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Results for “solar coronal loops”

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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Stability of a diffuse linear pinch with axial boundaries

A formulation of the stability behavior of a finite-length pinch is presented. A general initial perturbation is expressed as a uniformly convergent sum over a complete discrete k set. A variational calculation is then performed, based on the energy principle, in which the end-boundary conditions appear as constraints. The requisite Lagrange multipliers mutually couple the elemental periodic excitations. The resulting extended form of delta-W still admits a proper second-variation treatment so that the minimization and stability considerations of Newcomb remain applicable. Comparison theorems are discussed as is the relevance of this end-effect model to the stability of solar coronal loops.

Einaudi, G.↗
NASA NTRS1984

The effect of line-tying on the radiative MHD stability of coronal plasmas with radial pressure profile

The role of photospheric line-tying, i.e., solar coronal loop structures, was investigated in terms of the effect on radiative modes and the influence that different radial pressure profiles exert on the effects of line-tying on radiative MHD stability. Energy is assumed dissipated by heat conduction and radiation and zero- and first-order solutions are obtained for the radiative time scales. Line-tying is a magnetic tension in the zero-order MHD mode and produces stability. Heat conduction occurs along bent field lines in first-order MHD modes when plasmas cross the field lines. Irradiated cool-core loops can experience MHD instabilities in the cylinder center, while line-tying can stabilize the plasma in the surrounding hot medium. Line-tying also adds stability to magnetosonic and condensation modes.

An, C.-H.↗
NASA NTRS1988

Magnetic tearing of plasma discharges due to nonuniform resistivity

The rearrangement of current in a plasma discharge in response to resistivity nonuniformities within a magnetic surface is studied. It is shown that macroscopic magnetic islands develop about those surfaces where the nonuniformity is aligned with the magnetic field. If the nonuniformity and the field are not aligned anywhere, there is no current rearrangement; instead, relatively large plasma flows are set up. Such resistivity inhomogeneities can obtain in solar coronal loops and, in some circumstances, in tokamak discharges.

Hassam, A. B.↗
NASA NTRS1989

Heat flux in a non-Maxwellian plasma

A hybrid numerical scheme is applied to solve the Landau equation for the electron distribution function over all velocity space. Evidence is presented for the first time of the degree and character of the failure of the classical Spitzer-Haerm heat flux approximation in a realistic solar coronal loop structure. In the loop model used, the failure is so severe at some points that the role of the heat flux in the plasma's energy balance is completely misinterpreted. In the lower corona the Spitzer-Haerm approximation predicts that the heat flux should act as an energy source, whereas the more accurate distribution functions calculated here show this to be an energy sink.

Ljepojevic, N. N.↗
NASA NTRS1988

Soft X-ray images of the solar corona using normal incidence optics

A solar coronal loop system has been photographed in soft X-rays using a normal incidence telescope based on multilayer mirror technology. The telescope consisted of a spherical objective mirror of 4 cm aperture and 1 m focal length, a film cassette, and a focal plane shutter. A metallized thin plastic film filter was used to exclude visible light. The objective mirror was covered with a multilayer coating consisting of alternating layers of tungsten and carbon whose combined thicknesses satisfied the Bragg diffraction condition for 44 A radiation. The image was recorded during a rocket flight on October 25, 1985 and was dominated by emission lines arising from the Si XII spectrum. The rocket also carried a high resolution soft X-ray spectrograph that confirmed the presence of Si XII line radiation in the source. This image represents the first successful use of multilayer technology for astrophysical observations.

Bruner, M. E.↗
NASA NTRS1990

Dynamical evolution of twisted magnetic flux tubes. I - Equilibrium and linear stability

The three-dimensional dynamical evolution of twisted magnetic flux tubes is studied using a time-dependent magnetohydrodynamic (MHD) model. The flux tubes are intended to model solar coronal loops, and include the stabilizing effect of photospheric line tying. The model permits the complete evolution of flux tubes to be followed self-consistently, including the formation, equilibrium, linear instability, and nonlinear behavior. Starting from an initial uniform background magnetic field, a twisted flux tube is created by the application of slow, localized photospheric vortex flows. The flux tube evolves quasi-statically through sequences of equilibria with increasing twist, until it becomes linearly unstable to an ideal MHD kink mode. In this paper, the equilibrium properties and the linear stability behavior are discussed. The application of the method to the uniform-twist, Gold-Hoyle field confirms the previous stability threshold for kink instability and provides estimates of the resulting growth rate.

Mikic, Zoran↗
NASA NTRS1990

The effects of nonequilibrium ionization on the radiative losses of the solar corona

The emissivity of the ions of carbon and oxygen has been recalculated for a set of solar coronal loop models with a steady state siphon flow. The ion densities were calculated from the plasma velocities, temperatures, and densities of the models, and large departures from equilibrium were found. For purposes of comparison, the emissivity was calculated with and without the approximation of ionization equilibrium. Considerable differences in the radiative loss function Lambda(T) curve between equilibrium and nonequilibrium conditions were found. The nonequilibrium Lambda(T) function was then used to solve again the steady state flow equations of the loop models. The differences in the structure of these models with respect to the models calculated adopting the Lambda(T) curve in equilibrium are discussed.

Spadaro, D.↗
NASA NTRS1990

Electron acceleration by magnetosonic waves in solar flares

Results of a simulation of electron acceleration by magnetosonic waves in a solar coronal loop are presented. The dependence of the energization rate on the strength and shape of the wave spectrum is discussed. Comparisons are made with the predictions of quasilinear theory and of a model describing stochastic acceleration by waves as isotropic diffusion in momentum space.

Zweibel, Ellen G.↗
NASA NTRS1995

Study of nonlinear MHD equations governing the wave propagation in twisted coronal loops

The solar corona, modelled by a low beta, resistive plasma slab, sustains MHD wave propagations due to shearing footpoint motions in the photosphere. By using a numerical algorithm the excitation and nonlinear development of MHD waves in twisted coronal loops are studied. The plasma responds to the footpoint motion by sausage waves if there is no twist. The twist in the magnetic field of the loop destroys initially developed sausage-like wave modes and they become kinks. The transition from sausage to kink modes is analyzed. The twist brings about mode degradation producing high harmonics and this generates more complex fine structures. This can be attributed to several local extrema in the perturbed velocity profiles. The Alfven wave produces remnants of the ideal 1/x singularity both for zero and non-zero twist and this pseudo-singularity becomes less pronounced for larger twist. The effect of nonlinearity is clearly observed by changing the amplitude of the driver by one order of magnitude. The magnetosonic waves also exhibit smoothed remnants of ideal logarithmic singularities when the frequency of the driver is correctly chosen. This pseudo-singularity for fast waves is absent when the coronal loop does not undergo any twist but becomes pronounced when twist is included. On the contrary, it is observed for slow waves even if there is no twist. Increasing the twist leads to a higher heating rate of the loop. The larger twist shifts somewhat uniformly distributed heating to layers inside the slab corresponding to peaks in the magnetic field strength.

Parhi, S.↗
NASA NTRS1983

VLA observations of a solar active region and coronal loops

In May 1981, observations of a solar active region at 6 cm and 20 cm were conducted with the aid of a Very Large Array (VLA). It was found that the 6 cm emission was associated with sunspots and transverse magnetic fields between spots. The 20 cm emission came from elongated sources connecting regions of opposite magnetic polarity. The sources of the emission at 20 cm could be identified as coronal loops. A model for hydrostatic loops proposed by Rosner et al. (1978) was employed to determine a likely mechanism for the emission at 20 cm. It was found that a two-component model of bremsstrahlung emission from the feet and gyroresonance emission from the top of the loops can satisfactorily explain the observations.

Mcconnell, D.↗
NASA NTRS2022

The Coronal Veil

Coronal loops, seen in solar coronal images, are believed to represent emission from magneticflux tubes withcompact cross sections. We examine the 3D structure of plasma above an active region in a radiativemagnetohydrodynamic simulation to locate volume counterparts for coronal loops. In many cases, a loop cannot belinked to an individual thin strand in the volume. While many thin loops are present in the synthetic images, thebright structures in the volume are fewer and of complex shape. We demonstrate that this complexity can formimpressions of thin bright loops, even in the absence of thin bright plasma strands. We demonstrate the difficulty ofdiscerning from observations whether a particular loop corresponds to a strand in the volume, or a projectionartifact. We demonstrate how apparently isolated loops could deceive observers, even when observations frommultiple viewing angles are available. While we base our analysis on a simulation, the mainfindings areindependent from a particular simulation setup and illustrate the intrinsic complexity involved in interpretingobservations resulting from line-of-sight integration in an optically thin plasma. We propose alternativeinterpretation for strands seen in Extreme Ultraviolet images of the corona. The“coronal veil”hypothesis ismathematically more generic, and naturally explains properties of loops that are difficult to address otherwise—such as their constant cross section and anomalously high density scale height. We challenge the paradigm ofcoronal loops as thin magneticflux tubes, offering new understanding of solar corona, and by extension, of othermagnetically confined bright hot plasmas

A Malanushenko↗
NASA NTRS2016

Simulations of Solar Jets Confined by Coronal Loops

Coronal jets are collimated, dynamic events that occur over a broad range of spatial scales in the solar corona. In the open magnetic field of coronal holes, jets form quasi-radial spires that can extend far out into the heliosphere, while in closed-field regions the jet outflows are confined to the corona. We explore the application of the embedded-bipole model to jets occurring in closed coronal loops. In this model, magnetic free energy is injected slowly by footpoint motions that introduce twist within the closed dome of the jet source region, and is released rapidly by the onset of an ideal kink-like instability. Two length scales characterize the system: the width (N) of the jet source region and the footpoint separation (L) of the coronal loop that envelops the jet source. We find that both the conditions for initiation and the subsequent dynamics are highly sensitive to the ratio L/N. The longest-lasting and most energetic jets occur along long coronal loops with large L/N ratios, and share many of the features of open-field jets, while smaller L/N ratios produce shorter-duration, less energetic jets that are affected by reflections from the far-loop footpoint. We quantify the transition between these behaviors and show that our model replicates key qualitative and quantitative aspects of both quiet Sun and active-region loop jets. We also find that there connection between the closed dome and surrounding coronal loop is very extensive: the cumulative reconnected flux at least matches the total flux beneath the dome for small L/N, and is more than double that value for large L/N.

Wyper, P. F.↗
NASA NTRS1983

Electrodynamic coupling in magnetically confined stellar X-ray loops

The solar coronal complex X-ray structure is now known to involve radiation loops that coincide spatially with the magnetic loops confining the radiating plasma. An effort is presently made to identify primary submodels involved in the global coupling between a mechanical energy reservoir of beta value greater than 1 and a contiguous site of X-ray activity whose beta value is lower than 1. The 'dynamo' model invoked establishes a quantitative connection between mechanical driver properties and the dimensions, field strength, and number density distribution of elemental magnetic loops.

Ionson, J. A.↗
NASA NTRS1986

The propagation of solar flare particles in a coronal loop

A time-dependent diffusion equation with velocity-dependent diffusion and energy-loss coefficients was solved for the case where energetic solar particles are injected into a coronal loop and then diffuse out the ends of the loop into the lower corona/chromosphere. The solution yields for the case of relativistic electrons, precipitation rates and populations which are necessary for calculating thick and thin target X-ray emission. It follows that the thick target emission is necessarily delayed with respect to the particle acceleration on injection by more than the mere travel time of the particle over the loop length. In addition the time-dependent electron population at the top of the loop is calculated. This is useful in estimating the resulting micron-wave emission. The results show relative timing differences in the different emission processes which are functions of particle species, energy and the point of injection of the particles into the loop. Equivalent quantities are calculated for non-relativistic protons.

Ryan, J. M.↗
NASA NTRS2019

A Two-sided-Loop X-Ray Solar Coronal Jet and a Sudden Photospheric Magnetic-field Change, Both Driven by a Minifilament Eruption

Most of the commonly discussed solar coronal jets are of the type consisting of a single spire extending approximately vertically from near the solar surface into the corona. Recent research of a substantial number of events shows that eruption of a miniature filament (minifilament) drives at least many such single-spire jets, and concurrently generates a miniflare at the eruption site. A different type of coronal jet, identified in X-ray images during the Yohkoh era, are two-sided-loop jets, which extend from a central excitation location in opposite directions, along two opposite low-lying coronal loops that are more-or-less horizontal to the surface. We observe such a two-sided-loop jet from the edge of active region (AR) 12473, using data from Hinode XRT and EIS, and SDO AIA and HMI. Similar to single-spire jets, this two-sided-loop jet results from eruption of a minifilament, which accelerates to over 140 km/s before abruptly stopping upon striking overlying nearlyhorizontal magnetic field at ∼30,000 km altitude and producing the two-sided-loop jet via interchange reconnection. Analysis of EIS raster scans show that a hot brightening, consistent with a small flare, develops in the aftermath of the eruption, and that Doppler motions (∼40 km/s) occur near the jet-formation region. As with many single-spire jets, the trigger of the eruption here is apparently magnetic flux cancelation, which occurs at a rate of ∼4×10^18 Mx/hr, comparable to the rate observed in some single-spire AR jets. This example of a two-sided jet, along with numerous examples of single-spire jets, supports that essentially all coronal jets result from eruptions of minifiaments, and frequently the eruption of the minifilment is triggered by magnetic flux cancelation. (Details are in Sterling et al. 2019, ApJ, 871, 220.)

Sterling, Alphonse C.↗
DOE OSTI2025

On the Stability Analysis of Astrophysical Cooling Functions

To model the temperature evolution of optically thin astrophysical environments at MHD scales, radiative and collisional cooling rates are typically either pretabulated or fit into a functional form and then input into MHD codes as a radiative loss function. Thermal balance requires estimates of the analogous heating rates, which are harder to calculate, and due to uncertainties in the underlying dissipative heating processes these rates are often simply parameterized. The resulting net cooling function defines an equilibrium curve that varies with density and temperature. Such cooling functions can make the gas prone to thermal instability (TI), which will cause departures from equilibrium. There has been no systematic study of thermally unstable parameter space for nonequilibrium states. Motivated by our recent finding that there is a related linear instability, catastrophic cooling instability, that can dominate over TI, here we carry out such a study. We show that Balbus instability criteria for TI can be used to define a critical cooling rate, Λ c , that permits a nonequilibrium analysis of cooling functions through the mapping of TI zones. We furthermore illustrate how thermal conduction modifies the shape of TI zones. Upon applying a Λ c -based stability analysis to coronal loop simulations, we find that loops undergoing periodic episodes of coronal rain formation are linearly unstable to catastrophic cooling instability, while TI is stabilized by thermal conduction.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2023

Deriving Large Coronal Magnetic Loop Parameters Using LOFAR J Burst Observations

Large coronal loops around one solar radius in altitude are an important connection between the solar wind and the low solar corona. However, their plasma properties are ill-defined, as standard X-ray and UV techniques are not suited to these low-density environments. Diagnostics from type J solar radio bursts at frequencies above 10 MHz are ideally suited to understand these coronal loops. Despite this, J-bursts are less frequently studied than their type III cousins, in part because the curvature of the coronal loop makes them unsuited for using standard coronal density models. We used LOw-Frequency-ARray (LOFAR) and Parker Solar Probe (PSP) solar radio dynamic spectrum to identify 27 type III bursts and 27 J-bursts during a solar radio noise storm observed on 10 April 2019. We found that their exciter velocities were similar, implying a common acceleration region that injects electrons along open and closed magnetic structures. We describe a novel technique to estimate the density model in coronal loops from J-burst dynamic spectra, finding typical loop apex altitudes around 1.3 solar radius. At this altitude, the average scale heights were 0.36 solar radius, the average temperature was around 1 MK, the average pressure was 0.7mdyn cm -2 , and the average minimum magnetic field strength was 0.13 G. We discuss how these parameters compare with much smaller coronal loops.

Jinge Zhang↗