306 research outputs found
Discovery of a transient radiation belt at Saturn
Radiation belts have been detected in situ at five planets. Only at Earth however has any variability in their intensity been heretofore observed, in indirect response to solar eruptions and high altitude nuclear explosions. The Cassini spacecraft's MIMI/LEMMS instrument has now detected systematic radiation belt variability elsewhere. We report three sudden increases in energetic ion intensity around Saturn, in the vicinity of the moons Dione and Tethys, each lasting for several weeks, in response to interplanetary events caused by solar eruptions. However, the intensifications, which could create temporary satellite atmospheres at the aforementioned moons, were sharply restricted outside the orbit of Tethys. Unlike Earth, Saturn has almost unchanging inner ion radiation belts: due to Saturn's near-symmetrical magnetic field, Tethys and Dione inhibit inward radial transport of energetic ions, shielding the planet's main, inner radiation belt from solar wind influences
Irreversible and reversible modes of operation of deterministic ratchets
We discuss a problem of optimization of the energetic efficiency of a simple
rocked ratchet. We concentrate on a low-temperature case in which the
particle's motion in a ratchet potential is deterministic. We show that the
energetic efficiency of a ratchet working adiabatically is bounded from above
by a value depending on the form of ratchet potential. The ratchets with
strongly asymmetric potentials can achieve ideal efficiency of unity without
approaching reversibility. On the other hand we show that for any form of the
ratchet potential a set of time-protocols of the outer force exist under which
the operation is reversible and the ideal value of efficiency is also achieved.
The mode of operation of the ratchet is still quasistatic but not adiabatic.
The high values of efficiency can be preserved even under elevated
temperatures
Energetic Ion Moments and Polytropic Index in Saturnâs Magnetosphere using Cassini/MIMI Measurements: A Simple Model Based on ÎșâDistribution Functions
Moments of the charged particle distribution function provide a compact way of studying the transport, acceleration, and interactions of plasma and energetic particles in the magnetosphere. We employ Îșâdistributions to describe the energy spectra of H+ and O+, based on >20 keV measurements by the three detectors of Cassiniâs Magnetospheric Imaging Instrument, covering the time period from DOY 183/2004 to 016/2016, 5 < L < 20. From the analytical spectra we calculate the equatorial distributions of energetic ion moments inside Saturnâs magnetosphere and then focus on the distributions of the characteristic energy (Ec=IE/In), temperature, and Îșâindex of these ions. A semiempirical model is utilized to simulate the equatorial ion moments in both local time and Lâshell, allowing the derivation of the polytropic index (Î) for both H+ and O+. Primary results are as follows: (a) The âŒ9 < L < 20 region corresponds to a local equatorial acceleration region, where subadiabatic transport of H+ (ÎâŒ1.25) and quasiâisothermal behavior of O+ (ÎâŒ0.95) dominate the ion energetics; (b) energetic ions are heavily depleted in the inner magnetospheric regions, and their behavior appears to be quasiâisothermal (Î<1); (c) the (quasiâ) periodic energetic ion injections in the outer parts of Saturnâs magnetosphere (especially beyond 17â18 RS) produce durable signatures in the energetic ion moments; (d) the plasma sheet does not seem to have a ground thermodynamic state, but the extended neutral gas distribution at Saturn provides an effective cooling mechanism that does not allow the plasma sheet to behave adiabatically.Key PointsDerivation of energetic ion moments, Îșâindex, characteristic energy, temperature, and polytropic index in Saturnâs magnetospherePresentation of a semiempirical analytical model for the 20 keV energetic ion Pressure, density, and temperatureThe neutral gas at Saturn provides an effective cooling mechanism and does not allow the plasma sheet to behave adiabaticallyPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146558/1/jgra54546.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146558/2/jgra54546_am.pd
Current reversal with type-I intermittency in deterministic inertia ratchets
The intermittency is investigated when the current reversal occurs in a
deterministic inertia ratchet system. To determine which type the intermittency
belongs to, we obtain the return map of velocities of particle using
stroboscopic recording, and numerically calculate the distribution of average
laminar length . The distribution follows the scaling law of , the characteristic relation of type-I
intermittency.Comment: 4 pages, 7 figure
On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium
We report on the energy dependence of galactic cosmic rays (GCRs) in the very
local interstellar medium (VLISM) as measured by the Low Energy Charged
Particle (LECP) instrument on the Voyager 1 (V1) spacecraft. The LECP
instrument includes a dual-ended solid state detector particle telescope
mechanically scanning through 360 deg across eight equally-spaced angular
sectors. As reported previously, LECP measurements showed a dramatic increase
in GCR intensities for all sectors of the >=211 MeV count rate (CH31) at the V1
heliopause (HP) crossing in 2012, however, since then the count rate data have
demonstrated systematic episodes of intensity decrease for particles around
90{\deg} pitch angle. To shed light on the energy dependence of these GCR
anisotropies over a wide range of energies, we use V1 LECP count rate and pulse
height analyzer (PHA) data from >=211 MeV channel together with lower energy
LECP channels. Our analysis shows that while GCR anisotropies are present over
a wide range of energies, there is a decreasing trend in the amplitude of
second-order anisotropy with increasing energy during anisotropy episodes. A
stronger pitch-angle scattering at the higher velocities is argued as a
potential cause for this energy dependence. A possible cause for this velocity
dependence arising from weak rigidity dependence of the scattering mean free
path and resulting velocity-dominated scattering rate is discussed. This
interpretation is consistent with a recently reported lack of corresponding GCR
electron anisotropies
Multiple current reversals in forced inhomogeneous ratchets
Transport properties of overdamped Brownian paricles in a rocked thermal
ratchet with space dependent friction coefficient is studied. By tuning the
parameters, the direction of current exhibit multiple reversals, both as a
function of the thermal noise strength as well as the amplitude of rocking
force. Current reversals also occur under deterministic conditions and exhibits
intriguing structure. All these features arise due to mutual interplay between
potential asymmetry,noise, driving frequency and inhomogeneous friction.Comment: 6 figure
Molecular motor that never steps backwards
We investigate the dynamics of a classical particle in a one-dimensional
two-wave potential composed of two periodic potentials, that are
time-independent and of the same amplitude and periodicity. One of the periodic
potentials is externally driven and performs a translational motion with
respect to the other. It is shown that if one of the potentials is of the
ratchet type, translation of the potential in a given direction leads to motion
of the particle in the same direction, whereas translation in the opposite
direction leaves the particle localized at its original location. Moreover,
even if the translation is random, but still has a finite velocity, an
efficient directed transport of the particle occurs.Comment: 4 pages, 5 figures, Phys. Rev. Lett. (in print
IL-4-secreting CD4+ T cells are crucial to the development of CD8+ T-cell responses against malaria liver stages.
CD4+ T cells are crucial to the development of CD8+ T cell responses against hepatocytes infected with malaria parasites. In the absence of CD4+ T cells, CD8+ T cells initiate a seemingly normal differentiation and proliferation during the first few days after immunization. However, this response fails to develop further and is reduced by more than 90%, compared to that observed in the presence of CD4+ T cells. We report here that interleukin-4 (IL-4) secreted by CD4+ T cells is essential to the full development of this CD8+ T cell response. This is the first demonstration that IL-4 is a mediator of CD4/CD8 cross-talk leading to the development of immunity against an infectious pathogen
Characterisation of monoclonal antibodies specific for hamster leukocyte differentiation molecules
Flow cytometry was used to identify mAbs that recognize conserved epitopes on hamster leukocyte differentiation molecules (hLDM) and also to characterize mAbs developed against hLDM. Initial screening of mAbs developed against LDMs in other species yielded mAbs specific for the major histocompatibility (MHC) II molecule, CD4 and CD18. Screening of sets of mAbs developed against hLDM yielded 22 new mAbs, including additional mAbs to MHC II molecules and mAbs that recognize LDMs expressed on all leukocytes, granulocytes, all lymphocytes, all T cells, a subset of T cells, or on all B cells. Based on comparison of the pattern of expression of LDMs expressed on all hamster leukocytes with the patterns of expression of known LDMs in other species, as detected by flow cytometry (FC), four mAbs are predicted to recognize CD11a, CD44, and CD45. Cross comparison of mAbs specific for a subset of hamster T cells with a cross reactive mAb known to recognize CD4 in mice and one recognising CD8 revealed they recognize CD4. The characterization of these mAbs expands opportunities to use hamsters as an additional model species to investigate the mechanisms of immunopathogenesis of infectious diseases
- âŠ