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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Resonant decay of global MHD modes at 'thick' interfaces

A global surface mode can exist on a 'surface' which is not a true discontinuity, but without sufficient dissipation it is not a normal mode and it decays in time via resonance absorption. The decay rate can be calculated analytically when the 'surface' is thin. The goal of this paper is to numerically estimate the decay rate when the surface is not thin. A cold plasma in a uniform magnetic field is considered in which density varies linearly across the 'surface'. In this linearized calculation, the global surface mode is driven in steady state by an antenna located in one of the uniform regions external to the surface. The frequency is a free parameter, and resonance curves are computed numerically without approximations. The widths of the resonance curves are used to estimate the free decay times of undriven surface modes, via the uncertainty principle. When applied to the active solar corona, the decay rates are large enough to account for the coronal heating, but it should be kept in mind that the role of nonlinearity in resonance absorption is still undetermined.

Hollweg, Joseph V.↗
NASA NTRS1990

Linear tearing modes of a forced current-sheet equilibrium

Linear tearing modes are studied in a nearly singular forced current-sheet equilibrium, such as could result from global magnetic forces in the solar corona. Growth rates for the tearing modes, determined by solving the linearized reduced MHD (Strauss) equations numerically, were found to scale as (gamma)tau(d) = about S exp 4/5 k(y) exp 4/5, where S is the Lundquist number, k(y) is the wavenumber, and tau(d) is the classical resistive diffusion time. This scaling is in agreement with predictions from analytical theory. Because of the faster S scaling of these modes compared to the tearing modes of a diffuse current-sheet equilibrium, the modes have much higher growth rates (by a factor of about 10,000) for coronal values of S (about 10 to the 12th). For coronal parameters, the growth times of these new modes are estimated to be on the order of several hours to days, as compared to growth times of months to years for the tearing modes in a diffuse current-shear equilibrium. The growth times are comparable to reconnection times scales required in models of coronal heating by magnetic field dissipation.

Liewer, Paulett C.↗
NASA NTRS1990

Magnetohydrodynamic modeling of the solar corona

The ideal and resistive magnetohydrodynamic (MHD) model is used to examine the dynamics and structure of the solar corona. When the coronal magnetic field is deformed by photospheric flow it can evolve to states that become unstable to ideal MHD modes. The nonlinear evolution of these instabilities can lead to the generation of current sheets, field line reconnection, and energy release. The disruption of an arcade field and the kinking of coronal loops is described. The braiding of the large-scale coronal field by convective photospheric motions develops fine-scale structure in the magnetic field and leads to the development of intense current filaments. The resistive dissipation of these currents can provide an efficient coronal heating mechanism.

Mikic, Zoran↗
NASA NTRS1991

Corona: Coordinated research on non-thermal processes in astrophysics

Several problems in astrophysics are examined where departures from purely thermal behavior are expected to produce effects which are detectable by a variety of NASA satellites. The problems identified as worthy of study include coronal heating in stars, deposition of non-thermal particle energy in stars from external sources, and turbulence in atmospheres and winds of stars. The progress in solving these problems is summarized.

Mullan, Dermott J.↗
NASA NTRS1990

Quasilinear saturation of forced current sheet tearing modes

Numerical studies of tearing modes in a nearly singular forced current sheet equilibrium (Liewer and Payne, 1990) show that the modes saturate quasilinearly when the width of the magnetic island formed by the reconnection is on the order of several times the linear mode width which scales as approximately (kS) exp -2/5, where S is the Lundquist number and k is the wavenumber. The modes saturate quasilinearly by flattening the current profile, converting magnetic energy into plasma energy. The longer wavelength modes, which saturate at higher levels, release the most energy. These modes may, nonlinearly, play a role in coronal heating when sharp current sheets form as a result of global magnetic stresses.

Liewer, Paulett C.↗
NASA NTRS1991

The quasi-stationary and transient states of the solar wind

While the last 30 years' probing of the transient and quasi-stationary states of the solar wind by interplanetary spacecraft have yielded a clear differentiation of the physical properties of the two types of wind, the processes involved in their acceleration remains unclear; neither is the coronal heating mechanism implicated in the solar wind's existence entirely clear. Attention is presently given to recent evidence for a connection between heating and acceleration mechanisms. The transient wind undergoes greater expansion between the sun and 1 AU than the quasi-stationary wind. The quasi-stationary wind is driven by energy and momentum that arise from solar convection and are carried throughout a region that extends from the base of the corona to beyond the critical point at which the flow becomes supersonic.

Neugebauer, M.↗
NASA NTRS1991

Convection in stars and heating of coronae

The properties of convection in the sun and other cool stars are summarized. Recent studies of convection which have involved the use of supercomputers to model the flow of compressible gas in three dimensions are discussed. It is shown how the results of these computations may eventualy provide an understanding of how nonthermal processes heat coronal gas to temperatures of millions of degrees.

Mullan, D. J.↗
NASA NTRS1991

Coronal dynamics of a quiescent active region

Post-SMM repair Flat Crystal Spectrometer (FCS) observations of coronal X-ray line broadening in a nonflaring active region are discussed. The resulting map of FCS line profiles across the central X-ray bright portion of a large active region shows a clear variation in line width for the Mg XI resonance line at 9.17 A, but no evidnece of line shifts. Line ratios yield T(e) of about 3 x 10 to the 6th K within 10 percent for most of the map. The brightest, broadest profiles are consistent with emission from the tops of loops overlying the magnetic neutral line. For the spatially averaged Mg XI resonance line profile, the 'excess velocity' v(ex) corresponding to the excess width above the thermal value equals the thermal velocity for Mg ions, 45 km/s; for some individual bright pixels v(ex) is 60 km/s or more. The observed coronal line broadening is interpreted as a signature of the mechanisms responsible for coronal heating in active regions.

Saba, Julia L. R.↗
NASA NTRS1991

SOHO - A high resolution solar physics mission

The present study discusses SOHO (Solar and Heliospheric Observatory), a satellite with a pointing stability of 1 arcsec that will be launched in 1995 to the L1 Lagrangian point between the sun and the earth. In situ measurements, at approximately 1 AU, are to trace the results of solar wind expansion. The SOHO mission aims are discussed: to obtain long and uninterrupted series of solar oscillation measurements either in flux or in velocity in order to sound the solar interior using high resolution in the frequency domain; to probe the upper atmosphere with high-resolution UV/EUV imager/spectrometers and UV/visible coronagraphs and photometers in order to bring new constraints on the coronal heating; and to measure in situ the solar wind flow in front of the earth and thus establish the link between the sun and the near-earth environment. The satellite and instrumentation are reviewed in relation to the specific capabilities that they offer.

Lemaire, P.↗
NASA NTRS1991

Avalanches and the distribution of solar flares

The solar coronal magnetic field is proposed to be in a self-organized critical state, thus explaining the observed power-law dependence of solar-flare-occurrence rate on flare size which extends over more than five orders of magnitude in peak flux. The physical picture that arises is that solar flares are avalanches of many small reconnection events, analogous to avalanches of sand in the models published by Bak and colleagues in 1987 and 1988. Flares of all sizes are manifestations of the same physical processes, where the size of a given flare is determined by the number of elementary reconnection events. The relation between small-scale processes and the statistics of global-flare properties which follows from the self-organized magnetic-field configuration provides a way to learn about the physics of the unobservable small-scale reconnection processes. A simple lattice-reconnection model is presented which is consistent with the observed flare statistics. The implications for coronal heating are discussed and some observational tests of this picture are given.

Lu, Edward T.↗
NASA NTRS1992

Research in solar plasma theory

The main thrust and significance of our research results are presented. The topics covered include: (1) coronal structure and dynamics; (2) coronal heating; (3) filament formation; and (4) flare energy release.

Vanhoven, Gerard↗
NASA NTRS1991

Flares on the sun and other stars

Research on solar and stellar flares is reviewed. The characteristics of flares in various spectral regions are described, and the physics of flares is discussed, including the two-ribbon flare model, coronal mass ejections, flare dynamics, white light flare production, and electron beams and proton beams in hard X-ray flares. The analysis of the flare energy budget, magnetic field measurements, multispectral stellar observations, plasma emissivity, conditions in flare loops, cooling curves, and proton beam signatures is addressed. Microflaring and coronal heating are examined.

Haisch, Bernhard↗
NASA NTRS1992

Quasi-static evolution of coronal magnetic fields

A formalism is developed to describe the purely quasi-static part of the evolution of a coronal loop driven by its footpoints. This is accomplished under assumptions of a long, thin loop. The quasi-static equations reveal the possibility for sudden 'loss of equilibrium' at which time the system evolves dynamically rather than quasi-statically. Such quasi-static crises produce high-frequency Alfven waves and, in conjunction with Alfven wave dissipation models, form a viable coronal heating mechanism. Furthermore, an approximate solution to the quasi-static equations by perturbation method verifies the development of small-scale spatial current structure.

Longcope, D. W.↗
NASA NTRS1992

The Solar Wind Ion Composition Spectrometer

The Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses is designed to determine uniquely the elemental and ionic-charge composition, and the temperatures and mean speeds of all major solar-wind ions, from H through Fe, at solar wind speeds ranging from 175 km/s (protons) to 1280 km/s (Fe(8+)). The instrument, which covers an energy per charge range from 0.16 to 59.6 keV/e in about 13 min, combines an electrostatic analyzer with postacceleration, followed by a time-of-flight and energy measurement. The measurements made by SWICS will have an impact on many areas of solar and heliospheric physics, in particular providing essential and unique information on: (1) conditions and processes in the region of the corona where the solar wind is accelerated; (2) the location of the source regions of the solar wind in the corona; (3) coronal heating processes; (4) the extent and causes of variations in the composition of the solar atmosphere; (5) plasma processes in the solar wind; (6) the acceleration of energetic particles in the solar wind; (7) the thermalization and acceleration of interstellar ions in the solar wind, and their composition; and (8) the composition, charge states, and behavior of the plasma in various regions of the Jovian magnetosphere.

Gloeckler, G.↗
NASA NTRS1991

Data evaluation, analysis, and scientific study

Extensive work was performed in data analysis and modeling of solar active phenomena. The work consisted in the study of UV data from the Ultraviolet Spectrometer and Polarimeter (UVSP) instrument on board the Solar Maximum Mission satellite. These data were studied in conjunction with X-rays from the Hard X-ray Imaging Spectrometer (HXIS) instrument, and with H-alpha and magnetographic data from ground-based observatories. The processes we studied are the active phenomena which result from the interaction of the solar magnetic fields with the plasma in the outer regions of the solar atmosphere. These processes include some very dynamic processes such as the prominence eruptions and the 'microflares'. Our research aimed at characterizing the following: the observed phenomena, the possible physical models, and the relevance to the chromospheric and coronal heating.

Wu, S. T.↗
NASA NTRS1993

ROSAT investigation of flaring and activity on Prox Cen

The objective of this program was to investigate with high sensitivity the low-level flare activity which may underlie coronal heating. This was done. The ROSAT observations of Prox Cen were scheduled for 50 ks spread out from 26 Feb. - 10 Mar. 1992. Unfortunately because of spacecraft problems many of these pointings turned out to contain no useful data or extremely truncated valid data sets. Considerable time was spent trying to determine which of the data would be scientifically useful and which would not. Fortunately, several developments took place to augment the original data in such a way that the scientific goal of advancing the study of flaring and variability was able to be achieved after all. These are as follows: (1) a second round of ROSAT observations was carried out in Feb. 1993 which only came to the attention of the PI in Apr. 1993 when a new data tape arrived; (2) simultaneous IUE observations were requested and obtained; (3) data from the UK WFC are available via the collaboration with Dr. G. Bromage; and (4) the 'cleaned-up' original data set was found to include one major flare and 2 moderate flares. Because of the problems with the original data set, the unexpected acquisition of new data only two months ago, and the availability of IUE and WFC data, an article on Prox Cen for publication is not ready at this time. Such an article is being developed and can be completed as part of ongoing ROSAT research efforts on stellar coronae and flaring.

Haisch, Bernhard↗
NASA NTRS1993

Physics of solar activity

The aim of the research activity was to increase our understanding of solar activity through data analysis, theoretical analysis, and computer modeling. Because the research subjects were diverse and many researchers were supported by this grant, a select few key areas of research are described in detail. Areas of research include: (1) energy storage and force-free magnetic field; (2) energy release and particle acceleration; (3) radiation by nonthermal electrons; (4) coronal loops; (5) flare classification; (6) longitude distributions of flares; (7) periodicities detected in the solar activity; (8) coronal heating and related problems; and (9) plasma processes.

Sturrock, Peter A.↗
NASA NTRS1992

Simulated O VI Doppler dimming measurements of coronal outflow velocities

The possibility of determining O(5+) outflow velocities by using a Doppler dimming analysis of the resonantly scattered intensities of O VI lambda 1031.9 and lambda 1037.6 is addressed. The technique is sensitive to outflow velocities, W, in the range W greater than 30 and less than 250 km/s and can be used for probing regions of the inner solar corona, where significant coronal heating and solar wind acceleration may be occurring. These velocity measurements, when combined with measurements of other plasma parameters (temperatures and densities of ions and electrons) can be used to estimate the energy and mass flux of O(5+). In particular, it may be possible to locate where the flow changes from subsonic to supersonic and to identify source regions for the high and low speed solar wind. The velocity diagnostic technique is discussed with emphasis placed on the requirements needed for accurate outflow velocity determinations. Model determinations of outflow velocities based on simulated Doppler observations are presented.

Strachan, Leonard↗