19,724 research outputs found
Flux emergence in a magnetized convection zone
We study the influence of a dynamo magnetic field on the buoyant rise and
emergence of twisted magnetic flux-ropes, and their influence on the global
external magnetic field. We ran 3D MHD numerical simulations using the ASH code
and analysed the dynamical evolution of such buoyant flux-ropes from the bottom
of the convection zone until the post-emergence phases. The global nature of
this model represents very crudely and inaccurately the local dynamics of the
buoyant rise, but allows to study the influence of global effects such as
self-consistently generated differential rotation, meridional circulation and
Coriolis forces. Although motivated by the solar context, this model cannot be
thought of as a realistic model of the rise of magnetic structures and their
emergence in the Sun where the local dynamics are completely different. The
properties of initial phases of the buoyant rise in good agreement with
previous studies. However, the effects of the interaction of the background
dynamo field become increase as the flux-ropes evolve. During the buoyant rise
across the CZ, the flux-rope's magnetic field strength and scales as ,
with . An increase of velocity, density and current precedes flux
emergence at all longitudes. The geometry, latitude and relative orientation of
the flux-ropes with respect to the background magnetic field influences the
rise speeds, zonal flow amplitudes (which develop within the flux-ropes) and
the corresponding surface signatures. This influences the morphology, duration
and amplitude of the associated surface shearing and Poynting flux. The emerged
flux influences the system's global polarity, leading in some cases to a
polarity reversal while inhibiting background dynamo from doing so in some
others. The emerged magnetic flux is slowly advected poleward, while being
diffused and assimilated by the background dynamo field.Comment: Accepted for publication in Ap
Coupling the solar surface and the corona: coronal rotation, Alfv\'en wave-driven polar plumes
The dynamical response of the solar corona to surface and sub-surface
perturbations depends on the chromospheric stratification, and specifically on
how efficiently these layers reflect or transmit incoming Alfv\'en waves. While
it would be desirable to include the chromospheric layers in the numerical
simulations used to study such phenomena, that is most often not feasible. We
defined and tested a simple approximation allowing the study of coronal
phenomena while taking into account a parametrised chromospheric reflectivity.
We addressed the problems of the transmission of the surface rotation to the
corona and that of the generation of polar plumes by Alfv\'en waves (Pinto et
al., 2010, 2011). We found that a high (yet partial) effective chromospheric
reflectivity is required to properly describe the angular momentum balance in
the corona and the way the surface differential rotation is transmitted
upwards. Alfv\'en wave-driven polar plumes maintain their properties for a wide
range of values for the reflectivity, but they become bursty (and eventually
disrupt) when the limit of total reflection is attained.Comment: Solar Wind 13: Proceedings of the Thirteenth International Solar Wind
Conferenc
Multi-Step Knowledge-Aided Iterative ESPRIT for Direction Finding
In this work, we propose a subspace-based algorithm for DOA estimation which
iteratively reduces the disturbance factors of the estimated data covariance
matrix and incorporates prior knowledge which is gradually obtained on line. An
analysis of the MSE of the reshaped data covariance matrix is carried out along
with comparisons between computational complexities of the proposed and
existing algorithms. Simulations focusing on closely-spaced sources, where they
are uncorrelated and correlated, illustrate the improvements achieved.Comment: 7 figures. arXiv admin note: text overlap with arXiv:1703.1052
Soft X-ray emission in kink-unstable coronal loops
Solar flares are associated with intense soft X-ray emission generated by the
hot flaring plasma. Kink unstable twisted flux-ropes provide a source of
magnetic energy which can be released impulsively and account for the flare
plasma heating. We compute the temporal evolution of the thermal X-ray emission
in kink-unstable coronal loops using MHD simulations and discuss the results of
with respect to solar flare observations. The model consists of a highly
twisted loop embedded in a region of uniform and untwisted coronal magnetic
field. We let the kink instability develop, compute the evolution of the plasma
properties in the loop (density, temperature) without accounting for mass
exchange with the chromosphere. We then deduce the X-ray emission properties of
the plasma during the whole flaring episode. During the initial phase of the
instability plasma heating is mostly adiabatic. Ohmic diffusion takes over as
the instability saturates, leading to strong and impulsive heating (> 20 MK),
to a quick enhancement of X-ray emission and to the hardening of the thermal
X-ray spectrum. The temperature distribution of the plasma becomes broad, with
the emission measure depending strongly on temperature. Significant emission
measures arise for plasma at temperatures T > 9 MK. The magnetic flux-rope then
relaxes progressively towards a lower energy state as it reconnects with the
background flux. The loop plasma suffers smaller sporadic heating events but
cools down conductively. The total thermal X-ray emission slowly fades away
during this phase, and the high temperature component of emission measure
distribution converges to the power-law distribution . The
amount of twist deduced directly from the X-ray emission patterns is
considerably lower than the maximum magnetic twist in the simulated flux-ropes.Comment: submitted to A&
The Redner - Ben-Avraham - Kahng cluster system
We consider a coagulation model first introduced by Redner, Ben-Avraham and
Krapivsky in [Redner, Ben-Avraham, Kahng: Kinetics of 'cluster eating', J.
Phys. A: Math. Gen., 20 (1987), 1231-1238], the main feature of which is that
the reaction between a j-cluster and a k-cluster results in the creation of a
|j-k|-cluster, and not, as in Smoluchowski's model, of a (j+k)-cluster. In this
paper we prove existence and uniqueness of solutions under reasonably general
conditions on the coagulation coefficients, and we also establish
differenciability properties and continuous dependence of solutions. Some
interesting invariance properties are also proved. Finally, we study the
long-time behaviour of solutions, and also present a preliminary analysis of
their scaling behaviour.Comment: 24 pages. 2 figures. Dedicated to Carlos Rocha and Luis Magalhaes on
the occasion of their sixtieth birthday
Flux-tube geometry and solar wind speed during an activity cycle
The solar wind speed at 1 AU shows variations in latitude and in time which
reflect the evolution of the global background magnetic field during the
activity cycle. It is commonly accepted that the terminal wind speed in a
magnetic flux-tube is anti-correlated with its expansion ratio, which motivated
the definition of widely-used semi-empirical scaling laws relating one to the
other. In practice, such scaling laws require ad-hoc corrections. A predictive
law based solely on physical principles is still missing. We test whether the
flux-tube expansion is the controlling factor of the wind speed at all phases
of the cycle and at all latitudes using a very large sample of wind-carrying
open magnetic flux-tubes. We furthermore search for additional physical
parameters based on the geometry of the coronal magnetic field which have an
influence on the terminal wind flow speed. We use MHD simulations of the corona
and wind coupled to a dynamo model to provide a large statistical ensemble of
open flux-tubes which we analyse conjointly in order to identify relations of
dependence between the wind speed and geometrical parameters of the flux-tubes
which are valid globally (for all latitudes and moments of the cycle). Our
study confirms that the terminal speed of the solar wind depends very strongly
on the geometry of the open magnetic flux-tubes through which it flows. The
total flux-tube expansion is more clearly anti-correlated with the wind speed
for fast rather than for slow wind flows, and effectively controls the
locations of these flows during solar minima. Overall, the actual asymptotic
wind speeds attained are also strongly dependent on field-line inclination and
magnetic field amplitude at the foot-points. We suggest ways of including these
parameters on future predictive scaling-laws for the solar wind speed.Comment: Accepted for publicaton on Astronomy & Astrophysic
The Redner - Ben-Avraham - Kahng coagulation system with constant coefficients: the finite dimensional case
We study the behaviour as of solutions to the
Redner--Ben-Avraham--Kahng coagulation system with positive and compactly
supported initial data, rigorously proving and slightly extending results
originally established in [4] by means of formal arguments.Comment: 13 pages, 1 figur
Towards gauge theories in four dimensions
The abundance of infrared singularities in gauge theories due to unresolved
emission of massless particles (soft and collinear) represents the main
difficulty in perturbative calculations. They are typically regularized in
dimensional regularization, and their subtraction is usually achieved
independently for virtual and real corrections. In this paper, we introduce a
new method based on the loop-tree duality (LTD) theorem to accomplish the
summation over degenerate infrared states directly at the integrand level such
that the cancellation of the infrared divergences is achieved simultaneously,
and apply it to reference examples as a proof of concept. Ultraviolet
divergences, which are the consequence of the point-like nature of the theory,
are also reinterpreted physically in this framework. The proposed method opens
the intriguing possibility of carrying out purely four-dimensional
implementations of higher-order perturbative calculations at next-to-leading
order (NLO) and beyond free of soft and final-state collinear subtractions.Comment: Final version to appear in JHE
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