3,263 research outputs found
Inverse Bremsstrahlung in Shocked Astrophysical Plasmas
There has recently been interest in the role of inverse bremsstrahlung, the
emission of photons by fast suprathermal ions in collisions with ambient
electrons possessing relatively low velocities, in tenuous plasmas in various
astrophysical contexts. This follows a long hiatus in the application of
suprathermal ion bremsstrahlung to astrophysical models since the early 1970s.
The potential importance of inverse bremsstrahlung relative to normal
bremsstrahlung, i.e. where ions are at rest, hinges upon the underlying
velocity distributions of the interacting species. In this paper, we identify
the conditions under which the inverse bremsstrahlung emissivity is significant
relative to that for normal bremsstrahlung in shocked astrophysical plasmas. We
determine that, since both observational and theoretical evidence favors
electron temperatures almost comparable to, and certainly not very deficient
relative to proton temperatures in shocked plasmas, these environments
generally render inverse bremsstrahlung at best a minor contributor to the
overall emission. Hence inverse bremsstrahlung can be safely neglected in most
models invoking shock acceleration in discrete sources such as supernova
remnants. However, on scales > 100pc distant from these sources, Coulomb
collisional losses can deplete the cosmic ray electrons, rendering inverse
bremsstrahlung, and perhaps bremsstrahlung from knock-on electrons, possibly
detectable.Comment: 13 pages, including 2 figures, using apjgalley format; to appear in
the January 10, 2000 issue, of the Astrophysical Journa
Cocaine self-administration in the mouse: A low-cost, chronic catheter preparation
Intravenous drug self-administration is the most valid animal model of human addiction because it allows volitional titration of the drug in the blood based on an individual’s motivational state together with the pharmacokinetic properties of the drug. Here we describe a reliable low-cost mouse self-administration catheter assembly and protocol that that can be used to assess a variety of drugs of abuse with a variety of protocols. We describe a method for intravenous catheter fabrication that allows for efficient and long-lasting intravenous drug delivery. The intravenous catheters remained intact and patent for several weeks allowing us to establish stable maintenance of cocaine acquisition. This was followed by a dose response study in the same mice. For collaborators interested in premade catheters for research please make a request at www.neuro-cloud.net/nature-precedings/pomerenze
Radio to Gamma-Ray Emission from Shell-type Supernova Remnants: Predictions from Non-linear Shock Acceleration Models
Supernova remnants (SNRs) are widely believed to be the principal source of
galactic cosmic rays. Such energetic particles can produce gamma-rays and lower
energy photons via interactions with the ambient plasma. In this paper, we
present results from a Monte Carlo simulation of non-linear shock structure and
acceleration coupled with photon emission in shell-like SNRs. These
non-linearities are a by-product of the dynamical influence of the accelerated
cosmic rays on the shocked plasma and result in distributions of cosmic rays
which deviate from pure power-laws. Such deviations are crucial to acceleration
efficiency and spectral considerations, producing GeV/TeV intensity ratios that
are quite different from test particle predictions. The Sedov scaling solution
for SNR expansions is used to estimate important shock parameters for input
into the Monte Carlo simulation. We calculate ion and electron distributions
that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron
emission, yielding complete photon spectra from radio frequencies to gamma-ray
energies. The cessation of acceleration caused by the spatial and temporal
limitations of the expanding SNR shell in moderately dense interstellar regions
can yield spectral cutoffs in the TeV energy range; these are consistent with
Whipple's TeV upper limits on unidentified EGRET sources. Supernova remnants in
lower density environments generate higher energy cosmic rays that produce
predominantly inverse Compton emission at super-TeV energies; such sources will
generally be gamma-ray dim at GeV energies.Comment: 62 pages, AASTeX format, including 1 table and 11 figures, accepted
for publication in The Astrophysical Journal (Vol 513, March 1, 1999
High fidelity optogenetic control of individual prefrontal cortical pyramidal neurons in vivo
Precise spatial and temporal manipulation of neural activity in specific
genetically defined cell populations is now possible with the advent of
optogenetics. The emerging field of optogenetics consists of a set of
naturally-occurring and engineered light-sensitive membrane proteins that are
able to activate (e.g., channelrhodopsin-2, ChR2) or silence (e.g.,
halorhodopsin, NpHR) neural activity. Here we demonstrate the technique and the
feasibility of using novel adeno-associated viral (AAV) tools to activate
(AAV-CaMKll{\alpha}-ChR2-eYFP) or silence (AAV-CaMKll{\alpha}-eNpHR3.0-eYFP)
neural activity of rat prefrontal cortical prelimbic (PL) pyramidal neurons in
vivo. In vivo single unit extracellular recording of ChR2-transduced pyramidal
neurons showed that delivery of brief (10 ms) blue (473 nm) light-pulse trains
up to 20 Hz via a custom fiber optic-coupled recording electrode (optrode)
induced spiking with high fidelity at 20 Hz for the duration of recording (up
to two hours in some cases). To silence spontaneously active neurons we
transduced them with the NpHR construct and administered continuous green (532
nm) light to completely inhibit action potential activity for up to 10 seconds
with 100% fidelity in most cases. These versatile photosensitive tools combined
with optrode recording methods provide experimental control over activity of
genetically defined neurons and can be used to investigate the functional
relationship between neural activity and complex cognitive behavior.Comment: 4 pages, 4 figures F1000Research articl
Direct Acceleration of Pickup Ions at The Solar Wind Termination Shock: The Production of Anomalous Cosmic Rays
We have modeled the injection and acceleration of pickup ions at the solar wind termination shock and investigated the parameters needed to produce the observed Anomalous Cosmic Ray (ACR) fluxes. A non-linear Monte Carlo technique was employed, which in effect solves the Boltzmann equation and is not restricted to near-isotropic particle distribution functions. This technique models the injection of thermal and pickup ions, the acceleration of these ions, and the determination of the shock structure under the influence of the accelerated ions. The essential effects of injection are treated in a mostly self-consistent manner, including effects from shock obliquity, cross- field diffusion, and pitch-angle scattering. Using recent determinations of pickup ion densities, we are able to match the absolute flux of hydrogen in the ACRs by assuming that pickup ion scattering mean free paths, at the termination shock, are much less than an AU and that modestly strong cross-field diffusion occurs. Simultaneously, we match the flux ratios He(+)/H(+) or O(+)/H(+) to within a factor approx. 5. If the conditions of strong scattering apply, no pre-termination-shock injection phase is required and the injection and acceleration of pickup ions at the termination shock is totally analogous to the injection and acceleration of ions at highly oblique interplanetary shocks recently observed by the Ulysses spacecraft. The fact that ACR fluxes can be modeled with standard shock assumptions suggests that the much-discussed "injection problem" for highly oblique shocks stems from incomplete (either mathematical or computer) modeling of these shocks rather than from any actual difficulty shocks may have in injecting and accelerating thermal or quasi-thermal particles
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