Three-minute Sunspot Oscillations Driven by Magnetic Reconnection in a Light Bridge [SSA]

We report a different type of three-minute chromospheric oscillations above a sunspot in association with a small-scale impulsive event in a light bridge. During our observations, we found a transient brightening in the light bridge. The brightening was composed of elementary bursts that may be a manifestation of fast repetitive magnetic reconnections in the light bridge. Interestingly, the oscillations in the nearby sunspot umbra were impulsively excited when the intensity of the brightening reached its peak. The initial period of the oscillations was about 2.3 minutes and then gradually increased to 3.0 minutes with time. In addition, we found that the amplitude of the excited oscillations was twice the amplitude of oscillations before the brightening. Based on our results, we propose that magnetic reconnection occurring in a light bridge can excite of oscillations in the nearby sunspot umbra.

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D. Song, J. Chae, H. Kwak, et. al.
Mon, 20 Nov 17

Comments: 13 pages, 4 figures, accepted for publication in ApJL

Flare activity of the Sun and variations in its UV emission during cyrcle 24 [SSA]

The flare activity and the ultraviolet emission of the sun during its 24-th cycle are analysed. As compared to cycles 21-23, where the most powerful flares were observed during the decay phase, in cycle 24 the greatest number of powerful flares (>X2.7) occurred in the rising phase and at the maximum with the exception of the two largest flares of cycle 24 X9.3 and X8.2 in September 2017. We showed that regression fits of solar UV indices to the overall radiation level from the sun are substantially different for cycle 24 compared to cycles 21-23. It is found that for the flare of August 9, 2011 (SDO and GOES-15 observations), the flare propagates in a direction from the upper corona to the transition region and to the chromosphere. A study of the N-S asymmetry in the distribution of the flares in cycle 24 reveals a strong predominance of flares in the N-hemisphere in 2011 and in the S-hemisphere in 2014. It is also found that during cycles 23 and 24, the delays in the onset of proton events relative to the onset of the flares that cause them have a distribution with a distinct maximum corresponding to a delay of 2 hours for protons with energies >10 MeV, as well as for those with energies >100 MeV.

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E. Bruevich and G. Yakunina
Mon, 20 Nov 17

Comments: 18 pages, 9 figures,

Statistical analysis on Dynamic Fibrils observed from NST/BBSO observations [SSA]

We present the results obtained from the analysis of dynamic fibrils in NOAA active region (AR) 12132, using high resolution H$\alpha$ observations from New Solar Telescope operating at BBSO. The dynamic fibrils are seen to be moving up and down, and most of these dynamic fibrils are periodic and have jet like appearance. We found from our observations that the fibrils follows perfect parabolic paths at the most in many cases. A statistical measure on the properties of the parabolic paths showing an analysis on deceleration, maximum velocity, duration and kinetic energy of these fibrils is presented here. We found the average maximum velocity to be around 15 km s$^{-1}$ and mean deceleration to be around 100 m s$^{-2}$. The deceleration observed appears to be a fraction of gravity of sun and is not compatible with the path of ballistic at the gravity of sun. We found positive correlation between the deceleration and the maximum velocity. This correlation is consistent with the simulations done earlier on magnetoacoustic shock waves propagating upward.

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T. Priya, S. Jiangtao, J. Chen, et. al.
Mon, 20 Nov 17

Comments: 13 pages, 8 figures, accepted for publication in RAA

The evolution of the eccentricity in the eclipsing binary system AS Camelopardalis [SSA]

In 2002, 2004, and 2017 we conducted high precision CCD photometry observations of the eclipsing binary system AS~Cam. By the analysis of the light curves from 1967 to 2017 (our data + data from the literature) we obtained photometric elements of the system and found the change of the system’s orbital eccentricity by $\Delta e=0.03 \pm 0.01$. This change can indicate that there is a third companion in the system in a highly inclined orbit with respect to the orbital plane of the central binary, and its gravitational influence may cause the discrepancy between the observed and theoretical apsidal motion rates of AS~Cam.

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V. Kozyreva, A. Kusakin and A. Bogomazov
Mon, 20 Nov 17

Comments: 12 pages, 5 figures, 3 tables; accepted by Research in Astronomy and Astrophysics

Non-equilibrium ionization by a periodic electron beam II. Synthetic Si IV and O IV transition region spectra [SSA]

Transition region (TR) spectra typically show the Si IV 1402.8 A line to be enhanced by a factor of 5 or more compared to the neighboring O IV 1401.2 A, contrary to predictions of ionization equilibrium models and the Maxwellian distribution of particle energies. Non-equilibrium effects in TR spectra are therefore expected. To investigate the combination of non-equilibrium ionization and high-energy particles, we apply the model of the periodic electron beam, represented by a kappa-distribution that recurs at periods of several seconds, to plasma at chromospheric temperatures of 10^4 K. This simple model can approximate a burst of energy release involving accelerated particles. Instantaneous time-dependent charge states of silicon and oxygen were calculated and used to synthesize the instantaneous and period-averaged spectra of Si IV and O IV. The electron beam drives the plasma out of equilibrium. At electron densities of N_e = 10^10 cm^-3, the plasma is out of ionization equilibrium at all times in all cases we considered, while for a higher density of N_e = 10^11 cm^-3, ionization equilibrium can be reached toward the end of each period, depending on the conditions. In turn, the character of the period-averaged synthetic spectra also depends on the properties of the beam. While the case of kappa = 2 results in spectra with strong or even dominant O IV, higher values of kappa can approximate a range of observed TR spectra. Spectra similar to typically observed spectra, with the Si IV 1402.8 A line about a factor 5 higher than O IV 1401.2 A, are obtained for kappa = 3. An even higher value of kappa = 5 results in spectra that are exclusively dominated by Si IV, with negligible O IV emission. This is a possible interpretation of the TR spectra of UV bursts, although an interpretation that requires a density that is 1-3 orders of magnitude lower than for equilibrium estimates.

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E. Dzifcakova and J. Dudik
Mon, 20 Nov 17

Comments: Astronomy & Astrophysics, accepted

DAVs: Red edge and Outbursts [SSA]

As established by ground based surveys, white dwarfs with hydrogen atmospheres pulsate as they cool across the temperature range, $12500\,\mathrm{K} \gtrsim T_{\mathrm{eff}} \gtrsim 10800\,\mathrm{K}$. Known as DAVs or ZZ Ceti stars, their oscillations are attributed to overstable g-modes excited by convective driving. The effective temperature at the blue edge of the instability strip is slightly lower than that at which a surface convection zone appears. The temperature at the red edge is a two-decade old puzzle. Recently, {\it Kepler} discovered a number of cool DAVs which pulsate at higher frequencies and with much smaller photometric amplitudes than expected based on trends extrapolated from DAVs found by ground based observations. Remarkably, some of them exhibit sporadic outbursts separated by days, each lasting several hours, and releasing $\sim 10^{33}-10^{34}\,\mathrm{erg}$. We provide quantitative explanations for both the red edge and the outbursts. The minimal frequency for overstable modes rises abruptly near the red edge. Although high frequency overstable modes exist below the red edge, their photometric amplitudes are generally too small to be detected by ground based observations. Nevertheless, these overstable parent modes can manifest themselves through nonlinear mode couplings to damped daughter modes which generate limit cycles giving rise to photometric outbursts.

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J. Luan and P. Goldreich
Mon, 20 Nov 17

Comments: submitted to ApJ

The CARMENES search for exoplanets around M dwarfs: High-resolution optical and near-infrared spectroscopy of 324 survey stars [SSA]

The CARMENES radial velocity (RV) survey is observing 324 M dwarfs to search for any orbiting planets. In this paper, we present the survey sample by publishing one CARMENES spectrum for each M dwarf. These spectra cover the wavelength range 520–1710nm at a resolution of at least $R > 80,000$, and we measure its RV, H$\alpha$ emission, and projected rotation velocity. We present an atlas of high-resolution M-dwarf spectra and compare the spectra to atmospheric models. To quantify the RV precision that can be achieved in low-mass stars over the CARMENES wavelength range, we analyze our empirical information on the RV precision from more than 6500 observations. We compare our high-resolution M-dwarf spectra to atmospheric models where we determine the spectroscopic RV information content, $Q$, and signal-to-noise ratio. We find that for all M-type dwarfs, the highest RV precision can be reached in the wavelength range 700–900nm. Observations at longer wavelengths are equally precise only at the very latest spectral types (M8 and M9). We demonstrate that in this spectroscopic range, the large amount of absorption features compensates for the intrinsic faintness of an M7 star. To reach an RV precision of 1ms$^{-1}$ in very low mass M dwarfs at longer wavelengths likely requires the use of a 10m class telescope. For spectral types M6 and earlier, the combination of a red visual and a near-infrared spectrograph is ideal to search for low-mass planets and to distinguish between planets and stellar variability. At a 4m class telescope, an instrument like CARMENES has the potential to push the RV precision well below the typical jitter level of 3-4ms$^{-1}$.

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A. Reiners, M. Zechmeister, J. Caballero, et. al.
Mon, 20 Nov 17

Comments: accepted for publication in A&A, 13 pages plus 40 pages spectral atlas, first 10 atlas pages are reduced in quality to fit arXiv size limit; one CARMENES spectrum for each of the 324 stars is published in electronic format at this http URL