Dynamo/Solar Cycle Posters
Presiding: A Crouch, Universite de Montreal; H Jones, National Solar Observatory
SP43B-01 1330h
Turbulent Dynamos and the Minimum X-ray Flux in Solar-Type Main Sequence Stars
We investigate whether a small-scale turbulent dynamo can account quantitatively for the observed lower limit of X-ray surface flux in solar-type main sequence stars. Our approach is to use 3D numerical simulations of a turbulent dynamo driven by convection to characterize the dynamic behavior, magnetic field strengths, and filling factors in a non-rotating stratified medium, and to predict these magnetic properties at the surface of cool stars. We use simple applications of stellar structure theory for the convective envelopes of main-sequence stars to scale our simulations to the outer layers of stars in the F0--M0 spectral range, which allows us to estimate the unsigned magnetic flux on the surface of non-rotating reference stars. With these estimates we use the observed magnetic flux--X-ray flux correlation of Pevtsov et al. (2003) to predict the level of X-ray emission from such a turbulent dynamo, and find that our results compare well with observed lower limits of surface X-ray flux. This suggests that dynamo action from a convecting, non-rotating plasma is a viable alternative to acoustic heating models as an explanation for the basal emission level seen in chromospheric, transition region, and coronal diagnostics from late-type stars.
SP43B-02 1330h
3D MHD Shear instabilities in the Solar Tachocline
In the past decade, linear analysis has shown that the latitudinal differential rotation in the solar tachocline is unstable in the presence of a toroidal field under a wide range of field amplitudes and configurations. The most unstable modes are generally tipping modes whereby a band of toroidal field evolves such that its central axis tips away from the rotation axis. Previous studies of these instabilities have been based on two-dimensional (latitude-longitude) models or on the MHD shallow-water approximation in which horizontal velocities and fields are still independent of height but are no longer required to be divergenceless. Recently we have extended these linear analyses to 3D using a thin-shell model. Here we present nonlinear simulations of these 3D MHD shear instabilities.
SP43B-03 1330h
Kinetic Helicity in Solar Subsurface Layers and Flare Activity of Active Regions
We search for a relation between subsurface flows below active regions and flare events occuring in those regions. For this purpose, we use a ring-diagram analysis to determine the subsurface flows from high-resolution Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) data and derive the kinetic helicity as a measure of the topology of the subsurface flows. We compare it with X-ray flare data from Geostationary Operational Environmental Satellite (GOES). We study active regions in three Carrington rotations (CR~1982, 1988, and 2009), which represent different levels of flare activity. The maximum value of the unsigned kinetic helicity density associated with each active region correlates remarkably well with the total flare X-ray intensity of the active regions; active regions with strong flare activity show large values of kinetic helicity density in subsurface flows. This work has ben supported by NASA grant NAG 5-11703.
SP43B-04 1330h
Vorticity and Kinetic Helicity in Solar Subsurface Layers from GONG and MDI data
We use a ring-diagram analysis to determine the subsurface flows from high-resolution Global Oscillation Network Group (GONG) and Michelson Doppler Imager (MDI) data and derive the vorticity and the kinetic helicity of the subsurface flows in the upper 16~Mm of the convection zone. We separate the contributions of large-scale horizontal flows, such as differential rotation, from those of small-scale variations, such as the ones due to active regions, and analyze the large-scale and the residual component independently. We study the relation between magnetic activity and subsurface flows by comparing synoptic maps of the derived residual quantities with maps of photospheric magnetic activity. By comparing synoptic maps derived from GONG and MDI data, we are able to cross-validate the results. We will present the latest findings. This work has been supported by NASA and NSF.
SP43B-05 1330h
Quasi periodicities in the Fluctuations of the Axisymmetric Solar Magnetic Field from Independent Component Analysis
Observed solar, interplanetary and geomagnetic time series contain quasi periodicities on the order of 1 to 2.5 years. The further discovery of 1.3 year fluctuations in helioseismic observations suggests that a variety of signals may be related to the underlying dynamo in the Sun. We have applied the methods of principal component analysis (PCA) and independent component analysis (ICA) to search for the coherent structures (PCA) and independent global modes (ICA) of the axisymmetric solar magnetic field. While PCA was shown to be effective in identifying the coherent modes that describe the 22 yr solar cycle, ICA uncovers the independent global modes with characteristic 1 to 2.5 yr quasi periods observed in heliospheric and helioseismic time series. We found that five modes effectively describe the data in both spatial and temporal domains. Two modes describe the polar and high latitude fields, and present 1-1.5 year quasi periodicities. The other three modes correspond to low and mid-latitude phenomena and show both 1.3 year and 1.7 year variations. By comparing the characteristic timescales, dates of occurrence and heliocentric latitudes of these modes, we connect them to their manifestations in heliospheric time series.
SP43B-06 1330h
10 To 40 nHz Oscillations in Spherical Harmonic Representations of Sunspot Patterns Yield a Square-Root Dispersion Relationship With Respect to Harmonic Order.
The temporal changes in the 2-dimensional patterns of sunspots groups, spanning 1650 Carrington Rotations (CRs), were previously analyzed using surface spherical harmonics (SSHs). (Juckett, 2003, A&A, 399, 731.) The focus of that paper was to examine the common trends in the oscillations of both the amplitudes and spatial phases of sectoral combinations of SSHs. The amplitude analysis revealed strong evidence for the first and second harmonics of the 11-yr cycle across all SSHs. A further analysis of SSH amplitude variations is continued here. The Principle Components (PCs) located just after the second harmonic were extracted from individual sectoral SSHs. An apparent dispersion relationship appeared between the major frequencies of these PCs and SSH order, which has a near square-root dependence. The dispersion spans frequencies of approximately 10 to 40 nHz over the order range m=2 to m=18, indicating large-scale oscillations with very long periods (0.8 to 3 yr). Abrupt changes in the spatial phases of the SSHs, which are a hallmark of standing waves, also show a compatible dispersion relationship. In general, the square-root dependence would suggest g-mode oscillations, but the periods are far longer than predicted for such oscillations. While the source of these oscillations cannot be determined by this analysis, it suggests such mechanisms as the splitting of g-mode oscillations by differential rotation and the beating of the resultant closely spaced oscillations, or perhaps very long period internal waves, with an f-mode character, near the tachocline. In the latter case, the frequency range of the dispersion would require a reduced gravity of ~ 10-4 for such internal waves. This seems highly unlikely unless the local magnetic pressure is sufficient to generate such a region. These mechanisms as well as issues of data quality will be discussed.