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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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NASA NTRS1982

Mass loss from cool stars, facts, fads, and fallacies

The accumulation of observational material (ultraviolet spectroscopic measures, quantitative optical spectroscopy, and X-ray photometry) and its use in discerning the presence and character of mass loss across the cool half of the H-R diagram and establishing constraints on theoretical models are discussed. Analogies with closed and open solar magnetic structures are found. Two determinants of atmospheric wind structure, temperature and gravity, may suffice in a most superficial way to define the wind and atmospheric structure in a star, however it is apparent that there is still a missing parameter which may stem from magnetic activity and its particular configuration. Theories that appear successful in reproducing observed line profiles, wind temperatures, and terminal velocities incorporate Alfven wave heating and momentum deposition. Successive observations of an active binary (lambda and G8III-IV) and a supergiant star, alpha Aqr (G2 Ib) revealed that magnetic activity and perhaps mass loss occur on restricted regions of a stellar surface and that long-term structures are present in the wind. These phenomena are present in the solar atmosphere and wind and may be considered a general characteristic of stellar winds.

Dupree, A. K.↗
NASA NTRS1984

Measurements of solar total irradiance and its variability

The development of electrically self calibrated cavity pyrheliometric instrumentation that occurred in the early 20th century provided the technological base for experiments to detect variability of the solar total irradiance. Experiments from ground based observatories, aircraft and balloons during the 1st half of the 20th century were unable to achieve sufficient accuracy or long term precision to unambiguously detect irradiance variations of solar origin. Refinements in pyrheliometric technology during the 1960's and 1970's and the accessibility of extended experimental opportunities above the earth's atmosphere in recent years have provided the first direct observations of solar total irradiance variability and provided the cornerstone observations of a long term database on solar irradiance. A program of solar irradiance monitoring has evolved to sustain the database over at least 22 years, corresponding to a single cycle of solar magnetic activity, and the shortest well identified cycle of climate variation. Direct links between total irradiance variations, solar magnetic activity and the solar global '5 min' oscillation phenomena have been derived from recent space flight observations by the SMM/ACRIM I experiment.

Willson, R. C.↗
NASA NTRS1998

Brightness Changes in Sun-like Stars

Does the Sun's energy output vary with time? Are observable climatic changes on the earth caused by changes in the Sun? Can we gain greater insight into this relation-ship by studying other stars with properties similar to the Sun's? In recent years, satellite observations have shown that the solar irradiance varies in phase with the 1 l-year sunspot cycle. The Sun is brighter by about O.l% at the peak of the sunspot cycle when solar magnetic activity is at its maximum. Over longer intervals, changes in the cart h's climate and solar magnetic activity seem to be correlated. We are using automatic photoelectric telescopes to measure brightness changes in a sample of 150 Sun-like stars. Lowell Observatory astronomers have also observed about 30 of these same stars with a manual telescope in a program that began 10 years before ours. Since these two data sets were acquired with different instruments and so have significant systematic differences, we developed software to combine them accurately and, therefore, extend our observational time coverage. We show sample results of brightness variations over 14 years in several Sun-like stars with different ages. Longitudinal studies like these, combined with cross-sectional studies of the larger sample of stars, may eventually allow us to infer with confidence the Sun's long-term brightness history and its impact on the earth's climate.

Henry, Stephen M.↗
NASA NTRS1982

Active region magnetic fields inferred from simultaneous VLA microwave maps, X-ray spectroheliograms, and magnetograms

The VLA maps at 6-cm wavelength discussed here are generated from observations of a solar active region (NOAA 2363) on March 29 and 30, 1980. The X-ray spectroheliograms were acquired for this region in the lines of O VIII, Ne IX, Mg XI, Si XIII, S XV, and Fe XXV. It is found that the peaks of X-ray and 6-cm emission do not coincide but that the sources in the two wavelength domains tend to overlap. These facts are seen as evidence for the existence of opacity mechanisms other than thermal bremsstrahlung. To quantify this assertion, differential emission measures are computed to derive densities and temperatures. Using these and calculated force-free magnetic fields from Kitt Peak magnetograms, an assessment is made of the mechanism of gyroresonance absorption at low harmonics of the electron gyrofrequency as the source of opacity responsible for the microwave features. It is concluded that large-scale currents must be present in the active region loops to account for the bright 6-cm sources far from sunspots.

Schmahl, E. J.↗
NASA NTRS1976

The day-to-day variability in ionospheric electric fields and currents

The daily variations in ionospheric drift velocities are examined from incoherent scatter measurements at Millstone Hill. The data with summed Kp greater than 24 behave differently from those with low magnetic activity and basically follow the convection pattern but have large day-to-day variations. The influence of the magnetic conjugate point is discussed, and solar cycle variations are examined in conjunction with geomagnetic variations. Ionospheric currents calculated by using a semidiurnal neutral wind model are in good agreement with ground-based magnetograms for low magnetic activity, but the E region neutral wind model appears to be applicable only to this case.

Kirchhoff, V. W. J. H.↗
NASA NTRS1981

The global distribution of nitric oxide at 200 km

Measurements of nitric oxide at 200 km by the ultraviolet nitric oxide experiment on Atmosphere Explorer D are used to demonstrate the dependence of NO on latitude, longitude, and magnetic activity. NO is more abundant in the summer hemisphere than in the winter hemisphere and is more abundant during magnetically active times than during quiet times. A simple photochemical theory is used to show that the knowledge of local composition and temperature is sufficient to explain the variations of nitric oxide at this altitude.

Cravens, T. E.↗
NASA NTRS1985

Magnetic optical activity in intense laser fields. I - Self-rotation and Verdet constant

The paper is concerned with the Faraday rotation and circular dichroism of dilute gases in longitudinal magnetic fields subject to strong radiation fields. In particular, magnetic fields are considered which produce Zeeman shifts much smaller than inverse collision times but which may be greater or smaller then the multipole relaxation times. Doppler broadening is considered, and explicit expressions are given for dipole transitions in a two-level system with arbitrary angular momentum for strong and weak magnetic fields. The same system is then studied as a function of the external magnetic-field strength with a fixed laser frequency. It is shown that this experimental arrangement, referred to as forward scattering, has certain advantages that are not present in the more usual magnetic rotation spectroscopy.

Giraud-Cotton, S.↗
NASA NTRS1972

Thermal positive ions in the outer ionosphere and magnetosphere from Ogo 1.

Study of the positive thermal ion densities measured with a spherical electrostatic analyzer aboard OGO 1 between September and October, 1964. A high variability of charged-particle distributions is established within and beyond the plasmapause boundary. The density gradient was in an inverse relation to the plasmapause L position and was consistently about one order of magnitude higher for the afternoon sector orbits than for the nightside orbits. Irregularities of different types were observed in the positive thermal ion densities within the plasmasphere and outside it, showing no correlation with the magnetic activity level. A delay of 3 to 9 hr was observed in the movement of the plasmapause following an increase in the magnetic activity level on the nightside.

Ahmed, M.↗
NASA NTRS2017

Using Data Assimilation Methods for Prediction of Solar Activity

The variability of solar magnetic activity known as the 11-­‐year solar cycles has the longest history of observations. These solar cycles dramatically affect conditions in the heliosphere and the Earth’s space environment. Our current understanding of the physical processes that make up global solar dynamics and the dynamo that generates the magnetic fields is sketchy, resulting in unrealistic descriptions in theoretical and numerical models of the solar cycles. The absence of long-­‐term observations of solar interior dynamics and photospheric magnetic fields hinders development of accurate dynamo models and their calibration. In such situations, mathematical data assimilation methods provide an optimal approach for combining the available observational data and their uncertainties with theoretical models in order to estimate the state of the solar dynamo and predict future cycles. In this presentation, we will discuss the implementation and performance of an Ensemble Kalman Filter data assimilation method based on the Parker migratory dynamo model complemented by the equation of magnetic helicity conservation and long-­‐term sunspot data series. This approach has allowed us to reproduce the general properties of the solar cycles and has already demonstrated a good predictive capability for the current cycle, 24. We will discuss further development of this approach, which includes a more sophisticated dynamo model, synoptic magnetogram data, and employs the DART Data Assimilation Research Testbed.

Solar Activity↗
NASA NTRS1977

Global exospheric temperatures and densities under active solar conditions

Temperatures measured by the OGO-6 satellite using the 6300 A airglow spectrum are compared with temperatures derived from total densities and N2 densities. It is shown that while the variation of the total densities with latitude and magnetic activity agree well with values used for CIRA (1972), the temperature behavior is very different. While the temperatures derived from the N2 density were in much better agreement there were several important differences which radically affect the pressure gradients. The variation of temperature with magnetic activity showed seasonal and local time variations. Neutral temperature, density, pressure and boundary oxygen variations for the storm of 8 March 1970 are presented.

Nisbet, J. S.↗
NASA NTRS1994

Modelling and control of a rotor supported by magnetic bearings

In this paper we develop a dynamical model of a rotor and the active magnetic bearings used to support the rotor. We use this model to develop a stable state feedback control of the magnetic bearing system. We present the development of a rigid body model of the rotor, utilizing both Rotation Matrices (Euler Angles) and Euler Parameters (Quaternions). In the latter half of the paper we develop a stable state feedback control of the actively controlled magnetic bearing to control the rotor position under inbalances. The control law developed takes into account the variation of the model with rotational speed. We show stability over the whole operating range of speeds for the magnetic bearing system. Simulation results are presented to demonstrate the closed loop system performance. We develop the model of the magnetic bearing, and present two schemes for the excitation of the poles of the actively controlled magnetic bearing. We also present a scheme for averaging multiple sensor measurements and splitting the actuation forces amongst redundant actuators.

Gurumoorthy, R.↗
NASA NTRS2003

Validation of Spacecraft Active Cavity Radiometer Total Solar Irradiance (TSI) Long Term Measurement Trends Using Proxy TSI Least Squares Analyses

Long-term, incoming total solar irradiance (TSI) measurement trends were validated using proxy TSI values, derived from indices of solar magnetic activity. Spacecraft active cavity radiometers (ACR) are being used to measure longterm TSI variability, which may trigger global climate changes. The TSI, typically referred to as the solar constant, was normalized to the mean earth-sun distance. Studies of spacecraft TSI data sets confirmed the existence of a 0.1 %, long-term TSI variability component within a 10-year period. The 0.1% TSI variability component is clearly present in the spacecraft data sets from the 1984-2004 time frame. Typically, three overlapping spacecraft data sets were used to validate long-term TSI variability trends. However, during the years of 1978-1984, 1989-1991, and 1993-1996, three overlapping spacecraft data sets were not available in order to validate TSI trends. The TSI was found to vary with indices of solar magnetic activity associated with recent 10-year sunspot cycles. Proxy TSI values were derived from least squares analyses of the measured TSI variability with the solar indices of 10.7-cm solar fluxes, and with limb-darked sunspot fluxes. The resulting proxy TSI values were compared to the spacecraft ACR measurements of TSI variability to detect ACR instrument degradation, which may be interpreted as TSI variability. Analyses of ACR measurements and TSI proxies are presented primarily for the 1984-2004, Earth Radiation Budget Experiment (ERBE) ACR solar monitor data set. Differences in proxy and spacecraft measurement data sets suggest the existence of another TSI variability component with an amplitude greater than or equal to 0.5 Wm-2 (0.04%), and with a cycle of 20 years or more.

Lee, Robert Benjamin, III↗
NASA NTRS2006

SI: The Stellar Imager

The ultra-sharp images of the Stellar Imager (SI) will revolutionize our view of many dynamic astrophysical processes: The 0.1 milliarcsec resolution of this deep-space telescope will transform point sources into extended sources, and simple snapshots into spellbinding evolving views. SI s science focuses on the role of magnetism in the Universe, particularly on magnetic activity on the surfaces of stars like the Sun. SI s prime goal is to enable long-term forecasting of solar activity and the space weather that it drives in support of the Living With a Star program in the Exploration Era by imaging a sample of magnetically active stars with enough resolution to map their evolving dynamo patterns and their internal flows. By exploring the Universe at ultra-high resolution, SI will also revolutionize our understanding of the formation of planetary systems, of the habitability and climatology of distant planets, and of many magnetohydrodynamically controlled structures and processes in the Universe.

Carpenter, Kenneth G.↗
NASA NTRS1993

Long-term variations in dust production in R Coronae Borealis

Several searches for periodicities in the historical visible light curves of RCB stars have found that the intervals between declines are random. This paper reexamines 70 declines of R Coronae Borealis since 1853 using one homogeneous data set, the AAVSO light curve. In this data set, pairs of consecutive declines also show a random distribution of intervals. However, if the sequence of declines is examined, there are semiperiodic variations between times of high- and low-decline activity on a time scale of a few years. Near-IR photometry of RCB stars indicates that there are large semiperiodic variations in the amount of dust being produced which have similar time scales. Possible interpretations of a semiperiodic variation in dust formation rates in RCB stars are presented. One is a magnetic activity cycle resulting in 'spots' on the star over which dust formation takes place. Such a magnetic activity cycle is similar to the solar cycle. Another is the changes in the period and amplitude of the pulsations over several years affecting the efficiency of dust production.

Clayton, Geoffrey C.↗
NASA NTRS1972

The plasmapause as measured in positive ions.

Use of extensions of the existing theory of magnetospheric convection to describe the dynamics of the plasmasphere and the variation in the plasmapause location. The equatorial local time plane is divided into three separate and physically distinct regions, the bulge region, the nightside region, and the dayside region, corresponding to local times of 1500 to 2200 hrs, 2200 to 0600 hrs, and 0600 to 1500 hrs, respectively. The characteristics predicted by the model in the bulge region are: presence of the bulge at dusk, 1/R to the 4th power radial dependence of plasma concentration, and large fluctuations in plasma density at the plasmapause and plasma detachment. In the nightside region the predicted characteristics are: rapid response to magnetic activity changes, formative region for plasmasphere ripples, and the formative region for the dayside plasmapause location. The model also predicts the dayside plasmasphere characteristics, such as the slow response to magnetic activity changes, and filling of the plasmasphere from the ionosphere, including the formation of a 'double plasmapause.'

Sharp, G. W.↗
NASA NTRS1978

Global morphology of nitric oxide in the lower E region

Measurements of nitric oxide at 105 km by the ultraviolet nitric oxide experiment on Atmosphere Explorer C are presented. The amount of nitric oxide in the lower E region is shown to depend on latitude, longitude, and magnetic activity. Near the equator the density at the peak of the NO layer is typically about 2 x 10 to the 7th power/cu cm and varies little with longitude or magnetic activity, except during major storms. At high latitudes (up to 68 deg geographic latitude), peak densities are typically 2 or 3 times larger and much more variable. A longitudinal dependence is found in both geographic and geomagnetic coordinates, with minimum densities found near 45 deg E geomagnetic longitude and maxima near 135 deg W geomagnetic longitude. At 40 deg dip latitude the half amplitude is about 30%.

Cravens, T. E.↗
NASA NTRS1986

Observations of electric fields near the plasmapause at midnight

Static electric field measurements in the equatorial-plane from the 100-m double floating probe experiment carried on the SCATHA (P-78-2) satellite are reported. Eclipse measurements on auroral L shells are studied to better understand the change in magnetospheric convection at the plasmapause. The use of eclipse data allows the electric fields to be measured directly without contamination from the spacecraft photoelectron sheath; and it allows the thermal ion population in the outer plasmasphere to be observed, identifying the satellite location with respect to the plasmapause. (Such measurements are limited to near local midnight). It is found that, in the corotating frame, there is generally an electric field component in the antisolar direction during periods of low magnetic activity which begins at the edge of the plasmasphere and builds in amplitude as the satellite enters the plasma sheet. The amplitude of this electric field varies from 0.05 mV/m to 0.2 mV/m with increasing magnetic activity; the electric field switches to radially inward during periods of high activity with magnitudes up to 2 mV/m. The amplitude of the observed electric field increases with the amount of contribution from east-west currents in the earth's plasma sheet-magnetotail current system.

Olsen, R. C.↗
NASA NTRS2006

Why Do T Tauri Disks Accrete?

Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks scales roughly with the square of the central stellar mass. No dependence of accretion rate on stellar mass is predicted by the simplest version of the Gammie layered disk model, in which nonthermal ionization of upper disk layers allows accretion to occur via the magnetorotational instability. We show that a minor modification of Gaminie's model to include heating by irradiation from the central star yields a modest dependence of accretion on the mass of the central star. A purely viscous disk model could provide a strong dependence of accretion rate on stellar mass if the initial disk radius (before much viscous evolution has occurred) has a strong dependence on stellar mass. However, it is far from clear that at least the most massive pre-main-sequence disks can be totally magnetically activated by X-rays or cosmic rays. We suggest that a combination of effects are responsible for the observed dependence, with the lowest mass stars having the lowest mass disks, which can be thoroughly magnetically active, while the higher mass stars have higher mass disks that have layered accret,ion and relatively inactive or "dead" central zones at some radii. In such dead zones, we suggest that gravitational instabilities may play a role in allowing accretion to proceed. In this connection, we emphasize the uncertainty in disk masses derived from dust emission and argue that T Tauri disk masses have been systematically underestimated by conventional analyses. Furtlier study of accretion rates, especially in the lowest mass stars, would help to clarify the mechanisms of accretion in T Tauri stars.

Hartmann, Lee↗