Kinematic model of loop prominences formation

The kinematics of the material motion in a variable magnetic field in the MHD approximation of a strong field and cold plasma is investigated. The variation of magnetic moments of two dipole systems leads to the development of such phenomena as loop prominences, coronal rain and funnel prominences.     

A new model for X-ray emission from NGC 4151

We present an analysis of the reported spectral features of NGC 4151 in X-rays. It is shown that the origin of X-rays from the source is inconsistent with a single production mechanism. We suggest a new two-component model in which soft X-rays arise from the black-body emission of a tiny hot nucleus withT2×10⁷ K and the hard X-ray photons are generated in an extended region by inverse Compton scattering of electrons with the infrared photons.     

Splintering loop prominences

A new type of sunspot prominences (splintering loop prominences) is described. They have loop structure, but their material seems to originate from the lower layers. A tentative interpretation of the new type is given. In the splintering loop prominences on October 7, 1967 a phenomenon was observed which might be interpreted as the capture of an ejected prominence streamer by transverse magnetic fields of the loops.On leave from the Astronomical Institute of the Czechoslovak Academy of Sciences, Ondejov.     

X-ray emission associated with solar prominences,sprays and surges

Using H observations made at the Astronomical Observatory of Wroclaw University, and 3.5–5.5 keV X-ray data from the Hard X-ray Imaging Spectrometer on board the Solar Maximum Mission, sites of solar X-ray emission are identified which are associated with active H features, such as prominences, sprays and surges. The X-ray emission is found to be highly localized within the active (H) structures. For example, in the prominences examined, 3.5–5.5 keV X-rays were found only in compact sites near the feet of the prominences. Models predicting that, during the active phase of these structures, the energy release should be evenly distributed along the structure are clearly brought into question. It is argued that these X-ray sites are indicative of the cause of the expulsion and transport of chromospheric material. Models which satisfy these observations are discussed.This work was started during a visit to the High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, U.S.A. (NCAR is sponsored by the National Science Foundation).     

Hydrogen emission from moving solar prominences

Brightness variations of the lines arising from a five-level hydrogen model atom, depending upon prominence velocities, have been investigated using a combination of two non-LTE techniques. The importance of the Doppler brightening and/or Doppler dimming effects is demonstrated for the lines of the Lyman and Balmer series.On leave from Wroclaw University Observatory, Poland.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A model of polarized X-ray emission from twinkling synchrotron supernova shells

Synchrotron X-ray emission components were recently detected in many young supernova remnants (SNRs). There is even an emerging class – SN 1006, RX J1713.72−3946, Vela Jr and others – that is dominated by non-thermal emission in X-rays, also probably of synchrotron origin. Such emission results from electrons/positrons accelerated well above TeV energies in the spectral cut-off regime. In the case of diffusive shock acceleration, which is the most promising acceleration mechanism in SNRs, very strong magnetic fluctuations with amplitudes well above the mean magnetic field must be present. Starting from such a fluctuating field, we have simulated images of polarized X-ray emission of SNR shells and show that these are highly clumpy with high polarizations up to 50 per cent. Another distinct characteristic of this emission is the strong intermittency, resulting from the fluctuating field amplifications. The details of this 'twinkling' polarized X-ray emission of SNRs depend strongly on the magnetic field fluctuation spectra, providing a potentially sensitive diagnostic tool. We demonstrate that the predicted characteristics can be studied with instruments that are currently being considered. These can give unique information on magnetic field characteristics and high-energy particle acceleration in SNRs.     

Magnetic reconnection model for X-ray flare loop interaction

The subtle interactions between the magnetohydrodynamics (MHD) and transverse plasmons are investigated. It is shown that there is a resistive instability by the plasmon's soliton in a current sheet, which eventually turns into an eruptive instability at the magnetic field reconnection. In the case of ion-acoustic turbulence, the high temperature current sheet model must adopt the aromalous conductivity instead of the Coulomb conductivity. The numerical results are consistent with the observations obtained by Hanaoka (1994). Thus the flare caused by X-ray loop coalescence can be basically interpreted by this model of magnetic field reconnection driven by ponderomotive force.     

Linear current instability in loop prominences

By use of the dispersion equation given by Song, Wu, and Dryer (1987) for a cylinder plasma with mass motion and gravity included, we investigate the linear current instabilities developed in loop prominences. The results indicate that the mode of linear instability depends mainly on whetherv _s ² > or not, wherev _s is the sonic velocity at heightz, =GM/(R +z) is the gravity potential,G the gravitational constant,M andR the mass and the radius of the Sun respectively. Ifv _s ² > , then the sausage instability will be dominant. Otherwise, the kink instability will be more important. A possible explanation of knot structure, which appears sometimes in solar loop prominences has been given.     

A model for quiescent prominences with helical structure

We present a model for quiescent prominences with helical structure. The model is described by two magnetic fields, one produced by photospheric or subphotospheric currents, the other due to currents along the cylindrical model prominence.On leave from Max-Planck Institut für Physik und Astrophysik, München.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

The spatial distribution of 6 centimeter gyroresonance emission from a flaring X-ray loop

We compare simultaneous high resolution soft X-ray and 6 cm images of the decay phase of an M3 X-ray flare in Hale Region 16413. The photographic X-ray images were obtained on an AS & E sounding rocket flown 7 November, 1979, and the 6 cm observations were made with the VLA. The X-ray images were converted to arrays of line-of-sight emission integrals and average temperature throughout the region. The X-ray flare structure consisted of a large loop system of length 1.3 arc min and average temperature 8 × 10⁶ K. The peak 6 cm emission appeared to come from a region below the X-ray loop. The predicted 6 cm flux due to thermal bremsstrahlung calculated on the basis of the X-ray parameters along the loop was about an order of magnitude less than observed. We model the loop geometry to examine the expected gyroresonance absorption along the loop. We find that thermal gyroresonance emission requiring rather large azimuthal or radial field components, or nonthermal gyrosynchrotron emission involving continual acceleration of electrons can explain the observations. However, we cannot choose between these possibilities because of our poor knowledge of the loop magnetic field.     

A search for soft X-ray emission from stellar sources

With the soft X-ray detector (0.2–0.284 keV) aboard the Astronomical Netherlands Satellite (ANS) we have searched for X-ray emission from hot star coronae and peculiar stars. On Sirius ( CMa) and Capella ( Aur) X-ray emission has been measured at 6 and 5 level, respectively, above background. In all other cases the search revealed no evidence for soft X-ray emission. Upper limits to the luminosities of about 25 star coronae (main-sequence stars, (sub)giants, and supergiants) and of 4 peculiar stars ( Sco, Lyr, P Cyg, and Car) have been obtained.Paper presented at the COSPAR/IAU Symposium on Fast Transients in X-and Gamma-Rays, held at Varna, Bulgaria, 29–31 May, 1975.     

A model for quiescent solar prominences with normal polarity

A class of 2-D models of solar quiescent prominences, with normal polarity, is presented. These represent an extension to the Kippenhahn-Schlüter model for which the prominence configuration matches smoothly onto an external non-potential coronal solution of a constant field. Using typical prominence values a model is constructed which also matches the coronal conditions. It is found that the magnetic field component along the prominence influences the internal structure of the prominence. A simple extension to the basic models is indicated as a means of taking a lower boundary of the prominence and eliminating parasitic polarities in the photosphere.     

A search for radio emission from Galactic supersoft X-ray sources

We have made a deep search for radio emission from all the northern hemisphere supersoft X-ray sources using the Very Large Array (VLA) and multi-element radio-linked interferometer network (MERLIN) telescopes, at 5 and 8.4 GHz. Three previously undetected sources, T Pyx, V1974 Cygni and RX J0019.8+2156, were imaged in quiescence using the VLA in order to search for any persistent emission. No radio emission was detected in any of the VLA fields down to a typical 1 σ rms noise of 20 μJy beam⁻¹, however, 17 new point sources were detected in the fields with 5-GHz fluxes between 100 and 1500 μJy, giving an average 100-μJy source density of ∼200 deg⁻², comparable to what was found in the MERLIN Hubble Deep Field survey. The persistent source AG Draconis was observed by MERLIN to provide a confirmation of previous VLA observations and to investigate the source at a higher resolution. The core is resolved at the milliarcsec scale into two components that have a combined flux of ∼1 mJy. It is possible that we are detecting nebulosity, which is becoming resolved out by the higher MERLIN resolution. We have investigated possible causes of radio emission from a wind environment, both directly from the secondary star, and also consequently, of the high X-ray luminosity from the white dwarf. There is an order of magnitude discrepancy between observed and modelled values that can be explained by the uncertainty in fundamental quantities within these systems.     

On the mechanism of formation of loop prominences

A mechanism is suggested for the formation of loop-type prominences in solar-active regions following flare events. The mechanism is based on the already existing idea of compression of a coronal plasma element resulting in enhanced radiation and consequent cooling of the element. A model is suggested for such a compression based on the concept of a contracting, force-free filamentary structure. If the current in a filament increases with time, then there is a radial contraction of the filament. Since the coronal plasma is frozen into the magnetic field lines of the filament, a contraction of the filament causes a compression of the filamentary plasma. This model of compression is shown to be in approximate qualitative and quantitative agreement with observations.     

A method of analysing the emission lines of solar prominences

A new method of analysing the emission spectrum of solar prominences is presented, in which the source function is allowed to vary with optical depth. Least-squares fitting of the observed profile determines simultaneously the optical depth τ₀, the Doppler width Δλ_D and the factor characterising the variation of the source function. This method is applied to the early Balmer lines in ten prominences of Ref. [1]. The results show that the source function of the self-reversed H line increases towards the centre of the prominence, the value at the centre is 1.2–2.5 times the value at the edge. Neglect of this variation will give too large values of τ₀. The degree of attenuation by selfabsorption also depends on this variation. Discussion of the variation gives support to the view that the main exciting mechanism in solar prominences is the scattering of the incident radiation.     

A model for the fibril structure of normal-polarity solar prominences

A normal-polarity prominence is modelled as a series of cool fibrils set in the hotter corona. Equations of magnetostatic equilibrium are solved and each fibril corresponds to a dip in the mgnetic field. The ratio of fibril width to interfibril spacing is dependent on the prominence-coronal temperature ratio and the ratio of plasma to magnetic pressure. The prominence mass is found to depend on the square of the magnetic field strength. When variations along the prominence are allowed in addition to those across the prominence, an apparently random pattern of fibrils results.     

Thermal effects in the formation of loop prominences

Loop prominences that appear after some solar flares may owe their form and duration to the behavior of the cooling function of the material (Cox and Tucker, 1969) and to the magnetic field configuration. Computer simulation of a model event shows that small temperature contrasts in a medium of several million degrees temperature may be enhanced as the medium cools by line and recombination radiation, on a time scale of several minutes. The loop prominence systems could be the result of this process repeated through successively higher levels of the solar atmosphere.     

Simulated X-ray emission from a single-explosion model for a supernova remnant 3C 400.2

We present a single-explosion model study of the 3C 400.2 supernova remnant (SNR), made with two-dimensional axisymmetric simulations. The numerical simulations were made with the adaptive grid code yguazú-a . Several initial scenarios have been tested in order to reproduce the centrally peaked X-ray emission observed for this remnant. This study reveals that the explosion of the SN inside a pre-existing stellar wind bubble successfully generates the observed morphology, when thermal conduction is included in the model. The best morphological fit is obtained at an evolution time of 21 000 yr, when the total luminosity is  1.2 × 10³⁴ erg s⁻¹  .