7,523 research outputs found
On proximity and hierarchy : exploring and modelling space using multilevel modelling and spatial econometrics
Spatial econometrics and also multilevel modelling techniques are increasingly part of the regional scientists‟ toolbox. Both approaches are used to model spatial autocorrelation in a wide variety of applications. However, it is not always clear on which basis researchers make a choice between spatial econometrics and spatial multilevel modelling. Therefore it is useful to compare both techniques. Spatial econometrics incorporates neighbouring areas into the model design; and thus interprets spatial proximity as defined in Tobler‟s first law of geography. On the other hand, multilevel modelling using geographical units takes a more hierarchical approach. In this case the first law of geography can be rephrased as „everything is related to everything else, but things in the same region are more related than things in different regions‟. The hierarchy (multilevel) and the proximity (spatial econometrics) approach are illustrated using Belgian mobility data and productivity data of European regions. One of the advantages of a multilevel model is that it can incorporate more than two levels (spatial scales). Another advantage is that a multilevel structure can easily reflect an administrative structure with different government levels. Spatial econometrics on the other hand works with a unique set of neighbours which has the advantage that there still is a relation between neighbouring municipalities separated by a regional boundary. The concept of distance can also more easily be incorporated in a spatial econometrics setting. Both spatial econometrics and spatial multilevel modelling proved to be valuable techniques in spatial research but more attention should go to the rationale why one of the two approaches is chosen. We conclude with some comments on models which make a combination of both techniques
Quark deconfinement and neutrino trapping in compact stars
We study the role played by neutrino trapping on the hadron star (HS) to
quark star (QS) conversion mechanism proposed recently by Berezhiani and
collaborators. We find that the nucleation of quark matter drops inside hadron
matter, and therefore the conversion of a HS into a QS, is strongly inhibit by
the presence of neutrinos.Comment: 3 pages, 3 figures. Talk given at the VIII International Conference
on Strangeness in Quark Matter. Cape Town, South Africa, Septembre 200
Evidence of widespread hot plasma in a non-flaring coronal active region from Hinode/XRT
Nanoflares, short and intense heat pulses within spatially unresolved
magnetic strands, are now considered a leading candidate to solve the coronal
heating problem. However, the frequent occurrence of nanoflares requires that
flare-hot plasma be present in the corona at all times. Its detection has
proved elusive until now, in part because the intensities are predicted to be
very faint. Here we report on the analysis of an active region observed with
five filters by Hinode/XRT in November 2006. We have used the filter ratio
method to derive maps of temperature and emission measure both in soft and hard
ratios. These maps are approximate in that the plasma is assumed to be
isothermal along each line-of-sight. Nonetheless, the hardest available ratio
reveals the clear presence of plasma around 10 MK. To obtain more detailed
information about the plasma properties, we have performed Monte Carlo
simulations assuming a variety of non-isothermal emission measure distributions
along the lines-of-sight. We find that the observed filter ratios imply
bi-modal distributions consisting of a strong cool (log T ~ 6.3-6.5) component
and a weaker (few percent) and hotter (6.6 < log T < 7.2) component. The data
are consistent with bi-modal distributions along all lines of sight, i.e.,
throughout the active region. We also find that the isothermal temperature
inferred from a filter ratio depends sensitively on the precise temperature of
the cool component. A slight shift of this component can cause the hot
component to be obscured in a hard ratio measurement. Consequently, temperature
maps made in hard and soft ratios tend to be anti-correlated. We conclude that
this observation supports the presence of widespread nanoflaring activity in
the active region.Comment: 12 figures, accepted for publication on refereed journa
On the ultraviolet signatures of small scale heating in coronal loops
Studying the statistical properties of solar ultraviolet emission lines could
provide information about the nature of small scale coronal heating. We expand
on previous work to investigate these properties. We study whether the
predicted statistical distribution of ion emission line intensities produced by
a specified heating function is affected by the isoelectronic sequence to which
the ion belongs, as well as the characteristic temperature at which it was
formed. Particular emphasis is placed on the strong resonance lines belonging
to the lithium isoelectronic sequence. Predictions for emission lines observed
by existing space-based UV spectrometers are given. The effects on the
statistics of a line when observed with a wide-band imaging instrument rather
than a spectrometer are also investigated. We use a hydrodynamic model to
simulate the UV emission of a loop system heated by nanoflares on small,
spatially unresolved scales. We select lines emitted at similar temperatures
but belonging to different isoelectronic groups: Fe IX and Ne VIII, Fe XII and
Mg X, Fe XVII, Fe XIX and Fe XXIV. Our simulations confirm previous results
that almost all lines have an intensity distribution that follows a power-law,
in a similar way to the heating function. However, only the high temperature
lines best preserve the heating function's power law index (Fe XIX being the
best ion in the case presented here). The Li isoelectronic lines have different
statistical properties with respect to the lines from other sequences, due to
the extended high temperature tail of their contribution functions. However,
this is not the case for Fe XXIV which may be used as a diagnostic of the
coronal heating function. We also show that the power-law index of the heating
function is effectively preserved when a line is observed by a wide-band
imaging instrument rather than a spectromenter
Pre-Biblio, bibliografia georeferenziata della Preistoria e della Protostoria italiana: struttura del progetto e primi risultati
The paper presents the new relational database Pre-Biblio on the Prehistory and Quaternary geology of Italy. It will be mainly composed of two correlated databases, «sites» and «bibliography» and some others such as the biographies of the most important scholars, the taxa of fossil remains contained in the sites, the palaeobasins. Each bibliographic reference will be linked to sites, and these georeferenced on the 1:25.000 topographic maps of the «SIGEC» GIS system of the Ministry of Culture. An accurate survey of Italian and foreign literature regarding archaeology, vertebrate palaeontology, geology and related sciences from Villafranchian to Early Iron Age has been conducted in the most important libraries of Italian Institutions specialized in Quaternary studies and Prehistory. An estimated total of about 25.000 sites, 60.000 references and 800.000 links represents the core of the project, which could be concluded in five years with a team of eight specialists. The paper also provides a preliminary appraisal of the chronological distribution of published sites covering the whole Italian territory, according to which the protohistory (Bronze and First Iron Ages) accounts for half of the estimated body of data
An annotated checklist of fishes of the family Sciaenidae
A checklist of the croakers of the world, family Sciaenidae, is presented. A total of 584 nominal species belonging to 289 valid species and 69 genera is included. Four genera, Johnius with 32 species, Cynoscion 25, Stellifer 24, and Umbrina 17 contains 30% of the species, whereas 43% of the genera (31) are monotypic. Eques is a valid genus-name and should be used instead of Equetus. Fourteen nominal species remain unidentifiable and are placed in incertae sedis, whereas 12 nominal species currently described in sciaenid genera lay outside the family. Among this latter group two of them represent senior synonyms of well-established species: Sciaena guttata Bloch and Schneider, 1801 and Sciaena pallida Walbaum, 1792 predate Giuris margaritaceus (Valenciennes, 1837) and Cymolutes praetextatus (Quoy and Gaimard, 1824), respectively, and they are here declared nomina oblita
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