Level Of Flare Activity Research Articles

Dynamics of Electric Current’s Parameters in Active Regions on the Sun and Their Relation to the Flare Index

Data from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) on the components of the magnetic field vector in the photosphere for 39 active regions (AR) of the 24th solar activity cycle are used to calculate the parameters of the electric current. The time variations in the electric current parameters in the AR over the time the region is within ±35° of the central meridian are studied. An attempt is made to relate the parameters of the current in the photosphere and their dynamics to the level of flare activity of the region. These studies have yielded the following results: (1) The change in the total unsigned current in an AR is synchronous or quasi-synchronous with changes in the magnetic flux; (2) A relation between the total unsigned current and the flare index (FI) exists, but is weak (correlation k = 0.48); (3) The imbalance of the vertical electric currents for all 39 of the AR studied here was low and did not exceed a few percent, which indicates closure of the current structures on all scales within an AR; (4) The highest correlation (k = 0.72) with the flare index is observed for a time averaged absolute value of the density of the vertical electric current; (5) Statistical studies yield a critical level of equal to 2.7 mA/m2: when this level is exceeded in an AR, flares with higher x-ray classes (M and X) are observed and when the current density falls below the threshold value, there is a reduction in the flare productivity of the region; (6) For the example of two regions with an emerging magnetic flux it is shown that pumping magnetic energy into the corona requires some time; the time interval between the jump in and the onset of the development of the first powerful flares in x-ray classes M and X is at least 12-20 hours; (7) The character of the relation between the time variations in the average value of the density of the horizontal current and the imbalance in the vertical current ρjz with the level of flare activity of an AR is more complicated and requires separate, more detailed studies.

High-resolution spectroscopy of flares and CMEs on AD Leonis

Context.Flares and coronal mass ejections (CMEs) are important for the evolution of the atmospheres of planets and their potential habitability, particularly for planets orbiting M stars at a distance <0.4 AU. Detections of CMEs on these stars have been sparse, and previous studies have therefore modelled their occurrence frequency by scaling up solar relations. However, because the topology and strength of the magnetic fields on M stars is different from that of the Sun, it is not obvious that this approach works well.Aims.We used a large number of high-resolution spectra to study flares, CMEs, and their dynamics of the active M dwarf star AD Leo. The results can then be used as reference for other M dwarfs.Methods.We obtained more than 2000 high-resolution spectra (R~ 35 000) of the highly active M dwarf AD Leo, which is viewed nearly pole on. Using these data, we studied the behaviour of the spectral lines Hα, Hβ, and HeI5876 in detail and investigated asymmetric features that might be Doppler signatures of CMEs.Results.We detected numerous flares. The largest flare emitted 8.32 × 1031erg in Hβand 2.12 × 1032erg in Hα. Although the spectral lines in this and other events showed a significant blue asymmetry, the velocities associated with it are far below the escape velocity.Conclusions.Although AD Leo shows a high level of flare activity, the number of CMEs is relatively low. It is thus not appropriate to use the same flare-to-CME relation for M dwarfs as for the Sun.

Distributed Electric Currents in Solar Active Regions

Using magnetographic data provided by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we analyzed the structure of magnetic fields and vertical electric currents in six active regions (ARs) with different level of flare activity. We found that electric currents are well balanced over the entire AR: for all of them the current imbalance is below 0.1%, which means that any current system is closed within an AR. Decomposition of the transverse magnetic field vector into two components allowed us to reveal the existence of large-scale vortex structures of the azimuthal magnetic field component around main sunspots of ARs. In each AR, we found a large-scale electric current system occupying a vast area of an AR, which we call distributed electric current. For ARs obeying the Hale polarity law and the hemispheric helicity sign rule, the distributed current is directed upward in the leading part of an AR and it appears to be closing back to the photosphere in the following part of an AR through the corona and chromosphere. Our analysis of the time variations of the magnitude of the distributed electric currents showed that low-flaring ARs exhibit small variations of the distributed currents in the range of $\pm 20 \times 10^{12}$ A, whereas the highly flaring ARs exhibited significant slow variations of the distributed currents in the range of $30-95 \times 10^{12}$ A. Intervals of the enhanced flaring appear to be co-temporal with smooth enhancements of the distributed electric current.

ON THE CHROMOSPHERIC ACTIVITY NATURE OF A LOW-MASS CLOSE BINARY: KIC 12004834

We study the nature of the chromospheric activity of an eclipsing binary KIC 12004834, using Kepler data. We analyse the light curve of the system, the sinusoidal variations at out-of-eclipses and detected flare events. The secondary component’s temperature is found to be 4001±11 K, the mass ratio is 0.743±0.001, and the orbital inclination is 75◦.89±0◦.03. The analysis indicates a stellar spot effect on the variation. Moreover, the OPEA model has been derived over 149 flares. The saturation level called Plateau value, is found to be 2.093±0.236 s. The flare number per hour (known as flare frequency N1) is found to be 0.06644 h−1, while the flare-equivalent duration per hour (known as flare frequency N2) is found to be 0.59 second/hour. According to these results, KIC 12004834 is a very low-mass close binary system with high level of flare activity.

Hyperflares of M Dwarfs

Abstract From a study of the light curves of the M dwarfs observed by the Kepler space telescope in its primary mission, a number of flare events with the peak flux increases reaching more than the nominal stellar luminosity have been found. One of them, KIC 9201463, produced an extreme flare with the peak flux increase jumping to five times the quiet-time value. In relative terms, this class of hyperflares is much stronger than the superflares of the solar-type stars and could have a very important influence on the atmospheric evolution and the potential development of biospheres of habitable super-Earths orbiting around M dwarf stars. A cross-correlation of the flare activities of some of these M dwarf stars and their Hα equivalent width (EW) values derived from the LAMOST project indicates that the Hα EW values can be used to monitor the occurrence of hyperflares as well as the level of flare activity of different classes of M dwarfs with fast to slow rotations, and hence the long-term environmental effects of star–planet interaction of exoplanets.

Solar flares and solar subphotospheric vorticity

We explore the relation between surface magnetic flux of the sun and subsurface flow vorticity for flaring and nonflaring solar active regions. For this purpose, we use a data set consisting of 1009 active regions, including the vorticity measurements of their subsurface flows derived from high‐resolution global oscillation network group (GONG) helioseismology data and the corresponding X‐ray flare data from the geostationary operation environmental satellite (GOES). Using quantities averaged over the disk passage of active regions, we find that, while there is a considerable spread of the flux and vorticity values, they are more or less linearly related. We distinguish the level of flare activity by X‐ray flare class and find that large flux or large vorticity values are sufficient for an active region to produce low‐intensity C‐class flares. Active regions that produce high‐intensity X‐class flares are characterized by large values of both flux and vorticity. Active regions that produce M‐class flares of intermediate intensity are characterized by large vorticity values. The inclusion of solar subsurface vorticity thus helps to distinguish between flaring and nonflaring active regions.

Flaring Activity of Sagittarius A* at 43 and 22 GHz: Evidence for Expanding Hot Plasma

We have carried out Very Large Array (VLA) continuum observations to study the variability of Sgr A* at 43 GHz (λ = 7 mm) and 22 GHz (λ = 13 mm). A low level of flare activity has been detected with a duration of ~2 hr at these frequencies, showing the peak flare emission at 43 GHz leading the 22 GHz peak flare by ~20-40 minutes. The overall characteristics of the flare emission are interpreted in terms of the plasmon model of van der Laan by considering the ejection and adiabatic expansion of a uniform, spherical plasma blob due to flare activity. The observed peak of the flare emission with a spectral index, ν-α, of α = 1.6 is consistent with the prediction that the peak emission shifts toward lower frequencies in an adiabatically expanding self-absorbed source. We present the expected synchrotron light curves for an expanding blob, as well as the peak frequency emission, as a function of the energy spectral index constrained by the available flaring measurements in near-IR, submillimeter, millimeter, and radio wavelengths. We note that the blob model is consistent with the available measurements; however, we cannot rule out the jet of Sgr A*. If expanding material leaves the gravitational potential of Sgr A*, the total mass-loss rate of nonthermal and thermal particles is estimated to be ≤2 × 10-8 M☉ yr-1. We discuss the implication of the mass-loss rate, since this value matches closely the estimated accretion rate based on polarization measurements.

Solar MHD turbulence in regions with various levels of flare activity

The paper continues investigations of MHD turbulence in active solar regions. The statistical distributions of the increments (structure functions) of the turbulent field are studied analytically in the context of a refined Kolmogorov theory of turbulence. Since photospheric transport of the Bz component of the magnetic field is quite similar to that of a scalar field in a turbulent flow, the theory of transport of a passive scalar can be applied. This approach enables us to show that the structure functions are determined by the competition between the dissipation of the magnetic and kinetic energies and to obtain a number of relations between the structure-function parameters and energy characteristics of the MHD turbulence. Taking into account general conclusions that can be drawn on the basis of the refined Kolmogorov turbulence theory, the structure functions of the Bz field are calculated for eight active regions (from measurements of SOHO/MDI and the Huairou Solar Observing Station, China). These calculations show that the behavior of the structure functions is different for the Bz field of each active region. The energy-dissipation index of the fluctuation spectrum (which is uniquely determined by the structure functions) is closely related to the level of flare activity: the more activity, the less steep the dissipation spectrum for a given active region. This provides a means to test and, consequently, forecast the flare activity of active regions.