7,007 research outputs found
The Effect of Diffusion on the Particle Spectra in Pulsar Wind Nebulae
A possible way to calculate particle spectra as a function of position in
pulsar wind nebulae is to solve a Fokker-Planck transport equation. This paper
presents numerical solutions to the transport equation with the processes of
convection, diffusion, adiabatic losses, and synchrotron radiation included. In
the first part of the paper the steady-state version of the transport equation
is solved as a function of position and energy. This is done to distinguish the
various effects of the aforementioned processes on the solutions to the
transport equation. The second part of the paper deals with a time-dependent
solution to the transport equation, specifically taking into account the effect
of a moving outer boundary. The paper highlights the fact that diffusion can
play a significant role in reducing the amount of synchrotron losses, leading
to a modification in the expected particle spectra. These modified spectra can
explain the change in the photon index of the synchrotron emission as a
function of position. The solutions presented in this paper are not limited to
pulsar wind nebulae, but can be applied to any similar central source system,
e.g. globular clusters
Neutron monitors and muon detectors for solar modulation studies: Interstellar flux, yield function, and assessment of critical parameters in count rate calculations
Particles count rates at given Earth location and altitude result from the
convolution of (i) the interstellar (IS) cosmic-ray fluxes outside the solar
cavity, (ii) the time-dependent modulation of IS into Top-of-Atmosphere (TOA)
fluxes, (iii) the rigidity cut-off (or geomagnetic transmission function) and
grammage at the counter location, (iv) the atmosphere response to incoming TOA
cosmic rays (shower development), and (v) the counter response to the various
particles/energies in the shower. Count rates from neutron monitors or muon
counters are therefore a proxy to solar activity. In this paper, we review all
ingredients, discuss how their uncertainties impact count rate calculations,
and how they translate into variation/uncertainties on the level of solar
modulation (in the simple Force-Field approximation). The main
uncertainty for neutron monitors is related to the yield function. However,
many other effects have a significant impact, at the 5-10\% level on
values. We find no clear ranking of the dominant effects, as some depend on the
station position and/or the weather and/or the season. An abacus to translate
any variation of count rates (for neutron and detectors) to a variation
of the solar modulation is provided.Comment: 28 pages, 16 figures, 9 tables, match accepted version in AdSR (minor
corrections, Dorman (1974,2004,2009) reference textbooks added
Infestations of the cypress bark beetles Phloeosinus rudis, P. bicolor and P. thujae in The Netherlands (Coleoptera: Curculionidae: Scolytinae).
In de zomer van 2004 trad er grote sterfte op bij coniferen in Rotterdam en nabijgelegen steden zoals Ridderkerk, Sleeuwijk, Maassluis, Nieuw-Lekkerland en Dordrecht. De sterfte trad op bij een solitaire 60-jarige Thuja maar ook bij coniferenhagen van Thuja, Chamaecyparis en Juniperus. Na 2004 werd de kever nog maar enkele malen waargenomen. Kleine populaties kunnen echter overleven in verzwakte bomen zoals in 2008 bleek uit de vondst van kevers in een door parasitaire schimmels verzwakte boom (Rotterdam) en gebroken takken (Wageningen en Renkum). Het zijn zwakteparasieten die zich alleen in verzwakte bomen kunnen ontwikkelen. 2003 was een zeer droog jaar met veel droogtestress verzwakte bomen en struiken waardoor de aantastingen konden plaatsvinden
Drift and observations in cosmic-ray modulation, 1
It is illustrated that a relative simple drift model can, in contrast with no drift models, simultaneously fit proton and electron spectra observed in 1965-66 and 1977, using a single set of modulation parameters except for a change in the IMF polarity. This result is interpreted together with the observation of Evenson and Meyer that electrons are recovering more rapidly than protons after 1980, in contrast with what Burger and Swanenburg observed in 1968-72, as a charge sign dependent effect due to the occurrence of drift in cosmic ray modulation. The same set of parameters produces a shift in the phase and amplitude of the diurnal anisotropy vector, consistent with observations in 1969-71 and 1980-81
Record-setting Cosmic-ray Intensities in 2009 and 2010
We report measurements of record-setting intensities of cosmic-ray nuclei from C to Fe, made with the Cosmic Ray Isotope Spectrometer carried on the Advanced Composition Explorer in orbit about the inner Sun-Earth Lagrangian point. In the energy interval from ~70 to ~450 MeV nucleon^(â1), near the peak in the near-Earth cosmic-ray spectrum, the measured intensities of major species from C to Fe were each 20%-26% greater in late 2009 than in the 1997-1998 minimum and previous solar minima of the space age (1957-1997). The elevated intensities reported here and also at neutron monitor energies were undoubtedly due to several unusual aspects of the solar cycle 23/24 minimum, including record-low interplanetary magnetic field (IMF) intensities, an extended period of reduced IMF turbulence, reduced solar-wind dynamic pressure, and extremely low solar activity during an extended solar minimum. The estimated parallel diffusion coefficient for cosmic-ray transport based on measured solar-wind properties was 44% greater in 2009 than in the 1997-1998 solar-minimum period. In addition, the weaker IMF should result in higher cosmic-ray drift velocities. Cosmic-ray intensity variations at 1 AU are found to lag IMF variations by 2-3 solar rotations, indicating that significant solar modulation occurs inside ~20 AU, consistent with earlier galactic cosmic-ray radial-gradient measurements. In 2010, the intensities suddenly decreased to 1997 levels following increases in solar activity and in the inclination of the heliospheric current sheet. We describe the conditions that gave cosmic rays greater access to the inner solar system and discuss some of their implications
- âŠ