Reduction of steady-state forced vibrations of structures with dynamic absorbers

In the paper the problem of reduction of the steady-state response of a lightly damped structure to periodic excitation is considered. The primary structure, which has arbitrary distributions of mass, stiffness and damping, is subjected to periodic load with the fundamental frequency varying in a wide range that may include several resonant peaks of the amplitude-frequency response. A number of passive absorbers are applied simultaneously to the main structure. A general formulation of the dynamic absorber design methodology is presented, based on independent design of conventional absorbers, taking into account the selected modal systems of the original structure and the selected harmonic components of the excitation. In order to cover the losses in the vibration reduction, resulting from the couplings in the primary structure–the set of absorbers system and from the remaining harmonic components of the excitation, the mass of modal dynamic absorbers is increased properly. The methodology developed was verified on several numerical examples; one of them is presented in the study.     

An analysis of Skylab X-ray pictures of a giant coronal arch

The limb event of 13/14 August, 1973, imaged by Skylab in soft X-rays, proved to be a giant arch, quite similar to those observed in 1980–1986 on SMM. High spatial resolution (by a factor of 4–5 better than in SMM data) made it possible to see the internal structure of the arch. Its brightest part consisted of loops very similar to, but higher than, post-flare loops, surrounded by a rich system of weak loop structures extending up to altitudes of 260 000 km. While the main brightest structure of the arch was newly formed, the weak very large loops had existed above the active region before and were only enhanced during the event.Skylab data support the model proposed by Kopp and Poletto that the giant arch is formed by reconnections high in the corona, different from the reconnection process in the underlying flare. However, contrary to Kopp and Poletto's suggestion, the data strongly indicate that the field lines that reconnect in the arch did not open before, as in the Kopp and Pneuman model: more likely, we encounter here an interaction of large-scale loops high in the corona. (The interaction of two of them is clearly seen.) Thus, while post-flare loops are formed by the Kopp and Pneuman mechanism, giant arches above eruptive flares may originate through interactive reconnections of large-scale magnetic field lines which form loops high in the corona. These loops are brought close to each other in consequence of changes in the coronal structure caused by the eruptive flare phenomenon. The arch-associated enhancement of the pre-existing large-scale active-region loops may be caused by electrons accelerated during the reconnection process and diffusing across field lines, as suggested by Achterberg and Kuipers (1984).     

Optical Thickness of the 2–4.5 GHz Solar Plasma Emission

For radio emission at the frequency corresponding to the second harmonic of the local plasma frequency, the optical thickness in the solar atmosphere is calculated. Three types of models are assumed: the model with radio emission from the narrow transition region, and models with radio emission from a cool and dense plasma filament embedded in hotter plasma at the transition region and in the corona. The optical thickness is computed by integration of the collisional (free–free) absorption along a radio-ray path radial in the solar atmosphere. In all models considered the optical thickness can be sufficiently low for appropriate parameters. For example, in the narrow (<100 km) transition region where the density scale height is much less than that of the pressure one, the optical thickness can be lower than 1. Furthermore, the optical thickness can be decreased if the radio emission is generated in the cool and dense plasma filament surrounded by hotter and thinner plasma. But the models differ in density scale heights and thus in distances between plasma emission levels. This difference is essential for the interpretation of high-frequency type III radio bursts.     

The route to chaos during a pulsation event

A time series analysis of a pulsation event in solar radio emission provides an evolution from a regular doubly periodic phase to an irregular behaviour. Applying some techniques developed in the theory of nonlinear dynamic systems to this irregular stage suggests that there exists a low-dimensional attractor. Estimates of the maximum Lyapunov exponent give some evidence to deterministic chaos. The sudden transition from a regular to a chaotic structure is identified as a part of the Ruelle-Takens-Newhouse route to chaos which is typical in nonlinear systems. It is checked whether this pulsation event may be interpreted in terms of known pulsation models. Consequences for models, which are suitable to describe such an evolution, are discussed.     

Si‐bearing metal and niningerite in Almahata Sitta fine‐grained ureilites and insights into the diversity of metal on the ureilite parent body

A detailed mineralogical and chemical study of Almahata Sitta fine‐grained ureilites (MS‐20, MS‐165, MS‐168) was performed to shed light on the origin of these lithologies and their sulfide and metal. The Almahata Sitta fine‐grained ureilites (silicates <30 μm grain size) show textural and chemical evidence for severe impact smelting as described for other fine‐grained ureilites. Highly reduced areas in Almahata Sitta fine‐grained ureilites show large (up to ～1 mm) Si‐bearing metal grains (up to ～4.5 wt% Si) and niningerite [Mg_>0.5,(Mn,Fe)_<0.5S] with some similarities to the mineralogy of enstatite (E) chondrites. Overall, metal grains show a large compositional variability in Ni and Si concentrations. Niningerite grains probably formed as a by‐product of smelting via sulfidation. The large Si‐Ni variation in fine‐grained ureilite metal could be the result of variable degrees of reduction during impact smelting, inherited from coarse‐grained ureilite precursors, or a combination of both. Large Si‐bearing metal grains probably formed via coalescence of existing and newly formed metal during impact smelting. Bulk and in situ siderophile trace element abundances indicate three distinct populations of (1) metal crystallized from partial melts in MS‐20, (2) metal resembling bulk chondritic compositions in MS‐165, and (3) residual metal in MS‐168. Almahata Sitta fine‐grained ureilites developed their distinctive mineralogy due to severe reduction during smelting. Despite the presence of E chondrite and ureilite stones in the Almahata Sitta fall, a mixing relation of E chondrites or their constituents and ureilite material in Almahata Sitta can be ruled out based on isotopic, textural, and mineral‐chemical reasons.     

Chemical variations of sulfides and metal in enstatite chondrites—Introduction of a new classification scheme

In this study, the metal and sulfide compositions of 45 enstatite chondrites were analyzed to determine possible mineral-chemical trends correlated with the petrologic type. Data for 35 additional samples were taken from the literature. Considering the data from this huge number of different E chondrite samples (80 in total), none of the trends previously described in the literature could be clearly confirmed. Also, among the opaque phases of enstatite chondrites, no other “new” correlations between mineral chemistry and the petrologic type were found. However, major differences in the sulfide and metal chemistry became obvious. Specifically, a certain number of chondrites in the EH and the EL groups have Cr in troilite above 2 wt%, Fe in niningerite or alabandite above 20 wt%, and lack abundant daubréelite. Differences were also found for Ni concentrations in kamacite. Thus, we propose a system for classifying E chondrites by defining four major subgroups: EH_a, EL_a, EH_b, and EL_b. All subgroups show full petrologic sequences that are similar to each other. This observation, in combination with the differences in sulfide and metal chemistry, suggests an origin of the samples from different parent bodies. Considering the anomalous E chondrite samples that neither fit in the previous classification scheme nor in the new one described here, the samples investigated in this study require at least eight different parent bodies.     

Oscillations and Waves in Radio Source of Drifting Pulsation Structures

Drifting pulsation structures (DPSs) are considered to be radio signatures of the plasmoids formed during magnetic reconnection in the impulsive phase of solar flares. In the present paper we analyze oscillations and waves in seven examples of drifting pulsation structures, observed by the 800?–?2000 MHz Ond?ejov Radiospectrograph. For their analysis we use a new type of oscillation maps, which give us much more information as regards processes in DPSs than that in previous analyses. Based on these oscillation maps, made from radio spectra by the wavelet technique, we recognized quasi-periodic oscillations with periods ranging from about 1 to 108 s in all studied DPSs. This strongly supports the idea that DPSs are generated during a fragmented magnetic reconnection. Phases of most the oscillations in DPSs, especially for the period around 1 s, are synchronized (“infinite” frequency drift) in the whole frequency range of DPSs. For longer periods in some DPSs we found that the phases of the oscillations drift with the frequency drift in the interval from ?17 to \(+287~\mbox{MHz}\,\mbox{s}^{-1}\). We propose that these drifting phases can be caused (a) by the fast or slow magnetosonic waves generated during the magnetic reconnection and propagating through the plasmoid, (b) by a quasi-periodic structure in the plasma inflowing to the reconnection forming a plasmoid, and (c) by a quasi-periodically varying reconnection rate in the X-point of the reconnection close to the plasmoid.     

The acceleration of back-scattered beam electrons in a return-current electric field

Using a 1-D hybrid model of the electron beam bombardment in a flare loop, it is demonstrated that beam electrons, back-scattered in dense layers of the solar atmosphere, are accelerated in the return-current electric field. This effect is shown in two regimes of the electron beam bombardment: (a) with a monoenergetic quasi-steady beam, and (b) with a sequence of 4 pulse beams. It is suggested that the mirroring of electrons at loop magnetic mirrors can amplify this process. The role of such acceleration for the formation of a collisionless return-current, and thus for a decrease of return-current losses, is discussed.     

Particle tracking experiments in match-index-refraction porous media

A low-cost, noninvasive, three-dimensional (3D), particle tracking velocimetry system was designed and built to investigate particle movement in match-index-refraction porous media. Both a uniform load of the glass beads of the same diameter and a binary load of the glass beads of two diameters were used. The purpose of the experiments is to study the effect of the two loads on the trajectories, velocity distribution, and spreading of small physical particles. A total of 35 particles were released and tracked in the uniform load and 46 in the binary load. The 3D trajectory of each particle was recorded with two video camcorders and analyzed. It is found that the particle's velocity, trajectory, and spreading are very sensitive to its initial location and that the smaller pore size or heterogeneity in the binary load increases the particles' velocity and enhances their spreading as compared with the uniform load. The experiments also verified the previous finding that the distribution of the particle velocities are lognormal in the longitudinal direction and Gaussian in two transverse directions and that the particle spreading is much larger along the longitudinal direction than along the traverse directions.