853 research outputs found
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
Topological Quantification of the "Anemone" (Branching) Solar Flares
The so-called "anemone" solar flares are an interesting type of the space
plasma phenomena, where multiple null points of the magnetic field are
connected with each other and with the magnetic sources by the separators,
thereby producing the complex branching configurations. Here, using the methods
of dynamical systems and Morse-Smale theory, we derive a few universal
topological relations between the numbers of the null points and sources of
various kinds with arbitrary arrangement in the above-mentioned structures.
Such relations can be a valuable tool both for a quantification of the
already-observed anemone flares and for a prediction of the new ones in complex
magnetic configurations.Comment: LaTeX2e, elsarticle documentclass, 19 pages, 5 EPS figures; v2:
Theorem 3 substantially modified, minor changes in other parts of the text;
v3: Ref. 19 replaced, formulation of Theorem 1 extended, minor misprints
correcte
Magnetic field and unstable accretion during AM Herculis low states
A study of AM Her low states in September 1990 and 1991 and June-July 1997 is
reported from a coordinated campaign with observations obtained at the
Haute-Provence observatory, at the 6-m telescope of the Special Astrophysical
Observatory and at the 2.6m and 1.25m telescopes of the Crimean observatory.
Spectra obtained at different dates when the source was in low states at a
comparable V magnitude, show the presence of strong Zeeman absorption features
and marked changes in emission lines with a day-to-day reappearance of the HeII
(4686\AA) emission lines in 1991. Despite this variability, the magnetic field
inferred from the fitting of the absorption spectrum with Zeeman hydrogen
splitting, is remarkably constant with a best value of (12.50.5)MG.
Detailed analysis of the UBVRI light curves shows the presence of repetitive
moderate amplitude ( 0.3-0.5 mag) flares predominantly red in colour.
These flares are attributed to small accretion events and are compared to the
large ( 2 mag.) blue flare reported by Shakhovskoy et al. (1993). We
suggest that the general flaring activity observed during the low states is
generated by accretion events. The different characteristics of the flares
(colour and polarization) are the results of different shock geometries
depending on the net mass accretion flux.Comment: accepted in Astronomy & Astrophysics (Main Journal), 10 pages, 6
Figures, Late
On the trigger mechanisms for SGR giant flares
We examine two trigger mechanisms, one internal and the other external to the
neutron star, that give rise to the intense soft gamma-ray repeater (SGR) giant
flares. So far, three giant flares have been observed from the three out of the
seven confirmed SGRs on March 5, 1979, August 27, 1998, and December 27, 2004.
The last two events were found to be much more powerful than the first, and
both showcased the existence of a precursor, that we show to have had initiated
the main flare. In the internal mechanism, we propose that the strongly wound
up poloidal magnetic field develops tangential discontinuities and dissipates
its torsional energy in heating the crust. The timescale for the instability to
develop coincides with the duration of the quiescent state that followed the
precursor. Alternatively, we develop a reconnection model based on the
hypothesis that shearing motion of the footpoints causes the materialization of
a Sweet-Parker current layer in the magnetosphere. The thinning of this
macroscopic layer due to the development of an embedded super-hot turbulent
current layer switches on the impulsive Hall reconnection, which powers the
giant flare. Again, we show that the thinning time is on the order of the
preflare quiescent time. This model naturally explains the origin of the
observed nonthermal radiation during the flares, as well as the post flare
radio afterglows.Comment: 8 pages, 1 figure, version accepted by MNRAS. Changes: New
references, added a section to introduction, added a paragraph to discussion.
The results of this study remain unchanged
A broken solar type II radio burst induced by a coronal shock propagating across the streamer boundary
We discuss an intriguing type II radio burst that occurred on 2011 March 27.
The dynamic spectrum was featured by a sudden break at about 43 MHz on the
well-observed harmonic branch. Before the break, the spectrum drifted gradually
with a mean rate of about -0.05 MHz/s. Following the break, the spectrum jumped
to lower frequencies. The post-break emission lasted for about three minutes.
It consisted of an overall slow drift which appeared to have a few fast drift
sub-bands. Simultaneous observations from the Solar TErrestrial RElations
Observatory (STEREO) and the Solar Dynamics Observatory (SDO) were also
available and are examined for this event. We suggest that the slow-drift
period before the break was generated inside a streamer by a coronal eruption
driven shock, and the spectral break as well as the relatively wide spectrum
after the break is a consequence of the shock crossing the streamer boundary
where density drops abruptly. It is suggested that this type of radio bursts
can be taken as a unique diagnostic tool for inferring the coronal density
structure, as well as the radio emitting source region.Comment: 12 pages, 5 figures, accepted by ApJ 201
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