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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,441 records · Page 80
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DOE OSTI2017

Stacking caustic masses from galaxy clusters

Ongoing and future spectroscopic surveys will measure numerous galaxy redshifts within tens of thousands of galaxy clusters. However, the sampling within these clusters will be low, 15 1000 galaxies per ensemble phase space, the escape-velocity edge becomes readily identifiable and the presence of interloping galaxies is minimized. We develop and test an algorithm to trace the projected phase-space surface directly, which results in minimally biased dynamical mass estimates. We then quantify how binning and sampling affect the phase-space-based mass estimates when using an observational proxy that incorporates realistic mass scatter, like richness, and find the added uncertainty in the binning procedure has minimal influence on the resulting bias and scatter of the stacked mass estimates.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The sami galaxy survey: revisiting galaxy classification through high-order stellar kinematics

Recent cosmological hydrodynamical simulations suggest that integral field spectroscopy can connect the high-order stellar kinematic moments h{sub 3} (∼skewness) and h{sub 4} (∼kurtosis) in galaxies to their cosmological assembly history. Here, we assess these results by measuring the stellar kinematics on a sample of 315 galaxies, without a morphological selection, using two-dimensional integral field data from the SAMI Galaxy Survey. Proxies for the spin parameter (λ{sub R{sub e}}) and ellipticity (ϵ{sub e}) are used to separate fast and slow rotators; there exists a good correspondence to regular and non-regular rotators, respectively, as also seen in earlier studies. We confirm that regular rotators show a strong h{sub 3} versus V/σ anti-correlation, whereas quasi-regular and non-regular rotators show a more vertical relation in h{sub 3} and V/σ. Motivated by recent cosmological simulations, we develop an alternative approach to kinematically classify galaxies from their individual h{sub 3} versus V/σ signatures. Within the SAMI Galaxy Survey, we identify five classes of high-order stellar kinematic signatures using Gaussian mixture models. Class 1 corresponds to slow rotators, whereas Classes 2–5 correspond to fast rotators. We find that galaxies with similar λ{sub R{sub e}}--ϵ{sub e} values can show distinctly different h{sub 3}--V/σ signatures. Class 5 objects are previously unidentified fast rotators that show a weak h{sub 3} versus V/σ anti-correlation. From simulations, these objects are predicted to be disk-less galaxies formed by gas-poor mergers. From morphological examination, however, there is evidence for large stellar disks. Instead, Class 5 objects are more likely disturbed galaxies, have counter-rotating bulges, or bars in edge-on galaxies. Finally, we interpret the strong anti-correlation in h{sub 3} versus V/σ as evidence for disks in most fast rotators, suggesting a dearth of gas-poor mergers among fast rotators.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

THE ORIGIN OF THE HEAVIEST METALS IN MOST ULTRA-FAINT DWARF GALAXIES

The heaviest metals found in stars in most ultra-faint dwarf (UFD) galaxies in the Milky Way halo are generally underabundant by an order of magnitude or more when compared with stars in the halo field. Among the heavy elements produced by n -capture reactions, only Sr and Ba can be detected in red giant stars in most UFD galaxies. This limited chemical information is unable to identify the nucleosynthesis process(es) responsible for producing the heavy elements in UFD galaxies. Similar [Sr/Ba] and [Ba/Fe] ratios are found in three bright halo field stars, BD−18°5550, CS 22185–007, and CS 22891–200. Previous studies of high-quality spectra of these stars report detections of additional n -capture elements, including Eu. The [Eu/Ba] ratios in these stars span +0.41 to +0.86. These ratios and others among elements in the rare Earth domain indicate an r -process origin. These stars have some of the lowest levels of r -process enhancement known, with [Eu/H] spanning −3.95 to −3.32, and they may be considered nearby proxies for faint stars in UFD galaxies. Direct confirmation, however, must await future observations of additional heavy elements in stars in the UFD galaxies themselves.

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DOE OSTI2017

A Closer Look at the Alpha Persei Coronal Conundrum

A ROSAT survey of the Alpha Per open cluster in 1993 detected its brightest star, the mid-F supergiant α Persei: the X-ray luminosity and spectral hardness were similar to coronally active late-type dwarf members. Later, in 2010, a Hubble Cosmic Origins Spectrograph SNAPshot of α Per found the far-ultraviolet (FUV) coronal-proxy Si iv unexpectedly weak. This, and a suspicious offset of the ROSAT source, suggested that a late-type companion might be responsible for the X-rays. Recently, a multifaceted program tested that premise. Ground-based optical coronography and near-UV imaging with Hubble Space Telescope ( HST ) Wide-Field Camera 3 searched for any close-in faint candidate coronal objects, but without success. Then, a Chandra pointing found the X-ray source single and coincident with the bright star. Significantly, the Si iv emissions of α Per, in a deeper FUV spectrum collected by the HST Cosmic Origin Spectrograph as part of the joint program, are aligned well with chromospheric atomic oxygen (which must be intrinsic to the luminous star), within the context of cooler late-F and early-G supergiants, including Cepheid variables. This pointed to the X-rays as the fundamental anomaly. The overluminous X-rays still support the case for a hyperactive dwarf secondary, albeit now spatially unresolved. However, an alternative is that α Per represents a novel class of coronal source. Resolving the first possibility now has become more difficult, because the easy solution—a well-separated companion—has been eliminated. Testing the other possibility will require a broader high-energy census of the early-F supergiants.

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DOE OSTI2017

The Nearest Ultra Diffuse Galaxy: UGC 2162

We describe the structural, stellar population and gas properties of the nearest ultra diffuse galaxy discovered so far: UGC 2162 (z = 0.00392; R{sub e,g}=1.7(±0.2) kpc; μ{sub g}(0) = 24.4 ± 0.1 mag arcsec{sup −2}; g−i = 0.33 ± 0.02). This galaxy, located at a distance of 12.3(±1.7) Mpc, is a member of the M77 group. UGC 2162 has a stellar mass of ∼2({sub −1}{sup +2}) × 10{sup 7} M{sub ⊙} and is embedded within a cloud of HI gas ∼10 times more massive: ∼1.9(±0.6) × 10{sup 8} M{sub ⊙}. Using the width of its HI line as a dynamical proxy, the enclosed mass within the inner R ∼ 5 kpc is ∼4.6(±0.8) × 10{sup 9} M{sub ⊙} (i.e., M/L ∼ 200). The estimated virial mass from the cumulative mass curve is ∼8(±2)×10{sup 10} M {sub ⊙}. Ultra-deep imaging from the IAC Stripe82 Legacy Project show that the galaxy is irregular and has many star-forming knots, with a gas-phase metallicity around one-third of the solar value. Its estimated star-formation rate is ∼0.01 M{sub ⊙} yr{sup −1}. This SFR would double the stellar mass of the object in ∼2 Gyr. If the object were to stop forming stars at this moment, after a passive evolution, its surface brightness would become extremely faint: μ{sub g}(0) ∼ 27 mag arcsec{sup −2} and its size would remain large R{sub e,g} ∼ 1.8 kpc. Such faintness would make it almost undetectable to most present-day surveys. This suggests that there could be an important population of M{sub ⋆} ∼ 10{sup 7} M{sub ⊙} “dark galaxies” in rich environments (depleted of HI gas) waiting to be discovered by current and future ultra-deep surveys.

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DOE OSTI2017

The MOSDEF Survey: Metallicity Dependence of PAH Emission at High Redshift and Implications for 24 μm Inferred IR Luminosities and Star Formation Rates at z ∼ 2

We present results on the variation of 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission in galaxies spanning a wide range in metallicity at z ∼ 2. For this analysis, we use rest-frame optical spectra of 476 galaxies at 1.37 ≤ z ≤ 2.61 from the MOSFIRE Deep Evolution Field (MOSDEF) survey to infer metallicities and ionization states. Spitzer/MIPS 24 μm and Herschel/PACS 100 and 160 μm observations are used to derive rest-frame 7.7 μm luminosities (L{sub 7.7}) and total IR luminosities (L{sub IR}), respectively. We find significant trends between the ratio of L{sub 7.7} to L{sub IR} (and to dust-corrected star formation rate [SFR]) and both metallicity and [O iii]/[O ii] (O{sub 32}) emission line ratio. The latter is an empirical proxy for the ionization parameter. These trends indicate a paucity of PAH emission in low-metallicity environments with harder and more intense radiation fields. Additionally, L{sub 7.7}/L{sub IR} is significantly lower in the youngest quartile of our sample (ages of ≲500 Myr) compared to older galaxies, which may be a result of the delayed production of PAHs by AGB stars. The relative strength of L{sub 7.7} to L{sub IR} is also lower by a factor of ∼2 for galaxies with masses M{sub ∗}<10{sup 10} M{sub ⊙}, compared to the more massive ones. We demonstrate that commonly used conversions of L{sub 7.7} (or 24 μm flux density, f {sub 24}) to L{sub IR} underestimate the IR luminosity by more than a factor of 2 at M{sub ∗}∼10{sup 9.6--10.0} M{sub ⊙}. We adopt a mass-dependent conversion of L{sub 7.7} to L{sub IR} with L{sub 7.7}/L{sub IR} = 0.09 and 0.22 for M{sub ∗}⩽10{sup 10} and >10{sup 10} M{sub ⊙}, respectively. Based on the new scaling, the SFR–M {sub *} relation has a shallower slope than previously derived. Our results also suggest a higher IR luminosity density at z ∼ 2 than previously measured, corresponding to a ∼30% increase in the SFR density.

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DOE OSTI2017

X-Ray Temperatures, Luminosities, and Masses from XMM-Newton Follow-up of the First Shear-selected Galaxy Cluster Sample

We continue the study of the first sample of shear-selected clusters from the initial 8.6 square degrees of the Deep Lens Survey (DLS); a sample with well-defined selection criteria corresponding to the highest ranked shear peaks in the survey area. We aim to characterize the weak lensing selection by examining the sample’s X-ray properties. There are multiple X-ray clusters associated with nearly all the shear peaks: 14 X-ray clusters corresponding to seven DLS shear peaks. An additional three X-ray clusters cannot be definitively associated with shear peaks, mainly due to large positional offsets between the X-ray centroid and the shear peak. Here we report on the XMM-Newton properties of the 17 X-ray clusters. The X-ray clusters display a wide range of luminosities and temperatures; the L {sub X} − T {sub X} relation we determine for the shear-associated X-ray clusters is consistent with X-ray cluster samples selected without regard to dynamical state, while it is inconsistent with self-similarity. For a subset of the sample, we measure X-ray masses using temperature as a proxy, and compare to weak lensing masses determined by the DLS team. The resulting mass comparison is consistent with equality. The X-ray and weak lensing masses show considerable intrinsic scatter (∼48%), which is consistent with X-ray selected samples when their X-ray and weak lensing masses are independently determined.

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DOE OSTI2017

Rotation of Late-type Stars in Praesepe with K2

We have Fourier-analyzed 941 K2 light curves (LCs) of likely members of Praesepe, measuring periods for 86% and increasing the number of rotation periods (P) by nearly a factor of four. The distribution of P versus (V−K{sub s}), a mass proxy, has three different regimes: (V−K{sub s}) < 1.3, where the rotation rate rapidly slows as mass decreases; 1.3 < (V−K{sub s}) < 4.5, where the rotation rate slows more gradually as mass decreases; and (V−K{sub s}) > 4.5, where the rotation rate rapidly increases as mass decreases. In this last regime, there is a bimodal distribution of periods, with few between ∼2 and ∼10 days. We interpret this to mean that once M stars start to slow down, they do so rapidly. The K2 period–color distribution in Praesepe (∼790 Myr) is much different than that in the Pleiades (∼125 Myr) for late F, G, K, and early-M stars; the overall distribution moves to longer periods and is better described by two line segments. For mid-M stars, the relationship has a similarly broad scatter and is steeper in Praesepe. The diversity of LCs and of periodogram types is similar in the two clusters; about a quarter of the periodic stars in both clusters have multiple significant periods. Multi-periodic stars dominate among the higher masses, starting at a bluer color in Praesepe ((V−K{sub s}) ∼ 1.5) than in the Pleiades ((V−K{sub s}) ∼ 2.6). In Praesepe, there are relatively more LCs that have two widely separated periods, ΔP>6 days. Some of these could be examples of M star binaries where one star has spun down but the other has not.

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DOE OSTI2017

QSL Squasher: A Fast Quasi-separatrix Layer Map Calculator

Quasi-Separatrix Layers (QSLs) are a useful proxy for the locations where current sheets can develop in the solar corona, and give valuable information about the connectivity in complicated magnetic field configurations. However, calculating QSL maps, even for two-dimensional slices through three-dimensional models of coronal magnetic fields, is a non-trivial task, as it usually involves tracing out millions of magnetic field lines with immense precision. Thus, extending QSL calculations to three dimensions has rarely been done until now. In order to address this challenge, we present QSL Squasher—a public, open-source code, which is optimized for calculating QSL maps in both two and three dimensions on graphics processing units. The code achieves large processing speeds for three reasons, each of which results in an order-of-magnitude speed-up. (1) The code is parallelized using OpenCL. (2) The precision requirements for the QSL calculation are drastically reduced by using perturbation theory. (3) A new boundary detection criterion between quasi-connectivity domains is used, which quickly identifies possible QSL locations that need to be finely sampled by the code. That boundary detection criterion relies on finding the locations of abrupt field-line length changes, which we do by introducing a new Field-line Length Edge (FLEDGE) map. We find FLEDGE maps useful on their own as a quick-and-dirty substitute for QSL maps. QSL Squasher allows construction of high-resolution 3D FLEDGE maps in a matter of minutes, which is two orders of magnitude faster than calculating the corresponding 3D QSL maps. We include a sample of calculations done using QSL Squasher to demonstrate its capabilities as a QSL calculator, as well as to compare QSL and FLEDGE maps.

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DOE OSTI2017

Velocity Dispersion, Size, Sérsic Index, and D{sub n}4000: The Scaling of Stellar Mass with Dynamical Mass for Quiescent Galaxies

We examine the relation between stellar mass, velocity dispersion, size, Sérsic index, and D{sub n}4000 for a volume-limited sample of ∼40,000 quiescent galaxies in the SDSS. At a fixed stellar mass, galaxies with higher D{sub n}4000 have larger velocity dispersions and smaller sizes. D{sub n}4000 is a proxy for stellar population age, thus these trends suggest that older galaxies typically have larger velocity dispersions and smaller sizes. We combine velocity dispersion and size into a dynamical mass estimator, σ{sup 2}R. At a fixed stellar mass, σ{sup 2}R depends on D{sub n}4000. The Sérsic index is also correlated with D{sub n}4000. The dependence of σ{sup 2}R and Sérsic index on D{sub n}4000 suggests that quiescent galaxies are not structurally homologous systems. We derive an empirical correction for non-homology which is consistent with the analytical correction derived from the virial theorem. After accounting for non-homologous galactic structure, we measure M{sub ∗}∝M{sub d}{sup 0.998±0.004}, where M {sub *} is the stellar mass and M {sub d} is the dynamical mass derived from the velocity dispersion and size; stellar mass is directly proportional to dynamical mass. Quiescent galaxies appear to be in approximate virial equilibrium, and deviations of the fundamental plane parameters from the expected virial relation may result from mass-to-light ratio variations, selection effects, and the non-homology of quiescent galaxies. We infer the redshift evolution of velocity dispersion and size for galaxies in our sample assuming purely passive evolution. The inferred evolution is inconsistent with direct measurements at higher redshifts. Thus quiescent galaxies do not passively evolve. Quiescent galaxies have properties that are consistent with standard galaxy formation in ΛCDM. They form at different epochs and evolve modestly, increasing their size, velocity dispersion, and Sérsic index after they cease star formation.

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DOE OSTI2017

Calibrating the Planck Cluster Mass Scale with Cluster Velocity Dispersions

We measure the Planck cluster mass bias using dynamical mass measurements based on velocity dispersions of a subsample of 17 Planck-detected clusters. The velocity dispersions were calculated using redshifts determined from spectra that were obtained at the Gemini observatory with the GMOS multi-object spectrograph. We correct our estimates for effects due to finite aperture, Eddington bias, and correlated scatter between velocity dispersion and the Planck mass proxy. The result for the mass bias parameter, (1−b), depends on the value of the galaxy velocity bias, b{sub v}, adopted from simulations: (1−b)=(0.51±0.09)b{sub v}{sup 3}. Using a velocity bias of b{sub v}=1.08 from Munari et al., we obtain (1−b)=0.64±0.11, i.e., an error of 17% on the mass bias measurement with 17 clusters. This mass bias value is consistent with most previous weak-lensing determinations. It lies within 1σ of the value that is needed to reconcile the Planck cluster counts with the Planck primary cosmic microwave background constraints. We emphasize that uncertainty in the velocity bias severely hampers the precision of the measurements of the mass bias using velocity dispersions. On the other hand, when we fix the Planck mass bias using the constraints from Penna–Lima et al., based on weak-lensing measurements, we obtain a positive velocity bias of b{sub v}≳0.9 at 3σ.

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DOE OSTI2017

Galaxy Groups Within 3500 km s{sup −1}

A study of the group properties of galaxies in our immediate neighborhood provides a singular opportunity to observationally constrain the halo mass function, a fundamental characterization of galaxy formation. Detailed studies of individual groups have provided the coefficients of scaling relations between a proxy for the virial radius, velocity dispersion, and mass that usefully allow groups to be defined over the range 10{sup 10}--10{sup 15} M {sub ⊙}. At a second hierarchical level, associations are defined as regions around collapsed halos extending to the zero-velocity surface at the decoupling from cosmic expansion. The most remarkable result of the study emerges from the construction of the halo mass function from the sample. At ∼10{sup 12} M {sub ⊙}, there is a jog from the expectation Sheth-Tormen function, such that halo counts drop by a factor ∼3 in all lower mass bins.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

A Controlled Study of Cold Dust Content in Galaxies from z = 0–2

At z=1--3, the formation of new stars is dominated by dusty galaxies whose far-IR emission indicates they contain colder dust than local galaxies of a similar luminosity. We explore the reasons for the evolving IR emission of similar galaxies over cosmic time using (1) local galaxies from GOALS (L{sub IR}=10{sup 11}--10{sup 12} L{sub ⊙}), (2) galaxies at z∼0.1--0.5 from 5MUSES (L{sub IR}=10{sup 10}--10{sup 12} L{sub ⊙}), and (3) IR luminous galaxies spanning z=0.5--3 from GOODS and Spitzer xFLS (L{sub IR}>10{sup 11} L{sub ⊙}). All samples have Spitzer mid-IR spectra, and Herschel and ground-based submillimeter imaging covering the full IR spectral energy distribution, allowing us to robustly measure L{sub IR}{sup SF}, T{sub dust}, and M{sub dust} for every galaxy. Despite similar infrared luminosities, z>0.5 dusty star-forming galaxies (DSFG) have a factor of 5 higher dust masses and 5 K colder temperatures. The increase in dust mass is linked to an increase in the gas fractions with redshift, and we do not observe a similar increase in stellar mass or star formation efficiency. L{sub 160}{sup SF}/L{sub 70}{sup SF}, a proxy for T{sub dust}, is strongly correlated with L{sub IR}{sup SF}/M{sub dust} independently of redshift. We measure merger classification and galaxy size for a subsample, and there is no obvious correlation between these parameters and L{sub IR}{sup SF}/M{sub dust} or L{sub 160}{sup SF}/L{sub 70}{sup SF}. In DSFG, the change in L{sub IR}{sup SF}/M{sub dust} can fully account for the observed colder dust temperatures, suggesting that any change in the spatial extent of the interstellar medium is a second-order effect.

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DOE OSTI2017

The MUSCLES Treasury Survey. IV. Scaling Relations for Ultraviolet, Ca ii K, and Energetic Particle Fluxes from M Dwarfs

Characterizing the UV spectral energy distribution (SED) of an exoplanet host star is critically important for assessing its planet’s potential habitability, particularly for M dwarfs, as they are prime targets for current and near-term exoplanet characterization efforts and atmospheric models predict that their UV radiation can produce photochemistry on habitable zone planets different from that on Earth. To derive ground-based proxies for UV emission for use when Hubble Space Telescope ( HST ) observations are unavailable, we have assembled a sample of 15 early to mid-M dwarfs observed by HST and compared their nonsimultaneous UV and optical spectra. We find that the equivalent width of the chromospheric Ca ii K line at 3933 Å, when corrected for spectral type, can be used to estimate the stellar surface flux in ultraviolet emission lines, including H i Ly α . In addition, we address another potential driver of habitability: energetic particle fluxes associated with flares. We present a new technique for estimating soft X-ray and >10 MeV proton flux during far-UV emission line flares (Si iv and He ii) by assuming solar-like energy partitions. We analyze several flares from the M4 dwarf GJ 876 observed with HST and Chandra as part of the MUSCLES Treasury Survey and find that habitable zone planets orbiting GJ 876 are impacted by large Carrington-like flares with peak soft X-ray fluxes ≥10{sup −3} W m{sup −2} and possible proton fluxes ∼10{sup 2}–10{sup 3} pfu, approximately four orders of magnitude more frequently than modern-day Earth.

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DOE OSTI2017

The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B

The planets of the solar system are neatly divided between those with atmospheres and those without when arranged by insolation (I) and escape velocity (v{sub esc}). The dividing line goes at I∝v{sub esc}{sup 4}. Exoplanets with reported masses and radii are shown to crowd against the extrapolation of the solar system trend, making a metaphorical cosmic shoreline that unites all the planets. The I∝v{sub esc}{sup 4} relation may implicate thermal escape. We therefore address the general behavior of hydrodynamic thermal escape models ranging from Pluto to highly irradiated extrasolar giant planets (EGPs). Energy-limited escape is harder to test because copious XUV radiation is mostly a feature of young stars, and hence requires extrapolating to historic XUV fluences (I{sub xuv}) using proxies and power laws. An energy-limited shoreline should scale as I{sub xuv}∝v{sub esc}{sup 3}√ρ, which differs distinctly from the apparent I{sub xuv}∝v{sub esc}{sup 4} relation. Energy-limited escape does provide good quantitative agreement to the highly irradiated EGPs. Diffusion-limited escape implies that no planet can lose more than 1% of its mass as H{sub 2}. Impact erosion, to the extent that impact velocities v{sub imp} can be estimated for exoplanets, fits a v{sub imp}≈4--5 v{sub esc} shoreline. The proportionality constant is consistent with what the collision of comet Shoemaker–Levy 9 showed us we should expect of modest impacts in deep atmospheres. With respect to the shoreline, Proxima Centauri b is on the metaphorical beach. Known hazards include its rapid energetic accretion, high impact velocities, its early life on the wrong side of the runaway greenhouse, and Proxima Centauri’s XUV radiation. In its favor is a vast phase space of unknown unknowns.

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DOE OSTI2017

Scaling Relations of Starburst-driven Galactic Winds

Using synthetic absorption lines generated from 3D hydrodynamical simulations, we explore how the velocity of a starburst-driven galactic wind correlates with the star formation rate (SFR) and SFR density. We find strong correlations for neutral and low ionized gas, but no correlation for highly ionized gas. The correlations for neutral and low ionized gas only hold for SFRs below a critical limit set by the mass loading of the starburst, above which point the scaling relations flatten abruptly. Below this point the scaling relations depend on the temperature regime being probed by the absorption line, not on the mass loading. The exact scaling relation depends on whether the maximum or mean velocity of the absorption line is used. We find that the outflow velocity of neutral gas can be up to five times lower than the average velocity of ionized gas, with the velocity difference increasing for higher ionization states. Furthermore, the velocity difference depends on both the SFR and mass loading of the starburst. Thus, absorption lines of neutral or low ionized gas cannot easily be used as a proxy for the outflow velocity of the hot gas.

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DOE OSTI2017

The Slowly Varying Corona. I. Daily Differential Emission Measure Distributions Derived from EVE Spectra

Daily differential emission measure (DEM) distributions of the solar corona are derived from spectra obtained by the Extreme-ultraviolet Variability Experiment (EVE) over a 4 yr period starting in 2010 near solar minimum and continuing through the maximum of solar cycle 24. The DEMs are calculated using six strong emission features dominated by Fe lines of charge states viii, ix, xi, xii, xiv, and xvi that sample the nonflaring coronal temperature range 0.3–5 MK. A proxy for the non-Fe xviii emission in the wavelength band around the 93.9 Å line is demonstrated. There is little variability in the cool component of the corona (T < 1.3 MK) over the 4 yr, suggesting that the quiet-Sun corona does not respond strongly to the solar cycle, whereas the hotter component (T > 2.0 MK) varies by more than an order of magnitude. A discontinuity in the behavior of coronal diagnostics in 2011 February–March, around the time of the first X-class flare of cycle 24, suggests fundamentally different behavior in the corona under solar minimum and maximum conditions. This global state transition occurs over a period of several months. The DEMs are used to estimate the thermal energy of the visible solar corona (of order 10{sup 31} erg), its radiative energy loss rate ((2.5–8) × 10{sup 27} erg s{sup −1}), and the corresponding energy turnover timescale (about an hour). The uncertainties associated with the DEMs and these derived values are mostly due to the coronal Fe abundance and density and the CHIANTI atomic line database.

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A Database of Flare Ribbon Properties from the Solar Dynamics Observatory. I. Reconnection Flux

We present a database of 3137 solar flare ribbon events corresponding to every flare of GOES class C1.0 and greater within 45° from the central meridian, from 2010 April until 2016 April, observed by the Solar Dynamics Observatory. For every event in the database, we compare the GOES peak X-ray flux with the corresponding active region and flare ribbon properties. We find that while the peak X-ray flux is not correlated with the active region unsigned magnetic flux, it is strongly correlated with the flare ribbon reconnection flux, flare ribbon area, and the fraction of active region flux that undergoes reconnection. We find the relationship between the peak X-ray flux and the flare ribbon reconnection flux to be I{sub X,peak}∝Φ{sub ribbon}{sup 1.5}. This scaling law is consistent with earlier hydrodynamic simulations of impulsively heated flare loops. Using the flare reconnection flux as a proxy for the total released flare energy E, we find that the occurrence frequency of flare energies follows a power-law dependence: dN/dE∝E{sup −1.6} for 10{sup 31}<E<10{sup 33} erg, consistent with earlier studies of solar and stellar flares. The database is available online and can be used for future quantitative studies of flares.

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