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DOE OSTI2020

An Analysis of the Statistics and Systematics of Limb Anomaly Detections in HST/STIS Transit Images of Europa

Several recent studies derived the existence of plumes on Jupiter’s moon Europa. The only technique that provided multiple detections is the far-ultraviolet imaging observations of Europa in transit of Jupiter taken by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). In this study, we reanalyze the three HST/STIS transit images in which Sparks et al. identified limb anomalies as evidence for Europa’s plume activity. After reproducing the results of Sparks et al., we find that positive outliers are similarly present in the images as the negative outliers that were attributed to plume absorption. A physical explanation for the positive outliers is missing. We then investigate the systematic uncertainties and statistics in the images and identify two factors that are crucial when searching for anomalies around the limb. One factor is the alignment between the actual and assumed locations of Europa on the detector. A misalignment introduces distorted statistics, most strongly affecting the limb above the darker trailing hemisphere where the plumes were detected. The second factor is a discrepancy between the observation and the model used for comparison, adding uncertainty in the statistics. When accounting for these two factors, the limb minima (and maxima) are consistent with random statistical occurrence in a sample size given by the number of pixels in the analyzed limb region. The plume candidate features in the three analyzed images can be explained by purely statistical fluctuations and do not provide evidence for absorption by plumes.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2022

SDSS-IV MaNGA: Cannibalism Caught in the Act—On the Frequency of Occurrence of Multiple Cores in Brightest Cluster Galaxies

Although it is generally accepted that massive galaxies form in a two-phased fashion, beginning with a rapid mass buildup through intense starburst activities followed by primarily dry mergers that mainly deposit stellar mass at outskirts, the late time stellar mass growth of brightest cluster galaxies (BCGs), the most massive galaxies in the universe, is still not well understood. Several independent measurements have indicated a slower mass growth rate than predictions from theoretical models. We attempt to resolve the discrepancy by measuring the frequency of BCGs with multiple cores, which serve as a proxy of the merger rates in the central region and facilitate a more direct comparison with theoretical predictions. Using 79 BCGs at z = 0.06–0.15 with integral field spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) project, we obtain a multiple-core fraction of 0.11 ± 0.04 at z ≈ 0.1 within an 18 kpc radius from the center, which is comparable to the value of 0.08 ± 0.04 derived from mock observations of 218 simulated BCGs from the cosmological hydrodynamical simulation IllustrisTNG. We find that most cores that appear close to the BCGs from imaging data turn out to be physically associated systems. Anchoring on the similarity in the multiple-core frequency between the MaNGA and IllustrisTNG, we discuss the mass growth rate of BCGs over the past 4.5 Gyr.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1979

HEAO 1 observations of X-ray emission from flares on dMe stars

The paper reports the detection of two X-ray flares from each of the nearby dMe stars, AT Mic and AD Leo, with the A-2 experiment on board HEAO 1. A spectrum obtained during the brighter AT Mic flare, the first X-ray spectrum of a stellar flare, is well matched by a thermal model with a temperature 3 x 10 to the 7 K and an iron K-alpha emission line. The X-ray luminosities derived are in the range 1.3-16 x 10 to the 30th ergs/s, while emission measures are in the range 1.1-14 x 10 to the 53rd/cu cm. The estimated Lx/Lopt ratios exceed unity and are inconsistent with Mullan's flare model. Several scenarios to explain this discrepancy are proposed.

Kahn, S. M.↗

NASA NTRS1983

Theory of interstellar medium diagnostics

The theoretical interpretation of observed interplanetary resonance luminescence patterns is used as one of the must promising methods to determine the state of the local interstellar medium (LISM). However, these methods lead to discrepant results that would be hard to understand in the framework of any physical LISM scenario. Assuming that the observational data are reliable, two possibilities which could help to resolve these discrepancies are discussed: (1) the current modeling of resonance luminescence patterns is unsatisfactory and has to be improved, and (2) the extrapolated interstellar parameters are not indicative of the unperturbed LISM state, but rather designate an intermediate state attained in the outer regions of the solar system. It is shown that a quantitative treatment of the neutral gas-plasma interaction effects in the interface between the heliospheric and the interstellar plasmas is of major importance for the correct understanding of the whole complex.

Fahr, H. J.↗

NASA NTRS1984

Abrasion by aeolian particles: Earth and Mars

Estimation of the rate of aeolian abrasion of rocks on Mars requires knowledge of: (1) particle flux, (2) susceptibilities to abrasion of various rocks, and (3) wind frequencies on Mars. Fluxes and susceptibilities for a wide range of conditions were obtained in the laboratory and combined with wind data from the Viking meteorology experiment. Assuming an abundant supply of sand-sized particles, estimated rates range up to 2.1 x 10 to the minus 2 power cm of abrasion per year in the vicinity of Viking Lander 1. This rate is orders of magnitude too great to be in agreement with the inferred age of the surface based on models of impact crater flux. The discrepancy in the estimated rate of abrasion and the presumed old age of the surface cannot be explained easily by changes in climate or exhumation of ancient surfaces. The primary reason is thought to be related to the agents of abrasion. At least some sand-sized (approx. 100 micrometers) grains appear to be present, as inferred from both lander and orbiter observations. High rates of abrasion occur for all experimental cases involving sands of quartz, basalt, or ash. However, previous studies have shown that sand is quickly comminuted to silt- and clay-sized grains in the martian aeolian regime. Experiments also show that these fine grains are electrostatically charged and bond together as sand-sized aggregates. Laboratory simulations of wind abrasion involving aggregates show that at impact velocities capable of destroying sand, aggregates from a protective veneer on the target surface and can give rise to extremely low abrasion rates.

Greeley, R.↗

NASA NTRS1984

Ultraviolet Observations of Uranus and Neptune Below 3000 Angstrom

From 2000 to 3000 A, both Uranus and Neptune have albedos that are about two times higher than Jupiter or Saturn's, implying that the outer giants have stratospheres that are relatively free of aerosol absorption. Uncertainties in the absolute calibration procedure allow discrepancies of order 15% between conservative models and the observations. A small amount of aerosol absorption is therefore possible. Below 2000 A the derived albedo is highly dependent on the solar spectrum source used in the data reduction. The most recent result for Uranus, first reported here, is consistent with a secular change in C2H2 mixing ratio from approximately three times ten to the minus eight in 1980 to less than or equal to ten to the minus ninth in 1983. These values are approximately 2 orders of magnitude less than the mixing ratios of this gas on Saturn, and comparable to the amount on Jupiter.

Caldwell, J.↗

NASA NTRS1984

Ultraviolet observations of Uranus and Neptune below 3000 A

From 2000 A to 3000 A, both Uranus and Neptune have albedos that are about two times higher than Jupiter or Saturn's, implying that the outer giants have stratospheres that are relatively free of aerosol absorption. Uncertainties in the absolute calibration procedure allow discrepancies of order 15% between conservative models and the observations. A small amount of aerosol absorption is therefore possible. Below 2000 A, the derived albedo is highly dependent on the solar spectrum source used in the data reduction. The most recent result for Uranus is consistent with a secular change in C2H2 mixing ratio from approximately 3 x (10 to the -8 power) in 1980 to or = 10 to the -9 power in 1983. These values are approximately 2 orders of magnitude less than the mixing ratios of this gas on Saturn, and comparable to the amount on Jupiter.

Caldwell, J.↗

NASA NTRS1985

A comparison of precipitating electron energy flux on March 22, 1979 with an empirical model - CDAW 6

Data recorded by Defense Meteorological Satellite Program, Tiros and P-78-1 satellites for the CDAW 6 event on March 22, 1979, have been compared with a statistical model of precipitating electron fluxes. Comparisons have been made on both an orbit-by-orbit basis and on a global basis by sorting and binning the data by AE index, invariant latitude, and magnetic local time in a manner similar to which the model was generated. It is concluded that the model flux agrees with the data to within a factor of two, although small features and the exact locations of features are not consistently reproduced. In addition, the latitude of highest electron precipitation usually occurs about 3 deg more poleward in the model than in the data. This discrepancy is attributed to ring current inflation of the storm time magnetosphere.

Simons, S. L., Jr.↗

NASA NTRS1989

The global geochemistry of bomb-produced tritium - General circulation model compared to available observations and traditional interpretations

Observational evidence suggests that of the tritium produced during nuclear bomb tests that has already reached the ocean, more than twice as much arrived through vapor impact as through precipitation. In the present study, the Goddard Institute for Space Studies 8 x 10 deg atmospheric general circulation model is used to simulate tritium transport from the upper atmosphere to the ocean. The simulation indicates that tritium delivery to the ocean via vapor impact is about equal to that via precipitation. The model result is relatively insensitive to several imposed changes in tritium source location, in model parameterizations, and in model resolution. Possible reasons for the discrepancy are explored.

Koster, Randal D.↗

NASA NTRS1990

Influence of penetrating solar radiation on the heat budget of the equatorial Pacific Ocean

Recent satellite observations of ocean transparency, coupled with climatological surface heat fluxes and ocean density profiles, are used here to show that solar radiation in visible frequencies, usually assumed to be absorbed at the sea surface, in fact penetrates to a significant degree to below the upper mixed layer of the ocean which interacts actively with the atmosphere. The net effect is a reduction of the heat input into the upper layer; for a 20 m-thick mixed layer this is equivalent to an annual reduction in temperature of about 5-10 K. The results provide a natural explanation for the discrepancy between the SSTs predicted by models and those observed.

Lewis, Marlon R.↗

NASA NTRS1991

Conductivity and electric field variations with altitude in the stratosphere

Data regarding electric field, derived current density, and conductivity are presented for two balloons from the Electrodynamics of the Middle Atmosphere experiment which underwent the longest period of daily altitude variation. The magnetic L values range from 4.3 to 9.5 for the 18 days of Southern Hemisphere statistics, and the average conductivity and vertical electric fields are given. Simultaneous measurements of the average conductivity scale height and the vertical electric-field scale height indicate that vertical current density does not vary with altitude in the 10-28-km range. The measured conductivity varies significantly at a given altitude on a particular day, and some conductivity data sets are similar to other measurements between 10 and 30 km. Comparisons of the measured data to predictions from models of stratospheric conductivity demonstrate significant discrepancies.

Holzworth, Robert H.↗

NASA NTRS1992

Extreme ultraviolet filters for 58.4 and 83.4 nm

We report the design of an EUV filter for He I at 58.4 nm and the design and fabrication of an EUV filter for O II at 83.4 nm. Both filters are designed as combinations of three narrow-band reflection filters. The net transmittance through both EUV filters is close to 10 percent with bandwidths less than 10 nm, and blocking better than 0.005 percent for out-of-band wavelengths. A theoretical calculation of the 83.4 nm filter predicts higher values for the peak transmittance than the measured spectral performance of the fabricated filter. Since aluminum is one of the film materials used for the fabrication of EUV filters, the aluminum film oxidation can be modeled in order to explain the discrepancy between the theory and experiment. Since the filters operate at angles of incidence up to 50 deg, the optical components of a system can serve as both the filtering and imaging elements.

Zukic, Muamer↗

NASA NTRS1995

Predictions of LET spectra measured on LDEF

The linear energy transfer (LET) spectra measured by plastic (CR-39) detectors in Exp. P0006 on LDEF are much higher at high LET than expected from methods commonly used to predict LET spectra produced by the space ionizing radiation environment. This discrepancy is being investigated by examining modeling approximations used in the predictions, and some interim results are presented.

Armstrong, T. W.↗

NASA NTRS1994

A new determination of radii and limb parameters for Pluto and Charon from mutual event lightcurves

Over the past several years Pluto-Charon mutual events have yielded progressively more accurate estimates of Charon's orbital elements and the radii of Pluto and Charon (e.g., Buie, Tholen, and Horne, 1992). Analysis of the 1988 stellar occultation by Pluto indicates a radius for Pluto that is about 4%, or 50 km, larger than the mutual event radius of 1151 km. One possible explanation for the discrepancy is that the mutual event modeling treats Pluto and Charon as uniformly bright disks. If they are limb-darkened, the mutual event fits could underestimate their radii. In this paper we use an independent mutual event data set (Young and Binzel, 1992) to fit for Pluto and Charon's radii in a manner independent of either object's limb profile or albedo distribution. Our least-squares solution indicates that Pluto's radius is 1164 +/- 22.9 km and Charon's radius is 621 +/- 20.6 km.

Young, Eliot F.↗

NASA NTRS1996

Comment on 'Current Budget of the Atmospheric Electric Global Circuit'

In this paper, three major issues relevant to Kasemir's new model will be addressed. The first concerns Kasemir's assertion that there are significant differences between the potentials associated with the new model and the conventional model. A recalculation of these potentials reveals that both models provide equivalent results for the potential difference between the Earth and ionosphere. The second issue to be addressed is Kasemir's assertion that discrepancies in the electric potentials associated with both models can be attributed to modeling the Earth as a sphere, instead of as a planar surface. A simple analytical comparison will demonstrate that differences in the equations for the potentials of the atmosphere derived with a spherical and a planar Earth are negligible for applications to global current flow. Finally, the third issue to be discussed is Kasemir's claim that numerous aspects of the conventional model are incorrect, including the role of the ionosphere in global current flow as well as the significance of cloud-to-ground lightning in supplying charge to the global circuit. In order to refute these misconceptions, it will be shown that these aspects related to the flow of charge in the atmosphere are accurately described by the conventional model of the global circuit.

Driscoll, Kevin T.↗

NASA NTRS1998

Heat Transfer on a Film-Cooled Rotating Blade Using a Two Equation Turbulence Model

A three-dimensional Navier-Stokes code has been used to compare the heat transfer coefficient on a film-cooled, rotating turbine blade. The blade chosen is the ACE rotor with five rows containing 93 film cooling holes covering the entire span. This is the only film-cooled rotating blade over which experimental data is available for comparison. Over 2.278 million grid points are used to compute the flow over the blade including the tip clearance region, using Coakley's q-omega turbulence model. Results are also compared with those obtained by Garg and Abhari (1997) using the zero-equation Baldwin-Lomax (B-L) model. A reasonably good comparison with the experimental data is obtained on the suction surface for both the turbulence models. At the leading edge, the B-L model yields a better comparison than the q-omega model. On the pressure surface, however, the comparison between the experimental data and the prediction from either turbulence model is poor. A potential reason for the discrepancy on the pressure surface could be the presence of unsteady effects due to stator-rotor interaction in the experiments which are not modeled in the present computations. Prediction using the two-equation model is in general poorer than that using the zero-equation model, while the former requires at least 40% more computational resources.

Garg, Vijay K.↗

NASA NTRS2000

Alternative Methods of the Thermospheric Atomic Oxygen Density Determination

Atomic oxygen density in the upper thermosphere (approximately 300 km) can be calculated using ground based incoherent scatter radar and Fabry-Perot interferometer measurements. Burnside et al. [1991] was the first to try this method, but Buonsanto et al. provided an extensive treatment of the method in 1997. This paper further examines the method using 46 nights of data collected over six years and the latest information on the oxygen collision frequency. The method is compared with the MSIS-86 atomic oxygen prediction values, which are based upon in situ rocket born and satellite measurements from the 70's to the mid-80's In general, the method supports the MSIS-86 model, but indicates several areas of discrepancy. Furthermore, no direct correlation is found between the geomagnetic conditions and the difference between the method and MSIS-86 predictions.

Bennett. Adam C.↗

NASA NTRS2001

Alternative Method for the Thermospheric Atomic Oxygen Density Determination

Atomic oxygen density in the upper thermosphere (approximately 300 km) can be calculated using ground based incoherent scatter radar and Fabry-Perot interferometer measurements. Burnside et al. was the first to try this method, but Buonsanto et al. provided an extensive treatment of the method in 1997. This paper further examines the method using 46 nights of data collected over six years and the latest information on the oxygen collision frequency. The method is compared with the MSIS (Mass Spectrometer Incoherent Scatter)-86 atomic oxygen prediction values, which are based upon in situ rocket born and satellite measurements from the 70s to the mid-80s. In general, the method supports the MSIS-86 model, but indicates several areas of discrepancy. Furthermore, no direct correlation is found between the geomagnetic conditions and the difference between the method and MSIS-86 predictions.

Bennett, A. C.↗