by Kane and Anderson. The amount of S K radiation in particular depends sensitively on the lower-energy bound of the non-thermal electron distribution giving rise to the impulsive burst, offering a possible means of determining this. Thermal K emission is significant for only Fe ions. For S, Ar and Ca, the temperatures required for a sizeable number of electrons with energies greater than the K-ionization potential will also strip these elements to ionization stages too high for K transitions to be possible. Comparison of thermal K emission from iron during an intense solar flare leads to a very high emission measure on the basis of these calculations, but such a value seems to be compatible with an analysis of the 1–3 Å continuum during the same event.NAS/NRC Resident Research Associate.Visiting Scientist, High Altitude Observatory, NCAR, Boulder, Colo. 80302.     

Magnetic fields and flares in the region CMP 20 September 1963

The position of bright knots of 30 flares at their very beginning relative to the high-resolution isogauss maps of the longitudinal component (H ) and maps of the transverse component (H ) of magnetic field are considered for seven days during the passage of the active and large spot group in Sept. 1963 (see Table I and maps on Figures 1–8).The flare bright knots occur simultaneously in regions of opposite magnetic polarity, and the majority of these knots are adjacent to neutral line H = 0, although not coinciding precisely with this line (Figure 9). Lenticular form of flare knots and the motions of bright material of flares is restrained by transversal field H . Also flares are closely associated (83%) with so-called bifurcated regions, where specific crossing of transverse components takes place (Figures 4–5). There is well-expressed (80%) coincidence of flare knots with the strongest (positive or negative) electric currents as determined from the relation j = c/4 rot H. The relation of results obtained to some existing theories of flares is briefly discussed.U.S. Nat. Acad. of Science - U.S.S.R. Acad. Nauk. Exchange Scientist Program; now at CSIRO Division of Physics, Australia.     

Soft X-Ray Pre-Flare Emission Studied in Yohkoh-SXT Images

The goal of this study is to improve our knowledge of the spatial relation between pre-flare and flare X-ray sources, to find other connections between the two phenomena (if they exist) and to study the role of pre-flare heating in flare build-up. We selected all flares with available preflare data observed by Yohkoh during the period October 1993–October 1994 and thus created a data base of 32 flares. When studying the spatial relation we discovered that our events can be classified into 3 categories: Co-spatial, Adjacent/Overlapping and Distant according to the spatial separation between the pre-flare and flare source(s) in the same field of view. The 'Co-spatial class of events, of which we found 8 cases, refers to flares that had a visible pre-flare soft X-ray structure with the same size, shape, and orientation as the main flare loops at the flare site at least 5 min before the start of the impulsive phase. We suggest that this is strong evidence that for a significant number of flares the flare structure is active in soft X-rays several minutes or more before the flare begins. However, an analysis of the physical properties of the flare sites, including temperature and intensity variation found no consistent feature distinguishable from other non-flaring active region emission and hence no definite evidence of a special 'pre-flare or 'precursor phase in solar flares.     

Time distribution of flare events on UV Cet stars

A statistical analysis of the flare data on three UV Cet stars obtained at Catania Observatory in the indicated periods-UV Cet (1968–1976), EQ Peg (1969–1975) and YZ CMi (1968–1976)-is presented. The distributions of the four variables (i) , the time elapsed from the start of a continuous observation interval to the occurrence of the first flare; (ii) , the time interval between consecutive flares; (iii)k, the number of flares occurring in time intervals of given duration; (iv) , the ratio between the number of flares observed in each continuous observing interval to that expected for a Poisson process, indicate that the flares of the three stars considered are not randomly distributed in time.     

A solar flare observed with the SMM and Einstein satellites

We present X-ray observations of the 21 July, 1980 flare which was observed both with the Einstein Observatory Imaging Proportional Counter (IPC) and the X-Ray Polychromator (XRP) and Gamma-Ray Spectrometer onboard the SMM satellite. The Einstein observations were obtained in scattered X-ray light, i.e., in X-rays scattered off the Earth's atmosphere. In this way it is possible to obtain spatially unresolved X-ray data of a solar flare with the same instrument that observed many X-ray flares on other stars. This paper juxtaposes the results and implications of the stellar interpretation to those obtained from the far more detailed SMM observations. The result of this calibration observation is that the basic properties of the flaring plasma can be reliably determined from the stellar data, however, the basic physics issues can only be studied through models.     

Chromospheres of flare stars

The present paper contains an attempt to formulate a theory, based on fast electrons hypothesis, of the chromospheres of flare stars. At the same time we shall undertake a survey of observations on the emission lines of flare stars and comparisons with the theory. The general discussion in the introduction concerning the civilian right of fast electron hypothesis is followed by sections in which the following problems are tackled: the anatomy of the light curve of the flare in UV Cet-type stars or its division into two components; the sources of radiation, ionizing hydrogen and other elements, and the estimation of their power in cold dwarf stars with emission lines; conditions for the excitation of emission lines in the chromospheres of those stars; the problem of duration of luminescence of flare stars in the emission lines (observations and theory); the electron temperature and electron concentration in the chromospheres of flare stars; the problem of the luminescence of emission lines in the quiescent star; the degree of ionization and the role of inelastic collisions of the electrons in the chromosphere of flare stars; the profiles of emission lines, their broadening and intensification during the flare; the dependence of the infensity of emission lines on the flare amplitude; the nature and peculiarities of two types of the Haro flares; the impact of radiation dilution and the spectral class of the star on the equivalent width of the emission lines; the possibility of exciting the forbidden lines; the problem of generation of the emission lines of neutral and ionized helium; the possibility of the Lyman-alpha emission in flare stars, the expected parameters of such emission — the radiation power, the equivalent width and profile of the Lyman-alpha line; the possibility of the presence of the strongest (after the -line) emission line — of doublet 2800 Mgii in the spectra of flare stars, the expected value of the intensity and equivalent width of that line.     

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.     

Spatial development of X-ray emission during the impulsive phase of a solar flare

The flare of 11 November, 1980, 1725 UT occurred in a magnetically complex region. It was preceded by some ten minutes by a gradual flare originating over the magnetic inversion line, close to a small sunspot. This seems to have triggered the main flare (at 70 000 km distance) which originated between a large sunspot and the inversion line. The main flare started at 172320 UT with a slight enhancement of hard X-rays (E > 30 keV) accompanied by the formation of a dark loop between two H bright ribbons. In 3–8 keV X-rays a southward expansion started at the same time, with - 500 km s ^–1. At the same time a surge-like expansion started. It was observable slightly later in H, with southward velocities of 200 km s^–1. The dark H loop dissolved at 1724 UT at which time several impulsive phenomena started such as a complex of hard X-ray bursts localized in a small area. At the end of the impulsive phase at 172540 UT, a coronal explosion occurred directed southward with an initial expansion velocity of 1800 km s^–1, decreasing in 40 s to 500 km s^–1.Now at Fokker Aircraft Industries, Schiphol, The Netherlands.     

Plasma acceleration by radiation in X-ray pulsars

Charged particle acceleration is considered by a radiation flux from a star or hot spot in X-ray pulsars. It is shown that for any distance from the star there exists the upper velocity limit up to which a particle can be accelerated by radiation. This critical velocity does not depend on the luminosity of the spot. Near the hot spot surface the critical velocityv0.65c. These results are applied to plasma acceleration inX-ray pulsars. The mechanism is advanced, of -ray generation in the course of plasma accretion, onto a neutron star. It is shown that in the presence of a large magnetic field and high luminosity of the spot the relativistic electron-position avalanche may appear. The optical depth of the electron-positron cloud achieves the value of order one. The X-ray quanta emitted by the spot are scattered by relativistic (2.6) electron-positron pairs and are transformed into -radiation. Hard quanta with energy 1 MeV leave the generation region in the narrow cone 0.25.     

The Hard X-ray Imaging Spectrometer (HXIS)

The HXIS, a joint instrument of the Space Research Laboratory at Utrecht, The Netherlands, and the Department of Space Research of the University of Birmingham, U.K., images the Sun in hard X-rays: Six energy bands in energy range 3.5–30 keV, spatial resolution 8 over Ø 240 and 32 over Ø 624 field of view, and time resolution of 0.5–7 s depending on the mode of operation. By means of a flare flag it alerts all the other SMM instruments when a flare sets in and informs them about the location of the X-ray emission. The experiment should yield information about the position, extension and spectrum of the hard X-ray bursts in flares, their relation to the magnetic field structure and to the quasi-thermal soft X-rays, and about the characteristics and development of type IV electron clouds above flare regions.     

Radio emission from stellar flares

An overview is given of the observations of stellar radio flares, defined as radio emission which is both variable in time and created by explosive releases of magnetic energy. The main sources of such flares are late-type Main-Sequence stars, classic close binaries, X-ray binaries, and pre-Main-Sequence stars.We summarize the interpretations of these observations in terms of the various incoherent and coherent emission mechanisms. The possible importance of a coherent emission process in electrostatic double layers is pointed out.We briefly indicate the diagnostic importance of radio emission for the flare process in classic and compact stars. In particular we discuss the possible production of radio flares from interactions between an accretion disk and the magnetic field of the central object.     

Changes in coronal condensations emission after solar bursts at microwaves

The amount of circular polarization of the total solar radio emission at 7 GHz present permanent changes after the occurrence of certain radio bursts associated with larger flares. For isolated S-components, associated with such flares the changes of the polarization degree _sranges between 0.004 to 0.1, and appears to be a function of the flare importance. A semi-qualitative interpretation associates _swith magnetic field reductions at the S-component, agreeing fairly well with a flare mechanism based on collisionless dissipation of magnetic energy, corresponding to energies in the range of 10³⁰ to 10³² ergs, assuming an average model for the coronal condensations.     

Studies of solar flares using optical,X-ray and radio data

I have studied a number of flares for which good X-ray and optical data were available. An average lag of 5.5 s between hard X-ray (HXR) start and H start, and HXR peak and Ha peak was found for 41 flares for which determination was possible. Allowing for time constants the time lag is zero. The peak H lasts until 5–6 keV soft X-ray (SXR) peak. The level of H intensity is determined by the SXR flux.Multiple spikes in HXR appear to correspond to different occurrences in the flare development. Flares with HXR always have a fast H rise. Several flares were observed in the 3835 band; such emission appears when the 5.1–6.6 keV flux exceeds 5 × 10⁴ ph cm^-2 s^-1 at the Earth. Smaller flares produce no 3835 emission; we conclude that coronal back conduction cannot produce the bright chromospheric network of that wavelength.The nearly simultaneous growth of H emission at distant points means an agent travelling faster than 5 × 10³ km s^-1 is responsible, presumably electrons.In all cases near the limb an elevated Ha source is seen with the same time duration as HXR flux; it is concluded that this H source is almost always an elevated cloud which is excited by the fast electrons. A rough calculation is given. Another calculation of H emission from compressed coronal material shows it to be inadequate.In several cases homologous flares occur within hours with the same X-ray properties.Radio models fit, more or less, with field strengths on the order of 100G. A number of flares are discussed in detail.     

Spectrophotometry of bright Be stars

We present new measurements of the distribution of energy in the continuum for eight Be stars in the optical region (3200-7600 Å). The effective temperatures of these stars have been estimated from their observed fluxes. It is found that, in general, pole-on stars show near-infrared excess emission. It is interesting to note that the Balmer jumps for stars having an infrared excess are systematically smaller than for those lacking the infrared excess.Variability of ultraviolet and infrared excess emissions in these stars has been discussed. The stars 59 Cyg, 66 Cyg, 28 CMa, and 27 CMa show large variations in their continuum at ultraviolet (UV) and infrared (IR) regions.     

Dependence of radio emission in large Hα flares 1967–1970 upon the orientation of the local solar magnetic field

During 1967–1970, the greatness of 90 large flares (H importance 2) was influenced by the orientation of the large-scale ( 100 000 km) magnetic field structure over the flare site. Although the average X-ray and optical emissions are only slightly larger for flares with their overlying fields directed southward, as opposed to northward, the meter-wave-length prompt flux maxima are, on average, an order of magnitude greater for the flares with southward oriented magnetic fields. There is a comparable, but possibly smaller difference in the 10 cm- fluxes. We therefore conclude that, during this period, the orientation of the overlying magnetic field affects the amount of electromagnetic flare energy radiated promptly in the corona (10 cm- and m-), relative to that radiated in the chromosphere (X-ray and optical). We demonstrate that this statistical effect shows some variability in degree during the period, although the trend is consistent throughout.