3,596 research outputs found
Geomagnetic effects on cosmic ray propagation under different conditions for Buenos Aires and Marambio, Argentina
The geomagnetic field (Bgeo) sets a lower cutoff rigidity (Rc) to the entry
of cosmic particles to Earth which depends on the geomagnetic activity. From
numerical simulations of the trajectory of a proton using different models for
Bgeo (performed with the MAGCOS code), we use backtracking to analyze particles
arriving at the location of two nodes of the net LAGO (Large Aperture Gamma ray
burst Observatory) that will be built in the near future: Buenos Aires and
Marambio (Antarctica), Argentina. We determine the asymptotic trajectories and
the values of Rc for different incidence directions, for each node. Simulations
were done using several models for Bgeo that emulate different geomagnetic
conditions. The presented results will help to make analysis of future
observations of the flux of cosmic rays done at these two LAGO nodes.Comment: 9 page
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&
Coercivity reduction in a two-dimensional array of nanoparticles
We report on theoretical investigation of the magnetization reversal in
two-dimensional arrays of ferromagnetic nano-particles with parameters of
cobalt. The system was optimized for achieving the lowest coercivity in an
array of particles located in the nodes of triangular, hexagonal and square
grids. Based on the numerical solution of the non-stochastic
Landau-Lifshitz-Gilbert equation we show that each particle distribution type
is characterized with a proper optimal distance, allowing to lower the
coercivity values for approximately 30% compared with the reference value
obtained for a single nano-particle. It was shown that the reduction of
coercivity occurs even if the particle position in the array is not very
precise. In particular, the triangular particle arrangement maintained the same
optimal distance between the particles under up to 20% random displacements of
their position within the array.Comment: 7 pages, 5 figure
Disease-induced resource constraints can trigger explosive epidemics
Advances in mathematical epidemiology have led to a better understanding of
the risks posed by epidemic spreading and informed strategies to contain
disease spread. However, a challenge that has been overlooked is that, as a
disease becomes more prevalent, it can limit the availability of the capital
needed to effectively treat those who have fallen ill. Here we use a simple
mathematical model to gain insight into the dynamics of an epidemic when the
recovery of sick individuals depends on the availability of healing resources
that are generated by the healthy population. We find that epidemics spiral out
of control into "explosive" spread if the cost of recovery is above a critical
cost. This can occur even when the disease would die out without the resource
constraint. The onset of explosive epidemics is very sudden, exhibiting a
discontinuous transition under very general assumptions. We find analytical
expressions for the critical cost and the size of the explosive jump in
infection levels in terms of the parameters that characterize the spreading
process. Our model and results apply beyond epidemics to contagion dynamics
that self-induce constraints on recovery, thereby amplifying the spreading
process.Comment: 24 pages, 6 figure
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