| 1. |
- Ackermann, M., et al.
(författare)
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THE FIRST FERMI-LAT CATALOG OF SOURCES ABOVE 10 GeV
- 2013
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 209:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a catalog of gamma-ray sources at energies above 10 GeV based on data from the Large Area Telescope (LAT) accumulated during the first 3 yr of the Fermi Gamma-ray Space Telescope mission. The first Fermi catalog of > 10 GeV sources (1FHL) has 514 sources. For each source we present location, spectrum, a measure of variability, and associations with cataloged sources at other wavelengths. We found that 449 (87%) could be associated with known sources, of which 393 (76% of the 1FHL sources) are active galactic nuclei. Of the 27 sources associated with known pulsars, we find 20 (12) to have significant pulsations in the range > 10 GeV (> 25 GeV). In this work we also report that, at energies above 10 GeV, unresolved sources account for 27% +/- 8% of the isotropic. gamma-ray background, while the unresolved Galactic population contributes only at the few percent level to the Galactic diffuse background. We also highlight the subset of the 1FHL sources that are best candidates for detection at energies above 50-100 GeV with current and future ground-based gamma-ray observatories.
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| 2. |
- Harvey, V. Lynn, et al.
(författare)
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Improving ionospheric predictability requires accurate simulation of the mesospheric polar vortex
- 2022
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Ingår i: Frontiers in Astronomy and Space Sciences. - : Frontiers Media SA. - 2296-987X. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The mesospheric polar vortex (MPV) plays a critical role in coupling the atmosphere-ionosphere system, so its accurate simulation is imperative for robust predictions of the thermosphere and ionosphere. While the stratospheric polar vortex is widely understood and characterized, the mesospheric polar vortex is much less well-known and observed, a short-coming that must be addressed to improve predictability of the ionosphere. The winter MPV facilitates top-down coupling via the communication of high energy particle precipitation effects from the thermosphere down to the stratosphere, though the details of this mechanism are poorly understood. Coupling from the bottom-up involves gravity waves (GWs), planetary waves (PWs), and tidal interactions that are distinctly different and important during weak vs. strong vortex states, and yet remain poorly understood as well. Moreover, generation and modulation of GWs by the large wind shears at the vortex edge contribute to the generation of traveling atmospheric disturbances and traveling ionospheric disturbances. Unfortunately, representation of the MPV is generally not accurate in state-of-the-art general circulation models, even when compared to the limited observational data available. Models substantially underestimate eastward momentum at the top of the MPV, which limits the ability to predict upward effects in the thermosphere. The zonal wind bias responsible for this missing momentum in models has been attributed to deficiencies in the treatment of GWs and to an inaccurate representation of the high-latitude dynamics. In the coming decade, simulations of the MPV must be improved.
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| 3. |
- Hedin, Jonas, et al.
(författare)
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The MAGIC meteoric smoke particle sampler
- 2014
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 118, s. 127-144
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Tidskriftsartikel (refereegranskat)abstract
- Between a few tons to several hundred tons of meteoric material enters the Earth's atmosphere each day, and most of this material is ablated and vaporized in the 70-120 km altitude region. The subsequent chemical conversion, re-condensation and coagulation of this evaporated material are thought to form nanometre sized meteoric smoke particles (MSPs). These smoke particles are then subject to further coagulation, sedimentation and global transport by the mesospheric circulation. MSPs have been proposed as a key player in the formation and evolution of ice particle layers around the mesopause region, i.e. noctilucent clouds (NLC) and polar mesosphere summer echoes (PMSE). MSPs have also been implicated in mesospheric heterogeneous chemistry to influence the mesospheric odd oxygen/odd hydrogen (O-x/HOx) chemistry, to play an important role in the mesospheric charge balance, and to be a significant component of stratospheric aerosol and enhance the depletion of O-3. Despite their apparent importance, little is known about the properties of MSPs and none of the hypotheses can be verified without direct evidence of the existence, altitude and size distribution, shape and elemental composition. The aim of the MAGIC project (Mesospheric Aerosol - Genesis, Interaction and Composition) was to develop an instrument and analysis techniques to sample for the first time MSPs in the mesosphere and return them to the ground for detailed analysis in the laboratory. MAGIC meteoric smoke particle samplers have been flown on several sounding rocket payloads between 2005 and 2011. Several of these flights concerned non-summer mesosphere conditions when pure MSP populations can be expected. Other flights concerned high latitude summer conditions when MSPs are expected to be contained in ice particles in the upper mesosphere. In this paper we present the MAGIC project and describe the MAGIC MSP sampler, the measurement procedure and laboratory analysis. We also present the attempts to retrieve MSPs from these flights, the challenges inherent to the sampling of nanometre sized particles and the subsequent analysis of the sampled material, and thoughts for the future. Despite substantial experimental efforts, the MAGIC project has so far failed to provide conclusive results. While particles with elemental composition similar to what is to be expected from MSPs have been found, the analysis has been compromised by challenges with different types of contamination and uncertainties in the sticking efficiency of the particles on the sampling surfaces.
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| 4. |
- Russell, James M., III, et al.
(författare)
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Analysis of northern midlatitude noctilucent cloud occurrences using satellite data and modeling
- 2014
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X. ; 119:6, s. 3238-3250
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by numerous ground-based noctilucent cloud (NLC) sightings at latitudes as low as similar to 40 degrees N in recent years, we have conducted a study to determine if there have been any systematic NLC increases in the midnorthern latitudes. This question is addressed through investigating both the measured and modeled polar mesospheric cloud (PMC) occurrence frequencies. Temperature measured by the SABER instrument on the TIMED satellite over the 2002-2011 time period and a 7 year water vapor climatology developed from the data measured by the MLS instrument on the Aura satellite for the 2005-2011 period are used to simulate midlatitude PMCs. PMCs measured by the OSIRIS instrument on the Odin satellite and the SHIMMER instrument on the STPSat-1 satellite are used to extensively validate the model-generated PMC results. After validating the model against the PMC data, the model results were used to examine changes in the PMCs at midlatitudes between 2002 and 2011. The results show a statistically significant increase in the number of PMCs each season in the latitude range 40 degrees N-55 degrees N for the 10 year period examined. Increases in cloud frequency appear to be driven by the corresponding temperature decreases over the same time period. During this time, solar activity decreased from an active to a quiet period, which might have been partially responsible for the temperature decrease over this time period.
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| 5. |
- Mlynczak, Martin G., et al.
(författare)
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Atomic oxygen in the mesosphere and lower thermosphere derived from SABER : Algorithm theoretical basis and measurement uncertainty
- 2013
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Ingår i: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 118:11, s. 5724-5735
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Tidskriftsartikel (refereegranskat)abstract
- Atomic oxygen (O) is a fundamental component in chemical aeronomy of Earth's mesosphere and lower thermosphere region extending from approximately 50 km to over 100 km in altitude. Atomic oxygen is notoriously difficult to measure, especially with remote sensing techniques from orbiting satellite sensors. It is typically inferred from measurements of the ozone concentration in the day or from measurements of the Meinel band emission of the hydroxyl radical (OH) at night. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the NASA Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite measures OH emission and ozone for the purpose of determining the O-atom concentration. In this paper, we present the algorithms used in the derivation of day and night atomic oxygen from these measurements. We find excellent consistency between the day and night O-atom concentrations from daily to annual time scales. We also examine in detail the collisional relaxation of the highly vibrationally excited OH molecule at night measured by SABER. Large rate coefficients for collisional removal of vibrationally excited OH molecules by atomic oxygen are consistent with the SABER observations if the deactivation of OH(9) proceeds solely by collisional quenching. An uncertainty analysis of the derived atomic oxygen is also given. Uncertainty in the rate coefficient for recombination of O and molecular oxygen is shown to be the largest source of uncertainty in the derivation of atomic oxygen day or night.
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| 6. |
- Siskind, David E., et al.
(författare)
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Decreases in atomic hydrogen over the summer pole : Evidence for dehydration from polar mesospheric clouds?
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:13
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Tidskriftsartikel (refereegranskat)abstract
- Observations from the Sounding of the Atmosphere with Broadband Emission Radiometry (SABER) instrument on the NASA/Thermospheric Ionosphere Mesosphere Energetics and Dynamics satellite show a surprising decrease in the inferred atomic hydrogen (H) over the polar regions in the lowermost thermosphere during the summer. This contrasts with predictions by global models that H should peak in this region at this time. We suggest the decrease is a consequence of the sequestering of the water vapor by the formation of polar mesospheric clouds (PMCs) that redistributes the H2O thus reducing the chemical source of H. This decrease is more pronounced in the Northern rather than the Southern summer which is roughly consistent with the known morphology of PMCs. A model calculation which includes a PMC parameterization gives good qualitative agreement with the data suggesting that this process should be considered in global models of the coupling between the middle and upper atmosphere. Copyright 2008 by the American Geophysical Union.
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| 7. |
- Siskind, David E., et al.
(författare)
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Implications of odd oxygen observations by the TIMED/SABER instrument for lower D region ionospheric modeling
- 2015
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 124, s. 63-70
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Tidskriftsartikel (refereegranskat)abstract
- We document the variability in atomic oxygen inferred by the Sounding of the Atmosphere with Broadband Emission Radiometry (SABER) instrument on the NASA/TIMED satellite in the lower mesosphere (50-80 km altitude) according to its diurnal, latitudinal, seasonal and solar cycle components. The dominant variation is diurnal and latitudinal. Below 75 km, seasonal and solar cycle effects are less than 5%. Accordingly, we have developed a simple climatology that depends upon local time and latitude and applied it to a model of the D region of the ionosphere. Between 60 and 70 km, atomic oxygen is important in governing the ratio of negative ions to electrons. Using the SABER O climatology along with a previously published climatology of nitric oxide based upon UARS/HALOE data, we compare our model results both to previous calculations and to a profile of electron density [e(-)] acquired by a rocket launched from Kwajalein Atoll. The model results are shown to be consistent with previously published calculations, but the comparison with the data reveals a dramatic discrepancy whereby the calculated [e(-)] is over an order of magnitude less than the observations below 65 km. The most plausible explanation involves changing the partition of negative charge between molecules such as O-2 which rapidly dissociate in sunlight versus heavier, more stable negative ions. Although observations of [e(-)] below 70 km are difficult and infrequent, more research should be invested to evaluate the pervasiveness and the seasonal, latitudinal and diurnal morphology of this model [e(-)] deficit. This may have practical implications as empirical models of the ionosphere predict a secondary maximum in HF radio absorption in the 70 km altitude region.
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| 8. |
- Siskind, David E., et al.
(författare)
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Polar mesospheric cloud mass and the ice budget: 3. Application of a coupled ice-chemistry-dynamics model and comparison with observations
- 2007
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Ingår i: Journal of Geophysical Research. ; 112, s. D8303-
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Tidskriftsartikel (refereegranskat)abstract
- We have combined a two-dimensional chemical/dynamics model with a monodisperse parameterization of polar mesospheric clouds (PMCs) to study the interaction of PMCs with the climate of the summer mesopause region. First, we show that PMC absorption of terrestrial and solar IR radiation lead to atmospheric heating rates which can exceed 10 K/day. This heat is dissipated by increased upwelling above the cloud layer and by a 2–6 K temperature increase. We then calculate the global PMC ice mass and evaluate its sensitivity to IR heating, assumed particle size and level of solar activity. Inclusion of the temperature increase in the model can reduce the calculated ice mass by up to a factor of two. The calculated solar cycle range in the ice mass is also about a factor of two. The calculated latitude distribution and solar cycle range of PMC ice mass are in good agreement with recent analyses of PMC satellite data. Finally, we test the hypothesis that PMC formation leads to ozone changes by comparing our model with ozone data from the Halogen Occultation Experiment (HALOE). The data show a 20–30% ozone enhancement above PMCs. In the model, dehydration above the cloud layer leads to an ozone increase due to lowered HO x . However, this competes with the temperature increase from IR absorption that can damp out this ozone increase. Surprisingly, for realistic estimates of the terrestrial IR flux, the model ozone response is reduced to well below that observed by HALOE.
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