354 research outputs found
Monte Carlo simulations of a diffusive shock with multiple scattering angular distributions
We independently develop a simulation code following the previous dynamical
Monte Carlo simulation of the diffusive shock acceleration under the isotropic
scattering law during the scattering process, and the same results are
obtained. Since the same results test the validity of the dynamical Monte Carlo
method for simulating a collisionless shock, we extend the simulation toward
including an anisotropic scattering law for further developing this dynamical
Monte Carlo simulation. Under this extended anisotropic scattering law, a
Gaussian distribution function is used to describe the variation of scattering
angles in the particle's local frame. As a result, we obtain a series of
different shock structures and evolutions in terms of the standard deviation
values of the given Gaussian scattering angular distributions. We find that the
total energy spectral index increases as the standard deviation value of the
scattering angular distribution increases, but the subshock's energy spectral
index decreases as the standard deviation value of the scattering angular
distribution increases.Comment: This article include 10 pages, 8 figures, and accepted by Astronomy
and Astrophysic
Short term Variability of the Sun Earth System: An Overview of Progress Made during the CAWSES II Period
This paper presents an overview of results obtained during the CAWSES II
period on the short term variability of the Sun and how it affects the near
Earth space environment. CAWSES II was planned to examine the behavior of the
solar terrestrial system as the solar activity climbed to its maximum phase in
solar cycle 24. After a deep minimum following cycle 23, the Sun climbed to a
very weak maximum in terms of the sunspot number in cycle 24 (MiniMax24), so
many of the results presented here refer to this weak activity in comparison
with cycle 23. The short term variability that has immediate consequence to
Earth and geospace manifests as solar eruptions from closed field regions and
high speed streams from coronal holes. Both electromagnetic (flares) and mass
emissions (coronal mass ejections, CMEs) are involved in solar eruptions, while
coronal holes result in high speed streams that collide with slow wind forming
the so called corotating interaction regions (CIRs). Fast CMEs affect Earth via
leading shocks accelerating energetic particles and creating large geomagnetic
storms. CIRs and their trailing high speed streams (HSSs), on the other hand,
are responsible for recurrent small geomagnetic storms and extended (days) of
auroral zone activity, respectively. The latter lead to the acceleration of
relativistic magnetospheric killer electrons. One of the major consequences of
the weak solar activity is the altered physical state of the heliosphere that
has serious implications for the shock-driving and storm causing properties of
CMEs. Finally, a discussion is presented on extreme space weather events
prompted by the 2012 July 23 super storm event that occurred on the backside of
the Sun. Many of these studies were enabled by the simultaneous availability of
remote-sensing and in situ observations from multiple vantage points with
respect to the Sun Earth line.Comment: 85 pages, 30 figures, 2 tables, Accepted for publication in Progress
in Earth and Planetary Science on April 13, 201
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