80 research outputs found
Critical current anisotropy in Nd-1111 single crystals and the infuence of neutron irradiation
We report on angle-resolved magnetization measurements on
NdFeAsOF (Nd-1111) single crystals. The field dependence of
the critical current density, , is non-monotonous in these crystals at all
orientations and temperatures due to the fishtail effect, which strongly
influences the angular dependence of . The currents decrease as the field
is tilted from the crystallographic c-axis at low fields, but increase at high
fields. A peak occurs in the angular dependence of at intermediate
fields. The critical currents are significantly enhanced after irradiation with
fast neutrons and the fishtail disappears. The different current anisotropies
at low and high fields, however, persist. We discuss the data in the framework
of the anisotropic scaling approach and propose a transition from dominant
pinning by large defects of low density at low fields to pinning by small
defects of high density at high fields in the pristine crystal. Strong pinning
dominates at all fields after the irradiation, and the angular dependence of
can be described by anisotropic scaling only after an appropriate
extension to this pinning regime
Ground state correlations and structure of odd spherical nuclei
It is well known that the Pauli principle plays a substantial role at low
energies because the phonon operators are not ideal boson operators.
Calculating the exact commutators between the quasiparticle and phonon
operators one can take into account the Pauli principle corrections. Besides
the ground state correlations due to the quasiparticle interaction in the
ground state influence the single particle fragmentation as well. In this
paper, we generalize the basic QPM equations to account for both mentioned
effects. As an illustration of our approach, calculations on the structure of
the low-lying states in Ba have been performed.Comment: 12 pages, 1 figur
Interplanetary Protons versus Interacting Protons in the 2017 September 10 Solar Eruptive Event
We analyze the relativistic proton emission from the Sun during the eruptive event on 2017 September 10, which
caused a ground-level enhancement (GLE 72) registered by the worldwide network of neutron monitors. Using the
neutron monitor data and interplanetary transport modeling both along and across interplanetary magnetic field
(IMF) lines, we deduce parameters of the proton injection into the interplanetary medium. The inferred injection
profile of the interplanetary protons is compared with the profile of the >100 MeV Îł-ray emission observed by the
Fermi Large Area Telescope, attributed to pion production from the interaction of >300 MeV protons at the Sun.
GLE 72 started with a prompt component that arrived along the IMF lines. This was followed by a more prolonged
enhancement caused by protons arriving at the Earth across the IMF lines from the southwest. The interplanetary
proton event is modeled using two sources—one source at the root of the Earth-connected IMF line and another
source situated near the solar western limb. The maximum phase of the second injection of interplanetary protons
coincides with the maximum phase of the prolonged >100 MeV Îł-ray emission that originated from a small area at
the solar western limb, below the current sheet trailing the associated coronal mass ejection (CME). A possible
common source of interacting protons and interplanetary protons is discussed in terms of proton acceleration at the
CME bow shock versus coronal (re-)acceleration in the wake of the CME
Multi-Dimensional, Compressible Viscous Flow on a Moving Voronoi Mesh
Numerous formulations of finite volume schemes for the Euler and
Navier-Stokes equations exist, but in the majority of cases they have been
developed for structured and stationary meshes. In many applications, more
flexible mesh geometries that can dynamically adjust to the problem at hand and
move with the flow in a (quasi) Lagrangian fashion would, however, be highly
desirable, as this can allow a significant reduction of advection errors and an
accurate realization of curved and moving boundary conditions. Here we describe
a novel formulation of viscous continuum hydrodynamics that solves the
equations of motion on a Voronoi mesh created by a set of mesh-generating
points. The points can move in an arbitrary manner, but the most natural motion
is that given by the fluid velocity itself, such that the mesh dynamically
adjusts to the flow. Owing to the mathematical properties of the Voronoi
tessellation, pathological mesh-twisting effects are avoided. Our
implementation considers the full Navier-Stokes equations and has been realized
in the AREPO code both in 2D and 3D. We propose a new approach to compute
accurate viscous fluxes for a dynamic Voronoi mesh, and use this to formulate a
finite volume solver of the Navier-Stokes equations. Through a number of test
problems, including circular Couette flow and flow past a cylindrical obstacle,
we show that our new scheme combines good accuracy with geometric flexibility,
and hence promises to be competitive with other highly refined Eulerian
methods. This will in particular allow astrophysical applications of the AREPO
code where physical viscosity is important, such as in the hot plasma in galaxy
clusters, or for viscous accretion disk models.Comment: 26 pages, 21 figures. Submitted to MNRA
Investigating the origins of two extreme solar particle events: proton source profile and associated electromagnetic emissions
We analyze the high-energy particle emission from the Sun in two extreme solar particle events, in which protons are accelerated to relativistic energies and can cause a significant signal even in the ground-based particle detectors. Analysis of a relativistic proton event is based on modeling of the particle transport and interaction, from a near-Sun source through the solar wind and the Earth’s magnetosphere and atmosphere to a detector on the ground. This allows us to deduce the time profile of the proton source at the Sun and compare it with observed electromagnetic emissions. The 2 May 1998 event is associated with flare and coronal mass ejection (CME) well observed by the Nan¸cay Radioheliograph, so that the images of radio sources are available. For the 2 November 2003 event, there are available the low-corona images of the CME liftoff obtained at the Mauna Loa Solar Observatory. Those complementary data sets are analyzed jointly with the broadband dynamic radio spectra, EUV images and other data available for both events. We find a common scenario for both eruptions, including the flare’s dual impulsive phase, the CME-launch-associated decimetric-continuum burst, and the late, low-frequency type III radio bursts at the time of the relativistic proton injection into the interplanetary medium. The analysis supports the idea that the two considered events start with emission of relativistic protons previously accelerated during the flare and CME launch, then trapped in large-scale magnetic loops and later released by the expanding CME
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