Seismology of Prominence-Fine structures: Observations and Theory

Prominence seismology is a rapidly developing topic which seeks to infer the internal structure and properties of solar prominences from the study of its oscillations. Two-dimensional high-resolution observations suggest that filaments can be considered as made by small scale fibrils, having a cool region, stacked one after another in the vertical and horizontal directions. An extense observational background about oscillations in filaments has been gathered during the last 20 years and these observations point out that fibrils or groups of fibrils can oscillate independently. From the theoretical point of view, small amplitude oscillations in single and multifibril configurations have been studied as a first step to explain observational features.     

Prominence Seismology Using Small Amplitude Oscillations

Quiescent prominences can be modeled as thin slabs of cold, dense plasma embedded in the much hotter and rarer solar corona. Although their global shape is rather irregular, they are often characterised by an internal structure consisting of a large number of thin, parallel threads piled together. Prominences often display periodic disturbances mostly observed in the Doppler displacement of spectral lines and with an amplitude typically of the order of or smaller than 2–3 km?s^?1, a value which seems to be much smaller than the characteristic speeds of the prominence plasma (namely the Alfvén and sound velocities). Two particular features of these small amplitude prominence oscillations are that they seem to damp in a few periods and that they seem not to affect the whole prominence structure. In addition, in high spatial resolution observations, in which threads can be discerned, small amplitude oscillations appear to be clearly associated to these fine structure constituents. Prominence seismology tries to bring together the results from these observations (e.g. periods, wavelengths, damping times) and their theoretical modeling (by means of the magnetohydrodynamic theory) to gain insight into physical properties of prominences that cannot be derived from direct observation. In this paper we discuss works that have not been described in previous reviews, namely the first seismological application to solar prominences and theoretical advances on the attenuation of prominence oscillations.     

Large Amplitude Oscillations in Prominences

Since the first reports of oscillations in prominences in the 1930s, there have been major theoretical and observational developments to understand the nature of these oscillatory phenomena, leading to the whole new field of the so-called “prominence seismology”. There are two types of oscillatory phenomena observed in prominences; “small-amplitude oscillations” (2–3 km?s^?1), which are quite common, and “large-amplitude oscillations” (>20 km?s^?1) for which observations are scarce. Large-amplitude oscillations have been found as “winking filament” in Hα as well as motion in the plane-of-sky in Hα, EUV, micro-wave and He 10830 observations. Historically, it has been suggested that the large-amplitude oscillations in prominences were triggered by disturbances such as fast-mode MHD waves (Moreton wave) produced by remote flares. Recent observations show, in addition, that near-by flares or jets can also create such large-amplitude oscillations in prominences. Large-amplitude oscillations, which are observed both in transverse as well as longitudinal direction, have a range of periods varying from tens of minutes to a few hours. Using the observed period of oscillation and simple theoretical models, the obtained magnetic field in prominences has shown quite a good agreement with directly measured one and, therefore, justifies prominence seismology as a powerful diagnostic tool. On rare occasions, when the large-amplitude oscillations have been observed before or during the eruption, the oscillations may be applied to diagnose the stability and the eruption mechanism. Here we review the recent developments and understanding in the observational properties of large-amplitude oscillations and their trigger mechanisms and stability in the context of prominence seismology.     

Damping Mechanisms for Oscillations in Solar Prominences

Small amplitude oscillations are a commonly observed feature in prominences/filaments. These oscillations appear to be of local nature, are associated to the fine structure of prominence plasmas, and simultaneous flows and counterflows are also present. The existing observational evidence reveals that small amplitude oscillations, after excited, are damped in short spatial and temporal scales by some as yet not well determined physical mechanism(s). Commonly, these oscillations have been interpreted in terms of linear magnetohydrodynamic (MHD) waves, and this paper reviews the theoretical damping mechanisms that have been recently put forward in order to explain the observed attenuation scales. These mechanisms include thermal effects, through non-adiabatic processes, mass flows, resonant damping in non-uniform media, and partial ionization effects. The relevance of each mechanism is assessed by comparing the spatial and time scales produced by each of them with those obtained from observations. Also, the application of the latest theoretical results to perform prominence seismology is discussed, aiming to determine physical parameters in prominence plasmas that are difficult to measure by direct means.     

Filament Thread-like Structures and Their?Small-amplitude Oscillations

Thanks to gradually improving observational capabilities, both from space and ground-based observatories, it is now generally accepted that thin threads (width ??200 km) constitute the building blocks of solar filaments and prominences. At ultra-small scales, high quality image sequences show a non-static picture of filaments and reveal that their oscillatory behavior is an important dynamic feature of these structures. Filament seismology sheds light on the internal magnetic structures of filaments and their interactions with surrounding solar regions. Understanding the overall magnetic topology of solar filaments and prominences including their small-scale thread-like structures is essential in interpretation and understanding of their oscillations. For this reason we aim here to present an update of the dynamic and spatial structures of prominences and filaments as inferred from high resolution observations in the past decennia. Some constraints in high resolution observations are addressed. Our review focuses mainly on the observational aspects and aims to summarize recent oscillation studies of individual filament threads and groups of threads. Finally, some theoretical interpretations of oscillations of filament threads and the inferred physical conditions of filament plasma are also discussed.     

Plasma-physical aspects of the solar cycle

Mass motions below the photosphere drive the solar cycle which is associated with variations in the magnetic field structure and accompanying phenomena. In addition to semi-empirical models, dynamo theories have been used to explain the solar cycle. The emergence of magnetic field generated by these mechanisms and its expansion into the corona involves many plasma physical processes. Magnetic buoyancy aids the expulsion of magnetic flux. The corona may respond dynamically or by continually adjusting to a quasi-static force-free or pressure-balanced equilibrium. The formation and disruption of current sheets is significant for the overall structure of the coronal magnetic field and the physics of quiescent prominences. The corona has a fine structure consisting of magnetic loops. The structure and stability of these are important as they are one of the underlying elements which make up the corona.     

Physics of Solar Prominences: II—Magnetic Structure and Dynamics

Observations and models of solar prominences are reviewed. We focus on non-eruptive prominences, and describe recent progress in four areas of prominence research: (1) magnetic structure deduced from observations and models, (2) the dynamics of prominence plasmas (formation and flows), (3) Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and large-scale patterns of the filament channels in which prominences are located. Finally, several outstanding issues in prominence research are discussed, along with observations and models required to resolve them.     

Observations of interplanetary shocks: Recent progress

Interplanetary shock observations since the prior Solar Terrestrial Physics Symposium in 1978 are reviewed. Since the interval coincides with the recent solar maximum, emphasis is placed on shocks associated with transient solar phenomena, including coronal transients and eruptive prominences as well as flares. A good correlation between shocks and Storm Sudden Commencements has persisted into the recent maximum. Shocks have been identified that are associated with disappearing filaments and coronal transients rather than with flares. Significant progress has been made in the indirect observation of shocks near the Sun as a result of radio wave measurements in interplanetary space and measurement of the scintillation and spectral broadening of spacecraft radio transmissions. Preliminary results regarding the thickness of interplanetary shocks have appeared. Several quasi-parallel shocks propagating more nearly along, rather than across, the magnetic field have been identified. The plasma drivers accompanying interplanetary shocks have received increased attention and distinctive features have been found in electron, ion and magnetic field data.     

Longitudinal Waves in Coronal Loops

Outwardly propagating intensity disturbances are a common feature in large, quiescent coronal loop structures. In this paper, an overview is given of the observed properties and the theoretical modelling. As a large number of events have been observed and analysed, good statistical results on the estimated parameters have now been obtained. The theoretical modelling mainly focuses on two distinct aspects, namely the observed rapid damping of the perturbations, thought to be due to thermal conduction and the origin of the driver. Leakage of the solar surface p-modes is the main candidate to explain the observed periodicity, due to the strong correlation between loop position and period and the filamentary nature of the observed coronal intensity perturbations. Recent observational results appear to confirm the leakage and subsequent upward propagation of the solar surface 5 minute oscillations into the overlying atmospheric layers.     

基于分层思路的动态非线性气动力建模方法

很多非线性气动力模型难以精确预测系统的小扰动线性特征。针对这一局限,提出了一种非线性分层模型,用于辨识跨声速非线性非定常气动力。分层建模需要同时提供微幅振荡和大幅振荡两套训练样本,首先通过线性模型(如带外输入的自回归(ARX)模型)对微幅振荡样本进行建模,而后采用非线性模型(如径向基函数神经网络(RBFNN))对大幅振荡的样本与线性模型的差量进行建模,进而把线性模型和非线性模型的输出相叠加,得到分层非线性动力学模型。算例表明建立的分层气动力模型与单一自回归径向基函数(AR-RBF)神经网络模型相比不仅具有更高的数值精度,可以精确预测大幅运动中的非线性特征,而且在小扰动状态下自动退化为线性模型,能够精确刻画结构微幅振荡下的线性动力学特性。     

Theory and Modeling of Planetary Dynamos

Numerical dynamo models are increasingly successful in modeling many features of the geomagnetic field. Moreover, they have proven to be a useful tool for understanding how the observations connect to the dynamo mechanism. More recently, dynamo simulations have also ventured to explain the surprising diversity of planetary fields found in our solar system. Here, we describe the underlying model equations, concentrating on the Boussinesq approximations, briefly discuss the numerical methods, and give an overview of existing model variations. We explain how the solutions depend on the model parameters and introduce the primary dynamo regimes. Of particular interest is the dependence on the Ekman number which is many orders of magnitude too large in the models for numerical reasons. We show that a minor change in the solution seems to happen at $\mbox {E}=3\mbox {$\times 10^{-6}$}$ whose significance, however, needs to be explored in the future. We also review three topics that have been a focus of recent research: field reversal mechanisms, torsional oscillations, and the influence of Earth’s thermal mantle structure on the dynamo. Finally we discuss the possibility of tidally or precession driven planetary dynamos.     

Physics of Solar Prominences: I—Spectral Diagnostics and Non-LTE Modelling

This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (i.e. when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.     

Solar Cycle Forecasting

Predicting the behavior of a solar cycle after it is well underway (2–3 years after minimum) can be done with a fair degree of skill using auto-regression and curve fitting techniques that don’t require any knowledge of the physics involved. Predicting the amplitude of a solar cycle near, or before, the time of solar cycle minimum can be done using precursors such as geomagnetic activity and polar fields that do have some connection to the physics but the connections are uncertain and the precursors provide less reliable forecasts. Predictions for the amplitude of cycle 24 using these precursor techniques give drastically different values. Recently, dynamo models have been used directly with assimilated data to predict the amplitude of sunspot cycle 24 but have also given significantly different predictions. While others have questioned both the predictability of the solar cycle and the ability of current dynamo models to provide predictions, it is clear that cycle 24 will help to discriminate between some opposing dynamo models.     

Validation of numerical prediction of dynamic derivatives: The DLR-F12 and the Transcruiser test cases

The dynamic derivatives are widely used in linear aerodynamic models in order to determine the flying qualities of an aircraft: the ability to predict them reliably, quickly and sufficiently early in the design process is vital in order to avoid late and costly component redesigns. This paper describes experimental and computational research dealing with the determination of dynamic derivatives carried out within the FP6 European project SimSAC. Numerical and experimental results are compared for two aircraft configurations: a generic civil transport aircraft, wing-fuselage-tail configuration called the DLR-F12 and a generic Transonic CRuiser, which is a canard configuration. Static and dynamic wind tunnel tests have been carried out for both configurations and are briefly described within this paper. The data generated for both the DLR-F12 and TCR configurations include force and pressure coefficients obtained during small amplitude pitch, roll and yaw oscillations while the data for the TCR configuration also include large amplitude oscillations, in order to investigate the dynamic effects on nonlinear aerodynamic characteristics. In addition, dynamic derivatives have been determined for both configurations with a large panel of tools, from linear aerodynamic (Vortex Lattice Methods) to CFD. This work confirms that an increase in fidelity level enables the dynamic derivatives to be calculated more accurately. Linear aerodynamics tools are shown to give satisfactory results but are very sensitive to the geometry/mesh input data. Although all the quasi-steady CFD approaches give comparable results (robustness) for steady dynamic derivatives, they do not allow the prediction of unsteady components for the dynamic derivatives (angular derivatives with respect to time): this can be done with either a fully unsteady approach i.e. with a time-marching scheme or with frequency domain solvers, both of which provide comparable results for the DLR-F12 test case. As far as the canard configuration is concerned, strong limitations for the linear aerodynamic tools are observed. A key aspect of this work are the acceleration techniques developed for CFD methods, which allow the computational time to be dramatically reduced while providing comparable results.     

Eruptive prominences as sources of magnetic clouds in the solar wind

Large amounts of coronal material are propelled outward into interplanetary space by Coronal Mass Ejections (CMEs). Thus one might expect to find evidence for expanding flux ropes in the solar wind as well. To prove this assumption magnetic clouds were analyzed and correlated with H-observations of disappearing filaments. When clouds were found to be directly associated with a disappearing filament, the magnetic structure of the cloud was compared with that of the associated filament. Additionally the expansion of magnetic clouds was examined over a wide range of the heliosphere and compared with the expansion observed for erupting prominences.     

吸气式高超声速飞行器俯仰/滚转耦合运动特性

针对一种类似SR-72构型的吸气式高超声速飞机开展了进气道通流状态下俯仰/滚转耦合运动相关研究。通过数值模拟获得了滚转单自由度静稳定性、动稳定性以及强迫俯仰/自由滚转运动下的两自由度耦合动稳定性，研究了飞行器转动惯量以及俯仰运动频率对耦合运动的影响，简要分析了耦合运动的机理。研究发现虽然此飞行器具有滚转静稳定性和动稳定性，但是在强迫俯仰/自由滚转运动过程中，滚转通道却出现了小幅度振荡与大振幅振荡交替出现的情况，最大滚转角超过70°。小幅度振荡出现在正弦俯仰振荡的上半周期，其振荡频率随轴向转动惯量增加而降低，幅值随俯仰振荡频率增加而增大；大振幅振荡出现在下半周期，其幅值基本不变，而振荡频率与俯仰振荡一致。这种现象基本不受惯性耦合作用影响，可以认为是由气动力主导的。     

Effects of Magnetic Fields in the Solar Atmosphere on Global Oscillations

Helioseismology is practically the only efficient experimental way of probing the solar interior. Without it, the results of theoretical solar models would remain untested and, consequently, less reliable when applying them for investigating remote stars. Hence, having a firm understanding of the applicability and reliability of helioseismology and the awareness of its limits are essential in solar physics and also in astrophysics. One of the weaknesses of the currently popular helioseismic models is that they allow only limited interaction between the global acoustic oscillation modes and the magnetic lower solar atmosphere, although, observations confirm strong coupling of helioseismic oscillations to the atmospheric magnetic field. The present article overviews the attempts of taking into account atmospheric magnetic effects in the theoretical models of global solar oscillations.     

Magnetic Flux Ropes in the Martian Atmosphere: Global Characteristics

We report observations of magnetic fields amplitude, which consist of a series of individual spikes in the Martian atmosphere. A minimum variance analysis shows that these spikes form twisted cylindrical filaments. These small diameter magnetic filaments are commonly called magnetic flux ropes. We examine the global characteristics of magnetic flux ropes, which are observed on 5% of the elliptical orbits of Mars Global Surveyor. Flux ropes are more often observed in Venus' atmosphere (70% of the orbits). In this paper we report some of the global characteristics of the flux ropes identified in the Martian atmosphere. No flux ropes are observed in the southern hemisphere of Mars. Most of them occur at high solar zenith angles, close to the terminator plane, and at high latitude with altitudes below 400 km. The orientation of the flux ropes appears random while in the case of Venus the orientation is more horizontal near the terminator for altitudes greater than 200 km. We have identified fewer flux ropes for SZA between 40 to 60 deg and for SZA lower than 20 deg, like in the case of Venus (Elphic and Russell, 1983b). Statistically, Mars' ionosphere with SZA range between 40^circ to 60^circ is less magnetized than near the subsolar point. As the Martian ionosphere is quite often magnetized by the magnetic components of the crustal field, this crustal magnetic field seems to inhibit the flux ropes formation in the southern hemisphere. However, some orbits without crustal magnetic field, called magnetic cavities, were observed without flux ropes. So the flux ropes formation process seems to be uppressed by another factor, like the solar wind dynamic pressure for Venus (Krymskii and Breus, 1988).     

Magnetic Fields in the Inner Heliosphere

The structure of Heliospheric Magnetic Field (HMF) is a function of both the coronal conditions from which it originates and dynamic processes which take place in the solar wind. The division between the inner and outer regions of the heliosphere is the result of dynamic processes which form large scale structures with increasing heliocentric distance. The structure of the HMF is normally described in the reference frame based on Parker's geometric model, but is better understood as an extension of potential field models of the corona. The Heliospheric Current Sheet (HCS) separates the two dominant polarities in the heliosphere; its large scale geometry near solar minimum is well understood but its topology near solar maximum remains to be investigated by Ulysses. At solar minimum, Corotating Interaction Regions (CIRs) dominate the near-equatorial heliosphere and extend their influence to mid-latitudes; the polar regions of the heliosphere are dominated by uniform fast solar wind streams and large amplitude, long wavelength, mostly transverse magnetic fluctuations. Coronal Mass Ejections (CMEs) introduce transient variability into the large scale heliospheric structure and may dominate the inner heliosphere near solar maximum at all latitudes.     

Sensitivity of Low Frequency Oscillations to Updated Solar Models

A large number of acoustic frequencies have already been detected, leading to a "seismic" model of the Sun rather close to the actual standard solar models. The core however is not yet well constrained by these observations and frequencies of low degree, low frequency modes which penetrate deeply into the solar core are needed. We present here a study on the sensitivity of low degree low frequency (50 - 900 μHz) modes to the structure of the solar interior, in order to help their detection and identification in the low frequency spectrum observed by SoHO experiments like VIRGO and GOLF. The frequencies of p and g modes have been computed for a set of solar models with updated physics (Morel et al., 1997). We analyze their sensitivity to solar parameters like age and metallicity, and to various physical processes, like convective core overshoot and mass loss during the beginning of solar evolution. This revised version was published online in June 2006 with corrections to the Cover Date.