Oscillatory Modes of a Prominence – PCTR – Corona Slab Model

Oscillations of magnetic structures in the solar corona have often been interpreted in terms of magnetohydrodynamic waves. We study the adiabatic magnetoacoustic modes of a prominence plasma slab with a uniform longitudinal magnetic field, surrounded by a prominence – corona transition region (PCTR) and a coronal medium. Considering linear small-amplitude oscillations, we deduce the dispersion relation for the magnetoacoustic slow and fast modes by assuming evanescentlike perturbations in the coronal medium. In the system without PCTR, a classification of the oscillatory modes according to the polarisation of their eigenfunctions is made to distinguish modes with fastlike or slowlike properties. Internal and external slow modes are governed by the prominence and coronal properties, respectively, and fast modes are mostly dominated by prominence conditions for the observed wavelengths. In addition, the inclusion of an isothermal PCTR does not substantially influence the mode frequencies, but new solutions (PCTR slow modes) are present.     

Vector magnetic fields in prominences

Observations of linear polarization in two resolved components of HeI D₃ are interpreted using the Hanle effect to determine vector magnetic fields in thirteen prominences. As in all vector magnetic field measurements, there is a two-fold ambiguity in field direction that is symmetric to a 180° rotation about the line-of-sight. The polar angles of the fields show a pronounced preference to be close to 90° from the local solar radius, i.e., the field direction is close to horizontal. Azimuth angles show internal consistency from point to point in a given prominences, but because of the rotational symmetry, the fields may be interpreted, in most cases, as crossing the prominence either in the same sense as the underlying photospheric fields or in the opposite sense. An exceptionally well observed large prominence of approximately planar geometry exhibits no measurable change in the vector magnetic field either with height or with location along the prominence axis. A second well observed large prominence overlying a sharply curved magnetic neutral line, when interpreted assuming that the prominence field has the same sense as the photospheric field, shows a rotation in the azimuth angle of the field relative to the observer by about 150° and relative to the local plane of the prominence by about 65°. In the alternative interpretation in which the prominence field has the opposite sense of the photospheric field, the field still rotates by 150° relative to the observer but remains essentially constant with respect to the plane of the prominence. This prominence erupted shortly after the extended observations. One good quality observation during the course of the eruption gives a vector field fully consistent with the pre-eruption field in the same segment of the prominence.     

The small scale velocity field of a quiescent prominence

The small scale velocity field of a large quiescent prominence is studied from simultaneous filtergrams in the red and violet wings of the Ca ii K-line.The average spatial separation of peak velocities is about 3000 km. The duration of individual velocity elements appears to increase with increasing area of the elements. In some positions of the prominence the direction of the velocity persists through the 5 hrs of observations. It is suggested that emitting elements of gas, which flow along the magnetic lines of force, produce peak velocities when they pass through particular locations of the prominence region.There is no clear evidence in the data for oscillations or waves.     

Oscillatory motions in an active prominence

Different types of oscillatory motions were detected in the late phases of eruption of a prominence. We found oscillations of the prominence axis and diameter with periods of 4.3 and 9.1 min, corresponding to the eigenmodes m = 4 and m = 8 with a damping factor 4.6 × 10^–3 s^–1. A period about 4.5 min was found for oscillations of the pitch angle of the helically twisted filaments. The m = 2 and m = 3 eigenmodes could be also identified and they led to the final relaxation of the prominence axis. The observations are compared with a model in which we consider forces acting in a curved, cylindrical magnetic tube anchored at both ends in the photosphere and carrying an electric current. The stability of the prominence is discussed.     

The magnetic field in the prominences of the polar crown

The Hanle effect method has been applied to the determination of the magnetic field in 120 prominences of the polar crown observed during the 1974–1980 period, which is the ascending phase of cycle XXI. The average field strength which was about 6 G at the beginning of the cycle reached twice this value just before the maximum. There is also a clear trend for a increase of the prominence field with the altitude. We confirm the fact that the magnetic vector makes a small angle (25 °) with the long axis of the prominence. As to the field orientation, we show that the most striking feature lies in the regular pattern of the component which is parallel to the axis of the filament; its direction seems to depend closely on the polarities of the high latitude photospheric field.     

Current sheet models for inverse polarity prominences in twisted flux tubes

This paper treats the prominence model of Low (1993) to examine more complicated sheet currents than those used in the original model. Nonlinear force-free field solutions, in Cartesian coordinates, invariant in a given direction, are presented to show the possibility of an inverse-polarity prominence embedded in a large twisted flux tube. The force-free solution is matched to an external, unsheared, potential coronal magnetic field. These new solutions are mathematically interesting and allow an investigation of different profiles of the current intensity, magnetic field vector and mass density in the sheet. These prominence models show a general increase in magnetic field strength with height in agreement with observations. Other prominence properties are shown to match the observed values.     

Attenuation of small-amplitude oscillations in a prominence–corona model with a transverse magnetic field

Observations show that small-amplitude prominence oscillations are usually damped after a few periods. This phenomenon has been theoretically investigated in terms of non-ideal magnetoacoustic waves, non-adiabatic effects being the best candidates to explain the damping in the case of slow modes. We study the attenuation of non-adiabatic magnetoacoustic waves in a slab prominence embedded in the coronal medium. We assume an equilibrium configuration with a transverse magnetic field to the slab axis and investigate wave damping by thermal conduction and radiative losses. The magnetohydrodynamic equations are considered in their linearised form and terms representing thermal conduction, radiation and heating are included in the energy equation. The differential equations that govern linear slow and fast modes are numerically solved to obtain the complex oscillatory frequency and the corresponding eigenfunctions. We find that coronal thermal conduction and radiative losses from the prominence plasma reveal as the most relevant damping mechanisms. Both mechanisms govern together the attenuation of hybrid modes, whereas prominence radiation is responsible for the damping of internal modes and coronal conduction essentially dominates the attenuation of external modes. In addition, the energy transfer between the prominence and the corona caused by thermal conduction has a noticeable effect on the wave stability, radiative losses from the prominence plasma being of paramount importance for the thermal stability of fast modes. We conclude that slow modes are efficiently damped, with damping times compatible with observations. On the contrary, fast modes are less attenuated by non-adiabatic effects and their damping times are several orders of magnitude larger than those observed. The presence of the corona causes a decrease of the damping times with respect to those of an isolated prominence slab, but its effect is still insufficient to obtain damping times of the order of the period in the case of fast modes.     

Periodic structures in quiescent prominences

The observed structural periodicities in quiescent prominences and filaments are examined in terms of the instability of a plasma supported by a magnetic field against gravity. It is suggested that the spacing of arch-like structures may be identified with the most unstable wavelength of the interface between the prominence and the supporting magnetic field. The results of analysis further suggest that the observed spacing of periodic structures corresponds to the supporting magnetic field which lies at an angle 90° to 60° with respect to the long axis of the prominence.     

The MHD stability of a twisted flux tube prominence model

The ideal MHD stability of the 2D twisted magnetic flux tube prominence model of Cartledge and Hood (1993) is investigated. The model includes a temperature profile that varies from realistic prominence values up to typical coronal values. The prominence is considered to be of finite-width and finite height. The stability properties of the prominence models are studied by using a method that generates a separate necessary condition and a sufficient condition. These conditions give bounds on the parameters that define marginal stability. In many cases these bounds are quite close so that further, more detailed, stability calculations are not necessary. A number of parameter regimes are examined, corresponding to different profiles of the prominence temperatures, densities, and magnetic field shear. It is found that the model admits realistic stable and unstable loop lengths for observed prominence parameters when the axial magnetic field component does not vanish.     

Dynamics of a Quiescent Solar Prominence Observed with the SUMER/SOHO Instrument

We present the results obtained from analyzing SUMER/SOHO observational data of a quiescent solar prominence. The studied prominence is made of complex structures. From the 1-hr data set, we derive characteristic frequencies in terms of intensity and velocity oscillations, as measured in 4 transition-region lines. The presence of different types of frequencies is detected: chromospheric oscillations and intermediate periods (6 min to 12 min). This result suggests that these oscillations are transmitted by the magnetic fields from the chromosphere to the transition region.     

OSCILLATIONS IN PROMINENCE FINE-STRUCTURES

Oscillatory spectra of solar quiescent prominences highlight the importance of incorporating the effect of prominence fine-structure in the theory of prominence oscillations. We determine the magnetohydrodynamic modes of oscillation of an elementary, zero- model of a prominence fibril, arguing that the fast body kink modes, namely, the string and the internal magnetic Love modes, produce the observed short periodicities in prominence fine-structures. Estimates for the periods of these modes are presented: the modes are subject to testing in future high-resolution observations.     

On the possibility of investigation of the magnetic field structure in subphotospheric layers of the Sun according to observations of sunspot torsional oscillations

A method of investigation of the magnetic field structure in subphotospheric layers of the Sun has been developed. The method is based on observations of the torisonal oscillations of single sunspots. Characteristics of the torsional oscillations have been obtained from observations of the longitudinal magnetic field and radial velocities of seven single sunspots in the photospheric line Fe I λ5253 Å. The parameters of the torsional oscillations and magnetic tubes in the deep layers have been determined. The radius of the cross section of a magnetic flux tube forming a sunspot is greatest near the Sun’s surface and is approximately equal to the radius of a sunspot umbra. Down to the deeper layers, it decreases quite quickly. The longitudinal electric current appearing in the magnetic tube changes direction. The typical time of the current changes is determined by the period of the torsional oscillations. The intensity of the longitudinal magnetic field in the tube increases with depth. The Alfven wave velocity averaged over the length of a magnetic tube is tens or hundreds of times less than this velocity in a sunspot umbra. It decreases with an increase in the period of oscillations. A decrease in the Alfven wave velocity leads to an increase in the twisting angle of magnetic field lines.     

Prominence height and vertical gradient in magnetic field

The existence of a critical height for quiescent prominences and its relationship to parameters of the magnetic field of photospheric sources are discussed. In the inverse-polarity model, stable equilibrium of a filament with a current is possible only in the region where the external field decreases with height no faster than ～1/h. Calculations of the potential magnetic field above the polarity-inversion line are compared with the observed prominence height. The prominence height is shown to actually depend on the vertical field gradient and does not exceed the level at which the exponent of field decrease is equal to unity.     

Modulation of energetic particle fluxes due to long-period geomagnetic oscillations

Mechanism of flux modulations of energetic protons and electrons, associated with the long-period geomagnetic pulsations in the outer magnetosphere, is examined theoretically. In the first part, a linear perturbation theory of the guiding centre distribution function averaged over the bounce phase of an interacting particle is developed for the case of the three-dimensional magnetic oscillations with a sufficiently long period compared with the bounce time of the particle. Secondly we extend the formulation to include some effects of the perturbed drift orbit on the particle distribution such as the particle trapping in the wave field and the phase bunching process. The latter is important for the interaction with the coupling Alfvén mode of magnetic oscillations. Applying these results together with the basic characteristics of the coupling hydromagnetic oscillations in a non-uniform plasma, we discuss the possibilities for the observed particle flux modulations in two different cases, separately, i.e. flux oscillations due to the compressional magnetic perturbation and those from the nearly transverse magnetic variations.     

Present and Future Observing Trends in Atmospheric Magnetoseismology

With modern imaging and spectral instruments observing in the visible, EUV, X-ray, and radio wavelengths, the detection of oscillations in the solar outer atmosphere has become a routine event. These oscillations are considered to be the signatures of a wave phenomenon and are generally interpreted in terms of magnetohydrodynamic (MHD) waves. With multiwavelength observations from ground- and space-based instruments, it has been possible to detect waves in a number of different wavelengths simultaneously and, consequently, to study their propagation properties. Observed MHD waves propagating from the lower solar atmosphere into the higher regions of the magnetized corona have the potential to provide excellent insight into the physical processes at work at the coupling point between these different regions of the Sun. High-resolution wave observations combined with forward MHD modeling can give an unprecedented insight into the connectivity of the magnetized solar atmosphere, which further provides us with a realistic chance to reconstruct the structure of the magnetic field in the solar atmosphere. This type of solar exploration has been termed atmospheric magnetoseismology. In this review we will summarize some new trends in the observational study of waves and oscillations, discussing their origin and their propagation through the atmosphere. In particular, we will focus on waves and oscillations in open magnetic structures (e.g., solar plumes) and closed magnetic structures (e.g., loops and prominences), where there have been a number of observational highlights in the past few years. Furthermore, we will address observations of waves in filament fibrils allied with a better characterization of their propagating and damping properties, the detection of prominence oscillations in UV lines, and the renewed interest in large-amplitude, quickly attenuated, prominence oscillations, caused by flare or explosive phenomena.     

A model for an inverse-polarity prominence supported in a dip of a quadrupolar region

We investigate the formation and support of solar prominences in a quadrupolar magnetic configuration. The prominence is modeled as a current sheet with mass in equilibrium in a two-dimensional field. The model possesses an important property which is now thought to be necessary, namely that the prominence forms within the dip, rather than the dip being created by the prominence.The approach of two bipolar regions of the same sign gives a natural way to form a dip in the magnetic field in a horizontal band above the photospheric polarity inversion line. As the approach proceeds, the height of the dip region decreases but, in agreement with observations, a corridor, free of significant magnetic field, is needed in order to obtain a dip at low heights.Support is achieved locally just as for normal-polarity configurations, so the model avoids the strong self-pinching effect of several inverse-polarity configurations (such as the Kuperus and Raadu model). The role of the strong field component along the prominence axis, which is here modelled by a uniform field in that direction, may well be to provide the necessary thermal properties for prominence formation.The model thus has several attractive features which make it credible for inverse polarity prominences: (i) both the dip and the inverse orientation are naturally present; (ii) prominence formation is by converging rather than shearing motions, in agreement with observations; converging photospheric motions induce a horizontal upward motion in the filament; (iii) the orientation of the axial field, opposite to what is expected from differential rotation, is naturally accounted for; (iv) the observed relation between chromospheric and prominence magnetic field strengths is naturally reproduced; (v) the field configuration is more complex than a simple bipole, in agreement with observations.     

OSCILLATIONS IN QUIESCENT SOLAR PROMINENCES OBSERVATIONS AND THEORY – (Invited Review)

An extensive observational background about the existence of oscillations in quiescent solar prominences has been gathered during the last twenty years. From these observations, information about different oscillatory parameters such as period, wavelength, phase speed, damping time, etc., has been obtained. This observational background, combined with a growing number of theoretical studies about magneto-hydrodynamic waves in prominences, should allow the development of prominence seismology which, following helioseismology's approach, seeks to infer the internal structure and properties of solar prominences. The most recent observational and theoretical developments on prominence oscillations are reviewed here, with an emphasis on the aspects suitable to develop an observation versus theory feedback, but also pointing out key topics which should be the subject of future research for a further advancement of this field.     

Torsional oscillations of umbra and penumbra of sunspots

Torsional oscillations of seven single spots are studied based on the observations of the longitudinal magnetic field and the field of radial velocities in the photospheric Fe I λ 525.3 nm line. The periods of umbra and penumbra oscillations are 2.2–7.1 and 3.3–7.7 days, respectively. The spots at a greater solar latitude are characterized by a longer period of oscillations and a smaller axial strength of the magnetic field. The periods of umbra and penumbra oscillations increase with an increase in the period and amplitude of the sunspot umbra oscillations. The obtained results can point to a unitary mechanism of torsional oscillations of umbra and penumbra of single spots and a connection of these oscillations with the differential rotation of the Sun.     

1990年8月29日龙卷日珥的观测研究

１９９０年８月２９日在太阳东边缘（Ｅ９０；Ｓ１２，ＮＯＡＡ／ＵＳＡＦ６２４１）爆发了一个龙卷日珥，本文对这个龙卷日珥进行了详细的形态分析和光谱诊断，结果表明：（１）龙卷日珥有规则地螺旋运动上升，上升到最大高度后，日珥内物质无规则地纷纷下坠，这种螺旋运动可能是带电粒子漂移运动产生的电场与日珥内冻结磁场相互作用的结果；（２）龙卷日珥形态快速变化，最后呈规则的环状结构；（３）龙卷日珥的形态可能是一个重要的因素，它体现了局部区域磁场结构的变化。本文提出了一种可能的磁场结构模型，对观测结果给予了较好的解释。     

Electric field oscillations measured near an auroral arc

A sounding rocket, launched into the expansive phase of an auroral substorm, measured bursts of electric field oscillations with a typical period of one second and a magnitude exceeding 20 mV/m. The oscillations appear to be due to an MHD wave propagating along the magnetic field. The bursts were observed as the sounding rocket passed over the southern border of an auroral arc. The southern border coincided with an increase in 1–5 keV electron flux and an increase in field-aligned current.