6,793 research outputs found
Near-Earth solar wind forecasting using corotation from L5: the error introduced by heliographic latitude offset
Routine in‐situ solar wind observations from L5, located 60° behind Earth in its orbit, would provide a valuable input to space‐weather forecasting. One way to ulitise such observations is to assume that the solar wind is in perfect steady state over the 4.5 days it takes the Sun to rotate 60° and thus near‐Earth solar wind in 4.5‐days time would be identical to that at L5 today. This corotation approximation is most valid at solar minimum when the solar wind is slowly evolving. Using STEREO data, it has been possible to test L5‐corotation forecasting for a few months at solar minimum, but the various contributions to forecast error cannot be disentangled. This study uses 40+ years of magnetogram‐constrained solar wind simulations to isolate the effect of latitudinal offset between L5 and Earth due to the inclination of the ecliptic plane to the solar rotational equator. Latitudinal offset error is found to be largest at solar minimum, due to the latitudinal ordering of solar wind structure. It is also a strong function of time of year; maximum at the solstices and very low at equinoxes. At solstice, the latitudinal offset alone means L5‐corotation forecasting is expected to be less accurate than numerical solar wind models, even before accounting for time‐dependent solar wind structures. Thus, a combination of L5‐corotation and numerical solar wind modelling may provide the best forecast. These results also highlight that three‐dimensional solar wind structure must be accounted for when performing solar wind data assimilation
Early X-ray/UV Line Signatures of GRB Progenitors and Hypernovae
We calculate the X-ray/UV spectral line signatures expected from the
interaction of a gamma-ray burst afterglow and a dense pre-burst environment
produced by the progenitor. We explore the conditions under which Fe line and
edge equivalent widths of 1 keV can arise, and discuss the possibility
of gaining information about possible progenitor scenarios using X-ray metal
line spectra in the first few days of a burst. A wind or supernova shell around
the burst produces an X-ray absorption line spectrum and later emission lines,
while a hypernova funnel model produces mainly emission lines. The Fe \ked can
in some cases be more prominent than the Fe \kal line. Under simple assumptions
for the input continuum luminosity, current reports of observed Fe line
luminosities are compatible with an Fe-enriched funnel model, while lower
values are expected in shell models.Comment: revisions to ApJ ms first submitted 8/21/99; uses a higher and
flatter input spectrum, with modified implications suggesting preference for
funnel model
Anomalous prompt photon production in hadronic collisions at low-
We investigate the discrepancy that exists at low- between
the next--to--leading order QCD calculations of prompt photon production and
the measured cross section. The central values of the measured cross section
are of order 100\% larger than QCD predictions in this region. It has been
suggested that the bremsstrahlung contribution may account for this
discrepancy. The quark fragmentation function has not been
measured and an exactly known asymptotic form is normally used in calculations.
We examine the effect of much larger fragmentation functions on the QCD
predictions. After illustrating the effect of the large fragmentation functions
in some detail for recent CDF data at =1.8~TeV, we perform a
fit to 8 prompt photon data sets ranging in CMS energy from 24~GeV to 1.8~TeV.
While a large fragmentation function normalization may prove to play an
important role in resolving the discrepancy, the present theoretical and
experimental uncertainties prevent any definite normalization value from being
determined.Comment: 14 pages, LBL-33122 and UCB-PTH-92/38. 13 figures available by email,
specify postscript or topdrawe
New parton distributions from large-x and low-Q^2 data
We report results of a new global next-to-leading order fit of parton
distribution functions in which cuts on W and Q are relaxed, thereby including
more data at high values of x. Effects of target mass corrections (TMCs),
higher twist contributions, and nuclear corrections for deuterium data are
significant in the large-x region. The leading twist parton distributions are
found to be stable to TMC model variations as long as higher twist
contributions are also included. The behavior of the d quark as x-->1 is
particularly sensitive to the deuterium corrections, and using realistic
nuclear smearing models the d-quark distribution at large x is found to be
softer than in previous fits performed with more restrictive cuts.Comment: 31 pages, 8 figures. Minor corrections. References added. To appear
in Phys.Rev.
Temperature-dependent proximity magnetism in Pt
We experimentally demonstrate the existence of magnetic coupling between two
ferromagnets separated by a thin Pt layer. The coupling remains ferromagnetic
regardless of the Pt thickness, and exhibits a significant dependence on
temperature. Therefore, it cannot be explained by the established mechanisms of
magnetic coupling across nonmagnetic spacers. We show that the experimental
results are consistent with the presence of magnetism induced in Pt in
proximity to ferromagnets, in direct analogy to the well-known proximity
effects in superconductivity.Comment: 4 pages, 3 figure
Lambda Polarization in Polarized Proton-Proton Collisions at RHIC
We discuss Lambda polarization in semi-inclusive proton-proton collisions,
with one of the protons longitudinally polarized. The hyperfine interaction
responsible for the - and - mass splittings gives
rise to flavor asymmetric fragmentation functions and to sizable polarized
non-strange fragmentation functions. We predict large positive Lambda
polarization in polarized proton-proton collisions at large rapidities of the
produced Lambda, while other models, based on SU(3) flavor symmetric
fragmentation functions, predict zero or negative Lambda polarization. The
effect of and decays is also discussed. Forthcoming
experiments at RHIC will be able to differentiate between these predictions.Comment: 18 pages, 5 figure
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Global solar wind variations over the last four centuries
The most recent “grand minimum” of solar activity, the Maunder minimum (MM, 1650–1710), is of great interest both for understanding the solar dynamo and providing insight into possible future heliospheric conditions. Here, we use nearly 30 years of output from a data-constrained magnetohydrodynamic model of the solar corona to calibrate heliospheric reconstructions based solely on sunspot observations. Using these empirical relations, we produce the first quantitative estimate of global solar wind variations over the last 400 years. Relative to the modern era, the MM shows a factor 2 reduction in near-Earth heliospheric magnetic field strength and solar wind speed, and up to a factor 4 increase in solar wind Mach number. Thus solar wind energy input into the Earth’s magnetosphere was reduced, resulting in a more Jupiter-like system, in agreement with the dearth of auroral reports from the time. The global heliosphere was both smaller and more symmetric under MM conditions, which has implications for the interpretation of cosmogenic radionuclide data and resulting total solar irradiance estimates during grand minima
Compton Scattering by the Proton using a Large-Acceptance Arrangement
Compton scattering by the proton has been measured using the tagged-photon
facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The new
data are interpreted in terms of dispersion theory based on the SAID-SM99K
parameterization of photo-meson amplitudes. It is found that two-pion exchange
in the t-channel is needed for a description of the data in the second
resonance region. The data are well represented if this channel is modeled by a
single pole with mass parameter m(sigma)=600 MeV. The asymptotic part of the
spin dependent amplitude is found to be well represented by pi-0-exchange in
the t-channel. A backward spin-polarizability of
gamma(pi)=(-37.1+-0.6(stat+syst)+-3.0(model))x10^{-4}fm^4 has been determined
from data of the first resonance region below 455 MeV. This value is in a good
agreement with predictions of dispersion relations and chiral pertubation
theory. From a subset of data between 280 and 360 MeV the resonance
pion-photoproduction amplitudes were evaluated leading to a E2/M1 multipole
ratio of the p-to-Delta radiative transition of EMR(340
MeV)=(-1.7+-0.4(stat+syst)+-0.2(model))%. It was found that this number is
dependent on the parameterization of photo-meson amplitudes. With the MAID2K
parameterization an E2/M1 multipole ratio of EMR(340
MeV)=(-2.0+-0.4(stat+syst)+-0.2(model))% is obtained
kt Effects in Direct-Photon Production
We discuss the phenomenology of initial-state parton-kt broadening in
direct-photon production and related processes in hadron collisions. After a
brief summary of the theoretical basis for a Gaussian-smearing approach, we
present a systematic study of recent results on fixed-target and collider
direct-photon production, using complementary data on diphoton and pion
production to provide empirical guidance on the required amount of kt
broadening. This approach provides a consistent description of the observed
pattern of deviation of next-to-leading order QCD calculations relative to the
direct-photon data, and accounts for the shape and normalization difference
between fixed-order perturbative calculations and the data. We also discuss the
uncertainties in this phenomenological approach, the implications of these
results on the extraction of the gluon distribution of the nucleon, and the
comparison of our findings to recent related work.Comment: LaTeX, uses revtex and epsf, 37 pages, 15 figure
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