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Articles 1 - 19 of 19
Full-Text Articles in Physical Sciences and Mathematics
Wet Deposition In A Global Size-Dependent Aerosol Transport Model: 2. Influence Of The Scavenging Scheme On 210pb Vertical Profiles, Surface Concentrations, And Deposition, W Guelle, Y J. Balkanski, Jack E. Dibb, M Schulz, F Dulac
Wet Deposition In A Global Size-Dependent Aerosol Transport Model: 2. Influence Of The Scavenging Scheme On 210pb Vertical Profiles, Surface Concentrations, And Deposition, W Guelle, Y J. Balkanski, Jack E. Dibb, M Schulz, F Dulac
Earth Sciences
The main atmospheric sink for submicron aerosols is wet removal. Lead 210, the radioactive decay product of 222Rn, attaches immediately after being formed to submicron particles. Here we compare the effects of three different wet-scavenging schemes used in global aerosol simulations on the 210Pb aerosol distribution using an off-line, size-resolved, global atmospheric transport model. We highlight the merits and shortcomings of each scavenging scheme at reproducing available measurements, which include concentrations in surface air and deposition, as well as vertical profiles observed over North America and western and central North Pacific. We show that model-measurement comparison of total …
Soluble Species In Aerosol And Snow And Their Relationship At Glacier 1, Tien Shan, China, Junying Sun, Dahe Qin, Paul A. Mayewski, Jack E. Dibb, Sallie I. Whitlow, Zhongqin Lee, Qinzhao Yang
Soluble Species In Aerosol And Snow And Their Relationship At Glacier 1, Tien Shan, China, Junying Sun, Dahe Qin, Paul A. Mayewski, Jack E. Dibb, Sallie I. Whitlow, Zhongqin Lee, Qinzhao Yang
Earth Sciences
Simultaneous sampling of aerosol (n = 20) and snow (n = 114) was made at Glacier 1, Tien Shan, between May 19 and June 29, 1996. Similar temporal patterns of some major ion (calcium, magnesium, potassium, sodium, chloride, and sulfate) concentrations between snow and aerosol show that snow chemistry basically reflects changes in the chemistry of the atmosphere. This gives us confidence in the reconstruction of past atmospheric change using some snow data. There are no significant correlations between aerosol and snow samples for ammonium and nitrate. This suggests that post-depositional and/or post-collection processes may alter ammonium and nitrate concentrations …
Nitric Acid Scavenging By Mineral And Biomass Burning Aerosols, A Tabazadeh, M Z. Jacobson, H B. Singh, O Brian Toon, J S. Lin, R B. Chatfield, A N. Thakur, R. Talbot, Jack E. Dibb
Nitric Acid Scavenging By Mineral And Biomass Burning Aerosols, A Tabazadeh, M Z. Jacobson, H B. Singh, O Brian Toon, J S. Lin, R B. Chatfield, A N. Thakur, R. Talbot, Jack E. Dibb
Earth Sciences
The abundance of gas phase nitric acid in the upper troposphere is overestimated by global chemistry-transport models, especially during the spring and summer seasons. Recent aircraft data obtained over the central US show that mineral aerosols were abundant in the upper troposphere during spring. Chemical reactions on mineral dust may provide an important sink for nitric acid. In regions where the mineral dust abundance is low in the upper troposphere similar HNO3 removal processes may occur on biomass burning aerosols. We propose that mineral and biomass burning aerosols may provide an important global sink for gas phase nitric acid, …
Development Of A Hard X-Ray Polarimeter For Astrophysics, Mark L. Mcconnell, John R. Macri, M Mcclish, James M. Ryan, D J. Forrest, W T. Vestrand
Development Of A Hard X-Ray Polarimeter For Astrophysics, Mark L. Mcconnell, John R. Macri, M Mcclish, James M. Ryan, D J. Forrest, W T. Vestrand
Space Science Center
We have been developing a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (100-300 keV) from astrophysical sources. A laboratory prototype polarimeter has been used to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. Our design concept places a self-containedpolarimeter module on the front-end of a a 5-inch position sensitive PMT (PSPMT). We are currently working on the fabrication of a science model based on this PSPMT concept. Although the emphasis of our development effort is towards measuring hard X-rays from …
Cusp Energetic Ions: A Bow Shock Source, S. W. Chang, J. D. Scudder, S. A. Fuselier, J. F. Fennell, K. J. Trattner, J. S. Pickett, Harlan E. Spence, J. D. Menietti, W. K. Peterson, R. P. Lepping, R. Friedel
Cusp Energetic Ions: A Bow Shock Source, S. W. Chang, J. D. Scudder, S. A. Fuselier, J. F. Fennell, K. J. Trattner, J. S. Pickett, Harlan E. Spence, J. D. Menietti, W. K. Peterson, R. P. Lepping, R. Friedel
Physics & Astronomy
Recent interpretations of cusp energetic ions observed by the POLAR spacecraft have suggested a new energization process in the cusp [Chen et al., 1997; 1998]. Simultaneous enhancement of H+, He+2, and O>+2 fluxes indicates that they are of solar wind origin. In the present study, we examine H+ and He+2 energy spectra from 20 eV to several 100 keV measured by the Hydra, Toroidal Imaging Mass-Angle Spectrograph (TIMAS), and Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on POLAR. The combined spectrum for each species is shown to be continuous with a …
The Limits Of Spatial Resolution Achievable Using A 30khz Multibeam Sonar: Model Predictions And Field Results, John E. Hughes Clarke, James V. Gardner, Mike Torresan, Larry A. Mayer
The Limits Of Spatial Resolution Achievable Using A 30khz Multibeam Sonar: Model Predictions And Field Results, John E. Hughes Clarke, James V. Gardner, Mike Torresan, Larry A. Mayer
Center for Coastal and Ocean Mapping
A Simrad EM300 multibeam sonar was used to attempt to resolve small (-5m high) targets in 450m of water. The targets had previously been surveyed using a deeply towed 59 kHz sidescan sonar. Using multisector active yaw, pitch and roll compensation, together with dynamically altering angular sectors, the sonar is capable of maintaining sounding densities of as tight as 10m spacing in these water depths. This is significantly smaller than the largest dimension of the projected beam footprints (1 6-64m). The observed data suggest that the targets are intermittently resolved. The field results compare well to the output of a …
The Global Response Of Relativistic Radiation Belt Electrons To The January 1997 Magnetic Cloud, G. D. Reeves, D. N. Baker, R. D. Belian, J. B. Blake, T. E. Cayton, J. F. Fennell, R. H.W. Friedel, M. M. Meier, R. S. Selesnick, Harlan E. Spence
The Global Response Of Relativistic Radiation Belt Electrons To The January 1997 Magnetic Cloud, G. D. Reeves, D. N. Baker, R. D. Belian, J. B. Blake, T. E. Cayton, J. F. Fennell, R. H.W. Friedel, M. M. Meier, R. S. Selesnick, Harlan E. Spence
Physics & Astronomy
In January 1997 a large fleet of NASA and US military satellites provided the most complete observations to date of the changes in >2 MeV electrons during a geomagnetic storm. Observations at geosynchronous orbit revealed a somewhat unusual two-peaked enhancement in relativistic electron fluxes [ Reeves et al., 1998]. In the heart of the radiation belts at L ≈ 4, however, there was a single enhancement followed by a gradual decay. Radial profiles from the POLAR and GPS satellites revealed three distinct phases. (1) In the acceleration phase electron fluxes increased simultaneously at L ≈ 4–6. (2) During the …
Coronal Mass Ejections, Magnetic Clouds, And Relativistic Magnetospheric Electron Events: Istp, D. N. Baker, T. I. Pulkkinen, X. Li, S. G. Kanekal, J. B. Blake, R. S. Selesnick, M. G. Henderson, G. D. Reeves, Harlan E. Spence, G. Rostoker
Coronal Mass Ejections, Magnetic Clouds, And Relativistic Magnetospheric Electron Events: Istp, D. N. Baker, T. I. Pulkkinen, X. Li, S. G. Kanekal, J. B. Blake, R. S. Selesnick, M. G. Henderson, G. D. Reeves, Harlan E. Spence, G. Rostoker
Physics & Astronomy
The role of high-speed solar wind streams in driving relativistic electron acceleration within the Earth's magnetosphere during solar activity minimum conditions has been well documented. The rising phase of the new solar activity cycle (cycle 23) commenced in 1996, and there have recently been a number of coronal mass ejections (CMEs) and related “magnetic clouds” at 1 AU. As these CME/cloud systems interact with the Earth's magnetosphere, some events produce substantial enhancements in the magnetospheric energetic particle population while others do not. This paper compares and contrasts relativistic electron signatures observed by the POLAR, SAMPEX, Highly Elliptical Orbit, and geostationary …
The Discovery Of Trapped Energetic Electrons In The Outer Cusp, R. B. Sheldon, Harlan E. Spence, J. D. Sullivan, T. A, Fritz, Jiasheng Chen
The Discovery Of Trapped Energetic Electrons In The Outer Cusp, R. B. Sheldon, Harlan E. Spence, J. D. Sullivan, T. A, Fritz, Jiasheng Chen
Physics & Astronomy
We report on the POLAR/CEPPAD discovery of a trapped, 60°<θ<120° pitch angle electron population in the outer cusp (7−9+ Re), whose energetic electron component extends from below 30 keV to ∼2 MeV. Because the time variability in the outer cusp precludes mapping with POLAR, we have carried out test particle simulations using the Tsyganenko 1996 model (T96) to demonstrate the trapping of these energy electrons in the outer cusp region and the resonant frequencies of its trapped motion. We discuss the boundaries and regions of the cusp trap and show that it is analogous to the dipole trap. We show that the phase space densities observed there are equal or greater than the phase space densities observed in the radiation belts at constant magnetic moment, thus allowing the possibility of diffusive filling of the radiation belts from the cusp
Influence Of Vertical Transport On Free Tropospheric Aerosols Over The Central Usa In Springtime, R. Talbot, Jack E. Dibb, M Loomis
Influence Of Vertical Transport On Free Tropospheric Aerosols Over The Central Usa In Springtime, R. Talbot, Jack E. Dibb, M Loomis
Earth Sciences
Measurements of the atmospheric aerosol chemical composition during the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) indicate substantial vertical transport of boundary layer aerosol to the free troposphere over the south-central United States during springtime. Mixing ratios of water-soluble aerosol Ca 2+ at 6 - 12 km altitude exhibited a median mixing ratio of 20 pptv, with 15% of the measurements > 100 pptv and a maximum of ! 235 pptv. In air parcels with enhanced Ca 2+, the ratios K+/Ca 2+, Mg2+/Ca 2+, and Na+/Ca 2+ in the bulk aerosol were distinctly characteristic of those in limestone and/or …
Tropospheric Sulfate Distribution During Success: Contributions From Jet Exhaust And Surface Sources, Jack E. Dibb, R. Talbot, M Loomis
Tropospheric Sulfate Distribution During Success: Contributions From Jet Exhaust And Surface Sources, Jack E. Dibb, R. Talbot, M Loomis
Earth Sciences
The distribution of SO4= aerosol over the central US during SUCCESS indicates that surface sources of SO4= and SO2 in the western US caused SO4= enhancements up to 10 km altitude. The mean (median) SO4= mixing ratio in the mid- and upper-troposphere increased from 24 (16) pptv over the Pacific ocean to 58 (29) pptv over the central plains. Above 10 km the SO4=mixing ratio was essentially the same in both regions, and also when the geographic classifications were further partitioned into upper tropospheric and lower stratospheric categories …
Relationship Between Continuous Aerosol Measurements And Firn Core Chemistry Over A 10-Year Period At The South Pole, M H. Bergin, E A. Meyerson, Jack E. Dibb, Paul A. Mayewski
Relationship Between Continuous Aerosol Measurements And Firn Core Chemistry Over A 10-Year Period At The South Pole, M H. Bergin, E A. Meyerson, Jack E. Dibb, Paul A. Mayewski
Earth Sciences
Before ice core chemistry can be used to estimate past atmospheric chemistry it is necessary to establish an unambiguous link between concentrations of chemical species in the air and snow. For the first time a continuous long-term record of aerosol properties (aerosol light scattering coefficient, σsp, and Ångström exponent, å) at the South Pole are compared with the chemical record from a high resolution firn core (∼10 samples per year) covering the period from 1981 to 1991. Seasonal signals in å, associated with winter minima due to coarse mode seasalt and summer maxima due to accumulation mode sulfate …
Ampte/Cce‐Scatha Simultaneous Observations Of Substorm‐Associated Magnetic Fluctuations, S. Ohtani, K. Takahashi, T. Higuchi, A. T. Y. Lui, Harlan E. Spence, J. F. Fennell
Ampte/Cce‐Scatha Simultaneous Observations Of Substorm‐Associated Magnetic Fluctuations, S. Ohtani, K. Takahashi, T. Higuchi, A. T. Y. Lui, Harlan E. Spence, J. F. Fennell
Physics & Astronomy
This study examines substorm-associated magnetic field fluctuations observed by the AMPTE/CCE and SCATHA satellites in the near-Earth tail. Three tail reconfiguration events are selected, one event on August 28, 1986, and two consecutive events on August 30, 1986. The fractal analysis was applied to magnetic field measurements of each satellite. The result indicates that (1) the amplitude of the fluctuation of the north-south magnetic component is larger, though not overwhelmingly, than the amplitudes of the other two components and (2) the magnetic fluctuations do have a characteristic timescale, which is several times the proton gyroperiod. In the examined events the …
Air-Snow Exchange Of Hno3 And Noy At Summit, Greenland, Jack E. Dibb, R. Talbot, J W. Munger, Daniel J. Jacob, S M. Fan
Air-Snow Exchange Of Hno3 And Noy At Summit, Greenland, Jack E. Dibb, R. Talbot, J W. Munger, Daniel J. Jacob, S M. Fan
Earth Sciences
Ice core records of NO3− deposition to polar glaciers could provide unrivaled information on past photochemical status and N cycling dynamics of the troposphere, if the ice core records could be inverted to yield concentrations of reactive N oxides in the atmosphere at past times. Limited previous investigations at Summit, Greenland, have suggested that this inversion may be difficult, since the levels of HNO3 and aerosol-associated NO3− over the snow are very low in comparison with those of NO3− in the snow. In addition, it appears that some fraction of the NO3 …
Cusp Energetic Particle Events: Implications For A Major Acceleration Region Of The Magnetosphere, J. Chen, T. A. Fritz, R. B. Sheldon, Harlan E. Spence, W. N. Spjeldvik, J. F. Fennell, S. Livi, C. T. Russell, J. S. Pickett, D. A. Gurnett
Cusp Energetic Particle Events: Implications For A Major Acceleration Region Of The Magnetosphere, J. Chen, T. A. Fritz, R. B. Sheldon, Harlan E. Spence, W. N. Spjeldvik, J. F. Fennell, S. Livi, C. T. Russell, J. S. Pickett, D. A. Gurnett
Physics & Astronomy
The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on board the Polar spacecraft observed 75 energetic particle events in 1996 while the satellite was at apogee. All of these events were associated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Experiment (MFE) on Polar. These new events showed several unusual features: (1) They were detected in the dayside polar cusp near the apogee of Polar with about 79% of the total events in the afternoonside and 21% in the morningside; (2) an individual event could last for hours; (3) the measured …
The Pursuit Of Isotopic And Molecular Fire Tracers In The Polar Atmosphere And Cryosphere, L A. Currie, Jack E. Dibb, G A. Klouda, B A. Benner Jr., J M. Conny, S R. Biegalski, D B. Klinedinst, D R. Cahoon, N C. Hsu
The Pursuit Of Isotopic And Molecular Fire Tracers In The Polar Atmosphere And Cryosphere, L A. Currie, Jack E. Dibb, G A. Klouda, B A. Benner Jr., J M. Conny, S R. Biegalski, D B. Klinedinst, D R. Cahoon, N C. Hsu
Earth Sciences
We present an overview of recent multidisciplinary, multi-institutional efforts to identify and date major sources of combustion aerosol in the current and paleoatmospheres. The work was stimulated, in part, by an atmospheric particle 'sample of opportunity' collected at Summit, Greenland in August 1994, that bore the 14C imprint of biomass burning. During the summer field seasons of 1995 and 1996, we collected air filter, surface snow and snowpit samples to investigate chemical and isotopic evidence of combustion particles that had been transported from distant fires. Among the chemical tracers employed for source identification are organic acids, potassium and ammonium ions, …
Mev Measurements Of Gamma-Ray Bursts By Cgro-Comptel, A Connors, W Collmar, W Hermsen, S C. Kappadath, R M. Kippen, L Kuiper, Mark L. Mcconnell, F Pelaez, James M. Ryan, V Schonfelder, M Varendorff, O R. Williams, C Winkler, C A. Young
Mev Measurements Of Gamma-Ray Bursts By Cgro-Comptel, A Connors, W Collmar, W Hermsen, S C. Kappadath, R M. Kippen, L Kuiper, Mark L. Mcconnell, F Pelaez, James M. Ryan, V Schonfelder, M Varendorff, O R. Williams, C Winkler, C A. Young
Space Science Center
Since the launch of the Compton Gamma-Ray Observatory in April 1991, the imaging COMPTEL telescope has accumulated positions and 0.75–30 MeV spectra of more than thirty gamma-ray bursts within its ∼π sr field of view. In an ongoing collaboration with BACODINE/GCN, COMPTEL positions are relayed to a global network of multiwavelength observers in near real time (∼10 minutes). Here we summarize the MeV properties, and present spatial, spectral, and temporal data for the latest of these events, GRB 970807. In concurrence with earlier SMM and current BATSE, OSSE, and EGRET measurements, COMPTEL data add to the accumulating evidence that GRB …
Sontrac—A Low Background, Large Area Solar Neutron Spectrometer, James M. Ryan, D Holslin, John R. Macri, Mark L. Mcconnell, Cornelia B. Wunderer
Sontrac—A Low Background, Large Area Solar Neutron Spectrometer, James M. Ryan, D Holslin, John R. Macri, Mark L. Mcconnell, Cornelia B. Wunderer
Space Science Center
SONTRAC is a scintillating fiber neutron detector designed to measure solar flare neutrons from a balloon or spacecraft platform. The instrument is comprised of alternating orthogonal planes of scintillator fibers viewed by photomultiplier tubes and image intensifier/CCD camera optics. It operates by tracking the paths of recoil protons from the double scatter of 20 to 200 MeV neutrons off hydrogen in the plastic scintillator, thereby providing the necessary information to determine the incident neutron direction and energy. SONTRAC is also capable of detecting and measuring high-energy gamma rays >20 MeV as a “solid-state spark chamber.” The self-triggering and track imaging …
A Hard X-Ray Solar Flare Polarimeter Design Based On Scintillating Fibers, James M. Ryan, Mark L. Mcconnell, D J. Forrest, John R. Macri, M Mcclish, W T. Vestrand
A Hard X-Ray Solar Flare Polarimeter Design Based On Scintillating Fibers, James M. Ryan, Mark L. Mcconnell, D J. Forrest, John R. Macri, M Mcclish, W T. Vestrand
Space Science Center
We have developed a design for a Compton scatter polarimeter to measure the polarization of hard X-rays (50–300 keV) from solar flares. The modular design is based on an annular array of scintillating fibers coupled to a 5-inch position-sensitive PMT. Incident photons scatter from the fiber array into a small array of NaI detectors located at the center of the annulus. The location of the interactions in both the fiber array and in the NaI array can be used to measure the linear polarization of the incident flux. This compact design may be well-suited to a variety of astrophysical applications. …