Tangled magnetic field model of QPOs

The highly tangled magnetic field of a magnetar supports shear waves similar to Alfvén waves in an ordered magnetic field. Here we explore if torsional modes excited in the stellar interior and magnetosphere can explain the quasi-periodic oscillations (QPOs) observed in the tail of the giant flare of SGR 1900+14. We solve the initial value problem for a tangled magnetic field that couples interior shear waves to relativistic Alfvén shear waves in the magnetosphere. Assuming stellar oscillations arise from the sudden release of magnetic energy, we obtain constraints on the energetics and geometry of the process. If the flare energy is deposited initially inside the star, the wave energy propagates relatively slowly to the magnetosphere which is at odds with the observed rise time of the radiative event of ≲ 10 ms. Nor can the flare energy be deposited entirely outside the star, as most of the energy reflects off the stellar surface, giving surface oscillations of insufficient magnitude to produce detectable modulations of magnetospheric currents. Energy deposition in a volume that straddles the stellar surface gives agreement with the observed rise time and excites a range of modes with substantial amplitude at observed QPO frequencies. In general, localized energy deposition excites a broad range of modes that encompasses the observed QPOs, though many more modes are excited than the number of observed QPOs. If the flare energy is deposited axisymmetrically, as is possible for a certain class of MHD instabilities, the number of modes that is excited is considerably reduced.

CHOICE OF CONDITIONS FOR MHD SIMULATIONS ABOVE THE ACTIVE REGION, ALLOWING THE STUDY OF THE SOLAR FLARE MECHANISM

Primordial release of solar flare energy high in corona (at altitudes 1/40 - 1/20 of the solar radius) is explained by release of the magnetic energy of the current sheet. The observed manifestations of the flare are explained by the electrodynamical model of a solar flare proposed by I. M. Podgorny. To study the flare mechanism is necessary to perform MHD simulations above a real active region (AR). MHD simulation in the solar corona in the real scale of time can only be carried out thanks to parallel calculations using CUDA technology. Methods have been developed for stabilizing numerical instabilities that arise near the boundary of the computational domain. Methods are applicable for low viscosities in the main part of the domain, for which the flare energy is effectively accumulated near the singularities of the magnetic field. Singular lines of the magnetic field, near which the field can have a rather complex configuration, coincide or are located near the observed positions of the flare.

Superflares on the late-type giant KIC 2852961

Context. The most powerful superflares reaching 1039 erg bolometric energy are from giant stars. The mechanism behind flaring is thought to be the magnetic reconnection, which is closely related to magnetic activity (including starspots). However, it is poorly understood how the underlying magnetic dynamo works and how the flare activity is related to the stellar properties that eventually control the dynamo action. Aims. We analyze the flaring activity of KIC 2852961, a late-type giant star, in order to understand how its flare statistics are related to those of other stars with flares and superflares, and to understand the role of the observed stellar properties in generating flares. Methods. We searched for flares in the full Kepler dataset of KIC 2852961 using an automated technique together with visual inspection. We cross-matched the flare-like events detected by the two different approaches and set a final list of 59 verified flares during the observing term. We calculated flare energies for the sample and performed a statistical analysis. Results. The stellar properties of KIC 2852961 are revised and a more consistent set of parameters are proposed. The cumulative flare energy distribution can be characterized by a broken power law; that is to say, on the log-log representation the distribution function is fitted by two linear functions with different slopes, depending on the energy range fitted. We find that the total flare energy integrated over a few rotation periods correlates with the average amplitude of the rotational modulation due to starspots. Conclusions. Flares and superflares seem to be the result of the same physical mechanism at different energy levels, also implying that late-type stars in the main sequence and flaring giant stars have the same underlying physical process for emitting flares. There might be a scaling effect behind the generation of flares and superflares in the sense that the higher the magnetic activity, the higher the overall magnetic energy released by flares and/or superflares.

THE PROCESSES OF ENERGY RELEASE IN LOW-POWER SOLAR FLARES

Using flare patrol data for 1972–2010 [http://www.ngdc.noaa.gov/stp/space-weather/solar-data/solar-features/solar-flares/], we have conducted statistical studies of small solar flares. We have established a correlation between the flare brightness rise time and the total duration of small flares, and obtained evidence of the discreteness of relative rise times (Trel). The most significant Trel values are 0.2, 0.25, 0.33, and 0.5. As the area class and importance of flares increase, maxima of Trel distributions decrease, flatten, and completely disappear in case of large flares. We have found the discreteness of the area distribution of small flares. We have obtained distributions of solar flare energy, which exhibit significant overlap for flare energy of different area classes. The energy range of large solar flares contains 9.5 % of small flares. The energy range of flares of area class 1 has even a more significant overlap.

THE PROCESSES OF ENERGY RELEASE IN LOW-POWER SOLAR FLARES

Using flare patrol data for 1972–2010 [http://www.ngdc.noaa.gov/stp/space-weather/solar-data/solar-features/solar-flares/], we have conducted statistical studies of small solar flares. We have established a correlation between the flare brightness rise time and the total duration of small flares, and obtained evidence of the discreteness of relative rise times (Trel). The most significant Trel values are 0.2, 0.25, 0.33, and 0.5. As the area class and importance of flares increase, maxima of Trel distributions decrease, flatten, and completely disappear in case of large flares. We have found the discreteness of the area distribution of small flares. We have obtained distributions of solar flare energy, which exhibit significant overlap for flare energy of different area classes. The energy range of large solar flares contains 9.5 % of small flares. The energy range of flares of area class 1 has even a more significant overlap.