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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 127 records · Page 7
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NASA NTRS2010

High Spectral Resolution, High Cadence, Imaging X-Ray Microcalorimeters for Solar Physics

High spectral resolution, high cadence, imaging x-ray spectroscopy has the potential to revolutionize the study of the solar corona. To that end we have been developing transition-edge-sensor (TES) based x-ray micro calorimeter arrays for future solar physics missions where imaging and high energy resolution spectroscopy will enable previously impossible studies of the dynamics and energetics of the solar corona. The characteristics of these x-ray microcalorimeters are significantly different from conventional micro calorimeters developed for astrophysics because they need to accommodate much higher count rates (300-1000 cps) while maintaining high energy resolution of less than 4 eV FWHM in the X-ray energy band of 0.2-10 keV. The other main difference is a smaller pixel size (less than 75 x 75 square microns) than is typical for x-ray micro calorimeters in order to provide angular resolution less than 1 arcsecond. We have achieved at energy resolution of 2.15 eV at 6 keV in a pixel with a 12 x 12 square micron TES sensor and 34 x 34 x 9.1 micron gold absorber, and a resolution of 2.30 eV at 6 keV in a pixel with a 35 x 35 micron TES and a 57 x 57 x 9.1 micron gold absorber. This performance has been achieved in pixels that are fabricated directly onto solid substrates, ie. they are not supported by silicon nitride membranes. We present the results from these detectors, the expected performance at high count-rates, and prospects for the use of this technology for future Solar missions.

Bandler, Simon R.↗
NASA NTRS2022

Flows in Enthalpy-based Thermal Evolution of Loops

Plasma-filled loop structures are common in the solar corona. Because detailed modeling of the dynamical evolution of these structures is computationally costly, an efficient method for computing approximate but quick physics-based solutions is to rely on space-integrated 0D simulations. The enthalpy-based thermal evolution of loops (EBTEL) framework is a commonly used method to study the exchange of mass and energy between the corona and transition region. EBTEL solves for density, temperature, and pressure, averaged over the coronal part of the loop, velocity at coronal base, and the instantaneous differential emission measure distribution in the transition region. The current single-fluid version of the code, EBTEL2, assumes that at all stages the flows are subsonic. However, sometimes the solutions show the presence of supersonic flows during the impulsive phase of heat input. It is thus necessary to account for this effect. Here, we upgrade EBTEL2 to EBTEL3 by including the kinetic energy term in the Navier–Stokes equation. We compare the solutions from EBTEL3 with those obtained using EBTEL2, as well as the state-of-the-art field-aligned hydrodynamics code HYDRAD. We find that the match in pressure between EBTEL3 and HYDRAD is better than that between EBTEL2 and HYDRAD. Additionally, the velocities predicted by EBTEL3 are in close agreement with those obtained with HYDRAD when the flows are subsonic. However, EBTEL3 solutions deviate substantially from HYDRAD’s when the latter predicts supersonic flows. Using the mismatches in the solution, we propose a criterion to determine the conditions under which EBTEL can be used to study flows in the system.

Solar coronal heating↗
NASA NTRS2008

The Role of Magnetic Reconnection in Solar Activity

The central challenge in solar/heliospheric physics is to understand how the emergence and transport of magnetic flux at the photosphere drives the structure and dynamics that we observe in the corona and heliosphere. This presentation focuses on the role of magnetic reconnection in determining solar/heliospheric activity. We demonstrate that two generic properties of the photospheric magnetic and velocity fields are responsible for the ubiquitous reconnection in the corona. First, the photospheric velocities are complex, which leads to the injection of energy and helicity into the coronal magnetic fields and to the efficient, formation of small-scale structure. Second, the flux distribution at the photosphere is multi-polar, which implies that topological discontinuities and, consequently, current sheets, must be present in the coronal magnetic field. We: present numerical simulations showing that photospherically-driven reconnection is responsible for the heating and dynamics of coronal plasma, and for the topology of the coronal/heliospheric magnetic field.

Antiochos, Spiro↗
NASA NTRSDate not supplied

Flare Ribbon Signatures of Reconnection Plasmoids

Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration. Understanding the structure and dynamics of the magnetic reconnection that powers flares is critical for predicting the energy release. In particular, the amount of energy transferred to energetic particles is thought to be highly dependent on whether the reconnection is primarily turbulent (e.g., plasmoid dominated) or instead laminar. We present high-resolution MHD simulations of three-dimensional reconnection in an eruptive flare and compare the results to recent data. Although flare reconnection is challenging to observe directly in the corona, highly detailed constraints on its dynamics can be obtained from observations of flare ribbons that track the chromospheric footpoints of newly reconnected field lines. The analogues of flare ribbons in our simulations are identified by tracking discontinuous changes in field-line magnetic connectivity due to the reconnection. In our highest-resolution calculations, we find that these ribbon analogues are highly structured and exhibit many ‘whorl’ patterns that are linked to turbulent plasmoids in the reconnecting current sheet. Flare ribbon fine structure therefore reveals crucial information about the fundamental turbulent vs. laminar nature of the reconnection. We discuss the implications of these results for understanding reconnection-driven energy release throughout the heliosphere.

J. T. Dahlin↗
NASA NTRSDate not supplied

Flare Ribbon Signatures of Plasmoids

Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration. Understanding the structure and dynamics of the magnetic reconnection that powers flares is critical for predicting the energy release. In particular, the amount of energy transferred to energetic particles is thought to be highly dependent on whether the reconnection is primarily turbulent (e.g., plasmoid dominated) or instead laminar. We present new high-resolution MHD simulations of three-dimensional reconnection in an eruptive flare and compare the results to recent data. Although flare reconnection is challenging to observe directly in the corona, highly detailed constraints on its dynamics can be obtained from observations of flare ribbons that track the chromospheric footpoints of newly reconnected field lines. The analogues of flare ribbons in our simulations are identified by tracking discontinuous changes in field-line magnetic connectivity due to the reconnection. In our highest-resolution calculations, we find that these ribbon analogues are highly structured and exhibit many ‘whorl’ patterns that are linked to turbulent plasmoids in the reconnecting current sheet. Such flare ribbon fine structure therefore reveals crucial information about the fundamental turbulent vs. laminar nature of the reconnection. Plasmoid-driven particle acceleration models would therefore predict co-location of energetic particle signatures with plasmoid-associated ribbon fine structure.

J T Dahlin↗
NASA NTRS1972

Coronal survey in X-rays of O VII and Ne IX.

Results of a rocket experiment flown on Apr. 29, 1971, in which a survey of the solar corona was carried out with a pair of collimated Bragg spectrometers to study the resonance, intersystem, and forbidden line emission from the helium-like ions O vii (22 A) and Ne ix (13 A). The observed X-ray emission came from the general corona, seven plage regions, and one dynamic feature - the late stage of a small flare. From the intensity of the O vii and Ne ix resonance lines the electron temperature and emission measure of the individual emitting regions are derived on the basis of two models, one in which the region is assumed to be isothermal and another in which the emission measure decreases exponentially with increasing temperature. The latter model, which is the most adequate of the two, yields for the electron temperature of the time-varying feature 2,000,000 to 3,000,000 K, for the other active regions 1,500,000 to 2,500,000 K, and for the general corona 1,300,000 to 1,700,000 K. The Ne ix emitting regions are about 1.5 times as hot as the O vii regions.

Acton, L. W.↗
NASA NTRS2008

Recent Studies of the Behavior of the Sun's White-Light Corona Over Time

Predictions of upcoming solar cycles are often related to the nature and dynamics of the Sun's polar magnetic field and its influence on the corona. For the past 30 years we have a more-or-less continuous record of the Sun's white-light corona from groundbased and spacebased coronagraphs. Over that interval, the large scale features of the corona have varied in what we now consider a 'predictable' fashion--complex, showing multiple streamers at all latitudes during solar activity maximum; and a simple dipolar shape aligned with the rotational pole during solar minimum. Over the past three decades the white-light corona appears to be a better indicator of 'true' solar minimum than sunspot number since sunspots disappear for months (even years) at solar minimum. Since almost all predictions of the timing of the next solar maximum depend on the timing of solar minimum, the white-light corona is a potentially important observational discriminator for future predictors. In this contribution we describe recent work quantifying the large-scale appearance of the Sun's corona to correlate it with the sunspot record, especially around solar minimum. These three decades can be expanded with the HAO archive of eclipse photographs which, although sparse compared to the coronagraphic coverage, extends back to 1869. A more extensive understanding of this proxy would give researchers confidence in using the white-light corona as an indicator of solar minimum conditions.

SaintCyr, O. C.↗
NASA NTRS2021

The Dynamic Formation of Pseudostreamers

Streamers and pseudostreamers structure the corona at the largest scales, as seen in both eclipse and coronagraph white-light images. Their inverted-goblet appearance encloses broad coronal loops at the Sun and tapers to a narrow radial stalk away from the star. The streamer associated with the global solar dipole magnetic field is long-lived, predominantly contains a single arcade of nested loops within it, and separates opposite-polarity interplanetary magnetic fields with the heliospheric current sheet anchored at its apex. Pseudostreamers, on the other hand, are transient, enclose double arcades of nested loops, and separate like-polarity fields with a dense plasma sheet. We use numerical magnetohydrodynamic simulations to calculate, for the first time, the formation of pseudostreamers in response to photospheric magnetic-field evolution. Convective transport of a minority-polarity flux concentration, initially positioned under one side of a streamer, through the streamer boundary into the adjacent, pre-existing coronal hole forms the pseudostreamer. Interchange magnetic reconnection at the overlying coronal null point(s) governs the development of the pseudostreamer above – and of anew, satellite coronal hole behind – the moving minority polarity. The reconnection dynamics liberate coronal-loop plasma that can escape into the heliosphere along so-called separatrix-web (“S-Web”)arcs, which reach far from the heliospheric current sheet and the solar equatorial plane, and can explain the origin of high-latitude slow solar wind. We describe the implications of our results for in-situ and remote-sensing observations of the corona and heliosphere as obtained, most recently, by Parker Solar Probe and Solar OrbiteR.

Roger B. Scott↗
NASA NTRS1977

Dynamic modeling of coronal and interplanetary responses to solar events

Recent progress in the dynamic modeling of responses of the corona and interplanetary medium to solar events (such as surges, eruptive prominences, flares, etc.) is reviewed. In particular, coronal transients and wave phenomena are discussed in some detail including pertinent mathematical requirements. Within the context of hydrodynamics and magnetohydrodynamics, a summary of both one- and two-dimensional time-dependent models is presented. A comparison of theoretical results with ground-based optical, radio, ATM Skylab observations, Pioneer 9 and Pioneer 10 solar wind observations is also presented. It is illustrated that: (1) substantial progress has been made in the theoretical (i.e., numerical) modeling of coronal and interplanetary responses within the last few years; (2) two-dimensional, time-dependent modeling is needed to examine the details of nonlinear wave coupling; and (3) theoretical results of modeling appear to reproduce physical consequences successfully.

Wu, S. T.↗
NASA NTRS1990

The ultra high resolution XUV spectroheliograph: An attached payload for the Space Station Freedom

The principle goal of the ultra high resolution XUV spectroheliograph (UHRXS) is to improve the ability to identify and understand the fundamental physical processes that shape the structure and dynamics of the solar chromosphere and corona. The ability of the UHRXS imaging telescope and spectrographs to resolve fine scale structures over a broad wavelength (and hence temperature) range is critical to this mission. The scientific objectives and instrumental capabilities of the UHRXS investigation are reviewed before proceeding to a discussion of the expected performance of the UHRXS observatory.

Walker, Arthur B. C., Jr.↗
NASA NTRS1990

Solar x ray astronomy rocket program

The dynamics were studied of the solar corona through the imaging of large scale coronal structures with AS&E High Resolution Soft X ray Imaging Solar Sounding Rocket Payload. The proposal for this program outlined a plan of research based on the construction of a high sensitivity X ray telescope from the optical and electronic components of the previous flight of this payload (36.038CS). Specifically, the X ray sensitive CCD camera was to be placed in the prime focus of the grazing incidence X ray mirror. The improved quantum efficiency of the CCD detector (over the film which had previously been used) allows quantitative measurements of temperature and emission measure in regions of low x ray emission such as helmet streamers beyond 1.2 solar radii or coronal holes. Furthermore, the improved sensitivity of the CCD allows short exposures of bright objects to study unexplored temporal regimes of active region loop evolution.

Source record↗
NASA NTRS2009

What is New on the Sun?

A fundamental property of the Sun's corona is that it is violently dynamic. The most spectacular and most energetic manifestations of this activity are the giant magnetic disruptions that give rise to coronal mass ejections (CME) and eruptive flares. These major events are of critical importance, because they drive the most destructive forms of space weather at Earth and in the solar system, and they provide a unique opportunity to study, in revealing detail, the interaction of magnetic field and matter, in particular, magnetohydrodynamic instability and nonequilibrium - processes that are at the heart of laboratory and astrophysical plasma physics. Recent observations by a number of NASA space missions have given us new insights into the physical mechanisms that underlie coronal explosions. Furthermore, massively-parallel computations have now allowed us to calculate fully three-dimensional models for the Sun's activity. In this talk I will review some of the latest observations of the Sun, including those from the just-launched Hinode and STEREO mission, and discuss recent advances in the theory and modeling of explosive solar activity.

Antiochos, Spiro K.↗
NASA NTRS2012

Using the EUV to Weigh a Sun-Grazing Comet as it Disappears in the Solar Corona

On July 6,2011, the Atmospheric Imaging Assembly (AlA) on the Solar Dynamics Observatory (SDO) observed a comet in most of its EUY passbands. The comet disappeared while moving through the solar corona. The comet penetrated to 0.146 solar radii ($\simapprox.100,000 km) above the photosphere before its EUY faded. Before then, the comet's coma and a tail were observed in absorption and emission, respectively. The material in the variable tail quickly fell behind the nucleus. An estimate of the comet's mass based on this effect, one derived from insolation, and one using the tail's EUY brightness, all yield $\sim 50$ giga-grams some 10 minutes prior to the end of its visibility. These unique first observations herald a new era in the study of Sun-grazing comets close to their perihelia and of the conditions in the solar corona and solar wind. We will discuss the observations and interpretation of the comet by SDO as well as the coronagraph observations from SOHO and STEREO. A search of the SOHO comet archive for other comets that could be observed in the SDO; AlA EUY channels will be described

Pesnell, William Dean↗
NASA NTRS1998

Constraints on Nonlinear and Stochastic Growth Theories for Type 3 Solar Radio Bursts from the Corona to 1 AU

Existing, competing theories for coronal and interplanetary type III solar radio bursts appeal to one or more of modulational instability, electrostatic (ES) decay processes, or stochastic growth physics to preserve the electron beam, limit the levels of Langmuir-like waves driven by the beam, and produce wave spectra capable of coupling nonlinearly to generate the observed radio emission. Theoretical constraints exist on the wavenumbers and relative sizes of the wave bandwidth and nonlinear growth rate for which Langmuir waves are subject to modulational instability and the parametric and random phase versions of ES decay. A constraint also exists on whether stochastic growth theory (SGT) is appropriate. These constraints are evaluated here using the beam, plasma, and wave properties (1) observed in specific interplanetary type III sources, (2) predicted nominally for the corona, and (3) predicted at heliocentric distances greater than a few solar radii by power-law models based on interplanetary observations. It is found that the Langmuir waves driven directly by the beam have wavenumbers that are almost always too large for modulational instability but are appropriate to ES decay. Even for waves scattered to lower wavenumbers (by ES decay, for instance), the wave bandwidths are predicted to be too large and the nonlinear growth rates too small for modulational instability to occur for the specific interplanetary events studied or the great majority of Langmuir wave packets in type III sources at arbitrary heliocentric distances. Possible exceptions are for very rare, unusually intense, narrowband wave packets, predominantly close to the Sun, and for the front portion of very fast beams traveling through unusually dilute, cold solar wind plasmas. Similar arguments demonstrate that the ES decay should proceed almost always as a random phase process rather than a parametric process, with similar exceptions. These results imply that it is extremely rare for modulational instability or parametric decay to proceed in type III sources at any heliocentric distance: theories for type III bursts based on modulational instability or parametric decay are therefore not viable in general. In contrast, the constraint on SGT can be satisfied and random phase ES decay can proceed at all heliocentric distances under almost all circumstances. (The contrary circumstances involve unusually slow, broad beams moving through unusually hot regions of the Corona.) The analyses presented here strongly justify extending the existing SGT-based model for interplanetary type III bursts (which includes SGT physics, random phase ES decay, and specific electromagnetic emission mechanisms) into a general theory for type III bursts from the corona to beyond 1 AU. This extended theory enjoys strong theoretical support, explains the characteristics of specific interplanetary type III bursts very well, and can account for the detailed dynamic spectra of type III bursts from the lower corona and solar wind.

Cairns, Iver H.↗
NASA NTRS1990

Dynamic range considerations for EUV MAMA detectors

The multianode microchannel array (MAMA) has been chosen as the detector for two instruments on the ESA/NASA Solar Heliospheric Observatory. The response of the MAMA to the two extreme types of solar spectra, disk and corona, have been modeled with a view toward evaluating dynamic range effects present. The method of MAMA operation is discussed, with emphasis given to modeling the effect of electron cloud charge spreading to several detector anodes and amplifiers (n-fold events). Representative synthetic EUV spectra have been created. The detector response to these spectra is modeled by dissecting the input photon radiation field across the detector array into contributions to the various amplifier channels. The results of this dissection are shown for spectral regions across the entire wavelength region of interest. These results are used to identify regions in which total array photon counting rate or individual amplifier rate may exceed the design limits. This allows the design or operational modes to be tailored to eliminate the problem areas.

Illing, Rainer M. E.↗
NASA NTRS1984

Transient behavior of flare-associated solar wind. II - Gas dynamics in a nonradial open field region

Transient behavior of flare-associated solar wind in the nonradial open field region is numerically investigated, taking into account the thermal and dynamical coupling between the chromosphere and the corona. A realistic steady solar wind is constructed which passes through the inner X-type critical point in the rapidly diverging region. The wind speed shows a local maximum at the middle, O-type, critical point. The wind's density and pressure distributions decrease abruptly in the rapidly diverging region of the flow tube. The transient behavior of the wind following flare energy deposition includes ascending and descending conduction fronts. Thermal instability occurs in the lower corona, and ascending material flows out through the throat after the flare energy input ceases. A local density distribution peak is generated at the shock front due to the pressure deficit just behind the shock front.

Nagai, F.↗