452 research outputs found
Electron and Ion Heating during Magnetic Reconnection in Weakly Collisional Plasmas
Gyrokinetic simulations of magnetic reconnection are presented to investigate
plasma heating for strongly magnetized, weakly collisional plasmas. For a low
plasma beta case, parallel and perpendicular phase mixing strongly enhance
energy dissipation yielding electron heating. Heating occurs for a long time
period after a dynamical process of magnetic reconnection ended. For a higher
beta case, the ratio of ion to electron dissipation rate increases, suggesting
that ion heating (via phase-mixing) may become an important dissipation channel
in high beta plasmas.Comment: 9 pages, 3 figures, accepted for publication in JPSJ Suppl.
[Proceedings of the 12th Asia Pacific Physics Conference
Magnetohydrodynamic turbulence mediated by reconnection
Magnetic field fluctuations in MHD turbulence can be viewed as current sheets
that are progressively more anisotropic at smaller scales. As suggested by
Loureiro & Boldyrev (2017) and Mallet et al (2017), below a certain critical
thickness such current sheets become tearing-unstable. We propose
that the tearing instability changes the effective alignment of the magnetic
field lines in such a way as to balance the eddy turnover rate at all scales
smaller than . As a result, turbulent fluctuations become
progressively less anisotropic at smaller scales, with the alignment angle
increasing as , where
is the resistive dissipation scale. Here
is the outer scale of the turbulence, is the corresponding Lundquist
number, and {} is a parameter. The resulting Fourier energy
spectrum is , where is
the wavenumber normal to the local mean magnetic field, and the critical scale
is . The simplest model
corresponds to , in which case the predicted scaling formally agrees
with one of the solutions obtained in (Mallet et al 2017) from a discrete
hierarchical model of abruptly collapsing current sheets, an approach different
and complementary to ours. We also show that the reconnection-mediated interval
is non-universal with respect to the dissipation mechanism. Hyper-resistivity
of the form leads (in the simplest case of )
to the different transition scale
and the energy spectrum , where
is the corresponding hyper-resistive Lundquist number.Comment: submitted for publicatio
Role of reconnection in inertial kinetic-Alfvén turbulence
In a weakly collisional, low-electron-beta plasma, large-scale Alfvén turbulence transforms into inertial kinetic-Alfvén turbulence at scales smaller than the ion microscale (gyroscale or inertial scale). We propose that at such kinetic scales, the nonlinear dynamics tends to organize turbulent eddies into thin current sheets, consistent with the existence of two conserved integrals of the ideal equations, energy and helicity. The formation of strongly anisotropic structures is arrested by the tearing instability that sets a critical aspect ratio of the eddies at each scale a in the plane perpendicular to the guide field. This aspect ratio is defined by the balance of the eddy turnover rate and the tearing rate, and varies from (d [subscript]e / a)¹ / ² to d [subscript]e / a depending on the assumed profile of the current sheets. The energy spectrum of the resulting turbulence varies from k -⁸ / ³ to k -³ , and the corresponding spectral anisotropy with respect to the strong background magnetic field from [mathematical equation; see source] to [mathematical equation; see source].NSF (Grant no. NSF PHY-1707272)NASA (Grant no. NASA 80NSSC18K0646)DOE (Grant no. DE-SC0018266)NSF CAREER (Award no. 1654168)NSF-DOE Partnership in Basic Plasma Science and Engineering (Award no. DE-SC0016215
Magnetic Reconnection Onset via Disruption of a Forming Current Sheet by the Tearing Instability
The recent realization that Sweet-Parker current sheets are violently
unstable to the secondary tearing (plasmoid) instability implies that such
current sheets cannot occur in real systems. This suggests that, in order to
understand the onset of magnetic reconnection, one needs to consider the growth
of the tearing instability in a current layer as it is being formed. Such an
analysis is performed here in the context of nonlinear resistive MHD for a
generic time-dependent equilibrium representing a gradually forming current
sheet. It is shown that two onset regimes, single-island and multi-island, are
possible, depending on the rate of current sheet formation. A simple model is
used to compute the criterion for transition between these two regimes, as well
as the reconnection onset time and the current sheet parameters at that moment.
For typical solar corona parameters this model yields results consistent with
observations.Comment: 5 pages, no figures; accepted for publication in Physical Review
Letter
Operant EEG-based BMI: Learning and consolidating device control with brain activity
"Whether you are reading this thesis on paper or screen, it is easy to take
for granted all the highly specialized movements you are doing at this very
moment just to go through each page. Just to turn a page, you have to
reach for and grasp it, turn it and let go at the precise moment not to rip it.(...)
Visualyzart Project – The role in education
The VisualYzARt project intends to develop research on mobile platforms, web and social scenarios in order to bring augmented reality and natural interaction for the general public, aiming to study and validate the adequacy of YVision platform in various fields of activity such as digital arts, design, education, culture and leisure. The VisualYzARt project members analysed the components available in YVision platform and are defining new ones that allow the creation of applications to a chosen activity, effectively adding a new language to the domain YVision. In this paper we will present the role of the InstitutoPolitécnico de Santarém which falls into the field of education.VisualYzART is funded by QREN – Sistema de Incentivos à Investigação e Desenvolvimento Tecnológico (SI
I&DT), Project n. º 23201 - VisualYzARt (from January 2013 to December 2014). Partners: YDreams Portugal;
Instituto Politécnico de Santarém - Gabinete de e-Learning; Universidade de Coimbra - Centro de Informática e Sistemas; Instituto Politécnico de Leiria - Centro de Investigação em Informática e Comunicações; Universidade Católica do Porto - Centro de Investigação em Ciência e Tecnologia das Artes.info:eu-repo/semantics/publishedVersio
Photography, Land-Cover and Land-Use changes, and tourism urbanization: A narrative focused on Hotel do Garbe, Armação de Pêra, Algarve, Portugal
This article is focused on the use of photography to characterize land-cover and land-use
changes in a 7.59 km2
study area centered on Hotel do Garbe, in the village of Armação de Pêra,
Algarve, Portugal. Orthorectified vertical aerial, oblique aerial and ground-level photographs were
the main data sources required to carry out the analysis. In a preliminary approach, a conventional
research design was adopted. Based on the available orthorectified vertical aerial photographs, a
sixty-year time series, with four homogeneously distributed steps (1958, 1978, 1997 and 2018), was
constructed, and maps were produced to support the description of the changes that have taken
place. To deepen the analysis, photographs from fourteen picture postcards were recognized as a
useful source of information, and the authors of these photographs were considered “involuntary
or accidental photo-geographers” whose work was relevant to feed a case study in which human
geography and landscape biography sciences are the main narrative axes. The final result proved to
be richer than the interpretation only based on the orthorectified vertical aerial photographs, and the
importance of combining photographs taken from different points of view, with different aims and
for different recipients is highlighted.info:eu-repo/semantics/publishedVersio
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