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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1998

LOOPREF: A Fluid Code for the Simulation of Coronal Loops

This report documents the code LOOPREF. LOOPREF is a semi-one dimensional finite element code that is especially well suited to simulate coronal-loop phenomena. It has a full implementation of adaptive mesh refinement (AMR), which is crucial for this type of simulation. The AMR routines are an improved version of AMR1D. LOOPREF's versatility makes is suitable to simulate a wide variety of problems. In addition to efficiently providing very high resolution in rapidly changing regions of the domain, it is equipped to treat loops of variable cross section, any non-linear form of heat conduction, shocks, gravitational effects, and radiative loss.

deFainchtein, Rosalinda↗
NASA NTRS1981

Adiabatic and nonadiabatic processes in thermal models of solar hard X-ray bursts

The temporal evolution of thermal-bremsstrahlung-emitting material at the top of a solar coronal loop under various processes is examined as a model of solar hard X-ray bursts. The behavior of emission measure and temperature with time is analyzed for the hot plasma confined in a segment at the top of a loop, taking into account the effects or the time variation of the confining toroidal magnetic field, the longitudinal expansion of the source through the motion of a pair of collisionless conduction fronts and unspecified heating or cooling processes. Analytic expressions for the behavior of the toroidal magnetic field and the heating rate necessary to reproduce a given behavior of emission measure and temperature with time are obtained in terms of the initial magnetic field strength and density of the heated region and a characteristic longitudinal expansion time. Results are applied to observations of two impulsive solar hard X-ray bursts, and it is shown that the contribution of heating is negligible after the first few seconds of the events, although significant nonadiabatic heating occurs early in the events.

Emslie, A. G.↗
NASA NTRS1983

MHD thermal instabilities in cool inhomogeneous atmospheres

The formation of a coronal state in a stellar atmosphere is investigated. A numerical code is used to study the effects of atmospheric gradients and finite loop dimension on the scale of unstable perturbations, solving for oscillatory perturbations as eigenfunctions of a boundary value problem. The atmosphere is considered as initially isothermal, with density and pressure having scale heights fixed by the hydrostatic equations. Joule mode instability is found to be an efficient mechanism for current filamentation and subsequent heating in initially cool atmospheres. This instability is mainly effective at the top of magnetic loops and is not suppressed by thermal conduction.

Bodo, G.↗
NASA NTRS2004

Recent Progress in Understanding the Sun's Magnetic Dynamo

100 years ago we thought that the Sun and stars shone as a result of slow gravitational contraction over a few tens of millions of years - putting astronomers at odds with geologists who claimed that the Earth was much, much older. That mystery was solved in the 1920s and 30s with the discovery of nuclear energy (proving that the geologists had it right all along). Other scientific mysteries concerning the Sun have come and gone but three major mysteries remain: 1) How does the Sun produce sunspots with an 11-year cycle? 2) What produces the huge explosions that result in solar flares, prominence eruptions, and coronal mass ejections? and 3) Why is the Sun's outer atmosphere, the corona, so darned hot? Recent progress in solar astronomy reveals a single key to understanding all three of these mysteries.The 11-year time scale for the sunspot cycle indicates the presence of a magnetic dynamo within the Sun. For decades this dynamo was though to operate within the Sun's convection zone - the outmost 30% of the Sun where convective currents transport heat and advect magnetic lines of force. The two leading theories for the dynamo had very different models for the dynamics of the convection zone. Actual measurements of the dynamics using the techniques of helioseismology showed that both of these models had to be wrong some 20 years ago. A thin layer of strongly sheared flow at the base of the convection zone (now called the tachocline) was then taken to be the seat of the dynamo. Over the last 10 years it has become apparent that a weak meridional circulation within the convection zone also plays a key role in the dynamo. This meridional circulation has plasma rising up from the tachocline in the equatorial regions, spreading out toward the poles at a top speed of about 10-20 m/s at the surface, sinking back down to the tachocline in the polar regions, and then flowing back toward the equator at a top speed of about 1-2 m/s in the tachocline itself. Recent dynamo models that include this meridional flow now appear to have some power for predicting the size of future sunspot cycles.

Hathaway, David. H.↗
NASA NTRS1995

Electron temperature and density relationships in coronal mass ejections

We examine 10 coronal mass ejections from the in-ecliptic portion of the Ulysses mission. Five of these CMEs are magnetic clouds. In each case we observe an inverse relationship between electron temperature and density. For protons this relationship is less clear. Earlier work has shown a similar inverse relationship for electrons inside magnetic clouds and interpreted it to mean that the polytropic index governing the expansion of electrons is less than unity. This requires electrons to be heated as the CME expands. We offer an alternative view that the inverse relationship between electron temperature and density is caused by more rapid cooling of the denser plasma through collisions. More rapid cooling of denser plasma has been shown for 1 AU measurements in the solar wind. As evidence for this hypothesis we show that the denser plasma inside the CMEs tends to be more isotropic indicating a different history of collisions for the dense plasma. Thus, although the electron temperature inside CMEs consistently shows an inverse correlation with the density, this is not an indication of the polytropic index of the plasma but instead supports the idea of collisional modification of the electrons during their transit from the sun.

Hammond, C. M.↗
DOE OSTI2024

Coupled Experimental/Computational Investigation of the Dynamics of Interacting Magnetized Plasmas

The interaction, or interpenetration, of magnetized plasmas of different density and/or pressure occurs in a wide variety of natural and man-made systems. Such systems include extragalactic jets propagating into the intergalactic medium, solar coronal mass ejections into background solar wind, compact toroid (CT) fueling of magnetic fusion plasmas, and jets of capsule shell impurities into DT fusion fuel, which can lead to enhanced impurity mix in inertial fusion implosions. These plasmas may take the form of jets, with open, helical magnetic structure, or plasma “bubbles” with closed magnetic fields (B-fields), such as spheromaks or CT’s. Such structures, both open and closed B-field cases, can transport heat, particles and magnetic flux or magnetic helicity into background plasma regions. For example, the origin of extragalactic magnetic fields may be due, at least in part, to transport by astrophysical jets. The goal of this proposed work was to elucidate the detailed plasma and magnetic field dynamics of high-density plasma jets (open B-field) and bubbles (closed B-field) propagating into lower density background magnetized plasma through controlled laboratory experiments and closely coupled nonlinear MHD modeling. These experiments were conducted in the HelCat (Helicon-Cathode) linear plasma device at the University of New Mexico (UNM). Plasma jets and bubbles were launched via an existing compact coaxial plasma gun, mounted on the HelCat device. This gun produced plasmas tens of cm in scale and lasting tens of microseconds, thereby allowing detailed multipoint, space- and time-resolved measurements to be made routinely. The experiments were directly modeled using the extended magnetohydrodynamic (XMHD) PERSEUS code, developed at Cornell University [23,24]. Both experimental and numerical modeling work are ongoing. The main results to date are reported here. Additional supplemental funding for one year (8/1/2019 – 7/31/2020) supported numerical investigation of photoionization processes important in many low temperature plasmas, including the HelCat device. Initial results of this modeling work is also reported.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1989

Numerical simulations of impulsively heated solar flares

The response of a model solar atmosphere to heating by an electron beam has been studied for electron beam flux spectra which are power laws with low-energy 'knees' (rising linearly with time to a peak at 30 s and then falling linearly to 0 at 60 s) ranging from 10 to 20 keV. The results indicate that high peak electron beam fluxes, low-energy knees, and larger spectral indices all move the atmospheric response toward greater enhancements of the parameters in the coronal regions of the atmosphere. Coronal responses can thus be used as a diagnostic of the parameters of the electron beam.

Mariska, John T.↗
NASA NTRS1984

Strong electron heat flux modes in Jupiter's foreshock

Analysis of Voyager plasma wave data from the Jovian foreshock has revealed the existence of a new oscillation with center frequency below fp and typically lying above the upper limit Doppler shift frequency for ion acoustic waves. The properties of these waves are consistent with modes generated by a strong electron heat flux in an unmagnetized plasma. Such modes become unstable when the effective drift velocity exceeds n(cold)/(hot) exp 1/2 times the thermal speed of the hot component. The occasional simultaneous appearance of more than one frequency band is a possible indication of multiple magnetic connection with the shock.

Moses, S. L.↗
NASA NTRS1994

Accretion disk coronae in high-luminosity systems

We present the results of self-consistent models of Compton-heated accretion disk coronae. The models are calculated using a new method for computing monochromatic radiative transfer n two dimensions. The method splits the radiation into direct and scattered components. The direct radiation is computed by calculating the optical depth along rays, while transfer of the scattered radiation is approximated by flux-limited diffusion. The resulting code agrees with more accurate treatments to within 50%, and is highly efficient, making it practical for use in large hydrodynamic simulations. The coronal models are used to confirm the results of earlier work, and to extend it to higher luminosities. In contrast to earlier work, which found the outer disks to be shadowed by the inner corona at high luminosities, we find our results to form an almost continuous extension of the models at lower luminosities. This is due to the presence of multiply scattered radiation, which acts to partially offset the loss of direct radiation from the central source. Although the analytic methods derived at lower luminosities cannot be used to derive the coronal structure for L/L(sub Edd) approx. greater than 0.1, the results of the models are amenable to semiempirical fits. We also discuss possible observational consequences of the results for coronal veiling and line fluorescence from the disk.

Murray, Stephen D.↗
NASA NTRS2008

Flare Plasma Iron Abundance

The equivalent width of the iron-line complex at 6.7 keV seen in flare X-ray spectra suggests that the iron abundance of the hottest plasma at temperatures >approx.10 MK may sometimes be significantly lower than the nominal coronal abundance of four times the photospheric value that is commonly assumed. This conclusion is based on X-ray spectral observations of several flares seen in common with the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Solar X-ray Spectrometer (SOXS) on the second Indian geostationary satellite, GSAT-2. The implications of this will be discussed as it relates to the origin of the hot flare plasma - either plasma already in the corona that is directly heated during the flare energy release process or chromospheric plasma that is heated by flare-accelerated particles and driven up into the corona. Other possible explanations of lower-than-expected equivalent widths of the iron-line complex will also be discussed.

Dennis, Brian R.↗
NASA NTRS2013

Obscuration of Flare Emission by an Eruptive Prominence

We report on the eclipsing of microwave flare emission by an eruptive prominence from a neighboring region as observed by the Nobeyama Radioheliograph at 17 GHz. The obscuration of the flare emission appears as a dimming feature in the microwave flare light curve. We use the dimming feature to derive the temperature of the prominence and the distribution of heating along the length of the filament. We find that the prominence is heated to a temperature above the quiet Sun temperature at 17 GHz. The duration of the dimming is the time taken by the eruptive prominence in passing over the flaring region. We also find evidence for the obscuration in EUV images obtained by the Solar and Heliospheric Observatory (SOHO) mission.

coronal mass ejections↗
NASA NTRS2017

Observational Analysis of Coronal Fans

Coronal fans (see Figure 1) are bright observational structures that extend to large distances above the solar surface and can easily be seen in EUV (174 angstrom) above the limb. They have a very long lifetime and can live up to several Carrington rotations (CR), remaining relatively stationary for many months. Note that they are not off-limb manifestation of similarly-named active region fans. The solar conditions required to create coronal fans are not well understood. The goal of this research was to find as many associations as possible of coronal fans with other solar features and to gain a better understanding of these structures. Therefore, we analyzed many fans and created an overview of their properties. We present the results of this statistical analysis and also a case study on the longest living fan.

Corona↗
NASA NTRS1986

Definition and empirical structure of the range of stellar chromospheres-coronae across the H-R diagram: Cool stars

Major advances in our understanding of non-radiative heating and other activity in stars cooler than T sub eff = 10,000K has occured in the last few years. This observational evidence is reviewed and the trends that are now becoming apparent are discussed. The evidence for non-radiatively heated outer atmospheric layers (chromospheres, transition regions, and coronae) in dwarf stars cooler than spectral type A7, in F and G giants, pre-main sequence stars, and close bindary systems is unambiguous, as is the evidence for chromospheres in the K and M giants and supergiants. The existence of non-radiative heating in the outer layers of the A stars remains undetermined despite repeated searches at all wavelengths. Two important trends in the data are the decrease in plasma emission measure with age on the main sequence and decreasing rotational velocity. Variability and atmospheric inhomogeneity are commonly seen, and there is considerable evidence that magnetic fields define the geometry and control the energy balance in the outer atmospheric layers. In addition, the microwave observations imply that non-thermal electrons are confined in coronal magnetic flux tubes in at least the cool dwarfs and RS CVn systems. The chromospheres in the K and M giants and supergiants are geometrically extended, as are the coronae in the RS CVn systems and probably also in other stars.

Linsky, J. L.↗
NASA NTRS2016

Solar Coronal Jets: Observations, Theory, and Modeling

Coronal jets represent important manifestations of ubiquitous solar transients, which may be the source of significant mass and energy input to the upper solar atmosphere and the solar wind. While the energy involved in a jet-like event is smaller than that of "nominal" solar flares and coronal mass ejections (CMEs), jets share many common properties with these phenomena, in particular, the explosive magnetically driven dynamics. Studies of jets could, therefore, provide critical insight for understanding the larger, more complex drivers of the solar activity. On the other side of the size-spectrum, the study of jets could also supply important clues on the physics of transients close or at the limit of the current spatial resolution such as spicules. Furthermore, jet phenomena may hint to basic process for heating the corona and accelerating the solar wind; consequently their study gives us the opportunity to attack a broad range of solar-heliospheric problems.

Raouafi, N. E.↗
NASA NTRS1986

Particle propagation effects on wave growth in a solar flux tube

The evolution of a distribution of electrons is followed after they are injected impulsively at the top of a coronal magnetic loop, with the objective of studying the plasma instabilities which result. At early times the downgoing electrons have beamlike distributions and amplify electrostatic waves via the Cerenkov resonance; the anomalous Doppler resonance is found to be less important. Slightly later, while the electrons are still predominantly downgoing, they are unstable to cyclotron maser generation of z-mode waves with omega(p) much less than Omega, or to second harmonic x-mode waves. The energetics of these instabilities, including saturation effects and heating of the ambient plasma, are discussed. It is suggested that coalescence of two z-mode waves generated by cyclotron maser emission of the downgoing electrons may produce the observed microwave spike bursts.

White, S. M.↗
NASA NTRS1984

Detailed studies of the dynamics and energetics of coronal bullets

Coronal bullets are small ejecta of cool, dense plasma observed to accelerate through the solar atmosphere from 20 to 450 km/s. The NRL Dynamic Flux Tube Model has been used to simulate the evolving physical properties of these dynamic events. The present calculations utilize an adaptive-gridding technique to resolve the fine structure within and around the bullets. In this work, an identification was made of a component of shocked plasma which piles up ahead of the bullet and eventually dominates both the dynamics and heating of the original bullet mass. The observational consequences of this shocked component are discussed in terms of the available HRTS EUV data, and suggestions are made for optimizing future observations of this phenomenon. An investigation has also been conducted of the structure of the bullet material visible in EUV spectral lines and the observable characteristics of the EUV-emitting plasma. Finally, the most likely mechanisms for accelerating the bullets, as well as favorable sites of origin are evaluated.

Karpen, J. T.↗
NASA NTRS1993

X-ray/microwave relation of different types of active stars

Coronal active stars of seven classes between spectral types F and M, single and double, are compared in their quiescent radio and X-ray luminosities L(R) and L(X). We find, largely independent of stellar class, log L(X) is less than about log L(R) + 15.5. This general relation points to an intimate connection between the nonthermal, energetic electrons causing the radio emission and the bulk plasma of the corona responsible for thermal X-rays. The relation, observed over six orders of magnitude, suggests that the heating mechanism necessarily involves particle acceleration. We derive requirements for simple models based on optically thin gyrosynchrotron emission of mildly relativistic electrons and thermal X-rays from the bulk plasma. We discuss the possibility that a portion of the accelerated particles heats the ambient plasma by collisions. More likely, plasma heating and particle acceleration may occur in parallel and in the same process, but at a fixed ratio.

Guedel, Manuel↗
NASA NTRS1982

Magnetic field-related heating instabilities in the surface layers of the sun and stars

The stability of a magnetized low-density plasma to current-driven filamentation instabilities is investigated and the results are applied to the surface layers of stars. Unlike previous studies, the initial (i.e., precoronal) state of the stellar surface atmosphere is taken to be a low-density, optically thin magnetized plasma in radiative equilibrium. The linear analysis shows that the surface layers of main-sequence stars (including the sun) which are threaded by magnetic fields are unstable; the instabilities considered lead to structuring perpendicular to the ambient magnetic fields. These results suggest that relatively modest surface motions, in conjunction with the presence of magnetic fields, suffice to account for the presence of inhomogeneous chromospheric and coronal plasma overlying a star's surface.

Ferrari, A.↗