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A Kosterlitz-Thouless phase transition in the ground state of an antiferromagnetic spin-$\frac{1}{2}$ Heisenberg chain with nearest and next-nearest-neighbor interactions is re-investigated from a different perspective: An unequivocal correspondence is found between components of the scalar product $\langle \Psi_0| \Psi_0 \rangle$ and geometrical objects. One can classify these objects according to whether any two of them can be transformed into each other in a continuous way (belong to the same homotopy class). A finite size scaling of the "connection term`` $\langle \Psi_0|\partial \Psi_0 \rangle$ with respect to chain length (16, 18, 20, 22, 24 spins) for each homotopy class of above mentioned objects leads to the critical value of $\lambda$ with rather high accuracy., Comment: The paper contained some inaccuracies. Completely rewritten and extended is available as arXiv:2103.16203

Svestka (Solar Phys. 1989, 121, 399) on the basis of the Solar Maximum Mission observations introduced a new class of flares, the so-called flare hybrids. When they start, they look as typical compact flares (phase 1), but later on they look like flares with arcades of magnetic loops (phase 2). We summarize the features of flare hybrids in soft and hard X-rays as well as in extreme-ultraviolet; these allow us to distinguish them from other flares. Additional energy release or long plasma cooling timescales have been suggested as possible cause of phase 2. Estimations of frequency of flare hybrids have been given. Magnetic configurations supporting their origin have been presented. In our opinion, flare hybrids are quite frequent and a difference between lengths of two interacting systems of magnetic loops is a crucial parameter for recognizing their features., Comment: 15 pages, 4 figures, to appear in Solar Physics

We present a case study of the solar flare (SOL2001-10-02T17:31) that showed quasi-periodic pulsations (QPPs) in hard X-rays with two simultaneously excited periods, P_1 = 26-31 s and P_2 = 110 s. Complete evolution of the flare recorded by the Yohkoh telescopes, together with the patrol SOHO/EIT images, allowed us to identify magnetic structures responsible for particular periods and to propose an overall scenario which is consistent with the available observations. Namely, we suggest that emerging magnetic flux initiated the reconnection with legs of a large arcade of coronal loops that had been present in an active region for several days. The reconnection excited MHD oscillations in both magnetic structures simultaneously: period P_1 was generated in the emerging loop and in a loop being a result of the reconnection; period P_2 occurred in the arcade. Both resonators produced photons of different spectra. We anticipate that multiperiodicity in hard X-rays can be a common feature of flare hybrids, i.e. the events, in which magnetic structures of different sizes interact., Comment: 14 pages, 7 figures, to be printed in Central European Astrophysical Bulletin

We investigated the relationship between hot (~10 MK) and warm (~1 MK) plasma ejections observed in the solar corona during some flares from April 1998 to December 2001. Based on the catalog of X-ray Plasma Ejections (XPEs) observed by the Soft X-ray Telescope onboard the Yohkoh satellite, we have identified counterparts of XPEs in extreme-ultraviolet (EUV) images, derived by the TRACE satellite. We found that the XPEs are always associated with the EUV ejections. In most cases, ejections of warm plasma start earlier than ejections of hot plasma. The comprehensive analysis of kinematic and morphology evolution of the most interesting and well-observed events is also given

We present the results of the first attempt of statistical research devoted to the association between X-ray Plasma Ejections (XPEs) and prominences. For this aim, we compared contents of a catalogue of XPEs, observed by the Soft X-ray Telescope onboard Yohkoh, to H$\alpha$ reports from the Solar-Geophysical Data. We found that only less than one third of XPEs shows a low-temperature counterpart. A modest connection between hot and cool plasma motions in the corona is also supported by by a frequent discrepancy between morphology and kinematics of simultaneously occurring XPEs and prominences. We explain the poor correlation (20-30 %) between XPEs and prominences and high correlations (~70%) between these phenomena and CMEs as a proof of existence of two separate subclasses of CMEs., Comment: 11 pages, 1 figure, 5 tables, accepted by Central European Astrophysical Bulletin

Recently developed in Wroclaw, a new catalogue of X-ray Plasma Ejections (XPEs) observed by the Soft X-ray Telescope onboard Yohkoh, is a very useful tool for some statistical studies. We investigated the association of XPEs with solar flares and coronal mass ejections (CMEs). We found that particular subclasses of XPEs show different levels of association. Moreover, flares and CMEs associated with different subclasses of XPEs show distinctly different characteristics. We conclude that the event that we call 'X-ray Plasma Ejection' can be a manifestation of different physical processes., Comment: 10 pages, 3 figures, 2 tables, accepted for publication in Central European Astrophysical Bulletin

In our recent paper (Solar Physics 261, 233) we investigated quasi-periodic oscillations of hard X-rays during impulsive phase of solar flares. We have come to conclusion that they are caused by magnetosonic oscillations of magnetic traps within the volume of hard-X-ray (HXR) loop-top sources. In the present paper we investigate four flares which show clear quasi-periodic sequences of HXR pulses. We also describe our phenomenological model of oscillating magnetic traps to show that it can explain observed properties of HXR oscillations. Main results are the following: 1. We have found that low-amplitude quasi-periodic oscillations occur before impulsive phase of some flares. 2. We have found that quasi-period of the oscillations can change in some flares. We interpret this as being due to changes of the length of oscillating magnetic traps. 3. During impulsive phase a significant part of the energy of accelerated (non-thermal) electrons is deposited within the HXR loop-top source. 4. Our analysis suggests that quick development of impulsive phase is due to feedback between pulses of the pressure of accelerated electrons and the amplitude of magnetic-trap oscillation. 5. We have also determined electron number density and magnetic filed strength for HXR loop-top sources of several flares. The values fall within the limits of $N \approx (2 -15) \times 10^{10}$ cm$^{-3}$, $B \approx (45 - 130)$ gauss., Comment: 18 pages, 14 figures, submitted to Solar Physics

The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool for investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic traps within cusp-like magnetic structures. This is confirmed by a good correlation between periods of the oscillations and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations. During the compressions of a trap particles are accelerated, but during its expansions plasma, coming from chromospheric evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong impulses. The advantage of our model of oscillating magnetic traps is that it can explain both the impulses of electron acceleration and quasi-periodicity of their distribution in time., Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physics

Flares being partially occulted by the solar limb, are the best reservoir of our knowledge about hard X-ray loop-top sources. Recently, the survey of partially occulted flares observed by the RHESSI has been published (Krucker & Lin 2008). The extensive YOHKOH database still awaits such activities. This work is an attempt to fill this gap. Among from 1286 flares in the YOHKOH Hard X-ray Telescope Flare Catalogue, for which the hard X-ray images had been enclosed, we identified 98 events that occurred behind the solar limb. We investigated their hard X-ray spectra and spatial structure. We found that in most cases the hard X-ray spectrum of partially occulted flares consists of two components, non-thermal and thermal, which are co-spatial. The photon energy spectra of the partially occulted flares are systematically steeper than spectra of the non-occulted flares. Such a difference we explain as a consequence of intrinsically dissimilar conditions ruling in coronal parts of flares, in comparison with the footpoints which usually dominate the hard X-ray emission of disk flares. At least two reasons of the difference should be taken into consideration: (1) stronger contamination of hard X-rays with emission of thermal plasma, (2) different mechanism in which non-thermal electrons radiate their energy. A schematic picture, in which thin-target mechanism is responsible for hard X-ray emission of loop-top sources and thick-target mechanism -- for emission of footpoint sources, can be modified by the presence of some coronal thick-target sources. At least a part of them suggests a magnetic trapping. Investigated flares do not respond the global magnetic configuration of the solar corona. For their characteristics conclusive is rather the local magnetic configuration in which they were developed., Comment: 16 pages, 12 figures, 7 tables, submitted to A&A

The hard X-ray flux ratio R of the footpoint sources to the loop-top source has been used to investigate non-thermal electron trapping and precipitation in solar flares. Considering the mission-long Yohkoh Hard X-ray Telescope database, from which we selected 117 flares, we investigated a dependence of the ratio R on flare loop parameters like height h and column depth N. We used non-thermal electron beams as a diagnostic tool for magnetic convergence. The ratio R decreases with h which we interpret as an effect of converging field geometry. Two branches seen in the R-h diagram suggest that in the solar corona two kinds of magnetic loops can exist: a more-converged ones that are more frequent (above 80%) and less-converged loops that are less frequent (below 20%). A lack of correlation between the ratio R and N can be due to a more complex configuration of investigated events than seen in soft X-rays. Obtained values of the magnetic mirror ratio are consistent with previous works and suggest a strongly nonpotential configuration. Further investigation including RHESSI data is needed to verify our results., Comment: 12 pages, 6 figures, to be published in A&A