53 research outputs found
An off-shell I.R. regularization strategy in the analysis of collinear divergences
We present a method for the analysis of singularities of Feynman amplitudes
based on the Speer sector decomposition of the Schwinger parametric integrals
combined with the Mellin-Barnes transform. The sector decomposition method is
described in some details. We suggest the idea of applying the method to the
analysis of collinear singularities in inclusive QCD cross sections in the
mass-less limit regularizing the forward amplitudes by an off-shell choice of
the initial particle momenta. It is shown how the suggested strategy works in
the well known case of the one loop corrections to Deep Inelastic Scattering.Comment: 25 pages, 3 figure
Density of neutral interstellar hydrogen at the termination shock from Ulysses pickup ion observations
By reevaluating a 13-month stretch of Ulysses SWICS H pickup ion measurements
near 5 AU close to the ecliptic right after the previous solar minimum, this
paper presents a determination of the neutral interstellar H density at the
solar wind termination shock and implications for the density and ionization
degree of hydrogen in the LIC. The density of neutral interstellar hydrogen at
the termination shock was determined from the local pickup ion production rate
as obtained close to the cut-off in the distribution function at aphelion of
Ulysses. As shown in an analytical treatment for the upwind axis and through
kinetic modeling of the pickup ion production rate at the observer location,
with variations in the ionization rate, radiation pressure, and the modeling of
the particle behavior, this analysis turns out to be very robust against
uncertainties in these parameters and the modeling. Analysis using current
heliospheric parameters yields the H density at the termination shock equal to
cm, including observational and modeling uncertainties.Comment: Re-edited version, density revised downward due to data
re-processing, accepted by A&
Modelling light-cone distribution amplitudes from non-relativistic bound states
We calculate light-cone distribution amplitudes for non-relativistic bound
states, including radiative corrections from relativistic gluon exchange to
first order in the strong coupling constant. We distinguish between bound
states of quarks with equal (or similar) mass, m_1 ~ m_2, and between bound
states where the quark masses are hierarchical, m_1 >> m_2. For both cases we
calculate the distribution amplitudes at the non-relativistic scale and discuss
the renormalization-group evolution for the leading-twist and 2-particle
distributions. Our results apply to hard exclusive reactions with
non-relativistic bound states in the QCD factorization approach like, for
instance, (B_c -> eta_c l nu) or (e^+ e^- -> J/psi eta_c). They also serve as a
toy model for light-cone distribution amplitudes of light mesons or heavy B and
D mesons, for which certain model-independent properties can be derived. In
particular, we calculate the anomalous dimension for the B meson distribution
amplitude phi_B^-(w) in the Wandzura-Wilczek approximation and derive the
according solution of the evolution equation at leading logarithmic accuracy.Comment: 27 pages, 15 figures, discussion around Eq.(83,84) extende
A transonic collisionless model of the solar wind
Because of the semi-collisional nature of the solar wind, the collisionless
or exospheric approach as well as the hydrodynamic one are both inaccurate.
However, the advantage of simplicity makes them useful for enlightening some
basic mechanisms of solar wind acceleration. Previous exospheric models have
been able to reproduce winds that were already nearly supersonic at the
exobase, the altitude above which there are no collisions. In order to allow
transonic solutions, a lower exobase has to be considered, in which case the
protons are experiencing a non-monotonic potential energy profile. This is done
in the present work. In this model, the electron velocity distribution in the
corona is assumed non-thermal. Parametric results are presented and show that
the high acceleration obtained does not depend on the details of the
non-thermal distributions. This acceleration seems, therefore, to be a robust
result produced by the presence of a sufficient number of suprathermal
electrons. A method for improving the exospheric description is also given,
which consists in mapping particle orbits in terms of their invariants of
motion.Comment: 18 pages, 18 figures, accepted for publication in The Astrophysical
Journal (1 May 2004
Substorm triggering by new plasma intrusion: Incoherentâscatter radar observations
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95031/1/jgra20482.pd
Dust Processing in Disks around T Tauri Stars
The 8-14 micron emission spectra of 12 T Tauri stars in the Taurus/Auriga
dark clouds and in the TW Hydrae association obtained with the Infrared
Spectrograph (IRS; The IRS is a collaborative venture between Cornell
University and Ball Aerospace Corporation funded by NASA through the Jet
Propulsion Laboratory and the Ames Research Center.) on board Spitzer are
analyzed. Assuming the 10 micron features originate from silicate grains in the
optically thin surface layers of T Tauri disks, the 8-14 micron dust emissivity
for each object is derived from its Spitzer spectrum. The emissivities are fit
with the opacities of laboratory analogs of cosmic dust. The fits include small
nonspherical grains of amorphous silicates (pyroxene and olivine), crystalline
silicates (forsterite and pyroxene), and quartz, together with large fluffy
amorphous silicate grains. A wide range in the fraction of crystalline silicate
grains as well as large silicate grains among these stars are found. The dust
in the transitional-disk objects CoKu Tau/4, GM Aur, and DM Tau has the
simplest form of silicates, with almost no hint of crystalline components and
modest amounts of large grains. This indicates that the dust grains in these
objects have been modified little from their origin in the interstellar medium.
Other stars show various amounts of crystalline silicates, similar to the wide
dispersion of the degree of crystallinity reported for Herbig Ae/Be stars of
mass <2.5 solar masses. Late spectral type, low-mass stars can have significant
fractions of crystalline silicate grains. Higher quartz mass fractions often
accompany low amorphous olivine-to-amorphous pyroxene ratios. It is also found
that lower contrast of the 10 micron feature accompanies greater crystallinity.Comment: AASTEX, 39 pages text, 14 figures, 4 tables, scheduled to be
published July 2006 in the Astrophysical Journa
Solar parameters for modeling interplanetary background
The goal of the Fully Online Datacenter of Ultraviolet Emissions (FONDUE)
Working Team of the International Space Science Institute in Bern, Switzerland,
was to establish a common calibration of various UV and EUV heliospheric
observations, both spectroscopic and photometric. Realization of this goal
required an up-to-date model of spatial distribution of neutral interstellar
hydrogen in the heliosphere, and to that end, a credible model of the radiation
pressure and ionization processes was needed. This chapter describes the solar
factors shaping the distribution of neutral interstellar H in the heliosphere.
Presented are the solar Lyman-alpha flux and the solar Lyman-alpha resonant
radiation pressure force acting on neutral H atoms in the heliosphere, solar
EUV radiation and the photoionization of heliospheric hydrogen, and their
evolution in time and the still hypothetical variation with heliolatitude.
Further, solar wind and its evolution with solar activity is presented in the
context of the charge exchange ionization of heliospheric hydrogen, and in the
context of dynamic pressure variations. Also the electron ionization and its
variation with time, heliolatitude, and solar distance is presented. After a
review of all of those topics, we present an interim model of solar wind and
the other solar factors based on up-to-date in situ and remote sensing
observations of solar wind. Results of this effort will further be utilised to
improve on the model of solar wind evolution, which will be an invaluable asset
in all heliospheric measurements, including, among others, the observations of
Energetic Neutral Atoms by the Interstellar Boundary Explorer (IBEX).Comment: Chapter 2 in the planned "Cross-Calibration of Past and Present Far
UV Spectra of Solar System Objects and the Heliosphere", ISSI Scientific
Report No 12, ed. R.M. Bonnet, E. Quemerais, M. Snow, Springe
The Earth: Plasma Sources, Losses, and Transport Processes
This paper reviews the state of knowledge concerning the source of magnetospheric plasma at Earth. Source of plasma, its acceleration and transport throughout the system, its consequences on system dynamics, and its loss are all discussed. Both observational and modeling advances since the last time this subject was covered in detail (Hultqvist et al., Magnetospheric Plasma Sources and Losses, 1999) are addressed
Alfvenic velocity spikes and rotational flows in the near-Sun solar wind
The prediction of a supersonic solar wind1 was first confirmed by spacecraft near Earth2,3 and later by spacecraft at heliocentric distances as small as 62 solar radii4. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. AlfvĂ©nic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy5,6,7. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams8. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii9,10,11, well within the distance at which stream interactions become important. We find that AlfvĂ©n waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per secondâconsiderably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age12,13,14
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