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At least 37 records · Page 2
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NASA NTRS1979

Formation, support and stability of quiescent prominences

The most prevalent hypotheses and theories that treat the formation and support of quiescent prominences are reviewed, along with current ideas concerning the stability of such prominences. Observational clues to the formation of quiescent prominences are described, condensation of coronal material is examined, and injection mechanisms of prominence formation are considered. Questions pertaining to the support and stability of quiescent prominences are discussed in terms of magnetohydrostatic equilibrium, prominence equilibria, and breakdown of stability conditions.

Tandberg-Hanssen, E.↗
NASA NTRS1986

Condensation modes in sheared magnetic fields

The present study of the condensation modes in coronal cylindrical plasmas, with attention to magnetic shear effects on stability, notes that such shear is insignificant in the initiation of condensation in the case of low beta coronal plasmas. The effects of magnetic field shear, twist, and strength on condensation modes differ, depending on the wave vector. The stability of condensation modes strongly depends on the choice of equilibrium temperature and density profiles. If plasma temperature increases with twist but density does not, condensation modes are unstable for low field twist; by contrast, if plasma density increases with twist but temperature does not, condensation modes are unstable for high twist.

An, C.-H.↗
NASA NTRS1985

The physics of thermal instability in two dimensions

Previous studies of a thermal (radiative) instability in a sheared magnetic field have shown that, under solar coronal conditions, cool condensations can form in a small neighborhood about the shear layer. Such results have served to model the formation of solar filaments (or prominences) observed to occur above photospheric magnetic polarity-inversion lines. A surprising conclusion of these studies is that the width of the condensation does not depend on the thermal conductivity. By examining the mass-flow patterns of two-dimensional condensations in the absence of thermal conduction, it is demonstrated that local plasma dynamics and the constraints imposed by boundary conditions are together sufficient to explain the size of the condensation width. In addition the results of a series of numerical calculations are presented which illustrate the characteristic mode structure of sheared-field condensations.

Sparks, L.↗
NASA NTRS1992

Plasma dynamics and energetics in the solar atmosphere

Prominence condensation and support, the dynamics of coronal loops, and streamer structure and disconnection are briefly discussed. Prominence condensation and magnetic levitation in a coronal loop and simulations of coronal disconnection events are discussed.

Vanhoven, Gerard↗
NASA NTRS1980

The equatorial latitude of auroral activity during 1972-1977

The equatorial latitude of auroral activity has been derived from both electron and optical observations with the DMSP satellites. Virtually all of the observations obtained during the five-year interval June 1972-September 1977 have been used to construct a nearly continuous plot of invariant geomagnetic latitude versus time. This plot has two main characteristics: (1) a diurnal variation of approximately plus or minus 5 deg which is associated with the precession of the earth's magnetic dipole axis about the earth's rotation axis; and (2) an irregular variation of roughly 5-10 deg for intervals of one to several days associated with the occurrence of solar flares and coronal holes. Using a condensed, Bartels-type display of these measurements, it is concluded that: (a) modest auroral expansions (to latitude about 60 deg) occur during the main body of high-speed streams from coronal holes; (b) great expansions (to latitude less than 55 deg) occur only during intervals of intense interplanetary magnetic fields such as may occur at the leading edge of a high-speed stream or at a flare-produced interplanetary shock.

Sheeley, N. R., Jr.↗
NASA NTRS1985

On the formation of coronal cavities

The formation of a coronal cavity and its relation to a quiescent prominence is studied theoretically. The stability of condensation modes of a plasma in the coronal streamer model (Steinolfson et al., 1982) is considered using a two-dimensional time-dependent ideal MHD numerical simulation. It is found that a plasma with beta = 0.5 is unstable but one with beta = 4 is stable because the density enhancement of the plasma trapped by the closed fields increases with the strength of the magnetic field. The means by which condensation modes can produce a coronal cavity and/or initiate the formation of a prominence (depending on the field configuration) are discussed. It is argued that prominence and cavity material is all supplied from the chromospheric level in the form of spicules.

An, C.-H.↗
NASA NTRS1995

CME stimulated by eruptive prominence

A model of CME arising due to drift motion in the corona in the presence of an eruptive prominence is presented. Magnetic field configuration is in accordance with a model of inverse polarity. In a region where a magnetic pressure of the filament magnetic field is greater than a gas pressure, plasma motion can be assumed as a drift motion. Its characteristic is such that the further one gets from the filament, the higher is drift velocity of a plasma. That sort of motion leads to a rarefaction of plasma and formation of a cavity around the filament. But a current strength in eruptive prominences estimated from observations is such that the region b is less than 1 has a limited size. Near the boundary b = 1 plasma deceleration is occurred and as a result of it coronal density is increasing. Plasma condensation near the surface b = 1 leads to formation of a dense envelope which can be collated with an outer loop of CME. Two dimensional numerical MHD simulation displays a process of cavity and loop formation. If a current is large enough, two compact regions of compressed dense matter arise at both sides of the rising filament and two narrow jets are developed. This scenario, perhaps, corresponds to CMEs in which a top of the loop is faint or it is absent at all.

Filippov, B. P.↗
NASA NTRS1986

On the formation of coronal cavities

A theoretical study of the formation of a coronal cavity and its relation to a quiescent prominence is presented. It is argued that the formation of a cavity is initiated by the condensation of plasma which is trapped by the coronal magnetic field in a closed streamer and which then flows down to the chromosphere along the field lines due to lack of stable magnetic support against gravity. The existence of a coronal cavity depends on the coronal magnetic field strength; with low strength, the plasma density is not high enough for condensation to occur. Furthermore, we suggest that prominence and cavity material is supplied from the chromospheric level. Whether a coronal cavity and a prominence coexist depends on the magnetic field configuration; a prominence requires stable magnetic support.

An, C. H.↗
NASA NTRS1983

Flare loop radiative hydrodynamics. IV - Dynamic evolution of unstable semiempirical loop models

The evolution of the unstable solar atmosphere into the nonlinear phase, in response to various perturbations, is followed. The initial dynamic evolution of the atmosphere follows the predictions of linear stability analysis. In the nonlinear phase, rapid changes are confined to the transition region; these changes are manifested as a propagation of the transition region through the plasma, i.e., chromospheric evaporation or condensation. Global evolution therefore proceeds on the coronal conductive time scale. The rate of propagation of the transition region is determined by the imbalance between the energy supplied by thermal conduction from the corona and radiative cooling within the transition region itself. Flow velocities in the lower corona during evaporation or condensation are, in the cases studied, of order 3 km/s. The observed dynamic evolution is consistent with the existence of relatively long-lived coronal loops whose brightnesses vary on the evaporative time scale.

An, C.-H.↗
NASA NTRS1986

A siphon mechanism for supplying prominence mass

A siphonlike mechanism for moving mass from the chromosphere to a gravitational well at the top of a magnetic loop to form a prominence is examined. The calculations assume no a priori flow velocity at the loop base. Instead, heating in the loop legs drives the flow. The prominence formation process requires two steps. First, the background heating rate must be reduced to on the order of 1 percent of the initial heating rate required to maintain the coronal loop. This forms an initial condensation at the top of the loop. Second, the heating must take place only in the loop legs in order to produce a pressure differential which drives mass up into the well at the top of the loop. The heating rate in the loop must be increased once the prominence has begun to form, or full prominence densities cannot be achieved in a reasonable time. It is concluded that this heating driven siphonlike mechanism is feasible for producing and maintaining prominences.

Poland, A. I.↗
NASA NTRS2022

STITCH: A Subgrid-Scale Model for Energy Buildup in the Solar Corona

The solar corona routinely exhibits explosive activity, in particular coronal mass ejections and their accompanying eruptive ares, that have global-scale consequences. These events and their smaller counterparts, coronal jets, originate in narrow, sinuous lament channels. The key processes that form and evolve the channels operate on still smaller spatial scales and much longer time scales, culminating in a vast separation of characteristic lengths and times that govern these explosive phenomena. In this article, we describe implementation and tests of an efficient subgrid-scale model for generating eruptive structures in magnetohydrodynamics (MHD) coronal simulations. STITCH {STatistical InjecTion of Condensed Helicity { is a physics-based, reduced representation of helicity condensation: a process wherein small-scale vortical surface convection forms ubiquitous current sheets, and pervasive reconnection across the sheets mediates an inverse cascade of magnetic helicity and free energy, thereby forming the lament channels. We have developed a formalism, STITCH, that abstracts these complex processes into a single term in Ohm's law and the induction equation that directly injects tangential magnetic flux into the low corona. We show that our approach is in very good agreement with a full helicity-condensation calculation that treats all of the dynamics explicitly, while enabling substantial reductions in temporal duration and spatial resolution. In addition, we illustrate the flexibility of STITCH at forming localized lament channels and at energizing complex surface flux distributions that have sinuous boundaries. STITCH is simple to implement and computationally ecient, making it a powerful technique for physics-based modeling of solar eruptive events.

J T Dahlin↗
NASA NTRS1984

MHD stability of compressible coronal loops with radiative energy loss

The effect of radiative energy loss on the stability of compressible plasma in coronal loops is studied. By taking the limit as poloidal wavenumber m approaches infinity, stability conditions for local modes are derived. It was found that the radiation effect can trigger MHD instabilities of coronal loops which are in ideally marginally stable states. Compressibility is a stabilizing effect for ideal MHD local modes because the compression of magnetic field lines exerts a restoring force by increasing magnetic pressure. Compression of plasma induces two modes in a radiatively unstable plasma, magnetosonic and condensation modes. Compressibility affects the stability of ideally stable (or unstable) coronal plasmas through magnetosonic modes, which are a stabilizing (destabilizing) effect for ideally stable (unstable) plasmas. For coronal plasmas in ideally marginally stable states, condensation as well as magnetosonic modes can trigger MHD instability. Because of these two modes, the effect of radiation on compressible coronal plasmas is more destabilizing than it is on incompressible plasmas when the plasmas are in ideal MHD unstable or marginally stable states.

An, C.-H.↗
NASA NTRS1992

Dynamic formation and magnetic support of loop or arcade prominences

The results of model dynamic simulations of the formation and support of a narrow prominence at the apex of a coronal magnetic loop or arcade are described. The condensation process proceeds via an initial radiative cooling and pressure drop, and a secondary siphon flow from the dense chromospheric ends. The antibuoyancy effect as the prominence forms causes a bending of a confining magnetic field, which propagates toward the semirigid ends of the magnetic loop. Thus, a wide magnetic 'hammock' or well (of a normal polarity Kippenhahn-Schlueter type) is formed, which supports the prominence at or near the field apex.

Vanhoven, Gerard↗
NASA NTRS1992

The physics of solar prominences

The outstanding questions on the formation of quiescent prominences are discussed. One key issue is identified to be the formation of dips in coronal magnetic field lines. A model is presented which can account for such dipped field lines. The critical ingredients of the model are that the prominence magnetic field is a truly three dimensional structure with significant variation along the prominence length, and the magnetic field has strong shear concentrated at the photospheric neutral line. Simulations which demonstrate that these two features lead to dip formation and that the geometry of the dips are such that inverse polarity prominences can be explained are presented. Another key issue is identified to be the formation of prominence condensations on dipped field lines. It is argued that a spatially varying coronal heating rate which is maximum near the chromosphere explain these condensations.

Antiochos, Spiro K.↗
NASA NTRS2019

The Role of Magnetic Helicity in Coronal Heating

One of the greatest challenges in solar physics is understanding the heating of the Sun's corona. Most theories for coronal heating postulate that free energy in the form of magnetic twist/stress is injected by the photosphere into the corona where the free energy is converted into heat either through reconnection or wave dissipation. The magnetic helicity associated with the twist/stress, however, is expected to be conserved and appear in the corona. In previous works, we showed that the helicity associated with the small-scale twists undergoes an inverse cascade via stochastic reconnection in the corona and ends up as the observed large-scale shear of filament channels. Our "helicity condensation" model accounts for both the formation of filament channels and the observed smooth, laminar structure of coronal loops. In this paper, we demonstrate, using helicity- and energy-conserving numerical simulations of a coronal system driven by photospheric motions, that the model also provides a natural mechanism for heating the corona. We show that the heat generated by the reconnection responsible for the helicity condensation process is sufficient to account for the observed coronal heating. We study the role that helicity injection plays in determining coronal heating and find that, crucially, the heating rate is only weakly dependent on the net helicity preference of the photospheric driving. Our calculations demonstrate that motions with 100% helicity preference are least efficient at heating the corona; those with 0% preference are most efficient. We discuss the physical origins of this result and its implications for the observed corona.

Knizhnik, K. J.↗
NASA NTRS1990

Prominence formation in a coronal loop

A model is presented which depends on the preferential deposition of heating in the legs of a coronal loop and which produces a stable prominence-scale condensation at the loop top. Dynamic stability is attained by the subsequent adjustment of local parallel gravity by a magnetic inversion at the loop (or arcade) apex. A nonlinear numerical simulation of this process, which includes a deep chromosphere, a heating rate with a fixed dissipation length, and full solar gravity is described.

Mok, Y.↗
NASA NTRS1990

An equation for the evolution of solar and stellar flare loops

An ordinary differential equation describing the evolution of a coronal loop subjected to a spatially uniform but time-varying heating rate is discussed. It is assumed that the duration of heating is long compared to the sound transit time through the loop, which is assumed to have uniform cross section area. The form of the equation changes as the loop evolves through three states: 'strong evaporation', 'scaling law behavior', and 'strong condensation'. Solutions to the equation may be used to compute the time dependence of the average coronal temperature and emission measure for an assumed temporal variation of the flare heating rate. The results computed from the model agree reasonably well with recent published numerical simulations and may be obtained with far less computational effort. The model is then used to study the May 21, 1980, solar flare observed by SMM and the giant April 12, 1985, flare observed on the star AD Leo.

Fisher, George H.↗
NASA NTRS2008

The Formation of Filament Threads in a Sheared Magnetic Field

This slide presentation reviews the mechanism that form the filament threads in the sheared magnetic field of the solar photosphere. There are observations of the filaments, a chart showing the magnetic field configuration for the filaments, and charts that show the thermal instability, and thermal conduction. Discussion of the question "How is it possible for a magnetic field to inhibit heat flow in a coronal plasma without simultaneously restricting the mass flow required for the growth of density condensations?" is reviewed. Equations that are used in the model are given. The characteristics of dynamic condensations are reviewed. There are charts that show the density perturbations for principal dynamic condensation mode and 1st harmonic. The characteristics of the kinematic condensations are also discussed. The types of perturbations that were studied are: (1) Generic perturbation with T(sub 0) < T(sub c) (2) Generic perturbation with T(sub 0) = T(sub c) (3) Random perturbation with T(sub 0)< T(sub c).

condensational instability↗