Ray Sopa - Academia.edu (original) (raw)
Papers by Ray Sopa
The evolution of the high-energy electron population through the substorm cycle has been studied ... more The evolution of the high-energy electron population through the substorm cycle has been studied directly with in situ observations such as those provided by the LANL-SOPA instruments, and indirectly through precipitation seen via riometers, X-ray imagers, and other instruments. Ground-based and other remote sensing techniques provide the advantage of being able to track such disturbances in both space and time, with caveats including the fact that information about the energy of the precipitating particles is limited. Using data from the 13 instrument NORSTAR (formerly CANOPUS) riometer array in north-central Canada, we are examining a large number of substorm events. We are focussing on a transient ``spike'' of significant absorption that occurs during most substorm expansive phases. Seen from any one station, the spike lasts several minutes. More globally, it typically takes tens of minutes to propagate across the NORSTAR array. The propagation is in general a combination of azimuthal (ie., East or West) and poleward. In this paper, we present a statistical comparision between the propagation characteristics of the spike and other substorm features such as dipolarization, auroral bulge, Pi2s, and injections. As well, we present a detailed analysis of several events. Our objective is to determine the magnetospheric source of this transient high energy precipitation.
Prokaryotic chromosomes and plasmids encode partitioning systems that are required for DNA segreg... more Prokaryotic chromosomes and plasmids encode partitioning systems that are required for DNA segregation at cell division. The plasmid partitioning loci encode two proteins, ParA and ParB, and a cis-acting centromere-like site denoted parS. The chromosomally encoded homologues of ParA and ParB, Soj and Spo0J, play an active role in chromosome segregation during bacterial cell division and sporulation. Spo0J is a DNA-binding protein that binds to parS sites in vivo. We have solved the X-ray crystal structure of a C-terminally truncated Spo0J (amino acids 1-222) from Thermus thermophilus to 2.3 A resolution by multiwavelength anomalous dispersion. It is a DNA-binding protein with structural similarity to the helix-turn-helix (HTH) motif of the lambda repressor DNA-binding domain. The crystal structure is an antiparallel dimer with the recognition alpha-helices of the HTH motifs of each monomer separated by a distance of 34 A corresponding to the length of the helical repeat of B-DNA. Sedimentation velocity and equilibrium ultracentrifugation studies show that full-length Spo0J exists in a monomer-dimer equilibrium in solution and that Spo0J1-222 is exclusively monomeric. Sedimentation of the C-terminal domain of Spo0J shows it to be exclusively dimeric, confirming that the C-terminus is the primary dimerization domain. We hypothesize that the C-terminus mediates dimerization of Spo0J, thereby effectively increasing the local concentration of the N-termini, which most probably dimerize, as shown by our structure, upon binding to a cognate parS site.
Applied Physics Letters, 1987
The results of analyses of chemistry and microstructure of one of the secondary phases commonly o... more The results of analyses of chemistry and microstructure of one of the secondary phases commonly observed in samples of the new high-temperature superconductors are reported. The crystal structure has been examined by convergent beam electron diffraction and is confirmed to be orthorhombic, with space group Pbnm and lattice parameters a=0.71 nm, b=1.22 nm, and c=0.56 nm. The stoichiometry of the metal components was found by x-ray energy-dispersive spectroscopy to be Y1.0Ba0.5Cu0.5, which leads us to postulate that this phase is Y2BaCuO5−x.
Journal of Geophysical Research, 2005
1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamo... more 1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamos instruments, statistical properties of substorm-injected electron clouds are investigated, with special focus on the pitch angle distribution (PAD) of the electrons. The electron distributions at 6.6 R E do in general show some anisotropy, and their PADs are probably caused by the combined influence of drift orbits, different for each energy and pitch angle, and pitch angle diffusion due to waves. The statistical results of this paper indicate that the PADs during intervals of increased electron flux at energies greater than 10 keV from midnight till noon are dominated by pitch angle diffusion by interaction with waves. The strength of the pitch angle diffusion is seen to initially limit the growth of anisotropy from differential drift speeds and orbits and later on to increase the anisotropy when the diffusion strength decreases. After local noon we find evidence that pitch angle diffusion is no longer important and the PADs are evolving due to differential drift effects. Citation: Å snes, A., R. W. Friedel, J. Stadsnes, M. Thomsen, N. Østgaard, and T. Cayton (2005), Statistical pitch angle properties of substorm-injected electron clouds and their relation to dawnside energetic electron precipitation,
A local maximum in the energetic electron precipitation on the dawnside following substorm onsets... more A local maximum in the energetic electron precipitation on the dawnside following substorm onsets is a common feature in the PIXIE X-ray images. The cause of this maximum is thought to be whistler waves scattering electrons into the loss cone. The MPA instrument on the LANL satellites measures the full 3-dimensional particle distribution, and from the symmetry axis of the
1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamo... more 1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamos instruments, statistical properties of substorm-injected electron clouds are investigated, with special focus on the pitch angle distribution (PAD) of the electrons. The electron distributions at 6.6 R E do in general show some anisotropy, and their PADs are probably caused by the combined influence of drift orbits, different for each energy and pitch angle, and pitch angle diffusion due to waves. The statistical results of this paper indicate that the PADs during intervals of increased electron flux at energies greater than 10 keV from midnight till noon are dominated by pitch angle diffusion by interaction with waves. The strength of the pitch angle diffusion is seen to initially limit the growth of anisotropy from differential drift speeds and orbits and later on to increase the anisotropy when the diffusion strength decreases. After local noon we find evidence that pitch angle diffusion is no longer important and the PADs are evolving due to differential drift effects. Citation: Å snes, A., R. W. Friedel, J. Stadsnes, M. Thomsen, N. Østgaard, and T. Cayton (2005), Statistical pitch angle properties of substorm-injected electron clouds and their relation to dawnside energetic electron precipitation,
Space Weather-the International Journal of Research and Applications, 2004
1] The energetic charged particles of the Earth's magnetosphere are routinely detected by solid-s... more 1] The energetic charged particles of the Earth's magnetosphere are routinely detected by solid-state satellite instruments. Quantitative data are increasingly required for engineering dose calculations and for space weather science. However, the design of some energetic particle detectors can be severely constrained. Background effects must be accurately modeled in such cases to extract quantitative information. In particular, bremsstrahlung radiation from electrons impacting the detector and the satellite often dominates the background noise. Numerical tools are presented here to calculate total bremsstrahlung effects in energetic electron detectors. The calculations are illustrated for the low-energy particle detector of current Global Positioning System satellites.
The evolution of the high-energy electron population through the substorm cycle has been studied ... more The evolution of the high-energy electron population through the substorm cycle has been studied directly with in situ observations such as those provided by the LANL-SOPA instruments, and indirectly through precipitation seen via riometers, X-ray imagers, and other instruments. Ground-based and other remote sensing techniques provide the advantage of being able to track such disturbances in both space and time, with caveats including the fact that information about the energy of the precipitating particles is limited. Using data from the 13 instrument NORSTAR (formerly CANOPUS) riometer array in north-central Canada, we are examining a large number of substorm events. We are focussing on a transient ``spike'' of significant absorption that occurs during most substorm expansive phases. Seen from any one station, the spike lasts several minutes. More globally, it typically takes tens of minutes to propagate across the NORSTAR array. The propagation is in general a combination of azimuthal (ie., East or West) and poleward. In this paper, we present a statistical comparision between the propagation characteristics of the spike and other substorm features such as dipolarization, auroral bulge, Pi2s, and injections. As well, we present a detailed analysis of several events. Our objective is to determine the magnetospheric source of this transient high energy precipitation.
Prokaryotic chromosomes and plasmids encode partitioning systems that are required for DNA segreg... more Prokaryotic chromosomes and plasmids encode partitioning systems that are required for DNA segregation at cell division. The plasmid partitioning loci encode two proteins, ParA and ParB, and a cis-acting centromere-like site denoted parS. The chromosomally encoded homologues of ParA and ParB, Soj and Spo0J, play an active role in chromosome segregation during bacterial cell division and sporulation. Spo0J is a DNA-binding protein that binds to parS sites in vivo. We have solved the X-ray crystal structure of a C-terminally truncated Spo0J (amino acids 1-222) from Thermus thermophilus to 2.3 A resolution by multiwavelength anomalous dispersion. It is a DNA-binding protein with structural similarity to the helix-turn-helix (HTH) motif of the lambda repressor DNA-binding domain. The crystal structure is an antiparallel dimer with the recognition alpha-helices of the HTH motifs of each monomer separated by a distance of 34 A corresponding to the length of the helical repeat of B-DNA. Sedimentation velocity and equilibrium ultracentrifugation studies show that full-length Spo0J exists in a monomer-dimer equilibrium in solution and that Spo0J1-222 is exclusively monomeric. Sedimentation of the C-terminal domain of Spo0J shows it to be exclusively dimeric, confirming that the C-terminus is the primary dimerization domain. We hypothesize that the C-terminus mediates dimerization of Spo0J, thereby effectively increasing the local concentration of the N-termini, which most probably dimerize, as shown by our structure, upon binding to a cognate parS site.
Applied Physics Letters, 1987
The results of analyses of chemistry and microstructure of one of the secondary phases commonly o... more The results of analyses of chemistry and microstructure of one of the secondary phases commonly observed in samples of the new high-temperature superconductors are reported. The crystal structure has been examined by convergent beam electron diffraction and is confirmed to be orthorhombic, with space group Pbnm and lattice parameters a=0.71 nm, b=1.22 nm, and c=0.56 nm. The stoichiometry of the metal components was found by x-ray energy-dispersive spectroscopy to be Y1.0Ba0.5Cu0.5, which leads us to postulate that this phase is Y2BaCuO5−x.
Journal of Geophysical Research, 2005
1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamo... more 1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamos instruments, statistical properties of substorm-injected electron clouds are investigated, with special focus on the pitch angle distribution (PAD) of the electrons. The electron distributions at 6.6 R E do in general show some anisotropy, and their PADs are probably caused by the combined influence of drift orbits, different for each energy and pitch angle, and pitch angle diffusion due to waves. The statistical results of this paper indicate that the PADs during intervals of increased electron flux at energies greater than 10 keV from midnight till noon are dominated by pitch angle diffusion by interaction with waves. The strength of the pitch angle diffusion is seen to initially limit the growth of anisotropy from differential drift speeds and orbits and later on to increase the anisotropy when the diffusion strength decreases. After local noon we find evidence that pitch angle diffusion is no longer important and the PADs are evolving due to differential drift effects. Citation: Å snes, A., R. W. Friedel, J. Stadsnes, M. Thomsen, N. Østgaard, and T. Cayton (2005), Statistical pitch angle properties of substorm-injected electron clouds and their relation to dawnside energetic electron precipitation,
A local maximum in the energetic electron precipitation on the dawnside following substorm onsets... more A local maximum in the energetic electron precipitation on the dawnside following substorm onsets is a common feature in the PIXIE X-ray images. The cause of this maximum is thought to be whistler waves scattering electrons into the loss cone. The MPA instrument on the LANL satellites measures the full 3-dimensional particle distribution, and from the symmetry axis of the
1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamo... more 1] Using the existing large database of geosynchronous orbit particle measurements from Los Alamos instruments, statistical properties of substorm-injected electron clouds are investigated, with special focus on the pitch angle distribution (PAD) of the electrons. The electron distributions at 6.6 R E do in general show some anisotropy, and their PADs are probably caused by the combined influence of drift orbits, different for each energy and pitch angle, and pitch angle diffusion due to waves. The statistical results of this paper indicate that the PADs during intervals of increased electron flux at energies greater than 10 keV from midnight till noon are dominated by pitch angle diffusion by interaction with waves. The strength of the pitch angle diffusion is seen to initially limit the growth of anisotropy from differential drift speeds and orbits and later on to increase the anisotropy when the diffusion strength decreases. After local noon we find evidence that pitch angle diffusion is no longer important and the PADs are evolving due to differential drift effects. Citation: Å snes, A., R. W. Friedel, J. Stadsnes, M. Thomsen, N. Østgaard, and T. Cayton (2005), Statistical pitch angle properties of substorm-injected electron clouds and their relation to dawnside energetic electron precipitation,
Space Weather-the International Journal of Research and Applications, 2004
1] The energetic charged particles of the Earth's magnetosphere are routinely detected by solid-s... more 1] The energetic charged particles of the Earth's magnetosphere are routinely detected by solid-state satellite instruments. Quantitative data are increasingly required for engineering dose calculations and for space weather science. However, the design of some energetic particle detectors can be severely constrained. Background effects must be accurately modeled in such cases to extract quantitative information. In particular, bremsstrahlung radiation from electrons impacting the detector and the satellite often dominates the background noise. Numerical tools are presented here to calculate total bremsstrahlung effects in energetic electron detectors. The calculations are illustrated for the low-energy particle detector of current Global Positioning System satellites.