The Experts below are selected from a list of 312 Experts worldwide ranked by ideXlab platform
Yuri Shprits - One of the best experts on this subject based on the ideXlab platform.
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on the storm time evolution of relativistic electron phase Space Density in earth s outer radiation belt
Journal of Geophysical Research, 2013Co-Authors: D L Turner, Yuri Shprits, V Angelopoulos, W Li, M D Hartinger, M Usanova, I R Mann, J BortnikAbstract:[1] We report on internal, magnetospheric processes related to markedly different storm-time responses of phase Space Density (PSD) in invariant coordinates corresponding to equatorially mirroring, relativistic electrons in Earth's outer radiation belt. Two storms are studied in detail, selected from a database of 53 events (Dstmin < −40 nT) during the THEMIS era thus far (December 2007–August 2012). These storms are well covered by a number of in situ THEMIS Spacecraft and complemented by additional ground-based and in situ observatories, and they epitomize the divergent behaviors that the outer radiation belt electrons can exhibit during active periods, even during otherwise similar Dst and auroral electrojet (AE) profiles. From our statistical results with the full database, the changes in the radial profile peak in PSD reveal notably consistent behavior with prior studies: 58% of geomagnetic storms resulted in PSD peak enhancements, 17% resulted in PSD peak depletions, and 25% resulted in no significant change in the PSD peak after the storm. For the two case studies, we examined the PSD at multiple equatorial locations (using THEMIS), trapped and precipitating fluxes from low-Earth orbit (using POES), and chorus, hiss, EMIC, and ULF waves (using THEMIS Spacecraft, ground observatories, and the GOES Spacecraft). We show that (1) peaks in PSD were collocated with observed chorus waves outside of the plasmapause during the most active periods of the PSD-enhancing storm but not during the PSD-depleting storm, providing evidence for the importance of local acceleration by wave-particle interactions with chorus; (2) outer belt dropouts occurred following solar wind pressure enhancements during both storms and were consistent with losses from magnetopause shadowing and subsequent outward radial transport; during the PSD-enhancing storm, this revealed how the outer belt can replenish itself seemingly independently of the remnant of the pre-existing belt leftover after a dropout, which in this case resulted in a double-peaked outer belt distribution; (3) slow decay in PSD was associated with corresponding locations in L* and enhanced wave amplitudes of plasmaspheric hiss; (4) precipitation loss associated with wave-particle interactions with hiss and EMIC waves appeared to be significantly more important during the PSD-depleting storm than the PSD-enhancing storm; and (5) PSD transport during the recovery phase of both storms and throughout the PSD-enhancing storm was consistent with ULF-wave-driven radial diffusion away from maxima in PSD; this indicates the importance of ULF waves in redistributing outer belt PSD after local acceleration occurs. We conclude that these source, transport, and loss processes, individually well characterized by previous studies, do indeed appear to act in concert, leading to predominance of local acceleration in one case and loss in another. These processes can therefore conspire toward optimal source or loss of outer belt electrons under suitable external drivers, and the conditions resulting in wave growth for these acceleration and loss mechanisms are therefore an important area of future research.
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radial distributions of equatorial phase Space Density for outer radiation belt electrons
Geophysical Research Letters, 2012Co-Authors: D L Turner, Yuri Shprits, V Angelopoulos, A C Kellerman, P R Cruce, D E LarsonAbstract:[1] We present the first direct observations of equatorial electron phase Space Density (PSD) as a function of the three adiabatic invariants throughout the outer radiation belt using data from the Solid State Telescopes on THEMIS-D. We estimate errors in PSD that result from data fitting and uncertainty in the calculation of the second and third invariants based on performance-weighted results from seven different magnetic field models. The PSD gradients beyond geosynchronous orbit (GEO) are energy dependent, revealing different source regions for the relativistic and non-relativistic populations. Specifically, the PSD distribution of outer belt relativistic electrons is peaked near L* ≈ 5.5. These features are typical for the outer belt, based on a survey of a two-month period from 01 Feb.–31 Mar. 2010. The results are consistent with previous studies, which were based on off-equatorial observations, but remove the high uncertainties introduced from mapping by using truly equatorial measurements (i.e., within only a few degrees of the magnetic equator) and quantifying the error in PSD. The newly calibrated THEMIS-SST dataset forms a powerful tool for exploration of the near-Earth magnetosphere, especially when combined with the upcoming RBSP mission.
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statistical analysis of phase Space Density buildups and dropouts
Journal of Geophysical Research, 2012Co-Authors: Yuri Shprits, M Daae, Binbin NiAbstract:[1] The dynamics of the radiation belt Phase Space Density (PSD) is analyzed using measurements from four Spacecraft taken during two hundred days in 1990 and 1991. In situ measurements from CRRES, Akebono, GPS, and GEO and a realistic model of the magnetic field are used to infer values of PSD. The inferred values of PSD are assimilated into a radial diffusion model by means of Kalman filtering to produce a reanalysis of the relativistic electron PSD during this time interval. The statistical analysis shows that the plasmapause location is well correlated with the location of the peak of PSD. Positive innovation outside of the plasmasphere shows that local acceleration is present in the trough region. The peak of PSD and the local source, as inferred from the innovation, are displaced inward during times of increased geomagnetic activity. Analysis of non-adiabatic dropouts in PSD shows that the dropouts often coincide with sudden increases in the solar wind dynamic pressure. Approximately 73% of the dropouts can be associated with the simultaneous sudden jumps (>7 nPa over several hours) in the solar wind dynamic pressure, approximately 15% could be associated with small jumps or gradual increases in solar wind dynamic pressure, and the remaining 12%, which consists of only 3 events, occurred during relatively steady solar wind dynamic pressure.
Carlos S Frenk - One of the best experts on this subject based on the ideXlab platform.
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the phase Space Density of fermionic dark matter haloes
Monthly Notices of the Royal Astronomical Society, 2013Co-Authors: Shi Shao, Tom Theuns, Carlos S FrenkAbstract:We have performed a series of numerical experiments to investigate how the primordial thermal velocities of fermionic dark matter particles affect the physical and phase-Space Density profiles of the dark matter haloes into which they collect. The initial particle velocities induce central cores in both profiles, which can be understood in the framework of phase-Space Density theory. We find that the maximum coarse-grained phase-Space Density of the simulated haloes (computed in six-dimensional phase Space using the ENBID code is very close to the theoretical fine-grained upper bound, while the pseudo-phase-Space Density, Q ∼ ρ/σ3, overestimates the maximum phase-Space Density by up to an order of magnitude. The Density in the inner regions of the simulated haloes is well described by a ‘pseudo-isothermal’ profile with a core. We have developed a simple model based on this profile which, given the observed surface brightness profile of a galaxy and its central velocity dispersion, accurately predicts its central phase-Space Density. Applying this model to the dwarf spheroidal satellites of the Milky Way yields values close to 0.5 keV for the mass of a hypothetical thermal warm dark matter particle, assuming that the satellite haloes have cores produced by warm dark matter free streaming. Such a small value is in conflict with the lower limit of 1.2 keV set by the observations of the Lyman α forest. Thus, if the Milky Way dwarf spheroidal satellites have cores, these are likely due to baryonic processes associated with the forming galaxy, perhaps of the kind proposed by Navarro, Eke and Frenk and seen in the recent simulations of galaxy formation in the cold dark matter model.
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The Density and Pseudo-Phase-Space Density Profiles of CDM halos
arXiv: Cosmology and Nongalactic Astrophysics, 2011Co-Authors: Aaron D. Ludlow, Carlos S Frenk, Julio F Navarro, Michael Boylan-kolchin, Volker Springel, Adrian Jenkins, Simon D. M. WhiteAbstract:Cosmological N-body simulations indicate that the spherically-averaged Density profiles of cold dark matter halos are accurately described by Einasto profiles, where the logarithmic slope is a power-law of adjustable exponent, � dln�=dlnr / r � . The pseudo-phase-Space Density (PPSD) profiles of CDM halos also show remarkable regularity, and are well approximated by simple power laws, Q(r) � �=� 3 / r � . We show that this is expected from dynamical equilibrium considerations, since Jeans’ equations predict that the pseudo-phase-Space Density profiles of Einasto halos should
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secondary infall and the pseudo phase Space Density profiles of cold dark matter haloes
Monthly Notices of the Royal Astronomical Society, 2010Co-Authors: Aaron D. Ludlow, Julio F Navarro, Volker Springel, Adrian Jenkins, Simon D. M. White, Mark Vogelsberger, Jie Wang, Carlos S FrenkAbstract:We use N-body simulations to investigate the radial dependence of the Density, ρ, and velocity dispersion, σ, in cold dark matter (CDM) haloes. In particular, we explore how closely Q ≡ ρ/σ 3 , a surrogate measure of the phase-Space Density, follows a power law in radius. Our study extends earlier work by considering, in addition to spherically averaged profiles, local Q estimates for individual particles, Q i ; profiles based on the ellipsoidal radius dictated by the triaxial structure of the halo, Q i (r'); and by carefully removing substructures in order to focus on the profile of the smooth halo, Q s . The resulting Q s i (r') profiles follow closely a power law near the centre, but show a clear upturn from this trend near the virial radius, r 200 . The location and magnitude of the deviations are in excellent agreement with the predictions from Bertschinger's spherical secondary-infall similarity solution. In this model, Q ∝ r ― 1.875 in the inner, virialized regions, but departures from a power-law occur near r 200 because of the proximity of this radius to the location of the first shell crossing - the shock radius in the case of a collisional fluid. Particles there have not yet fully virialized, and so Q departs from the inner power-law profile. Our results imply that the power-law nature of Q profiles only applies to the inner regions and cannot be used to predict accurately the structure of CDM haloes beyond their characteristic scale radius.
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secondary infall and the pseudo phase Space Density profiles of cold dark matter halos
arXiv: Cosmology and Nongalactic Astrophysics, 2010Co-Authors: Aaron D. Ludlow, Julio F Navarro, Volker Springel, Adrian Jenkins, Simon D. M. White, Mark Vogelsberger, Jie Wang, Carlos S FrenkAbstract:We use N-body simulations to investigate the radial dependence of the Density and velocity dispersion in cold dark matter (CDM) halos. In particular, we explore how closely Q rho/sigma^3, a surrogate measure of the phase-Space Density, follows a power-law in radius. Our study extends earlier work by considering, in addition to spherically-averaged profiles, local Q-estimates for individual particles, Q_i; profiles based on the ellipsoidal radius dictated by the triaxial structure of the halo, Q_i(r'); and by carefully removing substructures in order to focus on the profile of the smooth halo, Q^s. The resulting Q_i^s(r') profiles follow closely a power law near the center, but show a clear upturn from this trend near the virial radius, r_{200}. The location and magnitude of the deviations are in excellent agreement with the predictions from Bertschinger's spherical secondary-infall similarity solution. In this model, Q \propto r^{-1.875} in the inner, virialized regions, but departures from a power-law occur near r_{200} because of the proximity of this radius to the location of the first shell crossing - the shock radius in the case of a collisional fluid. Particles there have not yet fully virialized, and so Q departs from the inner power-law profile. Our results imply that the power-law nature of $Q$ profiles only applies to the inner regions and cannot be used to predict accurately the structure of CDM halos beyond their characteristic scale radius.
R H A Iles - One of the best experts on this subject based on the ideXlab platform.
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phase Space Density analysis of the outer radiation belt energetic electron dynamics
Journal of Geophysical Research, 2006Co-Authors: R H A Iles, Nigel P Meredith, A N Fazakerley, Richard B HorneAbstract:[1] We present an analysis of the electron phase Space Density in the Earth's outer radiation belt during three magnetically disturbed periods to determine the likely roles of inward radial diffusion and local acceleration in the energization of electrons to relativistic energies. During the recovery phase of the 9 October 1990 storm and the period of prolonged substorms between 11 and 16 September 1990, the relativistic electron phase Space Density increases substantially and peaks in the phase Space Density occur in the region 4.0 ∼0.8 MeV. The peaks in the phase Space Density are associated with prolonged substorm activity, enhanced chorus amplitudes, and predominantly low values of the ratio between the electron plasma frequency, fpe, and the electron gyrofrequency, fce (fpe/fce < ∼4). The data provide further evidence for a local wave acceleration process in addition to radial diffusion operating in the heart of the outer radiation belt. During the recovery phase of the 9 October 1990 storm the peaks are more pronounced at large M (550 MeV/G) and large Kaufmann K (0.11 RE) than large M (700 MeV/G) and small K (0.025 RE), which suggests that radial diffusion is more effective below about 0.7 MeV for 5.0 < L* < 5.5 during this period. At low M (M ≤ 250 MeV/G), corresponding to energies, E < ∼0.8 MeV, there is no evidence for a peak in phase Space Density and the data are more consistent with inward radial diffusion and losses to the atmosphere by pitch angle scattering. During the 26 August 1990 storm there is a net loss in the relativistic electron phase Space Density for 3.3 < L* < 6.0. At low M (M ≤ 250 MeV/G) the phase Space Density decreases by almost a constant factor and the gradient remains positive for all L*, but at high M (M ≥ 550 MeV/G) the decrease in phase Space Density is greater at larger L* and the gradient changes from positive to negative. The data show that it is possible to have inward radial diffusion at low energies and outward radial diffusion at higher energies, which would fill the outer radiation belt.
Ezequiel Treister - One of the best experts on this subject based on the ideXlab platform.
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The Space Density of Compton‐thick AGN
arXiv: Cosmology and Nongalactic Astrophysics, 2010Co-Authors: Ezequiel Treister, Meg Urry, Carolin N. Cardamone, Shanil N. Virani, Kevin Schawinski, Eric GawiserAbstract:We constrain the number Density and evolution of Compton‐thick Active Galactic Nuclei (AGN), and their contribution to the extragalactic X‐ray background. In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on mid‐IR parameters. We present the properties of a sample of 211 heavily‐obscured AGN candidates in the Extended Chandra Deep Field‐South (ECDF‐S) selecting objects with f24μm/fR>1000 and R‐K>4.5. The X‐ray to mid‐IR ratios for these sources are significantly larger than that of star‐forming galaxies and ∼2 orders of magnitude smaller than for the general AGN population, suggesting column densities of NH>5×1024 cm−2. The Space Density of CT AGN at z∼2 derived from these observations is ∼10−5 Mpc−3, finding a strong evolution in the number of LX>1044 erg/s sources from z = 1.5 to 2.5.
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the Space Density of compton thick active galactic nucleus and the x ray background
The Astrophysical Journal, 2009Co-Authors: Ezequiel Treister, Meg Urry, S ViraniAbstract:We constrain the number Density and evolution of Compton-thick (CT) active galactic nuclei (AGNs). In the local universe, we use the wide-area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on a combination of X-ray and mid-infrared (mid-IR) parameters. We find a significantly lower Space Density of CT AGNs in the local universe than expected from published AGN population synthesis models to explain the X-ray background (XRB). This can be explained by the numerous degeneracies in the parameters of those models; we use the high-energy surveys described here to remove those degeneracies. We show that only direct observations of CT AGNs can currently constrain the number of heavily obscured supermassive black holes. At high redshift, the inclusion of IR-selected CT AGN candidates leads to a much higher Space Density, implying (1) a different (steeper) evolution for these sources compared to less-obscured AGNs, (2) that the IR selection includes a large number of interlopers, and/or (3) that there is a large number of reflection-dominated AGNs missed in the INTEGRAL and Swift observations. The contribution of CT AGN to the XRB is small, ~9%, with a comparable contribution to the total cosmic accretion, unless reflection-dominated CT AGNs significantly outnumber transmission-dominated CT AGNs, in which case their contribution can be much higher. Using estimates derived here for the accretion luminosity over cosmic time, we estimate the local mass Density in supermassive black holes and find a good agreement with available constraints for an accretion efficiency of ~10%. Transmission-dominated CT AGNs contribute only ~8% to total black hole growth.
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the Space Density of compton thick agn and the x ray background
arXiv: Cosmology and Nongalactic Astrophysics, 2009Co-Authors: Ezequiel Treister, Meg Urry, Shanil N. ViraniAbstract:We constrain the number Density and evolution of Compton-thick Active Galactic Nuclei (AGN). In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on a combination of X-ray and mid-IR parameters. We find a significantly lower Space Density of Compton-thick AGN in the local Universe than expected from published AGN population synthesis models to explain the X-ray background. This can be explained by the numerous degeneracies in the parameters of those models; we use the high-energy surveys described here to remove those degeneracies. We show that only direct observations of CT AGN can currently constrain the number of heavily-obscured supermassive black holes. At high redshift, the inclusion of IR-selected Compton-thick AGN candidates leads to a much higher Space Density, implying (a) a different (steeper) evolution for these sources compared to less-obscured AGN, (b) that the IR selection includes a large number of interlopers, and/or (c) that there is a large number of reflection-dominated AGN missed in the INTEGRAL and Swift observations. The contribution of CT AGN to the X-ray background is small, ~9%, with a comparable contribution to the total cosmic accretion, unless reflection-dominated CT AGN significantly outnumber transmission-dominated CT AGN, in which case their contribution can be much higher. Using estimates derived here for the accretion luminosity over cosmic time we estimate the local mass Density in supermassive black holes and find a good agreement with available constraints for an accretion efficiency of ~10%. Transmission-dominated CT AGN contribute only ~8% to total black hole growth.
Richard B Horne - One of the best experts on this subject based on the ideXlab platform.
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phase Space Density analysis of the outer radiation belt energetic electron dynamics
Journal of Geophysical Research, 2006Co-Authors: R H A Iles, Nigel P Meredith, A N Fazakerley, Richard B HorneAbstract:[1] We present an analysis of the electron phase Space Density in the Earth's outer radiation belt during three magnetically disturbed periods to determine the likely roles of inward radial diffusion and local acceleration in the energization of electrons to relativistic energies. During the recovery phase of the 9 October 1990 storm and the period of prolonged substorms between 11 and 16 September 1990, the relativistic electron phase Space Density increases substantially and peaks in the phase Space Density occur in the region 4.0 ∼0.8 MeV. The peaks in the phase Space Density are associated with prolonged substorm activity, enhanced chorus amplitudes, and predominantly low values of the ratio between the electron plasma frequency, fpe, and the electron gyrofrequency, fce (fpe/fce < ∼4). The data provide further evidence for a local wave acceleration process in addition to radial diffusion operating in the heart of the outer radiation belt. During the recovery phase of the 9 October 1990 storm the peaks are more pronounced at large M (550 MeV/G) and large Kaufmann K (0.11 RE) than large M (700 MeV/G) and small K (0.025 RE), which suggests that radial diffusion is more effective below about 0.7 MeV for 5.0 < L* < 5.5 during this period. At low M (M ≤ 250 MeV/G), corresponding to energies, E < ∼0.8 MeV, there is no evidence for a peak in phase Space Density and the data are more consistent with inward radial diffusion and losses to the atmosphere by pitch angle scattering. During the 26 August 1990 storm there is a net loss in the relativistic electron phase Space Density for 3.3 < L* < 6.0. At low M (M ≤ 250 MeV/G) the phase Space Density decreases by almost a constant factor and the gradient remains positive for all L*, but at high M (M ≥ 550 MeV/G) the decrease in phase Space Density is greater at larger L* and the gradient changes from positive to negative. The data show that it is possible to have inward radial diffusion at low energies and outward radial diffusion at higher energies, which would fill the outer radiation belt.
