5,087 research outputs found
Magnetic fluctuation power near proton temperature anisotropy instability thresholds in the solar wind
The proton temperature anisotropy in the solar wind is known to be
constrained by the theoretical thresholds for pressure anisotropy-driven
instabilities. Here we use approximately 1 million independent measurements of
gyroscale magnetic fluctuations in the solar wind to show for the first time
that these fluctuations are enhanced along the temperature anisotropy
thresholds of the mirror, proton oblique firehose, and ion cyclotron
instabilities. In addition, the measured magnetic compressibility is enhanced
at high plasma beta () along the mirror instability
threshold but small elsewhere, consistent with expectations of the mirror mode.
The power in this frequency (the 'dissipation') range is often considered to be
driven by the solar wind turbulent cascade, an interpretation which should be
qualified in light of the present results. In particular, we show that the
short wavelength magnetic fluctuation power is a strong function of
collisionality, which relaxes the temperature anisotropy away from the
instability conditions and reduces correspondingly the fluctuation power.Comment: 4 pages, 4 figure
On topological phases of spin chains
Symmetry protected topological phases of one-dimensional spin systems have
been classified using group cohomology. In this paper, we revisit this problem
for general spin chains which are invariant under a continuous on-site symmetry
group G. We evaluate the relevant cohomology groups and find that the
topological phases are in one-to-one correspondence with the elements of the
fundamental group of G if G is compact, simple and connected and if no
additional symmetries are imposed. For spin chains with symmetry
PSU(N)=SU(N)/Z_N our analysis implies the existence of N distinct topological
phases. For symmetry groups of orthogonal, symplectic or exceptional type we
find up to four different phases. Our work suggests a natural generalization of
Haldane's conjecture beyond SU(2).Comment: 18 pages, 7 figures, 2 tables. Version v2 corresponds to the
published version. It includes minor revisions, additional references and an
application to cold atom system
New measurements of total ionizing dose in the lunar environment
[1] We report new measurements of solar minimum ionizing radiation dose at the Moon onboard the Lunar Reconnaissance Orbiter (LRO) from June 2009 through May 2010. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on LRO houses a compact and highly precise microdosimeter whose design allows measurements of dose rates below 1 micro-Rad per second in silicon achieved with minimal resources (20 g, ∼250 milliwatts, and ∼3 bits/second). We envision the use of such a small yet accurate dosimeter in many future spaceflight applications where volume, mass, and power are highly constrained. As this was the first operation of the microdosimeter in a space environment, the goal of this study is to verify its response by using simultaneous measurements of the galactic cosmic ray ionizing environment at LRO, at L1, and with other concurrent dosimeter measurements and model predictions. The microdosimeter measured the same short timescale modulations in the galactic cosmic rays as the other independent measurements, thus verifying its response to a known source of minimum-ionizing particles. The total dose for the LRO mission over the first 333 days was only 12.2 Rads behind ∼130 mils of aluminum because of the delayed rise of solar activity in solar cycle 24 and the corresponding lack of intense solar energetic particle events. The dose rate in a 50 km lunar orbit was about 30 percent lower than the interplanetary rate, as one would expect from lunar obstruction of the visible sky
The Luminosity Function at z~8 from 97 Y-band dropouts: Inferences About Reionization
[Abbreviated] We present the largest search to date for Lyman break
galaxies (LBGs) based on 350 arcmin of HST observations in the V-, Y-, J-
and H-bands from the Brightest of Reionizing Galaxies (BoRG) survey. The BoRG
dataset includes 50 arcmin of new data and deeper observations of two
previous BoRG pointings, from which we present 9 new LBG candidates,
bringing the total number of BoRG LBGs to 38 with (AB system). We introduce a new Bayesian formalism for
estimating the galaxy luminosity function (LF), which does not require binning
(and thus smearing) of the data and includes a likelihood based on the formally
correct binomial distribution as opposed to the often used approximate Poisson
distribution. We demonstrate the utility of the new method on a sample of
LBGs that combines the bright BoRG galaxies with the fainter sources published
in Bouwens et al. (2012) from the HUDF and ERS programs. We show that the
LF is well described by a Schechter function with a characteristic
magnitude , a faint-end slope of , and a number density of . Integrated down to this
LF yields a luminosity density, . Our LF analysis
is consistent with previously published determinations within 1. We
discuss the implication of our study for the physics of reionization. By
assuming theoretically motivated priors on the clumping factor and the photon
escape fraction we show that the UV LF from galaxy samples down to
can ionize only 10-50% of the neutral hydrogen at . Full reionization
would require extending the LF down to .Comment: Accepted for publication in ApJ, 22 pages, 15 figure
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