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NASA NTRS2003

Large-Eddy Simulation of Conductive Flows at Low Magnetic Reynolds Number

In this paper we study the LES method with dynamic procedure in the context of conductive flows subject to an applied external magnetic field at low magnetic Reynolds number R(sub m). These kind of flows are encountered in many industrial applications. For example, in the steel industry, applied magnetic fields can be used to damp turbulence in the casting process. In nuclear fusion devices (Tokamaks), liquid-lithium flows are used as coolant blankets and interact with the surrounding magnetic field that drives and confines the fusion plasma. Also, in experimental facilities investigating the dynamo effect, the flow consists of liquid-sodium for which the Prandtl number and, as a consequence, the magnetic Reynolds number is low. Our attention is focused here on the case of homogeneous (initially isotropic) decaying turbulence. The numerical simulations performed mimic the thought experiment described in Moffatt in which an initially homogeneous isotropic conductive flow is suddenly subjected to an applied magnetic field and freely decays without any forcing. Note that this flow was first studied numerically by Schumann. It is well known that in that case, extra damping of turbulence occurs due to the Joule effect and that the flow tends to become progressively independent of the coordinate along the direction of the magnetic field. Our comparison of filtered direct numerical simulation (DNS) predictions and LES predictions show that the dynamic Smagorinsky model enables one to capture successfully the flow with LES, and that it automatically incorporates the effect of the magnetic field on the turbulence. Our paper is organized as follows. In the next section we summarize the LES approach in the case of MHD turbulence at low R(sub m) and recall the definition of the dynamic Smagorinsky model. In Sec. 3 we describe the parameters of the numerical experiments performed and the code used. Section 4 is devoted to the comparison of filtered DNS results and LES results. Conclusions are presented in Sec. 5.

Knaepen, B.↗

NASA NTRS1990

Polar thermospheric Joule heating, and redistribution of recombination energy in the upper mesosphere

Kellogg (1961), suggested that transport of atomic oxygen from the summer into the winter hemisphere and subsequent release of energy by three body recombination, O+O+N2 yields O2+N2+E, may contribute significantly to the so-called mesopause temperature anomaly. Earlier model calculations have shown that Kellogg's mechanism produces about a 10-percent increase in the temperature from summer to winter at 90 km. This process, however, is partly compensated by differential heating from absorption of UV radiation associated with dissociation of O2. In the auroral region of the thermosphere, there is a steady energy dissipation by Joule heating causing a redistribution and depletion of atomic oxygen due to wind-induced diffusion. With the removal of O, latent chemical energy normally released by three body recombination is also removed, and the result is that the temperature decreases by almost 2 percent near 90 km. Through dynamic feedback, this process reduces the depletion of atomic oxygen by about 25 percent and the temperature perturbation in the exosphere from 10 to 7 percent at polar latitudes. Under the influence of the internal dynamo interaction, the prevailing zonal circulation in the upper thermosphere changes direction when the redistribution of recombination energy is considered.

Mayr, H. G.↗

NASA NTRS2010

Initial Results of DC Electric Fields, Associated Plasma Drifts, Magnetic Fields, and Plasma Waves Observed on the C/NOFS Satellite

Initial results are presented from the Vector Electric Field Investigation (VEFI) on the Air Force Communication/Navigation Outage Forecasting System (C/NOFS) satellite, a mission designed to understand, model, and forecast the presence of equatorial ionospheric irregularities. The VEFI instrument includes a vector DC electric field detector, a fixed-bias Langmuir probe operating in the ion saturation regime, a flux gate magnetometer, an optical lightning detector, and associated electronics including a burst memory. Compared to data obtained during more active solar conditions, the ambient DC electric fields and their associated E x B drifts are variable and somewhat weak, typically < 1 mV/m. Although average drift directions show similarities to those previously reported, eastward/outward during day and westward/downward at night, this pattern varies significantly with longitude and is not always present. Daytime vertical drifts near the magnetic equator are largest after sunrise, with smaller average velocities after noon. Little or no pre-reversal enhancement in the vertical drift near sunset is observed, attributable to the solar minimum conditions creating a much reduced neutral dynamo at the satellite altitude. The nighttime ionosphere is characterized by larger amplitude, structured electric fields, even where the plasma density appears nearly quiescent. Data from successive orbits reveal that the vertical drifts and plasma density are both clearly organized with longitude. The spread-F density depletions and corresponding electric fields that have been detected thus far have displayed a preponderance to appear between midnight and dawn. Associated with the narrow plasma depletions that are detected are broad spectra of electric field and plasma density irregularities for which a full vector set of measurements is available for detailed study. Finally, the data set includes a wide range of ELF/VLF/HF oscillations corresponding to a variety of plasma waves, in particular banded ELF hiss, whistlers, and lower hybrid wave turbulence triggered by lightning-induced sferics. The VEFI data represents a new set of measurements that are germane to numerous fundamental aspects of the electrodynamics and irregularities inherent to the Earth's low latitude ionosphere.

Pfaff, R.↗

NASA NTRS2011

DC Electric Fields, Associated Plasma Drifts, and Irregularities Observed on the C/NOFS Satellite

Results are presented from the Vector Electric Field Investigation (VEFI) on the Air Force Communication/Navigation Outage Forecasting System (C/NOFS) satellite, a mission designed to understand, model, and forecast the presence of equatorial ionospheric irregularities. The VEFI instrument includes a vector DC electric field detector, a fixed-bias Langmuir probe operating in the ion saturation regime, a flux gate magnetometer, an optical lightning detector, and associated electronics including a burst memory. Compared to data obtained during more active solar conditions, the ambient DC electric fields and their associated E x B drifts are variable and somewhat weak, typically < 1 mV/m. Although average drift directions show similarities to those previously reported, eastward/outward during day and westward/downward at night, this pattern varies significantly with longitude and is not always present. Daytime vertical drifts near the magnetic equator are largest after sunrise, with smaller average velocities after noon. Little or no pre-reversal enhancement in the vertical drift near sunset is observed, attributable to the solar minimum conditions creating a much reduced neutral dynamo at the satellite altitude. The nighttime ionosphere is characterized by larger amplitude, structured electric fields, even where the plasma density appears nearly quiescent. Data from successive orbits reveal that the vertical drifts and plasma density are both clearly organized with longitude. The spread-F density depletions and corresponding electric fields that have been detected thus far have displayed a preponderance to appear between midnight and dawn. Associated with the narrow plasma depletions that are detected are broad spectra of electric field and plasma density irregularities for which a full vector set of measurements is available for detailed study. The VEFI data represents a new set of measurements that are germane to numerous fundamental aspects of the electrodynamics and irregularities inherent to the Earth s low latitude ionosphere.

Pfaff, R.↗

NASA NTRS1983

The origins of Birkeland currents

Fundamental observations and zero-order theories for Birkeland currents are employed, together with recent observational data, to formulate a first-order model for the currents. Five categories of Birkeland currents are detailed, including region 1 current sheets which follow the auroral oval, region 2 currents with opposite polarity forming on the equatorward side, cusp currents poleward of region 1, currents appearing after extended periods of the interplanetary magnetic field, and a three-layer overlap of current sheets in the Harang discontinuity. The region 1 currents near noon are found to be driven by the interplanetary electric field on open field lines, while away from noon the inner edge of the plasma sheet is the source. Charge separation drives the region 2 currents, and neutralization is effected in the ionosphere. The magnetopause is concluded to be an insignificant factor for the Birkeland currents. Convection reversal is predicted to occur in the region 1 interior. Finally, four lobed polar electric fields are considered to be generated by the boundary layer dynamo, and indicate a closed magnetospheric configuration.

Stern, D. P.↗

NASA NTRS2002

Chandra Observations of a Young Embedded Magnetic B Star in the p Ophiuchus Cloud

This paper reports the analysis of two Chandra X-ray observations of the young magnetic B star rho Ophiuchus S1. X-ray emission from the star was detected in both observations. The average flux is almost the same in both, but during each observation the flux shows significant time variations by a factor of two on timescales of 20-40 ksec. Each spectrum can be fit by either an absorbed power law model with a photon index of approx. -3 or a thin-thermal plasma model with a temperature of approx. 2 keV and an extremely low metal abundance (approx. less than 0.1 solar). The spectrum of the first observation has an apparent line feature at about 6.8 keV, which likely corresponds to highly ionized iron K alpha. In contrast, the spectrum of the second observation shows an anomalous edge absorption component at E approx. 1 keV. The continuum emission and log (L(sub X)/L(sub bol)) approx. -6 are similar to those of young intermediate-mass stars (Herbig Ae/Be stars) although the presence of the magnetic field inferred from the detection of non-thermal radio emission has drawn an analogy between rho Ophiuchus S1 and magnetic chemically peculiar (MCP) stars. If the X-ray emission is thermal, the highest plasma temperature observed is too high to be explained by the conventional theories of magnetic stars, and favors some kind of magnetic dynamo activity, while if the emission is nonthermal, it might be related to mass infall. The 6.8 keV line and 4 keV edge features are marginal but they give important information near the stellar body if they are real. Their physical interpretation is discussed.

Hamaguchi, Kenji↗

NASA NTRS1994

What we have learned about Mars from SNC meteorites

The SNC meteorites are thought to be igneous martian rocks, based on their young crystallization ages and a close match between the composition of gases implanted in them during shock and the atmosphere of Mars. A related meteorite, ALH84001, may be older and thus may represent ancient martian crust. These petrologically diverse basalts and ultramafic rocks are mostly cumulates, but their parent magmas share geochemical and radiogenic isotopic characteristics that suggest they may have formed by remelting the same mantle source region at different times. Information and inferences about martian geology drawn from these samples include the following: Planetary differentiation occured early at approximately 4.5 GA, probably concurrently with accretion. The martian mantle contains different abundances of moderately volatile and siderophile elements and is more Fe-rich than that of the Earth, which has implications for its mineralogy, density, and origin. The estimated core composition has a S abundance near the threshold value for inner core solidification. The former presence of a core dynamo may be suggested by remanent magnetization in Shergottite-Nakhlite-Chassignite (SNC) meteorites, although these rocks may have been magnetized during shock. The mineralogy of martian surface units, inferred from reflectance spectra, matches that of basaltic shergottites, but SNC lithologies thought to have crystallized in the subsurface are not presently recognized. The rheological properties of martian magmas are more accurately derived form these metorites than from observations of martian flow morphology, although the sampled range of magma compositions islimited. Estimates of planetary water abundance and the amount of outgassed water based on these meteorites are contridictory but overlap estimates based on geological observations and atmospheric measurements. Stable isotope measurements indicate that the martian hydrosphere experienced only limited exchange with the lithosphere, but it is in isotopic equilibrium with the atmosphere and has been since 1.3 Ga. The isotopically heavy atmosphere/hydrosphere composition deduced from these rocks reflects a loss process more severe than current atmospheric evolution models, and the occurence of carbonates in SNC meteorites suggest that they, rather than scapolite or hydrous carbonates, are the major crustal sink for CO2. Weathering products in SNC meteorites support the idea of limited alteration of the lithosphere by small volumes of saline, CO2-bearing water. Atmospheric composition and evolution are further constrained by noble gases in these meteorites, although Xe and Kr isotopes suggest different origins for the atmosphere. Planetary ejection of these rocks has promoted an advance in the understanding of impact physics, which has been accomplished by a model involving spallation during large cratering events. Ejection of all the SNC meteorites (except ALH84001) in one or two events may provide a plausible solution to most constraints imposed by chronology, geochemistry, and cosmic ray exposure, although problems remain with this scenario; ALH84001 may represent older martian crust sampled during a separate impact.

Mcsween, Harry Y., Jr.↗

NASA NTRS2004

Removing Activity-Related Radial Velocity Noise to Improve Extrasolar Planet Searches

We have made significant progress towards the proposal goals of understanding the causes and effects of magnetic activity-induced radial velocity (v_r) jitter and developing methods for correcting it. In the process, we have also made some significant discoveries in the fields of planet-induced stellar activity, planet detection methods, M dwarf convection, starspot properties, and magnetic dynamo cycles. We have obtained super high resolution (R approximately 200,000), high S / N (greater than 300) echelle study of joint line bisector and radial velocity variations using the McDonald 2-D coude. A long observing run in October 2002 in particular was quite successful (8 clear nights). We now have close to three years of data, which begins to sample a good fraction of the magnetic cycle timescales for some of our targets (e.g., kappa Ceti; P_cyc = 5.6 yrs). This will be very helpful in unraveling the complex relationships between plage and radial velocity (v-r) changes which we have uncovered. Preliminary analysis (Saar et al. 2003) of the data in hand, reveals correlations between median line bisector displacement and v_r. The correlation appears to be specific the the particular star being considered, probably since it is a function of both spectral type and rotation rate. Further analysis and interpretation will be in the context of evolving plage models and is in progress.

Saar, Steven↗

DOE OSTI2025

Hints of primordial magnetic fields at recombination and implications for the Hubble tension

Primordial Magnetic Fields (PMFs), long studied as relics of the early Universe, accelerate recombination and have been proposed as a way to relieve the Hubble tension. However, previous studies relied on simplified toy models. Here, in this study, we use recent evaluations of recombination with PMFs, incorporating full magnetohydrodynamic (MHD) simulations and detailed Lyman-alpha radiative transfer, to test PMF-enhanced recombination (bΛCDM) against observational data from the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), and Type Ia supernovae (SN). Focusing on non-helical PMFs with a Batchelor spectrum, we find a preference for presentday total field strengths of approximately 5-10 pico-Gauss. Depending on the dataset combination, this preference ranges from mild (∼ 1.8σ with Planck + DESI) to moderate (∼ 3σ with Planck + DESI + SH0ES-calibrated SN) significance. The bΛCDM has Planck + DESI χ 2 values equal to or better than ΛCDM while predicting a higher Hubble constant. Future high-resolution CMB temperature and polarization measurements will be crucial for confirming or further constraining PMFs at recombination. Field strengths of 5-10 pico-Gauss align closely with those required for cluster magnetic fields to originate entirely from primordial sources, without the need for additional dynamo amplification.

Jedamzik, Karsten [Centre National de la Recherche↗

NASA NTRS1998

The Coronae of Moderate-Mass Giants in the Hertzsprung Gap and the Clump

We have used the Roentgensatellit (ROSAT), the Extreme Ultraviolet Explorer (EUVE), and the Hubble Space Telescope (HST) to measure X-ray and ultraviolet emissions of moderate-mass (APPROX. 2-3 solar mass) giants in the Hertzsprung gap (spectral types early F to mid-G) and the post-helium flash "clump" (approx. G8-K0). Our motivation was to document the evolution of hot coronae (T greater than 10(exp 6)K) along the post-main-sequence trajectories traveled by such stars in order to gain insight concerning the "X-ray deficiency" of the F-GO giants and the strong braking of stellar rotation at the red edge of the Hertzsprung gap. With few exceptions, Hertzsprung gap and clump giants observed by ROSAT PSPC show hot (T approx. 10(exp 7)K) coronal energy distributions, regardless of any X-ray deficiency, EUVE spectra of gap star 31 Com (G0 111) indicate a broad coronal emission measure hump at approx. 10(exp 7.2)K, while the active clump giant beta Ceti (K0 III) displays a sharp peak at approx. 10(exp 6.8)K, as seen previously in the mixed clump/gap binary Capella (alpha Aur: G8 III + G0 III). The gap giants upsilon Peg (F8 III) and 24 UMa (G4 III) have EUV emissions of intermediate temperature (approx. 10(exp 7.0)K). The stars 31 Com, psi(sup 3) Psc (G0 III), and beta Cet exhibit redshifted transition zone (TZ: approx. 10(exp 5)K) lines in HST GHRS spectra, as reported earlier in Procyon (alpha CMi: F5 IV-V) and Capella G0. Such redshifts on the Sun are thought to signify flows in magnetic loops. beta Cas (F2 III)-a rare soft coronal source among the gap stars-displays blueshifts of C iv and 0 iv], although emissions at cooler and hotter temperatures are near the photospheric velocity. The remarkably broad line profiles of the fastest rotating gap giants suggest that the 10(exp 5) K "subcoronal " emission zones extend to h approx. R(sub *) above the photosphere, about 50 scale heights. In contrast to the TZ line redshifts, the upper chromospheric emissions (e.g., Mg II and Si III) of 31 Com and upsilon(sup 3) Psc have blueshifted cores. Blue-asymmetric peaks in the solar Mg a lines are thought to indicate dynamical heating in the chromosphere. Observations of the H(sub I) Ly(alpha) feature of 31 Com taken 9 months apart reveal striking profile changes, reminiscent of those noted previously in the Ly(alpha) blue peak of the Capella G0 star. We used the far-ultraviolet diagnostics, in combination with ROSAT X-ray photometry and EUVE high-excitation line strengths, to constrain physical models of the stellar outer atmospheres. Quasi-static magnetic loops can simulate the empirical coronal emission measures of the giant stars, but the inferred pressures for sensible loop lengths conflict with direct measurements of subcoronal densities. Furthermore, the high rate of emission at approx. 10(exp 5) K cannot be explained by thermal conduction down the legs of hot quasi-static loops. On the other hand, the possible existence of elongated (l - R(sub *) emission structures on the gap giants leads to a speculative scenario to explain the X-ray deficiency. It is based on the increased importance of the dynamical filling phase ("explosive evaporation") of the loop life cycle; conductive cooling, yielding TZ emissions at the footpoints, when the heating is interrupted; and the possibility for transitions between " hot " and " cool " energy balance solutions owing to dynamical suspension and centrifugal trapping of the cooling gas. The long loops might represent a vestigial global " magnetosphere " inherited from the main-sequence phase, which ultimately is disrupted near approx. G0 by the deepening convective envelope and growth of a more solar-like dynamo. Coronal emissions might be boosted temporarily as the X-ray deficiency is removed but soon would be quenched by wind braking previously inhibited by the magnetospheric "dead zone."

Ayres, Thomas R.↗

NASA NTRS1996

A Sundial-Atlas Precursor to the TIMED Mission: A Quick-Response Global Investigation into Coupled Lower Thermospheric, Ionospheric, and Mesospheric Physics

The SUNDIAL-ATLAS effort was a global-scale investigation which responded to the science priorities of the ITM Panel, the Integrated SPD Strategy Implementation Plan as a whole, and the need for potential cost-saving design criteria for the TIMED mission. The investigation focused on coupling processes in the ionospheric-thermospheric system, taking advantage of the timelines of the ATLAS-1 mission (March 1992), and the global-scale ground-based measurement and modeling activities of the SUNDIAL program. The collaborative SUNDIAL-ATLAS activity was the first opportunity for global measurements of the chemistry, kinetics, and electrodynamics which couple the E-, Fl-, and F2-regions into a single interactive system. As such, the program represented an important first step in studying global issues; and accordingly, was an important proof of concept experiment relevant to the strategic mission plans for the ITM community and the upcoming intermediate class satellite program called TIMED. To meet its projected goals, TIMED must perform a number of critical measurements and execute a number of correlations that were to be tried and tested for the first time in the SUNDIAL-ATLAS investigation. This was designed to include global correlations of thermospheric and ionospheric composition during quiet and disturbed conditions and the co-registration of global-scale ground-based measurements with along-track satellite diagnostics. The SUNDIAL component of the current investigation addressed this need by acquiring, reducing, and analyzing a multi-sensor database that complemented and extended that which was generated in the ATLAS mission (Atmospheric Laboratory for Applications and Science). The SUNDIAL data defined the state and condition of the global-scale ionosphere in the altitude range from 100 km to the F2-peak. These data specified the peak heights and densities of the E-, Fl-, and F2-regions, along with the global distributions of intermediate, descending, and sequential layers which play a critical role in the dynamo region of the lower ionospheric-thermospheric domain. The data were collected by the SUNDIAL network of more than 50 ground-based stations utilizing ionosondes, radars, photometers, Fabry-Perot interferometers, and total electron content measurements. The data were acquired during a three-week period centered on the eight-day ATLAS-1 mission, which provided image and photometric sensing of the altitude distributions of the major and minor ions and neutrals in the ITM system. This report focuses on the scientific contributions of the SUNDIAL component of the overall investigation. Specific findings are described in seven papers (attached) published in the Journal of Geophysical Research.

Szuszczewicz, E. P.↗

NASA NTRS2000

The DYNAMO Orbiter Project: High Resolution Mapping of Gravity/Magnetic Fields and In Situ Investigation of Mars Atmospheric Escape

Dynamo is a small Mars orbiter planned to be launched in 2005 or 2007, in the frame of the NASA/CNES Mars exploration program. It is aimed at improving gravity and magnetic field resolution, in order to better understand the magnetic, geologic and thermal history of Mars, and at characterizing current atmospheric escape, which is still poorly constrained. These objectives are achieved by using a low periapsis orbit, similar to the one used by the Mars Global Surveyor spacecraft during its aerobraking phases. The proposed periapsis altitude for Dynamo of 120-130 km, coupled with the global distribution of periapses to be obtained during one Martian year of operation, through about 5000 low passes, will produce a magnetic/gravity field data set with approximately five times the spatial resolution of MGS. Low periapsis provides a unique opportunity to investigate the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, therefore atmospheric escape, which may have played a crucial role in removing atmosphere, and water, from the planet. There is much room for debate on the importance of current atmosphere escape processes in the evolution of the Martian atmosphere, as early "exotic" processes including hydrodynamic escape and impact erosion are traditionally invoked to explain the apparent sparse inventory of present-day volatiles. Yet, the combination of low surface gravity and the absence of a substantial internally generated magnetic field have undeniable effects on what we observe today. In addition to the current losses in the forms of Jeans and photochemical escape of neutrals, there are solar wind interaction-related erosion mechanisms because the upper atmosphere is directly exposed to the solar wind. The solar wind related loss rates, while now comparable to those of a modest comet, nonetheless occur continuously, with the intriguing possibility of important cumulative and/or enhanced effects over the several billion years of the solar system's life. If the detailed history of the Martian internal field could be traced back, and the current escape processes could be understood well enough to model the expected stronger losses under early Sun conditions, one could go a long way toward constraining this part of the mysterious history of Mars' atmosphere.

Smrekar, S.↗

NASA NTRS1998

Turbulent Compressible Convection with Rotation: Mean Flows and Differential Rotation - 2

The effects of rotation on turbulent, compressible convection within stellar envelopes are studied through three-dimensional numerical simulations conducted within a local f-plane model. This work seeks to understand the types of differential rotation that can be established in convective envelopes of stars like the Sun, for which recent helioseismic observations suggest an angular velocity profile with depth and latitude at variance with many theoretical predictions. This paper analyzes the mechanisms that are responsible for the mean (horizontally averaged) zonal and meridional flows that are produced by convection influenced by Coriolis forces. The compressible convection is considered for a range of Rayleigh, Taylor, and Prandtl (and thus Rossby) numbers encompassing both laminar and turbulent flow conditions under weak and strong rotational constraints. When the nonlinearities are moderate, the effects of rotation on the resulting laminar cellular convection leads to distinctive tilts of the cell boundaries away from the vertical. These yield correlations between vertical and horizontal motions that generate Reynolds stresses that can drive mean flows, interpretable as differential rotation and meridional circulations. Under more vigorous forcing, the resulting turbulent convection involves complicated and contorted fluid particle trajectories, with few clear correlations between vertical and horizontal motions, punctuated by an evolving and intricate downflow network that can extend over much of the depth of the layer. Within such networks are some coherent structures of vortical downflow that tend to align with the rotation axis. These yield a novel turbulent alignment mechanism, distinct from the laminar tilting of cellular boundaries, that can provide the principal correlated motions and thus Reynolds stresses and subsequently mean flows. The emergence of such coherent structures that can persist amidst more random motions is a characteristic of turbulence with symmetries broken by rotation and stratification. Such structure is here found to play a crucial role in defining the mean zonal and meridional flows that coexist with the convection. Though they are subject to strong inertial oscillations, the strength and type of the mean flows are determined by a combination of the laminar tilting and the turbulent alignment mechanisms. Varying the parameters produces a wide range of mean motions. Among these, some turbulent solutions exhibit a mean zonal velocity profile that is nearly constant with depth, much as deduced by helioseismology at midlatitudes within the Sun. The solutions exhibit a definite handedness, with the direction of the persistent mean flows often prescribing a spiral with depth near the boundaries, also in accord with helioseismic deductions. The mean helicity has a profile that is positive in the upper portion of the domain and negative in the lower portion, a property bearing on magnetic dynamo processes that may be realized within such rotating layers of turbulent convection.

Brummell, Nicholas H.↗