2,092 research outputs found
Observational Quantification of the Energy Dissipated by Alfv\'en Waves in a Polar Coronal Hole: Evidence that Waves Drive the Fast Solar Wind
We present a measurement of the energy carried and dissipated by Alfv\'en
waves in a polar coronal hole. Alfv\'en waves have been proposed as the energy
source that heats the corona and drives the solar wind. Previous work has shown
that line widths decrease with height in coronal holes, which is a signature of
wave damping, but have been unable to quantify the energy lost by the waves.
This is because line widths depend on both the non-thermal velocity v_nt and
the ion temperature T_i. We have implemented a means to separate the T_i and
v_nt contributions using the observation that at low heights the waves are
undamped and the ion temperatures do not change with height. This enables us to
determine the amount of energy carried by the waves at low heights, which is
proportional to v_nt. We find the initial energy flux density present was 6.7
+/- 0.7 x 10^5 erg cm^-2 s^-1, which is sufficient to heat the coronal hole and
acccelerate the solar wind during the 2007 - 2009 solar minimum. Additionally,
we find that about 85% of this energy is dissipated below 1.5 R_sun,
sufficiently low that thermal conduction can transport the energy throughout
the coronal hole, heating it and driving the fast solar wind. The remaining
energy is roughly consistent with what models show is needed to provide the
extended heating above the sonic point for the fast solar wind. We have also
studied T_i, which we found to be in the range of 1 - 2 MK, depending on the
ion species.Comment: Accepted for the Astrophysical Journa
On the Energetics of the HCO + C CH + CO Reaction and Some Astrochemical Implications
We explore the energetics of the titular reaction, which current
astrochemical databases consider open at typical dense molecular (i.e., dark)
cloud conditions. As is common for reactions involving the transfer of light
particles, we assume that there are no intersystem crossings of the potential
energy surfaces involved. In the absence of any such crossings, we find that
this reaction is endoergic and will be suppressed at dark cloud temperatures.
Updating accordingly a generic astrochemical model for dark clouds changes the
predicted gas-phase abundances of 224 species by greater than a factor of 2. Of
these species, 43 have been observed in the interstellar medium. Our findings
demonstrate the astrochemical importance of determining the role of intersystem
crossings, if any, in the titular reaction.Comment: Accepted for publication in ApJ; 14 pages, 2 figures, and 1 tabl
Spectroscopic measurements of the ion velocity distribution at the base of the fast solar wind
In situ measurements of the fast solar wind reveal non-thermal distributions of electrons, protons, and minor ions extending from 0.3 au to the heliopause. The physical mechanisms responsible for these non-thermal properties and the location where these properties originate remain open questions. Here, we present spectroscopic evidence, from extreme ultraviolet spectroscopy, that the velocity distribution functions (VDFs) of minor ions are already non-Gaussian at the base of the fast solar wind in a coronal hole, at altitudes of <1.1 R ⊙. Analysis of Fe, Si, and Mg spectral lines reveals a peaked line-shape core and broad wings that can be characterized by a kappa VDF. A kappa distribution fit gives very small kappa indices off-limb of κ ≈ 1.9–2.5, indicating either (a) ion populations far from thermal equilibrium, (b) fluid motions such as non-Gaussian turbulent fluctuations or non-uniform wave motions, or (c) some combination of both. These observations provide important empirical constraints for the source region of the fast solar wind and for the theoretical models of the different acceleration, heating, and energy deposition processes therein. To the best of our knowledge, this is the first time that the ion VDF in the fast solar wind has been probed so close to its source region. The findings are also a timely precursor to the upcoming 2018 launch of the Parker Solar Probe, which will provide the closest in situ measurements of the solar wind at approximately 0.04 au (8.5 solar radii)
Laser Scheme for Doppler Cooling of the Hydroxyl Cation (OH)
We report on a cycling scheme for Doppler cooling of trapped OH ions
using transitions between the electronic ground state and the
first excited triplet state . We have identified relevant transitions
for photon cycling and repumping, have found that coupling into other
electronic states is strongly suppressed, and have calculated the number of
photon scatterings required to cool OH to a temperature where Raman
sideband cooling can take over. In contrast to the standard approach, where
molecular ions are sympathetically cooled, our scheme does not require
co-trapping of another species and opens the door to the creation of pure
samples of cold molecular ions with potential applications in quantum
information, quantum chemistry, and astrochemistry. The laser cooling scheme
identified for OH is efficient despite the absence of near-diagonal
Franck-Condon factors, suggesting that broader classes of molecules and
molecular ions are amenable to laser cooling than commonly assumed.Comment: 6 pages, 3 figure
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Shortcomings of the R-matrix method for treating dielectronic recombination
By performing radiation-damped R-matrix scattering calculations for the photorecombination of Fe17+ forming Fe16+, we demonstrate and discuss the difficulties and fundamental inaccuracies associated with the R-matrix method for treating dielectronic recombination (DR). Our R-matrix results significantly improve upon earlier R-matrix results for this ion. However, we show theoretically that all R-matrix methods are unable to account accurately for the phenomenon of radiative decay followed by autoionization. For Fe17+, we demonstrate numerically that this results in an overestimate of the DR cross section at the series limit, which tends to our analytically predicted amount of 40%. We further comment on the need for fine resonance resolution and the inclusion of radiation damping effects. Overall, slightly better agreement with experiment is still found with the results of perturbative calculations, which are computationally more efficient than R-matrix calculations by more than two orders of magnitude
Automated detection of laser cooling schemes for ultracold molecules
One of the demanding frontiers in ultracold science is identifying laser
cooling schemes for complex atoms and molecules, out of their vast spectra of
internal states. Motivated by a need to expand the set of available ultracold
molecules for applications in fundamental physics, chemistry, astrochemistry,
and quantum simulation, we propose and demonstrate an automated graph-based
search approach for viable laser cooling schemes. The method is time efficient
and the outcomes greatly surpass the results of manual searches used so far. We
discover new laser cooling schemes for C, OH, CN, YO, and CO that
can be viewed as surprising or counterintuitive compared to previously
identified laser cooling schemes. In addition, a central insight of this work
is that the reinterpretation of quantum states and transitions between them as
a graph can dramatically enhance our ability to identify new quantum control
schemes for complex quantum systems. As such, this approach will also be
applicable to complex atoms and, in fact, any complex many-body quantum system
with a discrete spectrum of internal states.Comment: 10 pages and 5 figures in the main text + 11 pages and 7 figures in
appendices. Comments and feedback are very welcome. Code is available at
https://github.com/Shmoo137/Detection-Of-Laser-Cooled-Molecule
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Collisional Ionization Equilibrium for Optically Thin Plasmas
Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have utilized state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He to Zn. We have also utilized state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the recommended electron impact ionization data of Mazzotta et al. (1998), we have calculated improved collisional ionization equilibrium calculations. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. (1998) for all elements from H to Ni, and with the fractional abundances derived from the modern DR and RR calculations of Gu (2003a,b, 2004) for Mg, Si, S, Ar, Ca, Fe, and Ni
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Dielectronic recombination data for dynamic finite-density plasmas VII. The neon isoelectronic sequence
Dielectronic recombination (DR) and radiative recombination (RR) data for neon-like ions forming sodium-like systems has been calculated as part of the assembly of a DR database necessary for modelling of dynamic and/or finite-density plasmas (Badnell et al. 2003). Dielectronic recombination coefficients for neon-like ions from Na+ to Zn20+, as well as Kr26+, Mo32+, Cd38+, and Xe44+, are presented and the results discussed
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Dielectronic recombination data for dynamic finite-density plasmas II. The oxygen isoelectronic sequence
Dielectronic recombination (DR) and radiative recombination (RR) data for oxygen-like ions forming fluorine-like ions have been calculated as part of the assembly of a level-resolved DR and RR database necessary for modelling of dynamic finite-density plasmas (Badnell et al. 2003). Total DR and RR rate coefficients for F+ to Zn22+ are presented and the results discussed. By comparison between perturbative and R-matrix results, we find that RR/DR interference effects are negligible even for the lowest-charged F+ member. We also find that the 2→2 low-temperature DR (no change in the principal quantum number of the core electrons) does not scale smoothly with nuclear charge Z due to resonances straddling the ionization limit, thereby making explicit calculations for each ion necessary. These RR and DR data are suitable for modelling of solar and cosmic plasmas under conditions of collisional ionization equilibrium, photoionization equilibrium, and non-equilibrium ionization
Thiol-yne \u27Click\u27 Chemistry As a Route to Functional Lipid Mimetics
Thiol-alkyne \u27click\u27 chemistry is a modular, efficient mechanism to synthesize complex A2B 3-arm star polymers. This general motif is similar to a phospholipid where the A blocks correspond to lypophilic chains and the B block represents the polar head group. In this communication we employ thiol-yne chemistry to produce polypeptide-based A2B lipid mimetics. The utility of the thiol-yne reaction is demonstrated by using a divergent and a convergent approach in the synthesis. These polymers self-assemble in aqueous solution into spherical vesicles with a relatively narrow size distribution independent of block composition over the range studied. Using the thiol-yne convergent synthesis, we envision a modular approach to functionalize proteins or oligopeptides with lipophilic chains that can imbed seamlessly into a cell membrane
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