453 research outputs found
Airglow Derived Measurements of Q-Branch Transition Probabilities for Several Hydroxyl Meinel Bands
Spectroscopic measurements of the hydroxyl (OH) airglow emissions are often used to infer neutral temperatures near the mesopause. Correct Einstein coefficients for the various transitions in the OH airglow are needed to calculate accurate temperatures. However, studies showed experimentally and theoretically that the most commonly used Einstein spontaneous emission transition probabilities for the Q-branch of the OH Meinel (6,2) transition are overestimated. Extending their work to several Delta v = 2 and 3 transitions from v' = 3 to 9, we have determined Einstein coefficients for the first four Q-branch rotational lines. These have been derived from high resolution, high signal to noise spectroscopic observations of the OH airglow in the night sky from the Nordic Optical Telescope. The Q-branch Einstein coefficients calculated from these spectra show that values currently tabulated in the HITRAN database overestimate many of the Q-branch transition probabilities. The implications for atmospheric temperatures derived from OH Q-branch measurements are discussed
Articulatory Tradeoffs Reduce Acoustic Variability During American English /r/ Production
Acoustic and articulatory recordings reveal that speakers utilize systematic articulatory tradeoffs to maintain acoustic stability when producing the phoneme /r/. Distinct articulator configurations used to produce /r/ in various phonetic contexts show systematic tradeoffs between the cross-sectional areas of different vocal tract sections. Analysis of acoustic and articulatory variabilities reveals that these tradeoffs act to reduce acoustic variability, thus allowing large contextual variations in vocal tract shape; these contextual variations in turn apparently reduce the amount of articulatory movement required. These findings contrast with the widely held view that speaking involves a canonical vocal tract shape target for each phoneme.National Institute on Deafness and Other Communication Disorders (1R29-DC02852-02, 5R01-DC01925-04, 1R03-C2576-0l); National Science Foundation (IRI-9310518
Leaf Herbivory Induces Resistance Against Florivores In \u3ci\u3eRaphanus sativus\u3c/i\u3e
Florivory can have significant negative effects on plant fitness, driving selection for resistance traits in flowers. In particular, herbivory to leaves may induce resistance in flowers because herbivores on leaves often become florivores on flowers as plant ontogeny proceeds. The literature on inducible resistance in floral tissues is limited, so we used a series of experiments to determine whether prior leaf damage by Spodoptera exigua (Hübner) caterpillars affected florivore preference and performance on wild radish (Raphanus sativus L.). We found that Spodoptera exigua larvae preferred petals from control plants versus petals from plants exposed to prior leaf damage, and that larvae gained more mass on petals from control plants, although this depended on the presence of anthocyanins in the petals. Our results suggest that leaf damage can induce changes in petals that reduce Spodoptera exigua larval fitness
ALOHA-93 Measurements of Intrinsic AGW Characteristics Using the Airborne Airglow Imager and Groundbased Na Wind/Temperature Lidar
Monochromatic Acoustic Gravity Waves (AGWs) with periods \u3c 1 hour are a prevalent feature in the mesospheric airglow layers. These waves are important dynamically and energetically to the region where their temporal and spatial morphology are not well established. The purpose of this study is establish the intrinsic AGW characteristics over an extended region (as flown by the NCAR Electra aircraft) and to present the data in terms of the predicted spectral domain defined by the Brunt‐Vaisala frequency and the diffusive filtering limit proposed by Gardner [1994]. On October 21, 1993, observations were made from the NCAR Electra aircraft during a 6 hour flight in a large triangle N and W of Maui, for a integral distance of ∼3000 km. The entire area observed [∼1 M km²] had a monochromatic AGW propagating toward the NW and the western half had a SW propagating wave superimposed. These waves were also observed with the Michelson interferometer on the aircraft and an airglow imager at the Haleakala location during this time. Intrinsic phase velocities were computed where the Na Wind/Temperature (W/T) lidar at Haleakala provided a measure of the mean wind to compensate phase velocities observed with the imager. The data were tabulated and plotted in an AGW spectral reference frame and compared to cutoff conditions predicted by diffusive filtering theory
Comment on ``Measurement of the He mass diffusion coefficient in superfluid He over the 0.45--0.95 K temperature range
The role of 3He-3He collisions in our diffusion experiment is addressed and
shown to not be relevant to the measurement of 3He diffusion against phonons in
superfluid helium.Comment: Two pages, in Europhysics Letters forma
Microtesla MRI of the human brain combined with MEG
One of the challenges in functional brain imaging is integration of
complementary imaging modalities, such as magnetoencephalography (MEG) and
functional magnetic resonance imaging (fMRI). MEG, which uses highly sensitive
superconducting quantum interference devices (SQUIDs) to directly measure
magnetic fields of neuronal currents, cannot be combined with conventional
high-field MRI in a single instrument. Indirect matching of MEG and MRI data
leads to significant co-registration errors. A recently proposed imaging method
- SQUID-based microtesla MRI - can be naturally combined with MEG in the same
system to directly provide structural maps for MEG-localized sources. It
enables easy and accurate integration of MEG and MRI/fMRI, because microtesla
MR images can be precisely matched to structural images provided by high-field
MRI and other techniques. Here we report the first images of the human brain by
microtesla MRI, together with auditory MEG (functional) data, recorded using
the same seven-channel SQUID system during the same imaging session. The images
were acquired at 46 microtesla measurement field with pre-polarization at 30
mT. We also estimated transverse relaxation times for different tissues at
microtesla fields. Our results demonstrate feasibility and potential of human
brain imaging by microtesla MRI. They also show that two new types of imaging
equipment - low-cost systems for anatomical MRI of the human brain at
microtesla fields, and more advanced instruments for combined functional (MEG)
and structural (microtesla MRI) brain imaging - are practical.Comment: 8 pages, 5 figures - accepted by JM
Measurement of the 3He mass diffusion coefficient in superfluid 4He over the 0.45-0.95 K temperature range
We have measured the mass diffusion coefficient D of 3He in superfluid 4He at
temperatures lower than were previously possible. The experimental technique
utilizes scintillation light produced when neutron react with 3He nuclei, and
allows measurement of the 3He density integrated along the trajectory of a
well-defined neutron beam. By measuring the change in 3He density near a heater
as a function of applied heat current, we are able to infer values of D with
20% accuracy. At temperatures below 0.7 K and for concentrations of order
10^{-4} we find D=(2.0+2.4-1.2)T^-(6.5 -/+ 1.2) cm^2/s, in agreement with a
theoretical approximation.Comment: 8 pages, 5 figures. Submitted to Europhysics Letters and prepared in
that journal's forma
Elastic Scattering of Pions From the Three-nucleon System
We examine the scattering of charged pions from the trinucleon system at a
pion energy of 180 MeV. The motivation for this study is the structure seen in
the experimental angular distribution of back-angle scattering for pi+ 3He and
pi- 3H but for neither pi- 3He nor pi+ 3H. We consider the addition of a double
spin flip term to an optical model treatment and find that, though the
contribution of this term is non-negligible at large angles for pi+ 3He and pi-
3H, it does not reproduce the structure seen in the experiment.Comment: 15 pages + 5 figure
Application of Resonance Perturbation Theory to Dynamics of Magnetization in Spin Systems Interacting with Local and Collective Bosonic Reservoirs
We apply our recently developed resonance perturbation theory to describe the
dynamics of magnetization in paramagnetic spin systems interacting
simultaneously with local and collective bosonic environments. We derive
explicit expressions for the evolution of the reduced density matrix elements.
This allows us to calculate explicitly the dynamics of the macroscopic
magnetization, including characteristic relaxation and dephasing time-scales.
We demonstrate that collective effects (i) do not influence the character of
the relaxation processes but merely renormalize the relaxation times, and (ii)
significantly modify the dephasing times, leading in some cases to a
complicated (time inhomogeneous) dynamics of the transverse magnetization,
governed by an effective time-dependent magnetic field
In vivo Observation of Tree Drought Response with Low-Field NMR and Neutron Imaging
Using a simple low-field NMR system, we monitored water content in a livingtree in a greenhouse over two months. By continuously running thesystem, we observed changes in tree water content on a scale of halfan hour. The data showed a diurnal change in water content consistentboth with previous NMR and biological observations. Neutron imaging experiments showthat our NMR signal is primarily due to water being rapidly transported through the plant, and not to other sources of hydrogen, such as water in cytoplasm, or water in cell walls. After accountingfor the role of temperature in the observed NMR signal, we demonstratea change in the diurnal signal behavior due to simulated drought conditionsfor the tree. These results illustrate the utility of our system toperform noninvasive measurements of tree water content outside of a temperature controlled environment
- …