49 research outputs found
Fundamental and harmonic emission in interplanetary type 2 radio bursts
Three interplanetary type II radio bursts which show two prominent and long duration bands in their dynamic spectra were analyzed in detail and compared to similar bands in meter wavelength type II events. These bands, which differ by a factor of about two in frequency, were interpreted in terms of fundamental and harmonic emission. The fundamental component has a greater average intensity than the harmonic, due largely to short intense brightenings. The fundamental spectral profile is more narrow than that of the harmonic, with harmonic band typically exhibiting a larger bandwidth to frequency ratio than the fundamental by a factor of two. The fundamental has a larger source size than the harmonic, 160 degrees versus 110 degrees, on average, as viewed from the Sun. Two of the events have source positions which correlate well with the associated flare positions
Spatially resolved observations of a split-band coronal type-II radio burst
Context. The origin of coronal type-II radio bursts and of their
band-splitting are still not fully understood. Aims. To make progress in
solving this problem on the basis of one extremely well observed solar eruptive
event. Methods. The relative dynamics of multi-thermal eruptive plasmas,
observed in detail by the SDO/AIA and of the harmonic type-II burst sources,
observed by the NRH at ten frequencies from 445 to 151 MHz, is studied for the
partially behind the limb event on 3 November 2010. Special attention is given
to the band-splitting of the burst. Analysis is supplemented by investigation
of coronal hard X-ray (HXR) sources observed by the RHESSI. Results. It is
found that the flare impulsive phase was accompanied by the formation of a
double coronal HXR source, whose upper part coincided with the hot (T~10 MK)
eruptive plasma blob. The leading edge (LE) of the eruptive plasmas (T~1-2 MK)
moved upward from the flare region with the speed of v=900-1400 km/s. The type
II burst source initially appeared just above the LE apex and moved with the
same speed and in the same direction. After about 20 s it started to move about
twice faster, but still in the same direction. At any given moment the low
frequency component (LFC) source of the splitted type-II burst was situated
above the high frequency component (HFC) source, which in turn was situated
above the LE. It is also found that at a given frequency the HFC source was
located slightly closer to the photosphere than the LFC source. Conclusions.
The shock wave, which could be responsible for the observed type-II radio
burst, was initially driven by the multi-temperature eruptive plasmas, but
later transformed to a freely propagating blast shock wave. The most preferable
interpretation of the type-II burst splitting is that its LFC was emitted from
the upstream region of the shock, whereas the HFC - from the downstream region.Comment: 14 pages, 10 figure
A Model of the Roles of Essential Kinases in the Induction and Expression of Late Long-Term Potentiation
The induction of late long-term potentiation (L-LTP) involves complex
interactions among second messenger cascades. To gain insights into these
interactions, a mathematical model was developed for L-LTP induction in the CA1
region of the hippocampus. The differential equation-based model represents
actions of protein kinase A (PKA), MAP kinase (MAPK), and CaM kinase II
(CAMKII) in the vicinity of the synapse, and activation of transcription by CaM
kinase IV (CAMKIV) and MAPK. L-LTP is represented by increases in a synaptic
weight. Simulations suggest that steep, supralinear stimulus-response
relationships between stimuli (elevations in [Ca2+]) and kinase activation are
essential for translating brief stimuli into long-lasting gene activation and
synaptic weight increases. Convergence of multiple kinase activities to induce
L-LTP helps to generate a threshold whereby the amount of L-LTP varies steeply
with the number of tetanic electrical stimuli. The model simulates tetanic,
theta-burst, pairing-induced, and chemical L-LTP, as well as L-LTP due to
synaptic tagging. The model also simulates inhibition of L-LTP by inhibition of
MAPK, CAMKII, PKA, or CAMKIV. The model predicts results of experiments to
delineate mechanisms underlying L-LTP induction and expression. For example,
the cAMP antagonist RpcAMPs, which inhibits L-LTP induction, is predicted to
inhibit ERK activation. The model also appears useful to clarify similarities
and differences between hippocampal L-LTP and long-term synaptic strengthening
in other systems.Comment: Accepted to Biophysical Journal. Single PDF, 7 figs include
Electromagnetic waves and electron anisotropies downstream of supercritical interplanetary shocks
We present waveform observations of electromagnetic lower hybrid and whistler
waves with f_ci << f < f_ce downstream of four supercritical interplanetary
(IP) shocks using the Wind search coil magnetometer. The whistler waves were
observed to have a weak positive correlation between \partialB and normalized
heat flux magnitude and an inverse correlation with T_eh/T_ec. All were
observed simultaneous with electron distributions satisfying the whistler heat
flux instability threshold and most with T_{perp,h}/T_{para,h} > 1.01. Thus,
the whistler mode waves appear to be driven by a heat flux instability and
cause perpendicular heating of the halo electrons. The lower hybrid waves show
a much weaker correlation between \partialB and normalized heat flux magnitude
and are often observed near magnetic field gradients. A third type of event
shows fluctuations consistent with a mixture of both lower hybrid and whistler
mode waves. These results suggest that whistler waves may indeed be regulating
the electron heat flux and the halo temperature anisotropy, which is important
for theories and simulations of electron distribution evolution from the sun to
the earth.Comment: 20 pages, 3 PDF figures, submitted to Journal of Geophysical Researc
Statistical Study of the Properties of Magnetosheath Lion Roars
Lion roars are narrowband whistler wave emissions that have been observed in several environments, such as planetary magnetosheaths, the Earth's magnetosphere, the solar wind, downstream of interplanetary shocks, and the cusp region. We present measurements of more than 30,000 such emissions observed by the Magnetospheric Multiscale spacecraft with high‐cadence (8,192 samples/s) search coil magnetometer data. A semiautomatic algorithm was used to identify the emissions, and an adaptive interval algorithm in conjunction with minimum variance analysis was used to determine their wave vector. The properties of the waves are determined in both the spacecraft and plasma rest frame. The mean wave normal angle, with respect to the background magnetic field (B0), plasma bulk flow velocity (Vb), and the coplanarity plane (Vb×B0) are 23°, 56°, and 0°, respectively. The average peak frequencies were ∼31% of the electron gyrofrequency (ωce) observed in the spacecraft frame and ∼18% of ωce in the plasma rest frame. In the spacecraft frame, ∼99% of the emissions had a frequency <ωce, while 98% had a peak frequency <0.72ωce in the plasma rest frame. None of the waves had frequencies lower than the lower hybrid frequency, ω. From the probability density function of the electron plasma βe, the ratio between the electron thermal and magnetic pressure, ∼99.6% of the waves were observed with βe<4 with a large narrow peak at 0.07 and two smaller, but wider, peaks at 1.26 and 2.28, while the average value was ∼1.25
Neuronal Oscillations Enhance Stimulus Discrimination by Ensuring Action Potential Precision
Although oscillations in membrane potential are a prominent feature of sensory, motor, and cognitive function, their precise role in signal processing remains elusive. Here we show, using a combination of in vivo, in vitro, and theoretical approaches, that both synaptically and intrinsically generated membrane potential oscillations dramatically improve action potential (AP) precision by removing the membrane potential variance associated with jitter-accumulating trains of APs. This increased AP precision occurred irrespective of cell type and—at oscillation frequencies ranging from 3 to 65 Hz—permitted accurate discernment of up to 1,000 different stimuli. At low oscillation frequencies, stimulus discrimination showed a clear phase dependence whereby inputs arriving during the trough and the early rising phase of an oscillation cycle were most robustly discriminated. Thus, by ensuring AP precision, membrane potential oscillations dramatically enhance the discriminatory capabilities of individual neurons and networks of cells and provide one attractive explanation for their abundance in neurophysiological systems
Highlighting the versatility of the Tracerlab synthesis modules. Part 2: fully automated production of [ 11 C]‐labeled radiopharmaceuticals using a Tracerlab FX C‐Pro
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/89445/1/jlcr1937.pd
Synaptic Transmission Optimization Predicts Expression Loci of Long-Term Plasticity
Long-term modifications of neuronal connections are critical for reliable memory storage in the brain. However, their locus of expression—pre- or postsynaptic—is highly variable. Here we introduce a theoretical framework in which long-term plasticity performs an optimization of the postsynaptic response statistics toward a given mean with minimal variance. Consequently, the state of the synapse at the time of plasticity induction determines the ratio of pre- and postsynaptic modifications. Our theory explains the experimentally observed expression loci of the hippocampal and neocortical synaptic potentiation studies we examined. Moreover, the theory predicts presynaptic expression of long-term depression, consistent with experimental observations. At inhibitory synapses, the theory suggests a statistically efficient excitatory-inhibitory balance in which changes in inhibitory postsynaptic response statistics specifically target the mean excitation. Our results provide a unifying theory for understanding the expression mechanisms and functions of long-term synaptic transmission plasticity
Gradient Enhanced Surrogate Models Based on Adjoint CFD Methods for the Design of a Counter Rotating Turbofan
This paper studies the use of adjoint CFD solvers in combination with surrogate modelling in order to reduce the computational cost of the optimization of complex 3D turbomachinery
components. The method is applied to a previously optimized
counter rotating turbofan, with a shape parameterized by 104
CAD parameters.
Through random changes on the reference design, a small
number of design variations are created to serve as training samples for the surrogate models. A steady RANS solver and its discrete adjoint are then used to calculate objective function values
and their corresponding sensitivities. Kriging and neural networks are used to build surrogate models from the training data.
To study the impact of the additional information provided by the
adjoint solver, each model is trained with and without the sensitivity information. The accuracy of the different surrogate model
predictions is assessed by comparison against CFD calculations.
The results show a considerable improvement of the fitness
function approximation when the sensitivity information is taken
into account. Through a gradient based optimization on one of
the surrogate models, a design with higher isentropic efficiency
at the aerodynamic design point is created. This application
demonstrates that the improved surrogate models can be used
for design and optimization