Analysis of two active prominences

We discuss the observations of two eruptive prominences, and the formation of condensations during the phenomena. The density and intensity variations of the condensations are analyzed spectroscopically in one of the events. Some hypothesis about the magnetic field configuration have been used in order to explain the observational data.     

Magnetic fields in flares and active prominences

We study the longitudinal magnetic field in a number of active limb prominences showing fields in excess of 30 G. The objects fall into three groups: surges, caps and active region prominences. There appears to be an upper limit of 150–200 G for the field strength in prominences.

A model of surges is presented in which a pre-surge axi-symmetric magnetic field is established by a line current in the corona. We observe particle acceleration in surges that indicates v×B≠0 in these objects during periods comparable to the Alfvén transit time.

The strong fields observed in caps seem to run between parts of active regions in accordance with Hale's law of sunspot group polarities.

     

Magnetic fields in flares and active prominences

The locations of flares and chromospheric absorption features on May 21 and 23, 1967, are compared with a series of H magnetograms. Each of the four major flares included in the study developed as double emission ribbons lying at positions of steep field gradient on opposite sides of the boundary between regions of opposite magnetic polarity. At certain stages, the flare outlines followed closely the isogauss contours of the longitudinal field. A fluctuating field of 75 gauss was measured directly in the importance two flare of May 21. Modifications in the magnetic structure of the active region followed the flares of May 23.     

The kinematic processes in solar prominences and flares and their spectral features

In this paper various models of mass motions in solar prominences and flares, such as expansion, contraction and rotation without or with depth gradients, are considered. The variation of the source function with depth is also taken into account. Under these conditions the profiles of the first Balmer lines are calculated. The various effects of mass motions on spectral lines are studied. We have established four methods for the derivation of the velocities of motions as well as their gradients from the corresponding spectral features. These methods have been preliminarily applied to observational data, mainly those of the spectra-spectroheliograph (SSHG) of the Yunnan Observatory.     

Asymmetry of solar active prominences separately at low and high latitudes from 1957 to 1998

The paper presents the results of a study of the asymmetry of the solar active prominences (SAP) at low (≤40°) and high (≥50°) latitudes, respectively, from 1957 through 1998 (solar cycles 19–22). A quantitative analysis of the hemispheric distribution of the SAP is given. We found that the annual hemispheric asymmetry indeed exists at low latitudes, but strangely, a similar asymmetry does not seem to occur for SAPs at high latitudes. We found that the north–south (N–S) asymmetry of the solar active prominences at high latitudes is always north dominated during solar cycles 19–22 while the N–S asymmetry of the SAPs at low latitudes is shifted to a dominance in the southern hemisphere for solar cycle 21 and remains south dominated even in cycle 22. Thus, the hemispheric asymmetry of the solar active prominences at high latitudes in a cycle appears to have little connection with the asymmetry of the solar activity at low latitudes.     

The fine structure of prominences

Faint, Doppler shifted, emission features are detected in high resolution spectra of limb prominences. Their average line-of-sight velocity is about 3 × 10⁶ cm s^-1, their average life time is 300 s, and their angular sizes are 10⁸ cm in our spectrograms. The emission line width of the spectral features increases with increasing line shift.Some implications on the stability of prominences by these structures are discussed briefly.Visiting Astronomer, on leave from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado.     

The fibril structure of prominences

In this paper we present several magneto-hydrostatic equilibrium models for prominences with fibril-like fine structure. For all the models ad hoc temperature profiles are used without discussing the energetics. For our models we assume fine structure to occur either across the prominence axis or along it. This approach is intended as a first step towards more realistic models based upon a series of vertical fibril structures.     

The fibril structure of prominences

We suggest that the fibril structure of prominences may be caused by filamentation during its formation by radiative instability. We also discuss the effects of other types of instability and give a mechanism for the formation of vertical threads. The models indicate that highly inhomogeneous density structures can exist in the presence of smooth profiles for the plasma pressure and magnetic field. In our particular models the plasma pressure of a fibril prominence is higher and the vertical magnetical field is weaker than in a uniform prominence model, while the mass is substantially smaller.     

The fine structure of prominences

High spatial resolution spectral observations of five hedgerow prominences were made in H, He i D₃ and Ca ii H and K.The observed relations between the lines were not the same in all prominences. The Ca ii H and K lines were 2–4 times brighter relative to H and D₃ than predicted theoretically. The optical thickness of H was less than for the H and K lines, the H was optically thin in medium faint prominence structures. Faint structures appeared slightly hotter than bright structures.On leave from Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, Oslo 3, Norway.     

The fine structure of prominences

Ions falling in vertically aligned magnetic structures of quiescent prominences may experience a vertical Lorentz force as flux ropes are distorted from the force-free condition. The terminal velocity of such ions may be sub-Alfvénic and may correspond to the 5–15 km s^–1 velocity of down falling material observed in many quiescent prominences. The higher velocities of down falling material found in active prominences and coronal rain may occur because of higher terminal velocities occurring in stronger magnetic fields.Visiting Astronomer, on leave from the Department of Astro-Geophysics, University of Colorado, Boulder, Colorado 80309.     

The evolution of isolated active regions

The decay of several active regions which emerged early in cycle 22 has been studied using daily magnetograms and synoptic plots obtained at the Vacuum Telescope at the National Solar Observatory, Kitt Peak. The observed patterns are compared with simulations using the flux transport equation and some discrepancies are noted. For one region it is shown that, by including the emergence of a non-random pattern of small magnetic bipoles during the decay, the correspondence between the observed and simulated patterns may be improved.     

The fine structure of prominences

The fine structure of nonspot prominences are studied from H filtergrams. The size of the smallest prominence structures increases with height above the chromosphere. Some prominences contain structures close to 1/2 arc second, which is the spatial resolution in the present data. The effective thickness of many nonspot prominences ranges between 4 × 10⁷ cm and 1.5 × 10⁸ cm. An apparent downward directed motion is observed in the majority of the prominences. No preferred direction of the motion is seen in regions composed of comparatively large diffuse structures. Some bright threads are visible for 1 hr and longer. Bright knots have an average observed lifetime of about 8 min. The process of condensation and subsequent destruction of prominence fine structure appears to take place on a very short time scale compared to the life time of the regions where prominences may exist. The observed H brightness of the prominences in the present data may be accounted for as scattered chromospheric radiation.     

The nature of quiescent solar prominences

Quiescent prominences occur as long-lasting cool sheets of matter in the surrounding hot corona at the base of coronal streamers. Seen on the disk they appear as dark filaments dividing regions of opposite magnetic polarity.In this paper a theoretical model is presented, which describes the general appearance of quiescent prominences.It is shown that the neutral sheet between two regions of oppositely directed magnetic fields is thermally unstable. This gives rise to compression and cooling of coronal material to prominence material in a characteristic time of the order of one day for a field strength of 0.5 gauss in the lower corona.It is assumed that due to the finite electrical resistivity of the plasma, filamentary structures are formed by the tearing-mode resistive plasma instability. These structures are thermally insulated from the hot surroundings by the newly formed closed azimuthal magnetic field configuration.It has been shown that for fine structures with a diameter of 300 km the growth rate of the tearing-mode instability is of the same order as the cooling time. The occurrence of fine structures within the prominence is of vital importance for their origin.On leave from the Observatory Sonnenborgh at Utrecht, The Netherlands.     

The internal motion of quiescent prominences

A study has been made of fine structure wavelength shift in the K line spectra from quiescent prominences. A persistent small scale motion is found in the prominence main body. In places where we see the characteristic thread like fine structure in the accompanying H filtergrams the average line-of-sight velocity amplitude is about 1 km s^–1. A higher velocity ( 4 km s^–1) is associated with a slightly coarser, mottled prominence fine structure. In the low lying regions, connecting the prominence body and the chromosphere, we do not detect any fine structure line shift (v 1/2 km s^–1).     

The evolution of active galaxies in clusters

The results of deep radio, sub-mm and X-ray observations of samples of high redshift (z∼1) clusters are presented. These reveal significant excesses of active galaxies associated with the clusters at all three wavelengths. The cluster radio source population shows evolution consistent with the (1+z)³ evolution typical of many AGN classes. A large fraction of the AGN are hosted by apparently passive early-type galaxies, often with a close companion. These results essentially constitute the detection of a counterpart of the Butcher-Oemler effect for both strongly star bursting galaxies and AGN. This revised version was published online in July 2006 with corrections to the Cover Date.     

The vertical filamentary structures of quiescent prominences

We present a simple magnetostatic theory of the thin vertical filaments that make up the quiescent prominence plasma as revealed by fine spatial resolution H photographs. A class of exact equilibrium solutions is obtained describing a horizontal row of long vertical filaments whose weights are supported by bowed magnetic field lines. A free function is available to generate different assortments of filament sizes and spacings, as well as different density and temperature variations. The classic Kippenhahn-Schlüter solution for a long sheet without filamentary structures is a particular member of this class of solutions. The role of the magnetic field in supporting and thermally shielding the filament plasma is illustrated. It is found that the filament can have a sharp transition perpendicular to the local field, whereas the transition in the direction of the local field is necessarily diffuse. A consequence of the filamentary structure is that its support by the Lorentz force requires the electric current to have a component along the magnetic field. This electric current flowing into the rarefied region around the prominence can contain substantial energy stored in the form of force-free magnetic fields. This novel feature has implications for the heating and the disruption of prominences.     

The evolution and orientation of early cycle 22 active regions

The evolution of six major active regions which appeared during the first phase of the present solar cycle (cycle 22) has been studied. It was found that the northern hemisphere regions exhibited a broad range of evolutionary behavior in which the commonly accepted normal pattern (whereby the follower flux moves preferentially polewards ahead of the leader flux) is represented only at one end of the range. At the other end of the range, the leader flux is displaced polewards of the follower flux. In the latter cases equatorward extensions of the polar coronal hole are noted.While it is emphasized that some of the regions in this study follow the more conventional pattern and that all regions in this study emerged during the early phase of cycle 22, the implications for theories of the solar polar field reversals are noted.     

The orientation of magnetic fields in quiescent prominences

We have measured the longitudinal component, B, of the magnetic field in quiescent prominences and obtained a relationship between B and , where is the angle between the long axis of the prominence and the north-south direction on the sun. From this relationship we deduce a distribution function for the magnetic field vector in quiescent prominences in terms of the angle between the field and the long axis of the prominence. The mean angle, , for our data is small, - 15°, indicating that the magnetic field traverses quiescent prominences under a small, but finite angle.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 relationship between UV and X-ray active region structures

Yohkoh and the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) jointly observed two brightenings in active region NOAA 7981 on 6 August 1996. Combining the UV data from CDS with information from the high time resolution coronal images obtained with the Soft X-ray Telescope (SXT) on Yohkoh, provides us with important information on the relationship between the transition region and corona. Our observations show that cool plasma (T_e = 2.2 x 10^-5 K) can lie at the same altitude as the hot coronal plasma (T_e = 1–4 x 10⁶ K). The lower temperature structure is not formed from the cooling of the hotter coronal loop. We are also able to observe a low temperature cut-off of T_e = 1–4 x 10⁶ K for a loop which repeatedly brightened over the period of approximately one day.