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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

Solar Surfing: Final Report on a Phase I NASA Innovative Advanced Concepts Study

The Sun sustains life on Earth and NASA has made its study one of the four pillars of the Science Mission Directorate. A specific area of study, the coronal heating problem, has been of significant concern for nearly 80years; namely how does the 5800 K surface of the Sun heat the nearby corona to over 1,000,000 K. Differing theories have been proposed to explain this process, but verification by actual measurement would not only resolve this issue, it would provide close-up measurements of the Sun never before obtained. However, this requires the development of a solar shield that can protect a satellite located less than 10,000 km from the Sun's surface. Steps towards that capability are the goal of this NIAC project. The current state-of-the-art in solar shielding is best shown by the upcoming Parker Solar Probe Mission, so the approach taken by that satellite is discussed and used as a starting point; allowing a distance of 9.5 solar radii from the Sun's center to be reached. It is then shown that state-of-the-art solar reflectors do not improve this performance. Next, we review the use of pressed powder as a better solar reflector and show that there is some improvement, but not sufficient to reach the Sun's surface. We spend some time on this architecture because the Parker Solar Probe has a thin scattering layer on its solar shield and it is important to discuss the advantages and disadvantages of this feature.

solar surfing↗
NASA NTRS2016

Suppression of Heating of Coronal Loops Rooted in Opposite Polarity Sunspot Umbrae

EUV observations of active region (AR) coronae reveal the presence of loops at different temperatures. To understand the mechanisms that result in hotter or cooler loops, we study a typical bipolar AR, near solar disk center, which has moderate overall magnetic twist and at least one fully developed sunspot of each polarity. From AIA 193 and 94 Å images we identify many clearly discernible coronal loops that connect plage or a sunspot of one polarity to an opposite-­polarity plage region. The AIA 94 Å images show dim regions in the umbrae of the spots. To see which coronal loops are rooted in a dim umbral area, we performed a non-linear force‐free field (NLFFF) modeling using photospheric vector magnetic field measurements obtained with the Heliosesmic Magnetic Imager (HMI) onboard SDO. The NLFFF model, validated by comparison of calculated model field lines with observed loops in AIA 193 and 94 Å, specifies the photospheric roots of the model field lines. Some model coronal magnetic field lines arch from the dim umbral area of the positive-polarity sunspot to the dim umbral area of a negative-polarity sunspot. Because these coronal loops are not visible in any of the coronal EUV and X-ray images of the AR, we conclude they are the coolest loops in the AR. This result suggests that the loops connecting opposite polarity umbrae are the least heated because the field in umbrae is so strong that the convective braiding of the field is strongly suppressed.

sunspots↗
NASA NTRS2016

Suppression of Heating of Coronal Loops Rooted in Opposite Polarity Sunspot Umbrae

EUV observations of active region (AR) coronae reveal the presence of loops at different temperatures. To understand the mechanisms that result in hotter or cooler loops, we study a typical bipolar AR, near solar disk center, which has moderate overall magnetic twist and at least one fully developed sunspot of each polarity. From AIA 193 and 94 Å images we identify many clearly discernible coronal loops that connect plage or a sunspot of one polarity to an opposite-­polarity plage region. The AIA 94 Å images show dim regions in the umbrae of the spots. To see which coronal loops are rooted in a dim umbral area, we performed a non-linear force‐free field (NLFFF) modeling using photospheric vector magnetic field measurements obtained with the Heliosesmic Magnetic Imager (HMI) onboard SDO. The NLFFF model, validated by comparison of calculated model field lines with observed loops in AIA 193 and 94 Å, specifies the photospheric roots of the model field lines. Some model coronal magnetic field lines arch from the dim umbral area of the positive-polarity sunspot to the dim umbral area of a negative-polarity sunspot. Because these coronal loops are not visible in any of the coronal EUV and X-ray images of the AR, we conclude they are the coolest loops in the AR. This result suggests that the loops connecting opposite polarity umbrae are the least heated because the field in umbrae is so strong that the convective braiding of the field is strongly suppressed.

sunspot↗
NASA NTRS1994

Correlative analysis of hard and soft x ray observations of solar flares

We have developed a promising new technique for jointly analyzing BATSE hard X-ray observations of solar flares with simultaneous soft X-ray observations. The technique is based upon a model in which electric currents and associated electric fields are responsible for the respective heating and particle acceleration that occur in solar flares. A useful by-product of this technique is the strength and evolution of the coronal electric field. The latter permits one to derive important flare parameters such as the current density, the number of current filaments composing the loop, and ultimately the hard X-ray spectrum produced by the runaway electrons. We are continuing to explore the technique by applying it to additional flares for which we have joint BATSE/Yohkoh observations. A central assumption of our analysis is the constant of proportionality alpha relating the hard X-ray flux above 50 keV and the rate of electron acceleration. For a thick-target model of hard X-ray production, it can be shown that cv is in fact related to the spectral index and low-energy cutoff of precipitating electrons. The next step in our analysis is to place observational constraints on the latter parameters using the joint BATSE/Yohkoh data.

Zarro, Dominic M.↗
NASA NTRS2004

Unified Models of Turbulence and Nonlinear Wave Evolution in the Extended Solar Corona and Solar Wind

The PI (Cranmer) and Co-I (A. van Ballegooijen) made substantial progress toward the goal of producing a unified model of the basic physical processes responsible for solar wind acceleration. The approach outlined in the original proposal comprised two complementary pieces: (1) to further investigate individual physical processes under realistic coronal and solar wind conditions, and (2) to extract the dominant physical effects from simulations and apply them to a 1D model of plasma heating and acceleration. The accomplishments in Year 2 are divided into these two categories: 1a. Focused Study of Kinetic Magnetohydrodynamic (MHD) Turbulence. lb. Focused Study of Non - WKB Alfven Wave Rejection. and 2. The Unified Model Code. We have continued the development of the computational model of a time-study open flux tube in the extended corona. The proton-electron Monte Carlo model is being tested, and collisionless wave-particle interactions are being included. In order to better understand how to easily incorporate various kinds of wave-particle processes into the code, the PI performed a detailed study of the so-called "Ito Calculus", i.e., the mathematical theory of how to update the positions of particles in a probabilistic manner when their motions are governed by diffusion in velocity space.

Cranmer, Steven R.↗
NASA NTRS1988

Helical magnetohydrodynamic turbulence and the coronal heating problem

Numerical simulations are used to investigate the relaxation of an unconfined, helically turbulent, fully three-dimensional magnetofluid, with conditions similar to those which are thought to result in the heating of the solar corona. In these simulations, the system evolves through a succession of force free states. After a relatively quiescent period of Ohmic decay, a phase of accelerated magnetic energy dissipation occurs. Some magnetic energy is transformed into kinetic energy, and the magnitude of entrophy created is a nontrival fraction of the mean square electric current. Concentrated vorticity structures are seen to play almost as important a role as electric current sheets in the heating process. Coincident with this accelerated dissipation process, a reorganization of the magnetic fields occurs, with transfer of magnetic energy to both shorter and longer wavelength modes than are initially present. The ratio of the magnetic field to the electric current density, alpha does not in general tend to assume a constant value in the force free regions during the evolution of the magnetofluid.

Dahlburg, R. B.↗
NASA NTRS2010

Constraints on CME Evolution from in situ Observations of Ionic Charge States

We present a novel procedure for deriving the physical properties of Coronal Mass Ejections (CMES) in the corona. Our methodology uses in-situ measurements of ionic charge states of C, O, Si and Fe in the heliosphere and interprets them in the context of a model for the early evolution of ICME plasma, between 2 - 5 R-solar. We find that the data can be fit only by an evolution that consists of an initial heating of the plasma, followed by an expansion that ultimately results in cooling. The heating profile is consistent with a compression of coronal plasma due to flare reconnect ion jets and an expansion cooling due to the ejection, as expected from the standard CME/flare model. The observed frozen-in ionic charge states reflect this time-history and, therefore, provide important constraints for the heating and expansion time-scales, as well as the maximum temperature the CME plasma is heated to during its eruption. Furthermore, our analysis places severe limits on the possible density of CME plasma in the corona. We discuss the implications of our results for CME models and for future analysis of ICME plasma composition.

Gruesbeck, Jacob R.↗
NASA NTRS2014

Investigating the Differential Emission Measure and Energetics of Microflares with Combined SDO/AIA and RHESSI Observations

An important question in solar physics is whether solar microflares, the smallest currently observable flare events in X-rays, possess the same energetic properties as large flares. Recent surveys have suggested that microflares may be less efficient particle accelerators than large flares, and hence contribute less non-thermal energy, which may have implications for coronal heating mechanisms. We therefore explore the energetic properties of microflares by combining EUV and X-ray measurements. We present forward-fitting differential emission measure (DEM) analysis of 10 microflares. The fitting is constrained by combining, for the first time, high-temperature Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations and flux data from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA). Two fitting models are tested for the DEM; a Gaussian distribution and a uniform DEM profile. A Gaussian fit proved unable to explain the observations for any of the studied microflares. However, 8 of 10 events studied were reasonably fit by a uniform DEM profile. Hence microflare plasma can be considered to be significantly multi-thermal, and may not be significantly peaked or contain resolvable fine structure, within the uncertainties of the observational instruments. The thermal and non-thermal energy is estimated for each microflare, comparing the energy budget with an isothermal plasma assumption. From the multi-thermal fits the minimum non-thermal energy content was found to average approximately 30% of the estimated thermal energy. By comparison, under an isothermal model the non-thermal and thermal energy estimates were generally comparable. Hence, multi-thermal plasma is an important consideration for solar microflares that substantially alters their thermal and non-thermal energy content.

Sun: flares↗
NASA NTRS2016

Prebiotic Chemistry and Atmospheric Warming of Early Earth by an Active Young Sun

Nitrogen is a critical ingredient of complex biological molecules. Molecular nitrogen, however, which was outgassed Into the Earth's early atmosphere, is relatively chemically inert and nitrogen fixation into more chemically reactive compounds requires high temperatures. Possible mechanisms of nitrogen fixation include lightning, atmospheric shock heating by meteorites, and solar ultraviolet radiation. Here we show that nitrogen fixation in the early terrestrial atmosphere can be explained by frequent and powerful coronal mass ejection events from the young Sun -- so-called superflares. Using magnetohydrodynamic simulations constrained by Kepler Space Telescope observations, we find that successive superflare ejections produce shocks that accelerate energetic particles, which would have compressed the early Earth's magnetosphere. The resulting extended polar cap openings provide pathways for energetic particles to penetrate into the atmosphere and, according to our atmospheric chemistry simulations, initiate reactions converting molecular nitrogen, carbon dioxide and methane to the potent greenhouse gas nitrous oxide as well as hydrogen cyanide, an essential compound for life. Furthermore, the destruction of N2, C02 and CH, suggests that these greenhouse gases cannot explain the stability of liquid water on the early Earth. Instead, we propose that the efficient formation of nitrous oxide could explain a warm early Earth.

Airapetian, V. S.↗
NASA NTRS1995

Solar coronal hole simulations compared with interpretations of Yohkoh SXT observations

Recent observations of coronal holes made with the soft X-ray telescope aboard Yohkoh have indicated a temperature of 1.8 approximately 2.1 x 10(exp 6) K and an emission measure of 10(exp 25.7 approximately 26.2) cm (exp -5). This is almost the same as in quiet regions of the Sun. Numerical simulations of the temperature density and velocity structure in a coronal hole. using a parameterized heating distribution have been used for a comparison with the Yohkoh observations. Models are obtained which fit the observed temperature and emission measure. with heating fluxes which are consistent with other measurements. However, the final velocity of the solar wind is very slow which indicates the necessity of another acceleration mechanism such as alfven waves.

Tziotziou, K.↗
NASA NTRS1993

Evidence for ion jets in the high-speed solar wind

Results of an investigation of shapes of proton and alpha particle velocity distribution functions in the high-speed solar wind for the time period from 1973 to 1987 are presented. It is suggested that a modest amount of heating results from the damping of waves, whereas additional heating and the bulk of the acceleration is associated with the direct conversion of magnetic energy flux to plasma convection and enthalpy flux through the process of magnetic reconnection. The present two-component model of heating and acceleration predicts that the ambient flow of plasma from coronal holes is characterized by a low-speed and a low-mass flux. The proton flux is insufficient to drag alpha particles out of the solar gravitational potential well.

Feldman, W. C.↗
NASA NTRS1980

Solar wind flows associated with hot heavy ions

Solar wind heavy ion spectra measured with the Vela instrumentation are studied to determine the solar origins of various solar wind structures which contain anomalously high ionization states. The spectra indicating hot coronal conditions were found to occur in 1/7th of all measurements and almost exclusively in postshock flows, nonshock related helium abundance enhancements, or noncompressive density enhancements. The observation of several flared-related helium abundance enhancements suggests that the flare-heated plasma can evolve into the solar wind without producing a noticeable shock at 1 AU.

Fenimore, E. E.↗
NASA NTRS1983

Solar wind minor ions: Recent observations

Systematic studies show that the minor ions generally travel with a common bulk speed and have temperatures roughly proportional to their masses. It was determined that (3)He(++) content varies greatly; (3)He(++)/(4)He(++) ranges from as high as 10(-12) values to below 2 x 10(-4). In some solar wind flows which can be related to energetic coronal events, the minor ions are found in unusual ionization states containing Fe(16+) as a prominent ion, showing that the states were formed at unusually high temperatures. Unexpectedly, in a few flows substantial quantities of (4)He(+) were detected, sometimes with ions identifiable as O(2+) and O(3+). Surprisingly, in some of these examples the ionization state is mixed showing that part of the plasma escaped the corona without attaining the usual million-degree temperatures while other parts were heated more nearly in the normal manner. Additionally, detailed studies of the minor ions increased our understanding of the coronal expansion. For example, such studies contributed to identifying near equatorial coronal streamers as the source of solar wind flows between high speed streams.

Bame, S. J.↗
DOE OSTI2022

Fundamental Studies in Basic Plasma Science: Experimental Investigations of Alfven Wave Damping Processes Relevant to the Solar Corona (Final Report)

Motivated by recent observations suggesting that the solar corona is heated by Alfvén wave damping at unexpectedly low heights, we performed experiments to study the basic plasma physics of Alfvén wave dissipation. Proposed coronal damping mechanisms rely on inhomogeneities in the plasma, but these theories have not been systematically studied in the laboratory. We performed experiments using the Large Plasma Device (LAPD) to test these theories.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1989

The International Heliospheric Study; Proceedings of Symposium 10 of the 27th COSPAR Plenary Meeting, Espoo, Finland, July 18-29, 1988

The conference presents papers on solar phenomena, coronal mass ejections, solar wind, heliospheric magnetic field, energetic particles, cosmic rays, and interaction with the interstellar medium. Topics include the NASA heliospheric program, Alfven wave heating of the solar atmosphere in the transition region, IPS imaging of heliospheric transients, CME associated forward-reverse shock pairs, and solar wind latitude/longitude variations from interplanetary scintillations. Consideration is also given to cosmic ray gradients in the heliosphere, reconnection at the heliopause, and the interaction of the solar wind with the local interstellar medium.

Shea, M. A.↗
NASA NTRS2010

Nanoflares, Spicules, and Other Small-Scale Dynamic Phenomena on the Sun

There is abundant evidence of highly dynamic phenomena occurring on very small scales in the solar atmosphere. For example, the observed pr operties of many coronal loops can only be explained if the loops are bundles of unresolved strands that are heated impulsively by nanoflares. Type II spicules recently discovered by Hinode are an example of small-scale impulsive events occurring in the chromosphere. The exist ence of these and other small-scale phenomena is not surprising given the highly structured nature of the magnetic field that is revealed by photospheric observations. Dynamic phenomena also occur on much lar ger scales, including coronal jets, flares, and CMEs. It is tempting to suggest that these different phenomena are all closely related and represent a continuous distribution of sizes and energies. However, this is a dangerous over simplification in my opinion. While it is tru e that the phenomena all involve "magnetic reconnection" (the changin g of field line connectivity) in some form, how this occurs depends s trongly on the magnetic geometry. A nanoflare resulting from the interaction of tangled magnetic strands within a confined coronal loop is much different from a major flare occurring at the current sheet form ed when a CME rips open an active region. I will review the evidence for ubiquitous small-scale dynamic phenomena on the Sun and discuss wh y different phenomena are not all fundamentally the same.

Klimchuk, James↗
NASA NTRS2010

SDO/AIA Light Curves and Implications for Coronal Heating: Model Predictions

It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Is it also impulsive or is it quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Nevertheless, the light curves exhibit predictable and understandable patterns. This presentation emphasizes the modeling aspects of our study. A companion presentation emphasizes the observations.

Klimchuk, James A.↗