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At least 1,585 records · Page 88
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

A STATISTICAL STUDY OF SOLAR ELECTRON EVENTS OVER ONE SOLAR CYCLE

We survey the statistical properties of 1191 solar electron events observed by the WIND 3DP instrument from <1 keV to {approx}>300 keV for a solar cycle (1995 through 2005). After taking into account times of high background, the corrected occurrence frequency of solar electron events versus peak flux exhibits a power-law distribution over three orders of magnitude with exponents between -1.0 and -1.6 for different years, comparable to the frequency distribution of solar proton events, microflares, and coronal mass ejections (CMEs), but significantly flatter than that of soft X-ray (SXR) flares. At 40 keV (2.8 keV), the integrated occurrence rate above {approx}0.29 ({approx}330) cm{sup -2} s{sup -1} sr{sup -1} keV{sup -1} near 1 AU is {approx}1000 year{sup -1} ({approx}600 year{sup -1}) at solar maximum and {approx}35 year{sup -1} ({approx}25 year{sup -1}) at solar minimum, about an order of magnitude larger than the observed occurrence rate. We find these events typically extend over {approx}45 Degree-Sign in longitude, implying the occurrence rate over the whole Sun is {approx}10{sup 4} year{sup -1} near solar maximum. The observed solar electron events have a 98.75% association with type III radio bursts, suggesting all type III bursts may be associated with a solar electron event. They have a close ({approx}76%) association with the presence of low-energy ({approx}0.02-2 MeV nucleon{sup -1}), {sup 3}He-rich ({sup 3}He/{sup 4}He {>=} 0.01) ion emissions measured by the ACE ULEIS instrument. For these electron events, only {approx}35% are associated with a reported GOES SXR flare, but {approx}60% appear to be associated with a CME, with {approx}50% of these CMEs being narrow. These electrons are often detected down to below 1 keV, indicating a source high in the corona.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

A STATISTICAL METHOD FOR MEASURING THE GALACTIC POTENTIAL AND TESTING GRAVITY WITH COLD TIDAL STREAMS

We introduce the Minimum Entropy Method, a simple statistical technique for constraining the Milky Way gravitational potential and simultaneously testing different gravity theories directly from 6D phase-space surveys and without adopting dynamical models. We demonstrate that orbital energy distributions that are separable (i.e., independent of position) have an associated entropy that increases under wrong assumptions about the gravitational potential and/or gravity theory. Of known objects, 'cold' tidal streams from low-mass progenitors follow orbital distributions that most nearly satisfy the condition of separability. Although the orbits of tidally stripped stars are perturbed by the progenitor's self-gravity, systematic variations of the energy distribution can be quantified in terms of the cross-entropy of individual tails, giving further sensitivity to theoretical biases in the host potential. The feasibility of using the Minimum Entropy Method to test a wide range of gravity theories is illustrated by evolving restricted N-body models in a Newtonian potential and examining the changes in entropy introduced by Dirac, MONDian, and f(R) gravity modifications.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

RHAPSODY. I. STRUCTURAL PROPERTIES AND FORMATION HISTORY FROM A STATISTICAL SAMPLE OF RE-SIMULATED CLUSTER-SIZE HALOS

We present the first results from the RHAPSODY cluster re-simulation project: a sample of 96 'zoom-in' simulations of dark matter halos of 10{sup 14.8{+-}0.05} h {sup -1} M {sub Sun }, selected from a 1 h {sup -3} Gpc{sup 3} volume. This simulation suite is the first to resolve this many halos with {approx}5 Multiplication-Sign 10{sup 6} particles per halo in the cluster mass regime, allowing us to statistically characterize the distribution of and correlation between halo properties at fixed mass. We focus on the properties of the main halos and how they are affected by formation history, which we track back to z = 12, over five decades in mass. We give particular attention to the impact of the formation history on the density profiles of the halos. We find that the deviations from the Navarro-Frenk-White (NFW) model and the Einasto model depend on formation time. Late-forming halos tend to have considerable deviations from both models, partly due to the presence of massive subhalos, while early-forming halos deviate less but still significantly from the NFW model and are better described by the Einasto model. We find that the halo shapes depend only moderately on formation time. Departure from spherical symmetry impacts the density profiles through the anisotropic distribution of massive subhalos. Further evidence of the impact of subhalos is provided by analyzing the phase-space structure. A detailed analysis of the properties of the subhalo population in RHAPSODY is presented in a companion paper.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

A STUDY OF FUNDAMENTAL LIMITATIONS TO STATISTICAL DETECTION OF REDSHIFTED H I FROM THE EPOCH OF REIONIZATION

In this paper, we explore for the first time the relative magnitudes of three fundamental sources of uncertainty, namely, foreground contamination, thermal noise, and sample variance, in detecting the H I power spectrum from the epoch of reionization (EoR). We derive limits on the sensitivity of a Fourier synthesis telescope to detect EoR based on its array configuration and a statistical representation of images made by the instrument. We use the Murchison Widefield Array (MWA) configuration for our studies. Using a unified framework for estimating signal and noise components in the H I power spectrum, we derive an expression for and estimate the contamination from extragalactic point-like sources in three-dimensional k-space. Sensitivity for EoR H I power spectrum detection is estimated for different observing modes with MWA. With 1000 hr of observing on a single field using the 128 tile MWA, EoR detection is feasible (S/N >1 for k ∼< 0.8 Mpc{sup –1}). Bandpass shaping and refinements to the EoR window are found to be effective in containing foreground contamination, which makes the instrument tolerant to imaging errors. We find that for a given observing time, observing many independent fields of view does not offer an advantage over a single field observation when thermal noise dominates over other uncertainties in the derived power spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Planetary populations in the mass-period diagram: A statistical treatment of exoplanet formation and the role of planet traps

The rapid growth of observed exoplanets has revealed the existence of several distinct planetary populations in the mass-period diagram. Two of the most surprising are (1) the concentration of gas giants around 1 AU and (2) the accumulation of a large number of low-mass planets with tight orbits, also known as super-Earths and hot Neptunes. We have recently shown that protoplanetary disks have multiple planet traps that are characterized by orbital radii in the disks and halt rapid type I planetary migration. By coupling planet traps with the standard core accretion scenario, we showed that one can account for the positions of planets in the mass-period diagram. In this paper, we demonstrate quantitatively that most gas giants formed at planet traps tend to end up around 1 AU, with most of these being contributed by dead zones and ice lines. We also show that a large fraction of super-Earths and hot Neptunes are formed as 'failed' cores of gas giants—this population being constituted by comparable contributions from dead zone and heat transition traps. Our results are based on the evolution of forming planets in an ensemble of disks where we vary only the lifetimes of disks and their mass accretion rates onto the host star. We show that a statistical treatment of the evolution of a large population of planetary cores caught in planet traps accounts for the existence of three distinct exoplanetary populations—the hot Jupiters, the more massive planets around r = 1 AU, and the short-period super-Earths and hot Neptunes. There are very few populations that feed into the large orbital radii characteristic of the imaged Jovian planet, which agrees with recent surveys. Finally, we find that low-mass planets in tight orbits become the dominant planetary population for low-mass stars (M {sub *} ≤ 0.7 M {sub ☉}).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Statistical properties of super-hot solar flares

We use Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) high-resolution imaging and spectroscopy observations from ∼6 to 100 keV to determine the statistical relationships between measured parameters (temperature, emission measure, etc.) of hot, thermal plasma in 37 intense (GOES M- and X-class) solar flares. The RHESSI data, most sensitive to the hottest flare plasmas, reveal a strong correlation between the maximum achieved temperature and the flare GOES class, such that 'super-hot' temperatures >30 MK are achieved almost exclusively by X-class events; the observed correlation differs significantly from that of GOES-derived temperatures, and from previous studies. A nearly ubiquitous association with high emission measures, electron densities, and instantaneous thermal energies suggests that super-hot plasmas are physically distinct from cooler, ∼10-20 MK GOES plasmas, and that they require substantially greater energy input during the flare. High thermal energy densities suggest that super-hot flares require strong coronal magnetic fields, exceeding ∼100 G, and that both the plasma β and volume filling factor f cannot be much less than unity in the super-hot region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Statistical equilibrium of copper in the solar atmosphere

Non-local thermodynamic equilibrium (NLTE) line formation for neutral copper in the one-dimensional solar atmospheres is presented for the atomic model, including 96 terms of Cu I and the ground state of Cu II. The accurate oscillator strengths for all the line transitions in model atom and photoionization cross sections were calculated using the R-matrix method in the Russell-Saunders coupling scheme. The main NLTE mechanism for Cu I is the ultraviolet overionization. We find that NLTE leads to systematically depleted total absorption in the Cu I lines and, accordingly, positive abundance corrections. Inelastic collisions with neutral hydrogen atoms produce minor effects on the statistical equilibrium of Cu I in the solar atmosphere. For the solar Cu I lines, the departures from LTE are found to be small, the mean NLTE abundance correction of ∼0.01 dex. It was found that the six low-excitation lines, with excitation energy of the lower level E {sub exc} ≤ 1.64 eV, give a 0.14 dex lower mean solar abundance compared to that from the six E {sub exc} > 3.7 eV lines, when applying experimental gf-values of Kock and Richter. Without the two strong resonance transitions, the solar mean NLTE abundance from 10 lines of Cu I is log ε{sub ☉}(Cu) = 4.19 ± 0.10, which is consistent within the error bars with the meteoritic value 4.25 ± 0.05 of Lodders et al. The discrepancy between E {sub exc} = 1.39-1.64 eV and E {sub exc} > 3.7 eV lines can be removed when the calculated gf-values are adopted and a mean solar abundance of log ε{sub ☉}(Cu) = 4.24 ± 0.08 is derived.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Helioseismology of pre-emerging active regions. III. Statistical analysis

The subsurface properties of active regions (ARs) prior to their appearance at the solar surface may shed light on the process of AR formation. Helioseismic holography has been applied to samples taken from two populations of regions on the Sun (pre-emergence and without emergence), each sample having over 100 members, that were selected to minimize systematic bias, as described in Paper I. Paper II showed that there are statistically significant signatures in the average helioseismic properties that precede the formation of an AR. This paper describes a more detailed analysis of the samples of pre-emergence regions and regions without emergence based on discriminant analysis. The property that is best able to distinguish the populations is found to be the surface magnetic field, even a day before the emergence time. However, after accounting for the correlations between the surface field and the quantities derived from helioseismology, there is still evidence of a helioseismic precursor to AR emergence that is present for at least a day prior to emergence, although the analysis presented cannot definitively determine the subsurface properties prior to emergence due to the small sample sizes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Comprehensive monitoring of gamma-ray bright blazars. I. Statistical study of optical, X-ray, and gamma-ray spectral slopes

We present γ-ray, X-ray, ultraviolet, optical, and near-infrared light curves of 33 γ-ray bright blazars over 4 years that we have been monitoring since 2008 August with multiple optical, ground-based telescopes and the Swift satellite, and augmented by data from the Fermi Gamma-ray Space Telescope and other publicly available data from Swift. The sample consists of 21 flat-spectrum radio quasars (FSRQs) and 12 BL Lac objects (BL Lacs). We identify quiescent and active states of the sources based on their γ-ray behavior. We derive γ-ray, X-ray, and optical spectral indices, α{sub γ}, α{sub X}, and α{sub o}, respectively (F{sub ν}∝ν{sup α}), and construct spectral energy distributions during quiescent and active states. We analyze the relationships between different spectral indices, blazar classes, and activity states. We find (1) significantly steeper γ-ray spectra of FSRQs than for BL Lacs during quiescent states, but a flattening of the spectra for FSRQs during active states while the BL Lacs show no significant change; (2) a small difference of α{sub X} within each class between states, with BL Lac X-ray spectra significantly steeper than in FSRQs; (3) a highly peaked distribution of X-ray spectral slopes of FSRQs at ∼ –0.60, but a very broad distribution of α{sub X} of BL Lacs during active states; (4) flattening of the optical spectra of FSRQs during quiescent states, but no statistically significant change of α{sub o} of BL Lacs between states; and (5) a positive correlation between optical and γ-ray spectral slopes of BL Lacs, with similar values of the slopes. We discuss the findings with respect to the relative prominence of different components of high-energy and optical emission as the flux state changes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Statistical techniques for detecting the intergalactic magnetic field from large samples of extragalactic Faraday rotation data

Rotation measure (RM) grids of extragalactic radio sources have been widely used for studying cosmic magnetism. However, their potential for exploring the intergalactic magnetic field (IGMF) in filaments of galaxies is unclear, since other Faraday-rotation media such as the radio source itself, intervening galaxies, and the interstellar medium of our Galaxy are all significant contributors. We study statistical techniques for discriminating the Faraday rotation of filaments from other sources of Faraday rotation in future large-scale surveys of radio polarization. We consider a 30° × 30° field of view toward the south Galactic pole, while varying the number of sources detected in both present and future observations. We select sources located at high redshifts and toward which depolarization and optical absorption systems are not observed so as to reduce the RM contributions from the sources and intervening galaxies. It is found that a high-pass filter can satisfactorily reduce the RM contribution from the Galaxy since the angular scale of this component toward high Galactic latitudes would be much larger than that expected for the IGMF. Present observations do not yet provide a sufficient source density to be able to estimate the RM of filaments. However, from the proposed approach with forthcoming surveys, we predict significant residuals of RM that should be ascribable to filaments. The predicted structure of the IGMF down to scales of 0.°1 should be observable with data from the Square Kilometre Array, if we achieve selections of sources toward which sightlines do not contain intervening galaxies and RM errors are less than a few rad m{sup –2}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Statistical detection of slow-mode waves in solar polar regions with SDO/AIA

Observations from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory are utilized to statistically investigate the propagating quasi-periodic oscillations in the solar polar plume and inter-plume regions. On average, the periods are found to be nearly equal in the three coronal channels of AIA 171 Å, 193 Å, and 211 Å, and the wavelengths increase with temperature from 171 Å, 193 Å, and 211 Å. The phase speeds may be inferred from the above parameters. Furthermore, the speed ratios of v {sub 193}/v {sub 171} and v {sub 211}/v {sub 171} are derived, e.g., 1.4 ± 0.8 and 2.0 ± 1.9 in the plume regions, respectively, which are equivalent to the theoretical ones for acoustic waves. We find that there are no significant differences for the detected parameters between the plume and inter-plume regions. To our knowledge, this is the first time that we have simultaneously obtained the phase speeds of slow-mode waves in the three channels in the open coronal magnetic structures due to the method adopted in the present work, which is able to minimize the influence of the jets or eruptions on wave signals.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Statistical eclipses of close-in Kepler sub-Saturns

We present a method to detect small atmospheric signals in Kepler's planet candidate light curves by averaging light curves for multiple candidates with similar orbital and physical characteristics. Our statistical method allows us to measure unbiased physical properties of Kepler's planet candidates, even for candidates whose individual signal-to-noise precludes the detection of their secondary eclipse. We detect a secondary eclipse depth of 3.83{sub −1.11}{sup +1.10} ppm for a group of 31 sub-Saturn (R < 6 R {sub ⊕}) planet candidates with the greatest potential for a reflected light signature ((R{sub p} /a){sup 2} > 10 ppm). Including Kepler-10b in this group increases the depth to 5.08{sub −0.72}{sup +0.71} ppm. For a control group with (R{sub p} /a){sup 2} < 1 ppm, we find a depth of 0.36 ± 0.37 ppm, consistent with no detection. We also analyze the light curve of Kepler-10b and find an eclipse depth of 7.08 ± 1.06 ppm. If the eclipses are due solely to reflected light, this corresponds to a geometric albedo of 0.22 ± 0.06 for our group of close-in sub-Saturns, 0.37 ± 0.05 if including Kepler-10b in the group, and 0.60 ± 0.09 for Kepler-10b alone. Including a thermal emission model does not change the geometric albedo appreciably, assuming A{sub B} = (3/2)*A{sub g} . Our result for Kepler-10b is consistent with previous works. Our result for close-in sub-Saturns shows that Kepler-10b is unusually reflective, but our analysis is consistent with the results of Demory for super-Earths. Our results also indicate that hot Neptunes are typically more reflective than hot Jupiters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Statistical study of reconnection exhausts in the solar wind

Magnetic reconnection is a fundamental process that changes magnetic field configuration and converts a magnetic energy to flow energy and plasma heating. This paper presents a survey of the plasma and magnetic field parameters inside 418 reconnection exhausts identified in the WIND data from 1995-2012. The statistical analysis is oriented on the re-distribution of the magnetic energy released due to reconnection between a plasma acceleration and its heating. The results show that both the portion of the energy deposited into heat as well as the energy spent on the acceleration of the exhaust plasma rise with the magnetic shear angle in accord with the increase of the magnetic flux available for reconnection. The decrease of the normalized exhaust speed with the increasing magnetic shear suggests a decreasing efficiency of the acceleration and/or the increasing efficiency of heating in high-shear events. However, we have found that the already suggested relation between the exhaust speed and temperature enhancement would be rather considered as an upper limit of the plasma heating during reconnection regardless of the shear angle.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STATISTICAL ANALYSIS OF INTERFEROMETRIC MEASUREMENTS OF AXIS RATIOS FOR CLASSICAL Be STARS

This work presents a novel method to estimate the effective opening angle of CBe star disks from projected axis ratio measurements, obtained by interferometry using a statistical approach. A Monte Carlo scheme was used to generate a large set of theoretical axis ratios from disk models using different distributions of disk densities and opening angles. These theoretical samples were then compared to observational samples, using a two-sample Kolmogorov-Smirnov test, to determine which theoretical distribution best reproduces the observations. The results suggest that the observed ratio distributions in the K, H, and N band can best be explained by the presence of thin disks, with opening half-angles of the order of 0.°15-4.°0. Results for measurements over the Hα line point toward slightly thicker disks, 3.°7-14°, which is consistent with a flaring disk predicted by the viscous disk model.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A COMPREHENSIVE STATISTICAL ASSESSMENT OF STAR-PLANET INTERACTION

We investigate whether magnetic interaction between close-in giant planets and their host stars produce observable statistical enhancements in stellar coronal or chromospheric activity. New Chandra observations of 12 nearby (d < 60 pc) planet-hosting solar analogs are combined with archival Chandra, XMM-Newton, and ROSAT coverage of 11 similar stars to construct a sample inoculated against inherent stellar class and planet-detection biases. Survival analysis and Bayesian regression methods (incorporating both measurements errors and X-ray upper limits; 13/23 stars have secure detections) are used to test whether ''hot Jupiter'' hosts are systematically more X-ray luminous than comparable stars with more distant or smaller planets. No significant correlations are present between common proxies for interaction strength (M {sub P}/a {sup 2} or 1/a) versus coronal activity (L {sub X} or L {sub X}/L {sub bol}). In contrast, a sample of 198 FGK main-sequence stars does show a significant (∼99% confidence) increase in X-ray luminosity with M {sub P}/a {sup 2}. While selection biases are incontrovertibly present within the main-sequence sample, we demonstrate that the effect is primarily driven by a handful of extreme hot-Jupiter systems with M {sub P}/a {sup 2} > 450 M {sub Jup} AU{sup –2}, which here are all X-ray luminous but to a degree commensurate with their Ca II H and K activity, in contrast to presented magnetic star-planet interaction scenarios that predict enhancements relatively larger in L {sub X}. We discuss these results in the context of cumulative tidal spin-up of stars hosting close-in gas giants (potentially followed by planetary infall and destruction). We also test our main-sequence sample for correlations between planetary properties and UV luminosity or Ca II H and K emission, and find no significant dependence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A STATISTICAL STUDY OF THE SOLAR WIND TURBULENCE AT ION KINETIC SCALES USING THE K-FILTERING TECHNIQUE AND CLUSTER DATA

Plasma turbulence at ion kinetic scales in the solar wind is investigated using the multi-point magnetometer data from the Cluster spacecraft. By applying the k-filtering method, we are able to estimate the full three-dimensional power spectral density P(ω{sub sc}, k) at a certain spacecraft frequency ω{sub sc} in wavevector (k) space. By using the wavevector at the maximum power in P(ω{sub sc}, k) at each sampling frequency ω{sub sc} and the Doppler shifted frequency ω{sub pla} in the solar wind frame, the dispersion plot ω{sub pla} = ω{sub pla}(k) is found. Previous studies have been limited to very few intervals and have been hampered by large errors, which motivates a statistical study of 52 intervals of solar wind. We find that the turbulence is predominantly highly oblique to the magnetic field k >> k {sub ∥}, and propagates slowly in the plasma frame with most points having frequencies smaller than the proton gyrofrequency ω{sub pla} < Ω{sub p}. Weak agreement is found that turbulence at the ion kinetic scales consists of kinetic Alfvén waves and coherent structures advected with plasma bulk velocity plus some minor more compressible components. The results suggest that anti-sunward and sunward propagating magnetic fluctuations are of similar nature in both the fast and slow solar wind at ion kinetic scales. The fast wind has significantly more anti-sunward flux than sunward flux and the slow wind appears to be more balanced.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STATISTICAL AND SCALING FEATURES OF FLUCTUATIONS IN THE DISSIPATION RANGE DURING A RECONNECTION EVENT

Reconnection events in space plasmas are accompanied by the occurrence of large-amplitude turbulent fluctuations of the magnetic and electric field, covering a wide range of temporal and spatial scales. Here, we study the scaling and statistical features of magnetic and electric field fluctuations below the ion-gyroperiod (i.e., in the dissipation domain) by carefully investigating the occurrence of local or global scaling features during a reconnection event studied by Eastwood et al . Our results point toward the presence of a global scale invariance, i.e., a mono-fractal nature, of fluctuations above the ion-cyclotron frequency and at spatial scales near the ion-inertial length.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STATISTICAL CHARACTERISTICS OF ELEMENTAL ABUNDANCE RATIOS: OBSERVATIONS FROM THE ACE SPACECRAFT

We statistically analyze the elemental galactic cosmic ray (GCR) composition measurements of elements 5 ≤ Z ≤ 28 within the energy range 30–500 MeV/nucleon from the CRIS instrument on board the ACE spacecraft in orbit about the L1 Lagrange point during the period from 1997 to 2014. Similarly to the last unusual solar minimum, the elevated elemental intensities of all heavy nuclei during the current weak solar maximum in 2014 are ∼40% higher than that of the previous solar maximum in 2002, which has been attributed to the weak modulation associated with low solar activity levels during the ongoing weakest solar maximum since the dawn of space age. In addition, the abundance ratios of heavy nuclei with respect to elemental oxygen are generally independent of kinetic energy per nucleon in the energy region 60–200 MeV/nuc, in good agreement with previous experiments. Furthermore, the abundance ratios of most relatively abundant species, except carbon, exhibit considerable solar-cycle variation, which are obviously positively correlated with the sunspot numbers with about one-year time lag. We also find that the percentage variation of abundance ratios for most elements are approximately identical. These preliminary results provide valuable insights into the characteristics of elemental heavy nuclei composition and place new and significant constraints on future GCR heavy nuclei propagation and modulation models.

79 ASTRONOMY AND ASTROPHYSICS↗