827 research outputs found
Does the spacecraft trajectory strongly affect the detection of magnetic clouds?
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections
(ICMEs) where a magnetic flux rope is detected. Is the difference between MCs
and ICMEs without detected flux rope intrinsic or rather due to an
observational bias? As the spacecraft has no relationship with the MC
trajectory, the frequency distribution of MCs versus the spacecraft distance to
the MCs axis is expected to be approximately flat. However, Lepping and Wu
(2010) confirmed that it is a strongly decreasing function of the estimated
impact parameter. Is a flux rope more frequently undetected for larger impact
parameter? In order to answer the questions above, we explore the parameter
space of flux rope models, especially the aspect ratio, boundary shape, and
current distribution. The proposed models are analyzed as MCs by fitting a
circular linear force-free field to the magnetic field computed along simulated
crossings.
We find that the distribution of the twist within the flux rope, the
non-detection due to too low field rotation angle or magnitude are only weakly
affecting the expected frequency distribution of MCs versus impact parameter.
However, the estimated impact parameter is increasingly biased to lower values
as the flux-rope cross section is more elongated orthogonally to the crossing
trajectory. The observed distribution of MCs is a natural consequence of a
flux-rope cross section flattened in average by a factor 2 to 3 depending on
the magnetic twist profile. However, the faster MCs at 1 AU, with V>550 km/s,
present an almost uniform distribution of MCs vs. impact parameter, which is
consistent with round shaped flux ropes, in contrast with the slower ones. We
conclude that either most of the non-MC ICMEs are encountered outside their
flux rope or near the leg region, or they do not contain any
Superposed epoch study of ICME sub-structures near Earth and their effects on galactic cosmic rays
Interplanetary coronal mass ejections (ICMEs) are the interplanetary
manifestations of solar eruptions. The overtaken solar wind forms a sheath of
compressed plasma at the front of ICMEs. Magnetic clouds (MCs) are a subset of
ICMEs with specific properties (e.g. the presence of a flux rope). When ICMEs
pass near Earth, ground observations indicate that the flux of galactic cosmic
rays (GCRs) decreases. The main aims of this paper are to find: common plasma
and magnetic properties of different ICME sub-structures, and which ICME
properties affect the flux of GCRs near Earth. We use a superposed epoch method
applied to a large set of ICMEs observed \insitu\ by the spacecraft ACE,
between 1998 and 2006. We also apply a superposed epoch analysis on GCRs time
series observed with the McMurdo neutron monitors. We find that slow MCs at 1
AU have on average more massive sheaths. We conclude that it is because they
are more effectively slowed down by drag during their travel from the Sun. Slow
MCs also have a more symmetric magnetic field and sheaths expanding similarly
as their following MC, while in contrast, fast MCs have an asymmetric magnetic
profile and a compressing sheath in compression. In all types of MCs, we find
that the proton density and the temperature, as well as the magnetic
fluctuations can diffuse within the front of the MC due to 3D reconnection.
Finally, we derive a quantitative model which describes the decrease of cosmic
rays as a function of the amount of magnetic fluctuations and field strength.
The obtained typical profiles of sheath/MC/GCR properties corresponding to
slow, mid, and fast ICMEs, can be used for forecasting/modelling these events,
and to better understand the transport of energetic particles in ICMEs. They
are also useful for improving future operative space weather activities.Comment: 13 pages, 6 figures, paper accepted in A&
Variation of practice and poor outcomes for extremely low gestation births: ordained before birth?
Editorial\uc9ditorialPeer reviewed: YesNRC publication: Ye
Etat Des Lieux De La Production De Sculptures Sur Bois Dans L’Atlantique Et Du Littoral Au Sud Du Benin
La sculpture sur bois est une activité artisanale qui se pratique dans certaines localités des départements de l’Atlantique et du Littoral au sud du Bénin. Son exercice, bien que pénible, continue de mobiliser quelques rares artisans passionnés. Malgré que cette activité soit progressivement délaissée, ses productions sont cependant d’un attrait pour les touristes. Pour la sauvegarde de cette activité, cette étude a visé à réaliser un état des lieux de sa pratique dans ces deux départements.L’approche méthodologique adoptée est axée sur la recherche documentaire, la collecte des données, leur traitement et l’analyse des résultats. La méthode de la boule de neige a permis d’enquêter 19 sculpteurs du bois de la zone d’étude. Les logiciels Excel, Sphinx Plus2 ont aidé au traitement des données.Les résultats obtenus de l’étude montrent que la sculpture sur bois continue à être pratiquée dans les départements de l’Atlantique et du Littoral. L’activité est favorisée par plusieurs facteurs. Il existe plusieurs espèces ligneuses permettant de disposer de bois variés. Ces départements disposent aussi d’une ressource humaine de qualité et d’expérience dans la sculpture du bois. Au plan technique et organisationnel, la mise en place par l’Etat du Fonds de Développement de l’Artisanat et l’institution du Certificat de Qualification aux Métiers (CQM) pour les artisans des différents corps de métiers en fin de formation au Bénin sont aussi d’un atout capital
La formation de Vitiacua (Permien moyen à supérieur-Trias ? inférieur, Bolivie du Sud) : stratigraphie, palynologie et paléontologie
Student perspectives on the relationship between a curve and its tangent in the transition from Euclidean Geometry to Analysis
The tangent line is a central concept in many mathematics and science courses. In this paper we describe a model of students’ thinking – concept images as well as ability in symbolic manipulation – about the tangent line of a curve as it has developed through students’ experiences in Euclidean Geometry and Analysis courses. Data was collected through a questionnaire administered to 196 Year 12 students. Through Latent Class Analysis, the participants were classified in three hierarchical groups representing the transition from a Geometrical Global perspective on the tangent line to an Analytical Local perspective. In the light of this classification, and through qualitative explanations of the students’ responses, we describe students’ thinking about tangents in terms of seven factors. We confirm the model constituted by these seven factors through Confirmatory Factor Analysis
Evolution of the magnetic field distribution of active regions
AIMS: Although the temporal evolution of active regions (ARs) is relatively well understood, the processes involved continue to be the subject of investigation. We study how the magnetic field of a series of ARs evolves with time to better characterise how ARs emerge and disperse. METHODS: We examined the temporal variation in the magnetic field distribution of 37 emerging ARs. A kernel density estimation plot of the field distribution was created on a log-log scale for each AR at each time step. We found that the central portion of the distribution is typically linear, and its slope was used to characterise the evolution of the magnetic field. RESULTS: The slopes were seen to evolve with time, becoming less steep as the fragmented emerging flux coalesces. The slopes reached a maximum value of ∼-1.5 just before the time of maximum flux before becoming steeper during the decay phase towards the quiet-Sun value of ∼-3. This behaviour differs significantly from a classical diffusion model, which produces a slope of -1. These results suggest that simple classical diffusion is not responsible for the observed changes in field distribution, but that other processes play a significant role in flux dispersion. CONCLUSIONS. We propose that the steep negative slope seen during the late-decay phase is due to magnetic flux reprocessing by (super)granular convective cells
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