277 research outputs found
Low-altitude measurements of 2ā6 MeV electron trapping lifetimes at 1.5 ā¤ L ā¤ 2.5
During the Halloween Storm period (OctoberāNovember 2003), a new Van Allen belt electron population was powerfully accelerated. The inner belt of electrons formed in this process decayed over a period of days to years. We have examined quantitatively the decay rates for electrons seen in the region of 1.5 ā¤ L ā¤ 2.5 using SAMPEX satellite observations. At L = 1.5 the e-folding lifetime for 2ā6 MeV electrons was Ļ ā¼ 180 days. On the other hand, for the half-dozen distinct acceleration (or enhancement) events seen during late-2003 through 2005 at L ā¼ 2.0, the lifetimes ranged from Ļ ā¼ 8 days to Ļ ā¼ 35 days. We compare these loss rates to those expected from prior studies. We find that lifetimes at L = 2.0 are much shorter than the average 100ā200 days that present theoretical estimates would suggest for the overall L = 2 electron population. Additional wave-particle interaction aspects must be included in theoretical treatments and we describe such possibilities here
Controlled exploration of chemical space by machine learning of coarse-grained representations
The size of chemical compound space is too large to be probed exhaustively.
This leads high-throughput protocols to drastically subsample and results in
sparse and non-uniform datasets. Rather than arbitrarily selecting compounds,
we systematically explore chemical space according to the target property of
interest. We first perform importance sampling by introducing a Markov chain
Monte Carlo scheme across compounds. We then train an ML model on the sampled
data to expand the region of chemical space probed. Our boosting procedure
enhances the number of compounds by a factor 2 to 10, enabled by the ML model's
coarse-grained representation, which both simplifies the structure-property
relationship and reduces the size of chemical space. The ML model correctly
recovers linear relationships between transfer free energies. These linear
relationships correspond to features that are global to the dataset, marking
the region of chemical space up to which predictions are reliable---a more
robust alternative to the predictive variance. Bridging coarse-grained
simulations with ML gives rise to an unprecedented database of drug-membrane
insertion free energies for 1.3 million compounds.Comment: 9 pages, 5 figure
In silico screening of drug-membrane thermodynamics reveals linear relations between bulk partitioning and the potential of mean force
The partitioning of small molecules in cell membranes---a key parameter for
pharmaceutical applications---typically relies on experimentally-available bulk
partitioning coefficients. Computer simulations provide a structural resolution
of the insertion thermodynamics via the potential of mean force, but require
significant sampling at the atomistic level. Here, we introduce high-throughput
coarse-grained molecular dynamics simulations to screen thermodynamic
properties. This application of physics based models in a large-scale study of
small molecules establishes linear relationships between partitioning
coefficients and key features of the potential of mean force. This allows us to
predict the structure of the insertion from bulk experimental measurements for
more than 400,000 compounds. The potential of mean force hereby becomes an
easily accessible quantity---already recognized for its high predictability of
certain properties, e.g., passive permeation. Further, we demonstrate how
coarse graining helps reduce the size of chemical space, enabling a
hierarchical approach to screening small molecules.Comment: 8 pages, 6 figures. Typos fixed, minor correction
CeREs: The Compact Radiation Belt Explorer
The Compact Radiation belt Explorer, CeREs, a 3U CubeSat, is expected to be launched July 2018. The primary science goal of CeREs will be to study the physics of the acceleration and loss of radiation belt electrons, in particular electron microbursts, an important process that contributes to loss of electrons. A secondary science objective of CeREs is to characterize solar energetic particles (SEP), specifically electrons and protons, accessing the near Earth environment via the open field lines over the poles. Solar electron observations will advance our understanding of electron acceleration mechanisms in solar flares and their transport in interplanetary and solar regions. The CeREs CubeSat will be in a low earth, high inclination orbit with a year-long prime-science phase. CeREs measurements complement and extend the science goals of the Van Allen Probes, a NASA flagship mission in a near-equatorial orbit. The MERiT instrument aboard CeREs will detect electrons (protons) at energy levels from ~5(100) keV to ~10 (100) MeV using a stack of 8 silicon solid-state detectors (SSDs) and four avalanche photo diodes (APDs), which are provided by Southwest Research Institute (SwRI), the co-I institute. The front-end electronics use an innovative Energy-4 ASIC developed at Goddard Space Flight Center. The onboard CHREC Space Processor card is multi-institutional effort funded by the NSF Center for High-Performance Reconfigurable Computing (CHREC). This paper will describe the CeREs spacecraft and its mission in detail and highlight advancements made in the development of the MERiT instrument and supporting hardware
Broad chemical transferability in structure-based coarse-graining
Compared to top-down coarse-grained (CG) models, bottom-up approaches are
capable of offering higher structural fidelity. This fidelity results from the
tight link to a higher-resolution reference, making the CG model chemically
specific. Unfortunately, chemical specificity can be at odds with
compound-screening strategies, which call for transferable parametrizations.
Here we present an approach to reconcile bottom-up, structure-preserving CG
models with chemical transferability. We consider the bottom-up CG
parametrization of 3,441 CO small-molecule isomers. Our approach
combines atomic representations, unsupervised learning, and a large-scale
extended-ensemble force-matching parametrization. We first identify a subset of
19 representative molecules, which maximally encode the local environment of
all gas-phase conformers. Reference interactions between the 19 representative
molecules were obtained from both homogeneous bulk liquids and various binary
mixtures. An extended-ensemble parametrization over all 703 state points leads
to a CG model that is both structure-based and chemically transferable.
Remarkably, the resulting force field is on average more structurally accurate
than single-state-point equivalents. Averaging over the extended ensemble acts
as a mean-force regularizer, smoothing out both force and structural
correlations that are overly specific to a single state point. Our approach
aims at transferability through a set of CG bead types that can be used to
easily construct new molecules, while retaining the benefits of a
structure-based parametrization.Comment: 15 pages, 7 figure
James van Allen and his namesake NASA mission
Abstract
In many ways, James A. Van Allen defined and āinventedā modern space research. His example showed the way for government-university partners to pursue basic research that also served important national and international goals. He was a tireless advocate for space exploration and for the role of space science in the spectrum of national priorities
The Radiation Belt Storm Probes Mission: Advancing Our Understanding of the Earth's Radiation Belts
We describe NASA's Radiation Belt Storm Probe (RBSP) mission, whose primary science objective is to understand, ideally to the point of predictability, the dynamics of relativistic electrons and penetrating ions in the Earth's radiation belts resulting from variable solar activity. The overarching scientific questions addressed include: 1. the physical processes that produce radiation belt enhancement events, 2. the dominant mechanisms for relativistic electron loss, and 3. how the ring current and other geomagnetic processes affect radiation belt behavior. The RBSP mission comprises two spacecraft which will be launched during Fall 2012 into low inclination lapping equatorial orbits. The orbit periods are about 9 hours, with perigee altitudes and apogee radial distances of 600 km and 5.8 RE respectively. During the two-year primary mission, the spacecraft orbits precess once around the Earth and lap each other twice in each local time quadrant. The spacecraft are each equipped with identical comprehensive instrumentation packages to measure, electrons, ions and wave electric and magnetic fields. We provide an overview of the RBSP mission, onboard instrumentation and science prospects and invite scientific collaboration
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