290 research outputs found
On mesogranulation, network formation and supergranulation
We present arguments which show that in all likelihood mesogranulation is not
a true scale of solar convection but the combination of the effects of both
highly energetic granules, which give birth to strong positive divergences
(SPDs) among which we find exploders, and averaging effects of data processing.
The important role played by SPDs in horizontal velocity fields appears in the
spectra of these fields where the scale 4 Mm is most energetic; we
illustrate the effect of averaging with a one-dimensional toy model which shows
how two independent non-moving (but evolving) structures can be transformed
into a single moving structure when time and space resolution are degraded.
The role of SPDs in the formation of the photospheric network is shown by
computing the advection of floating corks by the granular flow. The coincidence
of the network bright points distribution and that of the corks is remarkable.
We conclude with the possibility that supergranulation is not a proper scale of
convection but the result of a large-scale instability of the granular flow,
which manifests itself through a correlation of the flows generated by SPDs.Comment: 10 pages, 11 figures, to appear in Astronomy and Astrophysic
Hydrodynamical simulations of convection-related stellar micro-variability. II. The enigmatic granulation background of the COROT target HD49933
Local-box hydrodynamical model atmospheres provide statistical information
about a star's emergent radiation field which allows one to predict the level
of its granulation-related micro-variability. Space-based photometry is now
sufficiently accurate to test model predictions. We aim to model the
photometric granulation background of HD49933 as well as the Sun, and compare
the predictions to the measurements obtained by the COROT and SOHO satellite
missions. We construct hydrodynamical model atmospheres representing HD49933
and the Sun, and use a previously developed scaling technique to obtain the
observable disk-integrated brightness fluctuations. We further performed
exploratory magneto-hydrodynamical simulations to gauge the impact of small
scale magnetic fields on the synthetic light-curves. We find that the
granulation-related brightness fluctuations depend on metallicity. We obtain a
satisfactory correspondence between prediction and observation for the Sun,
validating our approach. For HD49933, we arrive at a significant
over-estimation by a factor of two to three in total power. Locally generated
magnetic fields are unlikely to be responsible, otherwise existing fields would
need to be rather strong to sufficiently suppress the granulation signal.
Presently suggested updates on the fundamental stellar parameters do not
improve the correspondence; however, an ad-hoc increase of the HD49933 surface
gravity by about 0.2dex would eliminate most of the discrepancy. We diagnose a
puzzling discrepancy between the predicted and observed granulation background
in HD49933, with only rather ad-hoc ideas for remedies at hand.Comment: 7 pages, 5 figures, accepted for publication in A&
Cognitive networks: brains, internet, and civilizations
In this short essay, we discuss some basic features of cognitive activity at
several different space-time scales: from neural networks in the brain to
civilizations. One motivation for such comparative study is its heuristic
value. Attempts to better understand the functioning of "wetware" involved in
cognitive activities of central nervous system by comparing it with a computing
device have a long tradition. We suggest that comparison with Internet might be
more adequate. We briefly touch upon such subjects as encoding, compression,
and Saussurean trichotomy langue/langage/parole in various environments.Comment: 16 page
Acoustic Events in the Solar Atmosphere from Hinode/SOT NFI observations
We investigate the properties of acoustic events (AEs), defined as spatially
concentrated and short duration energy flux, in the quiet sun using
observations of a 2D field of view (FOV) with high spatial and temporal
resolution provided by the Solar Optical Telescope (SOT) onboard
\textit{Hinode}. Line profiles of Fe \textsc{i} 557.6 nm were recorded by the
Narrow band Filter Imager (NFI) on a FOV during 75 min with a
time step of 28.75 s and 0.08 pixel size. Vertical velocities were computed
at three atmospheric levels (80, 130 and 180 km) using the bisector technique
allowing the determination of energy flux in the range 3-10 mHz using two
complementary methods (Hilbert transform and Fourier power spectra). Horizontal
velocities were computed using local correlation tracking (LCT) of continuum
intensities providing divergences.
The net energy flux is upward. In the range 3-10 mHz, a full FOV space and
time averaged flux of 2700 W m (lower layer 80-130 km) and 2000 W
m (upper layer 130-180 km) is concentrated in less than 1% of the solar
surface in the form of narrow (0.3) AE. Their total duration (including rise
and decay) is of the order of s. Inside each AE, the mean flux is W m (lower layer) and W m (upper). Each
event carries an average energy (flux integrated over space and time) of J (lower layer) to J (upper). More than events
could exist permanently on the Sun, with a birth and decay rate of 3500
s. Most events occur in intergranular lanes, downward velocity regions,
and areas of converging motions.Comment: 18 pages, 10 figure
High-resolution spectro-polarimetry of a flaring sunspot penumbra
We present simultaneous photospheric and chromospheric observations of the
trailing sunspot in NOAA 10904 during a weak flare eruption (GOES magnitude
B7.8), obtained with the Swedish Solar Telescope (SST) in La Palma, Canary
Islands. High-resolution \ion{Ca}{ii} images show a typical two-ribbon
structure that has been hitherto only known for larger flares, and the flare
appears in a confined region that is discernible by a bright border. The
underlying photosphere shows a disturbed penumbral structure with intersecting
branches of penumbral filaments. High-resolution Doppler- and
vector-magnetograms exhibit oppositely directed Evershed flows and magnetic
field vectors in the individual penumbral branches, resulting in several
regions of magnetic azimuth discontinuity and several islands where the
vertical magnetic field is reversed. The discontinuity regions are co-spatial
with the locations of the onset of the flare ribbons. From the results, we
conclude that the confined flare region is detached from the global magnetic
field structure by a separatrix marked by the bright border visible in
\ion{Ca}{ii} . We further conclude that the islands of reversed vertical
field appear because of flux emergence and that the strong magnetic shear
appearing in the regions of magnetic azimuth discontinuity triggers the flare.Comment: 20 pages + 1 online Figure for A&
The locus of legitimate interpretation in Big Data sciences : Lessons for computational social science from -omic biology and high-energy physics
This paper argues that analyses of the ways in which Big Data has been enacted in other academic disciplines can provide us with concepts that will help understand the application of Big Data to social questions. We use examples drawn from our Science and Technology Studies (STS) analyses of -omic biology and high energy physics to demonstrate the utility of three theoretical concepts: (i) primary and secondary inscriptions, (ii) crafted and found data, and (iii) the locus of legitimate interpretation. These help us to show how the histories, organisational forms, and power dynamics of a field lead to different enactments of big data. The paper suggests that these concepts can be used to help us to understand the ways in which Big Data is being enacted in the domain of the social sciences, and to outline in general terms the ways in which this enactment might be different to that which we have observed in the âhardâ sciences. We contend that the locus of legitimate interpretation of Big Data biology and physics is tightly delineated, found within the disciplinary institutions and cultures of these disciplines. We suggest that when using Big Data to make knowledge claims about âthe socialâ the locus of legitimate interpretation is more diffuse, with knowledge claims that are treated as being credible made from other disciplines, or even by those outside academia entirely
Evolution and Flare Activity of Delta-Sunspots in Cycle 23
The emergence and magnetic evolution of solar active regions (ARs) of
beta-gamma-delta type, which are known to be highly flare-productive, were
studied with the SOHO/MDI data in Cycle 23. We selected 31 ARs that can be
observed from their birth phase, as unbiased samples for our study. From the
analysis of the magnetic topology (twist and writhe), we obtained the following
results. i) Emerging beta-gamma-delta ARs can be classified into three
topological types as "quasi-beta", "writhed" and "top-to-top". ii) Among them,
the "writhed" and "top-to-top" types tend to show high flare activity. iii) As
the signs of twist and writhe agree with each other in most cases of the
"writhed" type (12 cases out of 13), we propose a magnetic model in which the
emerging flux regions in a beta-gamma-delta AR are not separated but united as
a single structure below the solar surface. iv) Almost all the "writhed"-type
ARs have downward knotted structures in the mid portion of the magnetic flux
tube. This, we believe, is the essential property of beta-gamma-delta ARs. v)
The flare activity of beta-gamma-delta ARs is highly correlated not only with
the sunspot area but also with the magnetic complexity. vi) We suggest that
there is a possible scaling-law between the flare index and the maximum umbral
area
Can we Determine Electric Fields and Poynting Fluxes from Vector Magnetograms and Doppler Measurements?
The availability of vector magnetogram sequences with sufficient accuracy and
cadence to estimate the time derivative of the magnetic field allows us to use
Faraday's law to find an approximate solution for the electric field in the
photosphere, using a Poloidal-Toroidal Decomposition (PTD) of the magnetic
field and its partial time derivative. Without additional information, however,
the electric field found from this technique is under-determined -- Faraday's
law provides no information about the electric field that can be derived the
gradient of a scalar potential. Here, we show how additional information in the
form of line-of-sight Doppler flow measurements, and motions transverse to the
line-of-sight determined with ad-hoc methods such as local correlation
tracking, can be combined with the PTD solutions to provide much more accurate
solutions for the solar electric field, and therefore the Poynting flux of
electromagnetic energy in the solar photosphere. Reliable, accurate maps of the
Poynting flux are essential for quantitative studies of the buildup of magnetic
energy before flares and coronal mass ejections.Comment: Solar Physics, in press. 14 pages, 3 figure
Overseeing the overseers: assessing compliance with municipal intervention rules in South Africa
Section 139 of the Constitution of South Africa empowers provinces to intervene into municipalities, an instrument to correct serious failures in local government. This article discusses the policy and legal framework for interventions and assesses whether the constitutional provisions that circumscribe it, are being adhered to. The starting point is that decentralisation, of which this instrument is part, is rules-based and that adherence to the rule of law is critical for its success. By its very nature, intervention represents an intrusion into the institutional integrity of the affected municipality and adherence to the constitutional safeguards surrounding the intervention is therefore critical. The article sets out the constitutional framework for interventions into municipalities which includes oversight roles for the Minister responsible for local government, the National Council of Provinces and the provincial legislature. It combines this with an assessment of 39 interventions that took place between 2008 and 2014. It presents a provincial breakdown and a breakdown of the legal basis of these 39 interventions. It concludes that provinces donât use the interventions envisaged in Section 139(4) and (5) but instead almost always intervene in terms of Section 139(1) of the Constitution. The interventions are assessed for compliance with constitutional prescripts, such as the need to establish a failure to fulfil an executive obligation, the timely submission of the intervention to the Minister and the NCOP and their timely approval. The article concludes that a significant number of interventions did not comply with the pro- visions pertaining to the timely submission and approval by the Minister and the NCOP. Furthermore, there is a need to accelerate the adoption of the legislation envisaged by Section 139(8) of the Constitution to further regulate interventions
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