Motions in a loop prominence

Motions of loop prominence knots have been studied on H-line filtergrams. By making use of contours of equal photographic density for entire cinegram it has been possible to significantly decrease the error in determining the locations of the knots. The method of mean velocities has been developed, which has permitted for the first time accurate determination of the laws of knot motion with sufficient accuracy. Two types of falling knots are distinguished: (1) those with a constant acceleration that is always below the gravitational acceleration, and (2) those with a constant velocity. The initial velocity of the falling knots is always different from zero. The gasdynamic theory has been developed to explain the deceleration of the two types of knots due to: the work done against the pressure force; pileup plasma raking; and shock-wave generation. The shock mechanism imparts a constant velocity to the motion. The temperature along the knot trajectories has been estimated. The ratio of the densities in the knots to the surrounding medium has been found.The aim of the present work is to gain new insight into the known observations of the motions in active prominences, in particular in the loop prominences, and to understand the reasons for the observed motions.A number of studies of prominence knot motions have been made using filter photographs (cinefilms). It seems to us, however, that the velocities (and especially acceleration) of individual knots have been determined within insufficient accuracy. It will be noted first of all that values of V(t) have been determined with a low accuracy by some authors even for high velocities (V 100 km s^–1). In fact, to achieve at least 10% accuracy in determining V by comparing two photographs obtained with a 30 s interval, it is necessary to measure the location of knots to 0.5 accuracy. The problem is the more complicated as the size of the knots can often reach 5 × 10. This is why the temporal dependence of the velocity of prominence knot motion is represented with a complicated curve.     

Kinematics of a loop prominence

The kinematics of a loop system has been studied from high resolution Ca ii K line spectra and H filtergrams recorded at Oslo Solar Observatory.Emission features are seen to fall at supersonic velocities from the top of the arches towards the chromosphere. Our data are in agreement with the assumption of matter falling freely along a dipole type magnetic field of maximum height 100–150 thousand km. There is a slight asymmetry between positive and negative line shifts which can be accounted for as a tilt of the individual loops relative to the plane of the sky of 5–10°. The planes of the loops are also inclined by a small angle of approximately 15°. It appears that matter starts from rest at the top of the loops. An observed tilt of some emission features in the K line spectra may be explained by a gradient in the line-of-sight velocity with height caused by the curvature of the dipole type loops.     

Spatial relationship between λ5303 and Hα components of a loop prominence system

There are remarkable similarities in the structure of loop prominences when observed in H and coronal lines, although the lines arise from extremely different excitation conditions. This leads to the consideration of a multi-component model, where different emission lines come from different elements of the structure. The late phases of a large west limb loop prominence system followed by a surge were recorded at Haleakala Observatory on March 6, 1970. Simultaneous filter-grams in H and 5303 were obtained, together with spectra at three heights in the prominence over the range 3850–5950 Å. The positions of bright knots of emission as seen in the green line are compared with associated H knots. With these relative positions determined, the filtergrams are superimposed to demonstrate the two dimensional spatial relationship between H and 5303 structures. The results support a model of cool loops within a closely associated hot loop system.     

Oscillations of a loop prominence preceding a limb flare

Observations of a limb flare and an associated loop prominence were obtained in H with the 512 channel magnetograph of the Kitt Peak National Observatory. Simultaneous radial and torsional oscillations with a period near 75 s, wavelength of 37 000 km, and amplitude of 1–2 km s^–1 were detected in the loop approximately 90 min before the onset of the flare. We interpret these coupled oscillations in terms of a kink instability of a current carrying flux tube. The magnitude of the steady-state component of current is estimated to be 6 × 10¹⁰ ampères.Visiting Astronomer, Kitt Peak National Observatory, operated by Aura, Inc., under contract with the National Science Foundation.Visiting Student, Kitt Peak National Observatory.     

Centimeter-wave radio emission of a high-latitude prominence

The results of observations of the radio emission of a high-latitude prominence located in the NE part of the solar limb are discussed. Observations were performed on the radio telescope RATAN-600 using its Northeastern sector and Southern sector with a periscope during the maximum phase (0.998) of the solar eclipse of March 29, 2006. The prominence was studied in the wavelength interval 1.03 ÷ 5.0 cm. The absence of the background illumination from the solar disk allowed us to study the parameters of the radio emission of the high-latitude prominence (? = 45°, NE limb of the Sun). Observations of the solar limb at the time of the maximum phase made it possible to record very small radio fluxes from the prominence, which amounted to 0.05 ÷ 0.01 s.f.u. in the wavelength interval from 1.84 to 5.0 cm. The position of the maximum of the radio emission of the prominence coincides, according to the results of observations performed on both sectors, with the summit of the prominence as seen on the solar image taken in the He II 304 Å line (SOHO, ? = 45°, NE limb of the Sun). The degree of polarization is P ≈ 7 ÷ 16% at 1.88 ÷ 5.0 cm. If interpreted in terms of the thermal mechanism considered here, polarized emission corresponds to a magnetic-field strength of H ? (550 ÷ 100) G in the prominence region.     

The analysis of XUV emission lines

A technique for analyzing measurements of XUV spectral line intensities is described. Application of the technique to OSO-4 and OSO-6 spectra indicates that the mean coronal temperature is 2.1 × 10⁶ K in typical active solar regions and that the mean coronal temperature in typical quiet regions ranges from 1.5 × 10⁶ to 2.1 × 10⁶ K. One active region spectrum shows evidence for substantial quantities of coronal material with 2 × 10⁶ < T < 3.5 × 10⁶ K. Measurements from limb spectra show evidence that (1) coronal abundances of N and O are low relative to heavier elements; or (2) that the ionization equilibrium calculations used may contain systematic errors; or (3) that the XUV intensity measurements may be incorrectly calibrated.     

Model calculations of the rising motion of a prominence loop

Model calculations are presented for the rising motion of the top section of a prominence loop, which is represented by a straight flux rope immersed in a coronal medium permeated with a bipolar magnetic field. Initially the prominence is at rest, in equilibrium with the surrounding coronal medium. When the magnetic monopoles that account for the source current for the bipolar field strengthen, the upward hydromagnetic buoyancy force overcomes the downward gravitational force so that the prominence is initiated into rising motion. The illustrative examples show that prominences can move away from the solar surface by the action of the hydromagnetic buoyancy force, which is preponderant with the diamagnetic force due to the current carried by the prominence interacting with the coronal magnetic field produced by the photospheric currents, if the changes in the photospheric magnetic field are sufficiently large.     

Visible coronal emission associated with a quiescent prominence

Emission-line coronagraph images of a high-latitude, nominally quiescent prominence, recorded at wavelengths of H, 6374 Å (Fex) and 5303 Å (Fe xiv), are analyzed. Over a two-day period, the coronal images, which are found to arise predominantly from coronal emission, evolve such that the emission becomes concentrated at locations corresponding to the outer regions of the prominence. This edge enhancement has similar characteristics to results inferred from EUV prominence observations. It is postulated that this coronal emission associated with the prominence results from MHD wave dissipation. Dissipation lengths for slow-mode, fast-mode and Alfvén waves are estimated for different prominence conditions. Of these, fast-mode waves appear to be the most physically realistic heating source if the prominence magnetic field is along the length of the prominence.Operated by the Association of Universities for Research in Astronomy, Inc., under contract AST 78-17292 with the National Science Foundation.     

Two prominence eruptions and the problem of emission

Two flare unconnected eruptions on January 15 and 29, 1968, are discussed. The first is a filament which turns bright and erupts upward, reappearing an hour later. The second is a large eruptive arch seen against the disk. The arch is bright at the top of its trajectory, turns dark, but produces chromospheric emission at the point of impact. The emission at the top of the arch is ascribed to the velocity shift of the illuminating chromospheric Ha line. It is shown that such emission will occur only if the motion is transverse to the line of sight and the prominence is optically thin.This research was supported by the National Science Foundation under Grant 1472, the Advanced Research Projects Agency under the Office of Naval Research Grant N00014-67-C-0140, and the National Aeronautics and Space Administration under Grant NGR 05 002 034.     

Magnetic reconnection in the corona and the loop prominence phenomenon

Many classes of transient solar phenomena, such as flares, flare sprays, and eruptive prominences, cause major disruptions in the magnetic geometry of the overlying corona. Typically, the results from Skylab indicate that pre-existing closed magnetic loops in the corona are torn open by the force of the disruption. We examine here some of the theoretical consequences to be expected during the extended relaxation phase which must follow such events. This phase is characterized by a gradual reconnection of the outward-distended field lines. In particular, the enhanced coronal expansion which occurs on open field lines just before they reconnect appears adequate to supply the large downward mass fluxes observed in Ha loop prominence systems that form during the post-transient relaxation. In addition, this enhanced flow may produce nonrecurrent high speed streams in the solar wind after such events. Calculations of the relaxation phase for representative field geometries and the resulting flow configurations are described.New address: Los Alamos Scientific Laboratory, Los Alamos, N.M. 87545, U.S.A.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.     

Studies on post-flare loop prominence of 1981 April 27

By use of the H observations of the Astrophysical Observatory in Catania, Italy and the Purple Mountain Observatory in Nanking, China as well as hard X-ray and gamma-ray burst data from the Solar Maximum Mission (SMM) Gamma-Ray Spectrometer (GRS), a major eruptive loop prominence was studied during the limb solar flare event of 1981 April 27.Our preliminary analysis shows that there seems to exist a second abrupt energy release for this event, almost 20 min after the end of the impulsive phase of the flare. This energy release is probably associated with the rapidity in upward motion or activation of the loop prominence.A possible candidate for such a process could be the reconnection of the old magnetic field with a newly emerging magnetic field.A theoretical gross estimate for the energy release and particle acceleration has also been made in this work. It appears that the proposed model for charged particle acceleration is very efficient.     

The oscillating loop prominence of July 17, 1981

Oscillatory motions of a loop prominence observed on July 17, 1981 are analysed. The oscillations were mainly horizontal, with a period of 8 min. Restoring force was a result of magnetic tension, and assuming a simplified magnetic field configuration the expression for frequencies of oscillations is derived and compared with the observations. Taking the observed period, the strength of the magnetic field permeating the prominence is estimated as 45 G. Finally, the stability of the prominence is discussed.     

Simultaneous monochromatic and spectral observations of two large loop prominence groups

Using the solar tower telescope of Nanjing University, we observed the two large loop prominence groups of 1982 Dec. 20 and 1983 Feb. 9. Hα photographs and spectra around the Hα and H and K lines were obtained simultaneously. From these data, we derived a line of sight velocity distribution, which agrees perfectly with the distribution for matter falling freely without viscosity. From the widths of the Hα and the K lines, we found the loop material to have a uniform kinetic temperature and a turbulent velocity that increases with height. From the central intensities of the lines we derived a density of n(H) ? 1.3 ? 2.6 × 10¹⁰cm^?3. A possible mechanism of the formation of loop prominence groups and their relation with flares are discussed.     

Calculations and physical properties of the D3 emission lines of a prominence

The D3 emission lines observed on a prominence over the solar east limb on 1984 May 5, which were found to consist of two components, i.e. the main and the broadened components, have been successfully calculated using the two-cloud model method with the multiplet of helium (3 ³D_3,2,1–2 ³P_2,1,0) taken into account. The results obtained show that the ejecta in front of the prominence are formed by the intermittent ejections of the matter from the plage region around the prominence, because there exist at least three distinguishable phases of the line-of-sight velocity, increasing during the observations. The turbulent velocity of the ejecta is rather large, about 29 km s⁻¹. The Doppler width of the ejecta is mainly the result of the non-thermal effect, and the thermal effect can be neglected compared with the non-thermal effect. The prominence is quiet and characterized by typical properties .     

On coronal Fe abundances and temperatures from XUV emission lines

Data obtained by the OSO-7 spectroheliograph on strong XUV lines of five, different Fe ions from the outer equatorial corona are presented. Interpretation of the data with a spherically symmetric model atmosphere gives average ion abundances for lines of sight at 0.3R from the limb. Fe xvi is usually more abundant than Fe xv, xiv, xii and ix, but there are times when Fe xii is more abundant than the other ions. The deviation of measured relative abundances of Fe xii, xiv, and xvi from predictions of ionization equilibrium at one temperature seems to indicate that there are appreciable temperature variations along lines of sight. Element abundances are very uncertain since they appear to depend so heavily on likely but unknown density irregularities along lines of sight.     

Spectral analysis and the two-dimensional distribution of physical parameters in a quiescent prominence

The profiles of H and Ca ii K lines of a arch quiescent prominence on April 1, 1971 have been analyzed and the two-dimensional distributions of electron temperature T _e , micro-turbulence velocity v _t and the column number density of hydrogen along the line-of-sight N _H have been obtained. T _e , _t , and N _H are found to be 7500 K, 6 km s^–1 and 2.2 ^× 10¹⁸ cm^–2 on an average, respectively. The electron temperature at the central part of the prominence and along the two arcades are greater than that at the edges, while the distribution of the micro-turbulence velocity in these regions is opposite. There is no systematic variation in T _e and v _t , from the center to the periphery as described by Hirayama (1971). The column number density in the central region is lower than that at the two edges.The contour lines of T _e , _t , and N _H are predominantly vertical rather than horizontal. This implies that the height-variation of physical parameters in filamentary structure is small. The arrangement of this structure in the prominence is likely to be arched and is probably in the direction of magnetic field lines.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The loop prominence of May 13, 1971 and its associated effects

A study is presented of the formation of a loop prominence system (LPS) on May 13, 1971. The development of the phenomenon is found to be consistent with the model of Jefferies and Orrall, and was associated with small radio bursts and several kind of SID. From the spectral analysis a temperature of 13200 K has been deduced and a density of hydrogen atoms n(H) 10¹⁰cm^-3 estimated. The existence of inclined emissions probably produced by rotatory motions is discussed. From the radio data analysis it was inferred that an impulsive and a thermal component were present; the length of the emitting plasma cloud was found to be about 56000 km. The LPS appeared, in accordance with previous results, in a region producing proton flares.     

A comprehensive analysis of the loop prominence of 1981 April 27

We make comprehensive analysis of morphological tracings and positional measurements of Hα images, white-light sunspot photographs and chromosphere velocity field, obtained at Yunnan Observatory, and hard X-ray images obtained by the Hinotori satellite, of the loop prominence of 1981 April 27. It seems likely that the observed loop is the projection of a post-flare loop system, and the associated flare occurred on the rear side of the solar disk. A two-ribbon flare such as in the Kopp-Pneuman model can satisfactorily explain all the observed features. The occurrence of such a flare seems likely from the evidence given by the data.     

Oscillations in EUV emission lines during a loop brightening

Oscillations in the emission in the ultraviolet lines of Cii, Oiv, and Mg x, detected by the Harvard College Observatory EUV spectroheliometer on Skylab are observed on August 7, 1973, during a loop brightening. The intensity of the EUV lines varies with a period of 141 s during the time of enhanced intensity of the coronal loop, lasting 10 min. The periodic oscillation is not only localized in the loop region but extends over a larger area of the active region, maintaining the same phase. We suggest that the intensity fluctuation of the EUV lines is caused by small-amplitude waves, propagating in the plasma confined in the magnetic loop and that size of the loop might be important in determining its perferential heating in the active region.On leave from the University of Torino, Italy.