1,250 research outputs found
Magnetic helicity transported by flux emergence and shuffling motions in Solar Active Region NOAA 10930
We present a new methodology which can determine magnetic helicity transport
by the passage of helical magnetic field lines from sub-photosphere and the
shuffling motions of foot-points of preexisting coronal field lines separately.
It is well known that only the velocity component which is perpendicular to the
magnetic field () has contribution to the helicity
accumulation. Here, we demonstrate that can be deduced
from horizontal motion and vector magnetograms, under a simple relation of
as suggested by
Dmoulin & Berger (2003). Then after dividing
into two components, as one is tangential and the other is normal to the solar
surface, we can determine both terms of helicity transport. Active region (AR)
NOAA 10930 is analyzed as an example during its solar disk center passage by
using data obtained by the Spectro-Polarimeter and the Narrowband Filter Imager
of Solar Optical Telescope on board Hinode. We find that in our calculation,
the helicity injection by flux emergence and shuffling motions have the same
sign. During the period we studied, the main contribution of helicity
accumulation comes from the flux emergence effect, while the dynamic transient
evolution comes from the shuffling motions effect. Our observational results
further indicate that for this AR, the apparent rotational motion in the
following sunspot is the real shuffling motions on solar surface
Pressure dependence of the superconducting transition and electron correlations in Na_xCoO_2 \cdot 1.3H_2O
We report T_c and ^{59}Co nuclear quadrupole resonance (NQR) measurements on
the cobalt oxide superconductor Na_{x}CoO_{2}\cdot 1.3H_{2}O (T_c=4.8 K) under
hydrostatic pressure (P) up to 2.36 GPa. T_c decreases with increasing pressure
at an average rate of -0.49\pm0.09 K/GPa. At low pressures P\leq0.49 GPa, the
decrease of T_c is accompanied by a weakening of the spin correlations at a
finite wave vector and a reduction of the density of states (DOS) at the Fermi
level. At high pressures above 1.93 GPa, however, the decrease of T_c is mainly
due to a reduction of the DOS. These results indicate that the
electronic/magnetic state of Co is primarily responsible for the
superconductivity. The spin-lattice relaxation rate 1/T_1 at P=0.49 GPa shows a
T^3 variation below T_c down to T\sim 0.12T_c, which provides compelling
evidence for the presence of line nodes in the superconducting gap function.Comment: published on 19, Sept. 2007 on Phys. Rev.
Correlation of Transmitter Release with Membrane Properties of the Presynaptic Fiber of the Squid Giant Synapse
Modeling the Transport and Deposition of ¹⁰Be Produced by the Strongest Solar Proton Event During the Holocene
Prominent excursions in the number of cosmogenic nuclides (e.g., ¹⁰Be) around 774 CE/775 document the most severe solar proton event (SPE) throughout the Holocene. Its manifestation in ice cores is valuable for geochronology, but also for solar-terrestrial physics and climate modeling. Using the ECHAM/MESSy Atmospheric Chemistry (EMAC) climate model in combination with the Warning System for Aviation Exposure to SEP (WASAVIES), we investigate the transport, mixing, and deposition of the cosmogenic nuclide ¹⁰Be produced by the 774 CE/775 SPE. By comparing the model results to the reconstructed ¹⁰Be time series from four ice core records, we study the atmospheric pathways of ¹⁰Be from its stratospheric source to its sink at Earth's surface. The reconstructed post-SPE evolution of the ¹⁰Be surface fluxes at the ice core sites is well captured by the model. The downward transport of the ¹⁰Be atoms is controlled by the Brewer-Dobson circulation in the stratosphere and cross-tropopause transport via tropopause folds or large-scale sinking. Clear hemispheric differences in the transport and deposition processes are identified. In both polar regions the ¹⁰Be surface fluxes peak in summertime, with a larger influence of wet deposition on the seasonal ¹⁰Be surface flux in Greenland than in Antarctica. Differences in the peak ¹⁰Be surface flux following the 774 CE/775 SPE at the drilling sites are explained by specific meteorological conditions depending on the geographic locations of the sites
Nonlinear force-free models for the solar corona I. Two active regions with very different structure
With the development of new instrumentation providing measurements of solar
photospheric vector magnetic fields, we need to develop our understanding of
the effects of current density on coronal magnetic field configurations. The
object is to understand the diverse and complex nature of coronal magnetic
fields in active regions using a nonlinear force-free model. From the observed
photospheric magnetic field we derive the photospheric current density for two
active regions: one is a decaying active region with strong currents (AR8151),
and the other is a newly emerged active region with weak currents (AR8210). We
compare the three-dimensional structure of the magnetic fields for both active
region when they are assumed to be either potential or nonlinear force-free.
The latter is computed using a Grad-Rubin vector-potential-like numerical
scheme. A quantitative comparison is performed in terms of the geometry, the
connectivity of field lines, the magnetic energy and the magnetic helicity
content. For the old decaying active region the connectivity and geometry of
the nonlinear force-free model include strong twist and strong shear and are
very different from the potential model. The twisted flux bundles store
magnetic energy and magnetic helicity high in the corona (about 50 Mm). The
newly emerged active region has a complex topology and the departure from a
potential field is small, but the excess magnetic energy is stored in the low
corona and is enough to trigger powerful flares.Comment: 11 pages, 11 figure
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