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.     

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.     

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.     

The generation of MHD shock waves during the impulsive phase of the February 27, 1992 flare

In this paper a unique 2.3–4.2 GHz radio spectrum of the flare impulsive phase, showing fast positively drifting bursts superimposed on a slowly negatively drifting burst, is presented. Analyzing this radio spectrum it was found that the flare started somewhere near the transition region, where upward propagating MHD waves were generated during the whole impulsive phase. Moreover, it was found that behind a front of these ascending MHD waves the downward propagating electron beams, which bombarded dense layers of the solar atmosphere, were accelerated. It seems that, simultaneously with the increase of beam bombardment intensity, the intensity of MHD waves was increasing and thus the MHD shock wave generation and the electron beam acceleration and bombardment formed a self-consistently amplifying flare process. At higher coronal heights this process was followed by a type II radio burst, i.e. by the MHD flare shock. To verify this concept, the numerical modeling of the shock-wave generation and propagation in space from a flare site near the transition region up to 3 solar radii was made. Comparing the thermal and magnetic field disturbances, it was found that those of magnetic origin are more relevant in this case. Combining the results of interpretation and numerical simulation, a model of the February 27, 1992 flare is suggested and new aspects of this model are discussed.     

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).     

Repeated activity of the 0.8–1.2 GHz radio source of March 20, 1993

During March 20, 1993, from 12:00 to 16:00 UT, repeated radio burst activity was observed in the 0.8–1.2 GHz frequency range. Periods in intervals 0.1–0.5, 0.7–1.0, 2.8–3.9, 75–170 s, and 15–25 min were recognized. This long-lasting narrowband activity consisted mainly of pulsations and continua. In some intervals it was accompanied not only by spikes, broadband pulsations, and fibers in the 1–2 GHz frequency range, but also by type III and U burst activity at lower frequencies as well as by hard X-ray bursts. From several radio bursts, two characterized by different fine structures were selected and compared. The observed differences are explained by different distribution functions of superthermal electrons. The position of the 0.8–1.2 GHz radio source above the photosphere and the magnetic field in the fiber burst source were estimated to be 66 000–75 000 km and 120–135 G, respectively.Presented at te CESRA-Workshop on Coronal Magnetic Energy Release at Caputh near Potsdam in May 1994.     

LNAWENR—linear nonadiabatic nonradial waves

This is a presentation of the pulsational model LNAWENR (linear nonadiabatic nonradial), included in the asteroseismological package ROMOSC which was implemented in the Bucharest Observatory to be used in the CoRoT mission.     

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.     

New Solar Broad-Band Hard X-Ray Spectrometer: First Results

The scientific and operational aims of the Czech-made Hard X-Ray Spectrometer (HXRS) launched onboard the U.S. Department of Energy Multispectral Thermal Imager satellite (MTI), on 12 March 2000 are discussed. The principal operating characteristics of the instrument such as the temporal resolution, energy band selection, spectral sensitivity, and the in-flight calibration procedure are described as well as the technical details of the spectrometer including detectors, shielding (against charged particles) and electronic design. The MTI host satellite and its orbit are briefly described. Recent observations by the 3 GHz Ondřejov radiometer are compared with HXRS data to demonstrate one example of the HXRS data utilization: the temporal relation between hard X-rays and radio emission. These results show relatively long time delays (2–14 s) of the GHz broadband radio pulses relative to the hard X-ray emission peaks. Access to the HXRS data base via the Internet is provided.