22 research outputs found
Disk Detective: Discovery of New Circumstellar Disk Candidates through Citizen Science
The Disk Detective citizen science project aims to find new stars with 22
micron excess emission from circumstellar dust using data from NASA's WISE
mission. Initial cuts on the AllWISE catalog provide an input catalog of
277,686 sources. Volunteers then view images of each source online in 10
different bands to identify false-positives (galaxies, background stars,
interstellar matter, image artifacts, etc.). Sources that survive this online
vetting are followed up with spectroscopy on the FLWO Tillinghast telescope.
This approach should allow us to unleash the full potential of WISE for finding
new debris disks and protoplanetary disks. We announce a first list of 37 new
disk candidates discovered by the project, and we describe our vetting and
follow-up process. One of these systems appears to contain the first debris
disk discovered around a star with a white dwarf companion: HD 74389. We also
report four newly discovered classical Be stars (HD 6612, HD 7406, HD 164137,
and HD 218546) and a new detection of 22 micron excess around a previously
known debris disk host star, HD 22128.Comment: 50 pages, accepted for publication in the Astrophysical Journa
Near-Earth injection of MeV electrons associated with intense dipolarization electric fields: Van Allen Probes observations.
Substorms generally inject tens to hundreds of keV electrons, but intense substorm electric fields have been shown to inject MeV electrons as well. An intriguing question is whether such MeVelectron injections can populate the outer radiation belt. Here we present observations of a substorm injection of MeV electrons into the inner magnetosphere. In the premidnight sector at L ∼ 5.5, Van Allen Probes (Radiation Belt Storm Probes)-A observed a large dipolarization electric field (50 mV/m) over ∼40 s and a dispersionless injection of electrons up to ∼3 MeV. Pitch angle observations indicated betatron acceleration of MeV electrons at the dipolarization front. Corresponding signals of MeV electron injection were observed at LANL-GEO, THEMIS-D, and GOES at geosynchronous altitude. Through a series of dipolarizations, the injections increased the MeV electron phase space density by 1 order of magnitude in less than 3 h in the outer radiation belt (L > 4.8). Our observations provide evidence that deep injections can supply significant MeV electrons
Tidal Wave-Driven Variability in the Mars Ionosphere-Thermosphere System
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited
https://creativecommons.org/licenses/by/4.0/In order to further evaluate the behavior of ionospheric variations at Mars, we investigate the Martian ionosphere-thermosphere (IT) perturbations associated with non-migrating thermal tides using over four years of Mars Atmosphere and Volatile Evolution (MAVEN) in situ measurements of the IT electron and neutral densities. The results are consistent with those of previous studies, namely strong correlation between the tidal perturbations in electron and neutral densities on the dayside at altitudes ~150–185 km, as expected from photochemical theory. In addition, there are intervals during which this correlation extends to higher altitudes, up to ~270 km, where diffusive transport of plasma plays a dominant role over photochemical processes. This is significant because at these altitudes the thermosphere and ionosphere are only weakly coupled through collisions. The identified non-migrating tidal wave variations in the neutral thermosphere are predominantly wave-1, wave-2, and wave-3. Wave-1 is often the dominant wavenumber for electron density tidal variations, particularly at high altitudes over crustal fields. The Mars Climate Database (MCD) neutral densities (below 300 km along the MAVEN orbit) shows clear tidal variations which are predominantly wave-2 and wave-3, and have similar wave amplitudes to those observed.© 2020 by the authors.The MAVEN project is supported by NASA through the Mars Exploration Program. Work at the Laboratory for Atmospheric and Space Physics, at Boston University and at the University of Michigan was done under the MAVEN project. F.G. is funded by the Spanish Ministerio de Ciencia, Innovaci?n y Universidades, the Agencia Estatal de Investigacion and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award to the Instituto de Astrof?sica de Andaluc?a (SEV-2017-0709). X.F. is funded by NASA grant 80NSSC19K0562.We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI)Peer reviewe
Scattering by whistler-mode waves during a quiet period perturbed by substorm activity
International audienceWe study the dynamics of radiation belt electrons during a 10-day quiet period perturbed by substorm activity and preceding a high-speed stream (HSS), aiming at a global description of the radiation belts in L-shell, L in [2, 6], and energy [0.1, 10] MeV. We combine Van Allen Probes observations and Fokker-Planck numerical simulations of pitch-angle diffusion. The Fokker-Planck model uses event-driven pitch angle diffusion coefficients from whistler-mode waves, built from the wave properties and the ambient plasma density measurements from the Van Allen Probes. We first find this event has some similar characteristics to regular quiet times previously studied; a widely extended plasmasphere within which we observe strong and varying whistler-mode waves. These ambient conditions lead to strong pitch-angle scattering, which contributes to the creation of a wide slot region as well as a significant decay of the outer radiation belts, which are observed and qualitatively well simulated. In addition, we find the substorm activity causes short duration (within ± 4h) decay of the plasma density and a lowering amplitude of the whistler-mode waves within the plasmasphere, both causing opposite effects in terms of pitch angle diffusion. This leads to a diminution of pitch-angle diffusion at the time of the main substorm activity. Conversely, whistler-mode waves become enhanced in the time periods between the substorm injections. All effects cumulated, we find an enhancement of pitch angle diffusion by whistler-mode waves above L~4.7 during the 10-day period. This directly relates to the combination of quietness and substorm activity which allows pitch angle diffusing of up to 1 MeV electrons in the outer belt. Relativistic electrons of 1–2 MeV remain trapped in the outer belt, from L~4.7 to L~5.2, forming, in both the observations and the simulations, a distinct pocket of remnant electrons
Van Allen Probes observation of plasmaspheric hiss modulated by injected energetic electrons
Expression of the homeobox Hox-4 genes and the specification of position in chick wing development
Vertebrate homeobox genes
In the former part of the review the principal available data about Hox genes, their molecular organisation and their expression in vertebrate embryos, with particular emphasis for mammals, are briefly summarized. In the latter part we analysed the expression of four mouse homeobox genes related to two Drosophila genes expressed in the developing head of the fly: Emx1 and Emx2, related to ems, and Otx1 and Otx2, related to otd. \ua9 1994 Kluwer Academic Publishers