Observation of "Topological" Microflares in the Solar Atmosphere

We report on observation of the unusual kind of solar microflares, presumably associated with the so-called "topological trigger" of magnetic reconnection, which was theoretically suggested long time ago by Gorbachev et al. (Sov. Ast. 1988, v.32, p.308) but has not been clearly identified so far by observations. As can be seen in pictures by Hinode SOT in CaII line, there may be a bright loop connecting two sunspots, which looks at the first sight just as a magnetic field line connecting the opposite poles. However, a closer inspection of SDO HMI magnetograms shows that the respective arc is anchored in the regions of the same polarity near the sunspot boundaries. Yet another peculiar feature is that the arc flashes almost instantly as a thin strip and then begins to expand and decay, while the typical chromospheric flares in CaII line are much wider and propagate progressively in space. A qualitative explanation of the unusual flare can be given by the above-mentioned model of topological trigger. Namely, there are such configurations of the magnetic sources on the surface of photosphere that their tiny displacements result in the formation and fast motion of a 3D null point along the arc located well above the plane of the sources. So, such a null point can quickly ignite a magnetic reconnection along the entire its trajectory. Pictorially, this can be presented as flipping the so-called two-dome magnetic-field structure (which is just the reason why such mechanism was called topological). The most important prerequisite for the development of topological instability in the two-dome structure is a cruciform arrangement of the magnetic sources in its base, and this condition is really satisfied in the case under consideration.Comment: LaTeX, rnaastex documentclass, 3 pages, 1 PDF figure, accepted for publication in the "Research Notes of the American Astronomical Society

Topological model of the anemone microflares in the solar chromosphere

Context: The chromospheric anemone microflares, which were discovered by Hinode satellite about a decade ago, are specific transient phenomena starting from a few luminous ribbons on the chromospheric surface and followed by an eruption upward. While the eruptive stage was studied in sufficient detail, a quantitative theory of formation of the initial multi-ribbon structure remains undeveloped until now. Aims: We construct a sufficiently simple but general model of the magnetic field sources that is able to reproduce all the observed types of luminous ribbons by varying only a single parameter. Methods: As a working tool, we employed the Gorbachev-Kel'ner-Somov-Shvarts (GKSS) model of the magnetic field, which was originally suggested about three decades ago to explain fast ignition of the magnetic reconnection over considerable spatial scales by tiny displacements of the magnetic sources. Quite unexpectedly, this model turns out to be efficient for the description of generic multi-ribbon structures in the anemone flares as well. Results: As follows from our numerical simulation, displacement of a single magnetic source (sunspot) with respect to three other sources results in a complex transformation from three to four ribbons and then again to three ribbons, but with an absolutely different arrangement. Such structures closely resemble the observed patterns of emission in the anemone microflares.Comment: LaTeX, aa documentclass, 4 pages, 4 EPS figures, submitted to Astronomy & Astrophysics; v2: 6 pages, 5 EPS figures, text substantially extended and modified, 1 figure replaced and 1 figure added, Appendix added; v3: minor textual correction

Indeterminacy and instability in Petschek reconnection

We explain two puzzling aspects of Petschek's model for fast reconnection. One is its failure to occur in plasma simulations with uniform resistivity. The other is its inability to provide anything more than an upper limit for the reconnection rate. We have found that previously published analytical solutions based on Petschek's model are structurally unstable if the electrical resistivity is uniform. The structural instability is associated with the presence of an essential singularity at the X-line that is unphysical. By requiring that such a singularity does not exist, we obtain a formula that predicts a specific rate of reconnection. For uniform resistivity, reconnection can only occur at the slow, Sweet-Parker rate. For nonuniform resistivity, reconnection can occur at a much faster rate provided that the resistivity profile is not too flat near the X-line. If this condition is satisfied, then the scale length of the nonuniformity determines the reconnection rate

The unipolar solar flares as a manifestation of the 'topological' magnetic reconnection

Solar flares - which are the most prominent manifestation of the solar activity - typically manifest themselves as a single or a set of luminous arcs (magnetic flux tubes) rooted in regions of opposite polarity in the photosphere. However, a careful analysis of the archival data by Hinode satellite sometimes reveals surprising cases of flaring arcs whose footpoints belong to regions of the same polarity or to areas without any appreciable magnetic field. Despite the counterintuitive nature of this phenomenon, it can be reasonably interpreted in the framework of the so-called 'topological model' of magnetic reconnection, where a magnetic null point is formed due to specific superposition of influences from remote sources rather than by local current systems. As a result, the energy release propagates along a separator of the flipping two-dome structure rather than along a fixed magnetic field line. Therefore, the luminous arc needs not to be associated anymore immediately with the magnetic sources. Here, we report both observational cases of the above-mentioned type as well as provide their theoretical model and numerical simulations.Comment: PDFLaTeX, mnras documentclass, 5 pages, 4 PDF figures, accepted for publication in MNRAS Letters. Animated figure_3 attached as mp4 video file. To view attachments, please download and extract the gzipped tar source file listed under "Other formats"; v2: moderate extension of text, minor corrections in figs. 3 and 4, four bibliographic references added; v3: minor textual correction