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Magnetic Activity (magnetic + activity)
Selected AbstractsMagnetic activity on 12 Cam and 29 Dra from long-term photometryASTRONOMISCHE NACHRICHTEN, Issue 4 2009M. Zboril Abstract We present newly discovered magnetic cycles of two late-type and long-period SB1 systems: 12 Cam and 29 Dra. The long-term photometry study revealed the presence of magnetic multiperiodic cycles on both stars, namely 14.8 and 8.5 yr for 12 Cam and 20.3, 11.1, and 7.6 yr for 29 Dra. Furthermore, the modelling of the V -band light curves revealed the existence of two active longitudes on 12 Cam and probably on 29 Dra as well. Both stars show changes of rotational period. The 12 Cam is the slowest rotating star whose activity cycle has been determined. The activity cycles determined by us allow us to extend to the slower rotation regime and to improve the significance of the empirical relation between rotation period and magnetic cycle length (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Photospheric and chromospheric activity on EY Dra,ASTRONOMISCHE NACHRICHTEN, Issue 9 2007H. Korhonen Abstract Magnetic activity in the photosphere and chromosphere of the M dwarf EY Dra is studied and possible correlations between the two are investigated using photometric observations in the V and R bands and optical and near infrared spectroscopy. The longitudinal spot configuration in the photosphere is obtained from the V band photometry, and the chromospheric structures are investigated using variations in the H, line profile and observations of the Paschen , line. The shape of the V band light-curve indicates two active regions on the stellar surface, about 0.4 in phase apart. The spectroscopic observations show enhanced H, emission observed close to the phases of the photometrically detected starspots. This could indicate chromospheric plages associated with the photospheric starspots. Some indications of prominence structures are also seen. The chromospheric pressure is limited to log mTR < ,4 based on the non-detection of emission in the Paschen , wavelength region. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Simulated geomagnetic reversals and preferred virtual geomagnetic pole pathsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004C. Kutzner SUMMARY The question of whether virtual geomagnetic poles (VGPs) recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists. One possible mechanism for such VGP clustering is the heterogeneity of heat flux at the core,mantle boundary (CMB). We use 3-D convection-driven numerical dynamo models with imposed non-uniform CMB heat flow that show stochastic reversals of the dipole field. We calculate transitional VGPs for a large number of token sites at the Earth's surface. In a model with a simple heat flux variation given by a Y22 harmonic, the VGP density maps for individual reversals differ substantially from each other, but the VGPs have a tendency to fall around a longitude of high heat flow. The mean VGP density for many reversals and excursions shows a statistically significant correlation with the heat flow. In a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean VGP density at American and East Asian longitudes, roughly consistent with the VGP paths seen in several palaeomagnetic studies. We find that low-latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated. They contribute to the equatorial dipole component and bias its orientation in longitude. During reversals the equatorial dipole part is not necessarily dominant at the Earth's surface, but is strong enough to explain the longitudinal preference of VGPs as seen from different sites. [source] Observation and modelling of main-sequence star chromospheres , X. Radiative budgets on Gl 867A and AU Mic (dM1e), and a two-component model chromosphere for Gl 205 (dM1),MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010E. R. Houdebine ABSTRACT We report on high-resolution observations of two dM1 stars: Gl 867A, an active dM1e star, and Gl 205, a less active dM1 star. The wavelength coverage is from 3890 to 6820 Å with a resolving power of about 45 000. The difference spectrum of these two stars allows us to make a survey of spectral lines sensitive to magnetic activity. We chose these two stars because, to within measurement errors, they have very close properties: Gl 867A has R= 0.726 R,, [M/H]= 0.080 dex and Teff= 3416 K, and Gl 205 has R= 0.758 R,, [M/H]= 0.101 dex and Teff= 3493 K. We find that besides traditional chromospheric lines, many photospheric lines are ,filled-in' in the active star spectrum. These differences are, most of the time, weak in absolute fluxes but can be large in terms of differences in the spectral-line equivalent widths. We calculate the differences in surface fluxes between these two stars for many spectral lines. We derive the radiative budgets for two dM1e stars: Gl 867A and AU Mic. We show that the sum of the numerous spectral lines represents a significant fraction of the radiative cooling of the outer atmosphere. We also re-investigate the cooling from the continuum from the visible to the extreme ultraviolet; we find that earlier predictions of the calculations of Houdebine et al. (Paper V) are in good agreement with observations. We emphasize that if this radiative cooling is chromospheric in character, then in chromospheric model calculations, we should include the radiative losses in Ca i, Cr i, V i, Ti i and Fe i. From simple constraints, we derive model chromospheres for quiescent and active regions on Gl 205. We show that the quiescent regions have a strong absorption H, profile. The plage regions show a filled-in intermediate activity H, profile. We also present possible spectral line profiles of quiescent and active regions on Gl 867A. [source] Long-term monitoring in IC4665: fast rotation and weak variability in very low mass objectsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009Alexander Scholz ABSTRACT We present the combined results of three photometric monitoring campaigns targeting very low mass (VLM) stars and brown dwarfs in the young open cluster IC4665 (age ,40 Myr). Each of our observing runs covers time-scales of ,5 d in the seasons 1999, 2001 and 2002, respectively. In all three runs, we observe ,100 cluster members, allowing us for the first time to put limits on the evolution of spots and magnetic activity in fully convective objects on time-scales of a few years. For 20 objects covering masses from 0.05 to 0.5 M,, we detect a periodic flux modulation, indicating the presence of magnetic spots co-rotating with the objects. The detection rate of photometric periods (,20 per cent) is significantly lower than in solar-mass stars at the same age, which points to a mass dependence in the spot properties. With two exceptions, none of the objects exhibits variability and thus spot activity in more than one season. This is contrary to what is seen in solar-mass stars and indicates that spot configurations capable of producing photometric modulations occur relatively rarely and are transient in VLM objects. The rotation periods derived in this paper range from 3 to 30 h, arguing for a lack of slow rotators among VLM objects. The periods fit into a rotational evolution scenario with pre-main sequence contraction and moderate (40,50 per cent) angular momentum losses due to wind braking. By combining our findings with literature results, we identify two regimes of rotational and magnetic properties, called C- and I-sequence. Main properties on the C-sequence are fast rotation, weak wind braking, H, emission and saturated activity levels, while the I-sequence is characterized by slow rotation, strong wind braking, no H, emission and linear activity-rotation relationship. Rotation rate and stellar mass are the primary parameters that determine in which regime an object is found. We outline a general scheme to understand rotational evolution for low-mass objects in the context of these two regimes and discuss the potential as well as the problems of this picture. [source] Graphite under the magnetic force microscopePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007T. L. Makarova Abstract In search for magnetically active carbon structures, we have undertaken the magnetic force microscopy study of intrinsic defects at the surface of highly oriented pyrolytic graphite. Most of the observed defects, such as ridges and cleavage edges, are found magnetically inert. However, some of the observed sharp cleavage edges do show magnetic activity , a built-in surface magnetization, which reveals itself as the magnetic force gradient signal sensitive to the polarity of the tip magnetization. These results indicate the existence of a defect related magnetism at room temperature on graphite surface. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Unconventional superconductivity and magnetism in Sr2RuO4 and related materialsANNALEN DER PHYSIK, Issue 3 2004I. Eremin Abstract We review the normal and superconducting state properties of the unconventional triplet superconductor Sr2RuO4 with an emphasis on the analysis of the magnetic susceptibility and the role played by strong electronic correlations. In particular, we show that the magnetic activity arises from the itinerant electrons in the Ru d -orbitals and a strong magnetic anisotropy occurs (,+- < ,zz) due to spin-orbit coupling. The latter results mainly from different values of the g -factor for the transverse and longitudinal components of the spin susceptibility (i.e. the matrix elements differ). Most importantly, this anisotropy and the presence of incommensurate antiferromagnetic and ferromagnetic fluctuations have strong consequences for the symmetry of the superconducting order parameter. In particular, reviewing spin fluctuation-induced Cooper-pairing scenario in application to Sr2RuO4 we show how p -wave Cooper-pairing with line nodes between neighboring RuO2 -planes may occur. We also discuss the open issues in Sr2RuO4 like the influence of magnetic and non-magnetic impurities on the superconducting and normal state of Sr2RuO4. It is clear that the physics of triplet superconductivity in Sr2RuO4 is still far from being understood completely and remains to be analyzed more in more detail. It is of interest to apply the theory also to superconductivity in heavy-fermion systems exhibiting spin fluctuations. [source] Dynamics of the solar magnetic field from SOHO/MDIASTRONOMISCHE NACHRICHTEN, Issue 1 2010E.E. Benevolenskaya Abstract The investigation of the dynamics of magnetic fields from small scales to the large scales is very important for the understanding of the nature of solar activity. It is also the base for producing adequate models of the solar cycle with the purpose to predict the level of solar activity. Since December 1995 the Michelson Doppler Imager (MDI) on board of the Solar and Heliospheric Observatory (SOHO) provides full disk magnetograms and synoptic maps which cover the period of solar cycle 23 and the current minimum. In this paper, I review the following important topics with a focus on the dynamics of the solar magnetic field. The synoptic structure of the solar cycle; the birth of the solar cycle (overlapping cycles 23 and 24); the relationship of the photospheric magnetic activity and the EUV solar corona, polar magnetic fields and dynamo theory (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Ultra-cool and extra-vigorous: Rotation and activity in Mand L dwarfsASTRONOMISCHE NACHRICHTEN, Issue 10 2007A. Reiners, Article first published online: 27 DEC 200 Abstract The study of rotation and activity in low-mass stars or brown dwarfs of spectral classes M and L has seen enormous progress during the last years. I summarize the results from different works that measured activity, rotation, and sometimes magnetic fields. The generation of magnetic activity seems to be unchanged at the threshold to completely convective stars, i.e. no change in the efficiency of the magnetic dynamos is observed. On the other hand, a sudden change in the strength of rotational braking appears at the threshold mass to full convection, and strong evidence exists for rotational braking weakening with lower mass. A probable explanation is that the field topology changes from dipolar to small scale structure as the objects become fully convective. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Helioseismology program for Solar Dynamics ObservatoryASTRONOMISCHE NACHRICHTEN, Issue 3-4 2007A. G. Kosovichev Abstract An overview of the science investigation program for the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory (SDO) space mission scheduled for launch in 2008 is presented. The HMI investigation encompasses three primary objectives of the Living With a Star Program:.rst, to determine how and why the Sun varies; second, to improve our understanding of how the Sun drives global change and space weather; and third, to determine to what extent predictions of space weather and global change can be made and to prototype predictive techniques. Helioseismology provides unique tools to study the basic mechanisms of the Sun's magnetic activity and variability. It plays a crucial role in all HMI investigations, which include convection-zone dynamics and the solar dynamo; origin and evolution of sunspots, active regions and complexes of activity; sources and drivers of solar activity and disturbances; links between the internal processes and dynamics of the corona and heliosphere; and precursors of solar disturbances for space-weather forecasts. We describe new unique opportunities for helioseismology studies with HMI data, in combination with data from the other SDO instruments, Atmospheric Imaging Assembly (AIA) and Extreme-ultraviolet Variability Experiment (EVE), and also from various space and ground-based observatories. The complete HMI science investigation and data analysis plan is available at http://hmi.stanford.edu. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Unsolved problems in observational astronomy.ASTRONOMISCHE NACHRICHTEN, Issue 6-8 2004Abstract We present the highlights of current observational programs in stellar optical spectroscopy carried out with 8-10 m class telescopes as well as with smaller telescopes. Topics discussed include: 1. light elements abundances and their cosmological implications; 2. search for Population III stars and spectroscopy of extremely metal deficient stars; 3. abundances of different stellar populations in the Galaxy; 4. spectroscopy of resolved stars in Local Group galaxies; 5. Li and Be abundances and internal mixing in stars; 6. spectroscopy of very-low mass stars and brown dwarfs; 7. radial velocity search of extrasolar planets; 8. stellar oscillations and asteroseismology; 9. stellar magnetic activity and Doppler imaging of stellar surface features. We also highlight the role that dedicated 1-2 m automatic telescopes with spectroscopic capabilities can play in several fields of stellar optical spectroscopy. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Quinoidal Oligothiophenes: Towards Biradical Ground-State SpeciesCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2010Ortiz Dr., Rocío Ponce Abstract A family of quinoidal oligothiophenes, from the dimer to the hexamer, with fused bis(butoxymethyl)cyclopentane groups has been extensively investigated by means of electronic and vibrational spectroscopy, electrochemical measurements, and density functional calculations. The latter predict that the electronic ground state always corresponds to a singlet state and that, for the longest oligomers, this state has biradical character that increases with increasing oligomer length. The shortest oligomers display closed-shell quinoidal structures. Calculations also predict the existence of very low energy excited triplet states that can be populated at room temperature. Aromatization of the conjugated carbon backbone is the driving force that determines the increasing biradical character of the ground state and the appearance of low-lying triplet states. UV/Vis, Raman, IR, and electrochemical experiments support the aromatic biradical structures predicted for the ground state of the longest oligomers and reveal that population of the low-lying triplet state accounts for the magnetic activity displayed by these compounds. [source] | ||||||||||