26,511 research outputs found
Magnetic-Island Contraction and Particle Acceleration in Simulated Eruptive Solar Flares
The mechanism that accelerates particles to the energies required to produce
the observed high-energy impulsive emission in solar flares is not well
understood. Drake et al. (2006) proposed a mechanism for accelerating electrons
in contracting magnetic islands formed by kinetic reconnection in multi-layered
current sheets. We apply these ideas to sunward-moving flux ropes (2.5D
magnetic islands) formed during fast reconnection in a simulated eruptive
flare. A simple analytic model is used to calculate the energy gain of
particles orbiting the field lines of the contracting magnetic islands in our
ultrahigh-resolution 2.5D numerical simulation. We find that the estimated
energy gains in a single island range up to a factor of five. This is higher
than that found by Drake et al. for islands in the terrestrial magnetosphere
and at the heliopause, due to strong plasma compression that occurs at the
flare current sheet. In order to increase their energy by two orders of
magnitude and plausibly account for the observed high-energy flare emission,
the electrons must visit multiple contracting islands. This mechanism should
produce sporadic emission because island formation is intermittent. Moreover, a
large number of particles could be accelerated in each
magnetohydrodynamic-scale island, which may explain the inferred rates of
energetic-electron production in flares. We conclude that island contraction in
the flare current sheet is a promising candidate for electron acceleration in
solar eruptions.Comment: Accepted for publication in The Astrophysical Journal (2016
The Soft Landing Problem: Minimizing Energy Loss by a Legged Robot Impacting Yielding Terrain
Enabling robots to walk and run on yielding terrain is increasingly vital to
endeavors ranging from disaster response to extraterrestrial exploration. While
dynamic legged locomotion on rigid ground is challenging enough, yielding
terrain presents additional challenges such as permanent ground deformation
which dissipates energy. In this paper, we examine the soft landing problem:
given some impact momentum, bring the robot to rest while minimizing foot
penetration depth. To gain insight into properties of penetration
depth-minimizing control policies, we formulate a constrained optimal control
problem and obtain a bang-bang open-loop force profile. Motivated by examples
from biology and recent advances in legged robotics, we also examine
impedance-control solutions to the dimensionless soft landing problem. Through
simulations, we find that optimal impedance reduces penetration depth nearly as
much as the open-loop force profile, while remaining robust to model
uncertainty. Through simulations and experiments, we find that the solution
space is rich, exhibiting qualitatively different relationships between impact
velocity and the optimal impedance for small and large dimensionless impact
velocities. Lastly, we discuss the relevance of this work to
minimum-cost-of-transport locomotion for several actuator design choices
Deformation Potentials for Excitons in Cuprous Halides
The hydrostatic-pressure shifts of the Z1,2 and Z3 exciton peaks were measured in thin films of cubic CuCl, CuBr, and CuI at 90 K. That of the E1 peak in CuI also was measured. The deformation potentials of all Z excitons and of the E1exciton in CuI, about -1 eV, are more than twice those of the Z excitons in CuCl and CuBr. This suggests the two valence bands in CuI may be considerably more mixed than in CuCl and CuBr
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