1,607 research outputs found
Interference between postural control and mental task performance in patients with vestibular disorder and healthy controls
OBJECTIVES - To determine whether interference between postural control and mental task performance in patients with balance system impairment and healthy subjects is due to general capacity limitations, motor control interference, competition for spatial processing resources, or a combination of these.METHOD - Postural stability was assessed in 48 patients with vestibular disorder and 24 healthy controls while they were standing with eyes closed on (a) a stable and (b) a moving platform. Mental task performance was measured by accuracy and reaction time on mental tasks, comprising high and low load, spatial and non-spatial tasks. Interference between balancing and performing mental tasks was assessed by comparing baseline (single task) levels of sway and mental task performance with levels while concurrently balancing and carrying out mental tasks.RESULTS - As the balancing task increased in difficulty, reaction times on both low load mental tasks grew progressively longer and accuracy on both high load tasks declined in patients and controls. Postural sway was essentially unaffected by mental activity in patients and controls.CONCLUSIONS - It is unlikely that dual task interference between balancing and mental activity is due to competition for spatial processing resources, as levels of interference were similar in patients with vestibular disorder and healthy controls, and were also similar for spatial and non-spatial tasks. Moreover, the finding that accuracy declined on the high load tasks when balancing cannot be attributed to motor control interference, as no motor control processing is involved in maintaining accuracy of responses. Therefore, interference between mental activity and postural control can be attributed principally to general capacity limitations, and is hence proportional to the attentional demands of both tasks
FIP Bias Evolution in a Decaying Active Region
Solar coronal plasma composition is typically characterized by first
ionization potential (FIP) bias. Using spectra obtained by Hinode's EUV Imaging
Spectrometer (EIS) instrument, we present a series of large-scale, spatially
resolved composition maps of active region (AR) 11389. The composition maps
show how FIP bias evolves within the decaying AR from 2012 January 4-6.
Globally, FIP bias decreases throughout the AR. We analyzed areas of
significant plasma composition changes within the decaying AR and found that
small-scale evolution in the photospheric magnetic field is closely linked to
the FIP bias evolution observed in the corona. During the AR's decay phase,
small bipoles emerging within supergranular cells reconnect with the
pre-existing AR field, creating a pathway along which photospheric and coronal
plasmas can mix. The mixing time scales are shorter than those of plasma
enrichment processes. Eruptive activity also results in shifting the FIP bias
closer to photospheric in the affected areas. Finally, the FIP bias still
remains dominantly coronal only in a part of the AR's high-flux density core.
We conclude that in the decay phase of an AR's lifetime, the FIP bias is
becoming increasingly modulated by episodes of small-scale flux emergence, i.e.
decreasing the AR's overall FIP bias. Our results show that magnetic field
evolution plays an important role in compositional changes during AR
development, revealing a more complex relationship than expected from previous
well-known Skylab results showing that FIP bias increases almost linearly with
age in young ARs (Widing Feldman, 2001, ApJ, 555, 426)
The role of sulfate-rich fluids in heavy rare earth enrichment at the Dashigou carbonatite deposit, Huanglongpu, China
The Huanglongpu carbonatites are located in the north-western part of the Qinling orogenic belt in central China. Calcite carbonatite dykes at the Dashigou open pit are unusual due to their enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE), leading to a flat REE pattern, and in that the majority of dykes have a quartz core. They also host economic concentrations of molybdenite. The calcite carbonatite dykes show two styles of mineralogy according to the degree of hydrothermal reworking, and these are reflected in REE distribution and concentration. The REE in the little-altered calcite carbonatite occur mostly in magmatic REE minerals, mainly monazite-(Ce), and typically have ΣLREE/(HREE+Y) ratios from 9.9 to 17. In hydrothermally altered calcite carbonatites, magmatic monazite-(Ce) is partially replaced to fully replaced by HREE-enriched secondary phases and the rocks have ΣLREE/(HREE+Y) ratios from 1.1 to 3.8. The fluid responsible for hydrothermal REE redistribution is preserved in fluid inclusions in the quartz lenses. The bulk of the quartz lacks fluid inclusions but is cut by two later hydrothermal quartz generations, both containing sulfate-rich fluid inclusions with sulfate typically present as multiple trapped solids, as well as in solution. The estimated total sulfate content of fluid inclusions ranges from 6 to >33 wt.% K2SO4 equivalent. We interpret these heterogeneous fluid inclusions to be the result of reaction of sulfate-rich fluids with the calcite carbonatite dykes. The final HREE enrichment is due to a combination of factors: (1) the progressive HREE enrichment of later magmatic calcite created a HREE-enriched source; (2) REE–SO42– complexing allowed the REE to be redistributed without fractionation; and (3) secondary REE mineralisation was dominated by minerals such as HREE-enriched fluorocarbonates, xenotime-(Y) and churchite-(Y) whose crystal structures tends to favour HREE
Widespread Occurrence of High-Velocity Upflows in Solar Active Regions
We performed a systematic study of 12 active regions (ARs) with a broad range
of areas, magnetic flux and associated solar activity in order to determine
whether there are upflows present at the AR boundaries and if these upflows
exist, whether there is a high speed asymmetric blue wing component present in
the upflows. To identify the presence and locations of the AR upflows we derive
relative Doppler velocity maps by fitting a Gaussian function to {\it
Hinode}/EIS Fe XII 192.394\,\AA\ line profiles. To determine whether there is a
high speed asymmetric component present in the AR upflows we fit a double
Gaussian function to the Fe XII 192.394\,\AA\ mean spectrum that is computed in
a region of interest situated in the AR upflows. Upflows are observed at both
the east and west boundaries of all ARs in our sample with average upflow
velocities ranging between -5 to -26~km s. A blue wing asymmetry is
present in every line profile. The intensity ratio between the minor high speed
asymmetric Gaussian component compared to the main component is relatively
small for the majority of regions however, in a minority of cases (8/30) the
ratios are large and range between 20 to 56~\%. These results suggest that
upflows and the high speed asymmetric blue wing component are a common feature
of all ARs.Comment: Accepted in A&A, 5 pages, 3 figure
Flux cancellation and the evolution of the eruptive filament of 2011 June 7
We investigate whether flux cancellation is responsible for the formation of
a very massive filament resulting in the spectacular 2011 June 7 eruption. We
analyse and quantify the amount of flux cancellation that occurs in NOAA AR
11226 and its two neighbouring ARs (11227 & 11233) using line-of-sight
magnetograms from the Heliospheric Magnetic Imager. During a 3.6-day period
building up to the filament eruption, 1.7 x 10^21 Mx, 21% of AR 11226's maximum
magnetic flux, was cancelled along the polarity inversion line (PIL) where the
filament formed. If the flux cancellation continued at the same rate up until
the eruption then up to 2.8 x 10^21 Mx (34% of the AR flux) may have been built
into the magnetic configuration that contains the filament plasma. The large
flux cancellation rate is due to an unusual motion of the positive polarity
sunspot, which splits, with the largest section moving rapidly towards the PIL.
This motion compresses the negative polarity and leads to the formation of an
orphan penumbra where one end of the filament is rooted. Dense plasma threads
above the orphan penumbra build into the filament, extending its length, and
presumably injecting material into it. We conclude that the exceptionally
strong flux cancellation in AR 11226 played a significant role in the formation
of its unusually massive filament. In addition, the presence and coherent
evolution of bald patches in the vector magnetic field along the PIL suggests
that the magnetic field configuration supporting the filament material is that
of a flux rope.Comment: 18 pages, 7 figures. Submitted to ApJ in December 2015, accepted in
June 201
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