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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Modeling solar flare conduction fronts. I - Homogeneous plasmas and ion-acoustic turbulence. II - Inhomogeneous plasmas and ambipolar electric fields

A one-dimensional, electrostatic, particle-in-cell simulation is used here to model the expansion of a heated electron population in a coronal loop during a solar flare and the characteristics of the associated X-ray emissions. The hot electrons expand outward from the localized region, creating an ambipolar electric field which accelerates a return current of cooler, ambient electrons. Ion-acoustic waves are generated by the return currents as proposed by Brown et al. (1979), but they play little or no role in containing energetic electrons and the conduction front proposed by Brown et al. does not form. The X-ray emission efficiency of the electrons is too low in the corona for them to be the source of hard X-ray bursts. The particle dynamics changes dramatically if the heated plasma is at low altitudes and expands upward into the more tenuous plasma at higher altitudes. Two important applications of this finding are the radio-frequency heating of the corona and the collisional heating of the chromosphere by precipitating energetic electrons. In both cases, the overlying plasma has a density that is too low to supply a balancing return current to the expanding hot electrons. As a result, an ambipolar electric field develops that tends to confine the energetic electrons behind a front that propagate outward at about the speed of sound.

Mckean, M. E.↗
NASA NTRS1982

Electron heating by fast mode magnetohydrodynamic waves in the solar wind emanating from coronal holes

It is shown that fast mode magnetohydrodynamic waves, propagating outwards from the sun in coronal hole regions, will dissipate primarily through collisionless interaction with electrons rather than with protons. This dissipation can lead to higher electron than proton temperatures in the accelerating region of the solar wind, provided the waves carry a sufficiently large energy flux.

Habbal, S. R.↗
NASA NTRS2014

Core and Wing Densities of Asymmetric Coronal Spectral Profiles: Implications for the Mass Supply of the Solar Corona

Recent solar spectroscopic observations have shown that coronal spectral lines can exhibit asymmetric profiles, with enhanced emissions at their blue wings. These asymmetries correspond to rapidly upflowing plasmas at speeds exceeding approximately equal to 50 km per sec. Here, we perform a study of the density of the rapidly upflowing material and compare it with that of the line core that corresponds to the bulk of the plasma. For this task, we use spectroscopic observations of several active regions taken by the Extreme Ultraviolet Imaging Spectrometer of the Hinode mission. The density sensitive ratio of the Fe(sub XIV) lines at 264.78 and 274.20 Angstroms is used to determine wing and core densities.We compute the ratio of the blue wing density to the core density and find that most values are of order unity. This is consistent with the predictions for coronal nanoflares if most of the observed coronal mass is supplied by chromospheric evaporation driven by the nanoflares. However, much larger blue wing-to-core density ratios are predicted if most of the coronal mass is supplied by heated material ejected with type II spicules. Our measurements do not rule out a spicule origin for the blue wing emission, but they argue against spicules being a primary source of the hot plasma in the corona. We note that only about 40% of the pixels where line blends could be safely ignored have blue wing asymmetries in both Fe(sub XIV) lines. Anticipated sub-arcsecond spatial resolution spectroscopic observations in future missions could shed more light on the origin of blue, red, and mixed asymmetries.

coronal spectral profiles↗
NASA NTRS2014

Simulated Time Lags of Hinode/XRT and SDO/AIA Lightcurves as an Indication of Loop Heating Scenario

The precise nature of the heating mechanism (location, duration) in coronal loops is still a matter of enormous research. We present results from a one-dimensional (1D) hydrodynamic loop simulation of a coronal loop which was run using different parameters such as loops length (50, 200 and 500 light-seconds), maximum temperature reached (3 million degrees Kelvin and 10 million degrees Kelvin), and abundances. For each scenario the model outputs were used to calculate the corresponding lightcurves as seen by X-ray telescope/Be-thin filter and various Extreme Ultra Violet Atmospheric Imaging Assembly channels. The lag time between the peak of these lightcurves was computed using cross-correlation and plotted as a function of loop length. Additional results were computed using the zero-dimensional Enthalpy-Based Thermal Evolution of Loops (EBTEL) code in order to test the compatibility of the two codes and to investigate additional loop lengths. Initial results indicate that the long (greater than 5000 seconds) lags observed in the approximately 100 light-seconds loops of active regions can only be reproduced using photospheric abundances and much longer loop lengths. This result suggests that the observed time lags cannot be completely explained by impulsive heating.

nanoflares↗
NASA NTRS1971

The release of energetic particles from the sun.

Development of a model to account for the release of solar cosmic rays from the sun. The solar atmosphere out to 3 to 5 solar radii above the photosphere is permeated with magnetic field lines which trap low-rigidity (less than or equal to 50 MV) flare particles. Plasma heated by the flare process disturbs the trapping field, and not until the disturbance reaches 3 to 5 solar radii can the low-rigidity flare particles have access to interplanetary space. If the plasma is not heated sufficiently to overcome the coronal field, flare particles are trapped efficiently. Subsequent leakage of these particles into interplanetary space forms corotating streams. Reference is made to satellite observations of solar electromagnetic radiation and charged particles.

Simnett, G. M.↗
NASA NTRS1985

The structure of high-temperature solar flare plasma in non-thermal flare models

Analytic differential emission measure distributions have been derived for coronal plasma in flare loops heated both by collisions of high-energy suprathermal electrons with background plasma, and by ohmic heating by the beam-normalizing return current. For low densities, reverse current heating predominates, while for higher densities collisional heating predominates. There is thus a minimum peak temperature in an electron-heated loop. In contrast to previous approximate analyses, it is found that a stable reverse current can dominate the heating rate in a flare loop, especially in the low corona. Two 'scaling laws' are found which relate the peak temperature in the loop to the suprathermal electron flux. These laws are testable observationally and constitute a new diagnostic procedure for examining modes of energy transport in flaring loops.

Emslie, A. G.↗
NASA NTRS1986

Solar burst precursors and energy build-up at microwave wavelengths

We summarize high-resolution microwave observations (VLA) of heating and magnetic triggering in coronal loops. Magnetic changes that precede solar eruptions on time scales of tens of minutes involve primarily emerging coronal loops and the interaction of two or more loops. Thermal cyclotron lines have been detected in coronal loops, suggesting the presence of hot current sheets that enhance emission from relatively thin layers of enhanced temperature and constant magnetic field. These current sheets may play a role in the excitation of solar bursts. A filament-associated source with a high brightness temperature and steep radiation spectrum occurs above a region of apparently weak photospheric field. This source might be attributed to currents that enhance coronal magnetic fields. Compact (phi=5 sec) transient sources with lifetimes of 30 to 60 minutes have also been detected in regions of apparently weak photospheric field. We conclude by comparing VLA observations of coronal loops with simultaneous SMM-XRP observations.

Lang, Kenneth R.↗
NASA NTRS1995

Solar coronal hole simulations compared with interpretations of Yohkoh SXT observations

Recent observations of coronal holes made with the soft X-ray telescope aboard Yohkoh have indicated a temperature of 1.8 approximately 2.1 x 10(exp 6) K and an emission measure of 10(exp 25.7 approximately 26.2) cm (exp -5). This is almost the same as in quiet regions of the Sun. Numerical simulations of the temperature density and velocity structure in a coronal hole. using a parameterized heating distribution have been used for a comparison with the Yohkoh observations. Models are obtained which fit the observed temperature and emission measure. with heating fluxes which are consistent with other measurements. However, the final velocity of the solar wind is very slow which indicates the necessity of another acceleration mechanism such as alfven waves.

Tziotziou, K.↗
NASA NTRS2014

Simulated Time Lags of Hinode/XRT and SDO/AIA Lightcurves as an Indication of Loop Heating Scenario

The precise nature of the heating mechanism (location, duration) in coronal loops is still a matter of enormous research. We present results from a 1D hydrodynamic loop simulation of a coronal loop which was run using different parameters such as loops length (50, 200, and 500 Mm), maximum temperature reached (3MK and 10MK), and abundances. For each scenario the model outputs were used to calculate the corresponding lightcurves as seen by XRT/Be-thin and various EUV AIA channels. The lag time between the peak of these lightcurves was computed using cross-correlation and plotted as a function of loop length. Additional results were computed using the 0D EBTEL code in order to test the compatibility of the two codes and to investigate additional loop lengths. Initial results indicate that the long (greater than 5000s) lags observed in the approx. 100Mm loops of active regions can only be reproduced using photospheric abundances and much longer loop lengths.

Alexander, Caroline↗
NASA NTRS1978

He-3-rich flares - A possible explanation

A plasma mechanism is proposed to explain the dramatic enhancements in He-3 observed in He-3-rich flares. It is shown that a common current instability in the corona may heat ambient He-3(2+) over any other ion and thus may preferentially inject He-3 into the flare acceleration process. This mechanism operates when the abundance of He-4 and heavier elements is larger than normal in the coronal plasma. It may also preferentially heat and thus inject certain ions of iron. The mechanism thus provides a possible explanation for the observed correlation between He-3 and heavy enhancements in He-3-rich flares.

Fisk, L. A.↗
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 NTRS1997

The Multi-Spectral Solar Telescope Array (MSSTA)

In 1987, our consortium pioneered the application of normal incidence multilayer X-ray optics to solar physics by obtaining the first high resolution narrow band, "thermally differentiated" images of the corona', using the emissions of the Fe IX/Fe X complex at ((lambda)lambda) approx. 171 A to 175 A, and He II Lyman (beta) at 256 A. Subsequently, we developed a rocket borne solar observatory, the Multi Spectral Solar Telescope Array (MSSTA) that pioneered multi-thermal imaging of the solar atmosphere, using high resolution narrow band X-ray, EUV and FUV optical systems. Analysis of MSSTA observations has resulted in four significant insights into the structure of the solar atmosphere: (1) the diameter of coronal loops is essentially constant along their length; (2) models of the thermal and density structure of polar plumes based on MSSTA observations have been shown to be consistent with the thesis that they are the source of high speed solar wind streams; (3) the magnetic structure of the footpoints of polar plumes is monopolar, and their thermal structure is consistent with the thesis that the chromosphere at their footpoints is heated by conduction from above; (4) coronal bright points are small loops, typically 3,500 - 20,000 km long (5 sec - 30 sec); their footpoints are located at the poles of bipolar magnetic structures that are are distinguished from other network elements by having a brighter Lyman a signature. Loop models derived for 26 bright points are consistent with the thesis that the chromosphere at their footpoints is heated by conduction from the corona.

Walker, A. B. C., Jr.↗
NASA NTRS2012

Enthalpy-Based Thermal Evolution of Loops: III. Comparison of Zero-Dimensional Models

Zero dimensional (0D) hydrodynamic models, provide a simple and quick way to study the thermal evolution of coronal loops subjected to time-dependent heating. This paper presents a comparison of a number of 0D models that have been published in the past and is intended to provide a guide for those interested in either using the old models or developing new ones. The principal difference between the models is the way the exchange of mass and energy between corona, transition region and chromosphere is treated, as plasma cycles into and out of a loop during a heating-cooling cycle. It is shown that models based on the principles of mass and energy conservation can give satisfactory results at some, or, in the case of the Enthalpy Based Thermal Evolution of Loops (EBTEL) model, all stages of the loop evolution. Empirical models can lead to low coronal densities, spurious delays between the peak density and temperature, and, for short heating pulses, overly short loop lifetimes.

Cargill, P. J.↗
NASA NTRS1978

The association of nonthermal electrons with non-flaring coronal transients

A close temporal and spatial association has been found between erupting filaments/coronal transients and radio noise storm continua. The three transients studied occurred away from active regions and are members of a class not usually accompanied by chromospheric emission. The data analyzed were from the S-054 soft X-ray telescope on Skylab and from one- and two-dimensional interferometers at meter and decameter wavelengths at Nancay and Clark Lake observatories. Calculations confirmed that observed microwave radiation from the transients is due to thermal bremsstrahlung. The results are consistent with an interpretation of heating of an increased amount of coronal plasma by nonthermal, 10-100 keV electrons. Three possibilities for the source of the material are described: (1) the filament material; (2) evaporation from the chromosphere or transition region; and (3) via a long-lived reconnection process.

Webb, D. F.↗
NASA NTRS1984

Resonances of coronal loops

It is pointed out that any theoretical demonstration that the solar corona can be heated by waves requires a demonstration that the required energies can actually be carried from the convection zone to the corona by waves. In addition, it must be shown that the waves can dissipate their energy into heat in the corona. The present investigation is concerned specifically with the heating of closed magnetic structures in the corona, taking into account coronal loops or active region loops. Attention is given to the MHD Alfven wave. It is shown analytically that coronal active region loops can behave much like interference filters. The coronal part of the loop acts like a resonant cavity for Alfven waves. When the resonances are excited, large energy fluxes can be carried into the loop by Alfven waves which are generated in the solar convection zone. It is estimated that the energy fluxes can power the observed loops.

Hollweg, J. V.↗
NASA NTRS1987

The eastward deflection of fast coronal mass ejecta in interplanetary space

Previous work has shown that a bidirectional solar wind electron heat flux is one of the more prominent signatures of a coronal mass ejection event in the solar wind at 1 AU. Using ISEE 3 solar wind electron measurements obtained during 1978 and 1979, this signature was used to identify the fast coronal mass ejecta driving 19 interplanetary shocks. In 17 of the 19 shock events an eastward deflection of the ejection plasma (apparent arrival from west of the sun) was observed. The average eastward deflection for all of the events was about 3 deg, corresponding to a typical transverse velocity of 25 km/s. Usually an oppositely directed (i.e., westward) flow deflection of comparable magnitude was observed within the compressed ambient plasma ahead of the ejecta. The sense of these deflections - first westward within the compressed ambient plasma and then eastward within the ejecta - is the same as is observed near the leading edges of quasi-stationary, corotating high speed streams.

Gosling, J. T.↗
NASA NTRS2019

Satellite Orbital Drag During Magnetic Storms

We investigate satellite orbital drag effects at lowEarth orbit associated with thermosphere heating during magnetic storms caused by coronal mass ejections. CHAllenge Minisatellite Payload (CHAMP) and Gravity Recovery And Climate Experiment (GRACE) neutral density data are used to compute orbital drag. Stormtoquiet density comparisons are performed with background densities obtained by the JacchiaBowman 2008 (JB2008) empirical model. Our storms are grouped in different categories regarding their intensities as indicated by minimum values of the SYMH index. We then perform superposed epoch analyses with storm main phase onset as zero epoch time. In general, we find that orbital drag effects are larger for CHAMP (lower altitudes) in comparison to GRACE (higher altitudes). Results show that storm time drag effects manifest first at high latitudes, but for extreme storms, particularly observed by GRACE, stronger orbital drag effects occur during early main phase at low/equatorial latitudes, probably due to heating propagation from high latitudes. We find that storm time orbital decay along the satellites' path generally increases with storm intensity, being stronger and faster for the most extreme events. For these events, orbital drag effects decrease faster probably due to elevated cooling effects caused by nitric oxide, which introduce modeled density uncertainties during storm recovery phase. Errors associated with total orbit decay introduced by JB2008 are generally the largest for the strongest storms and increase during storm times, particular during recovery phases. We discuss the implication of these uncertainties for the prediction of collision between space objects at lowEarth orbit during magnetic storms.

Oliveira, D. M.↗
NASA NTRS1995

Some constraints on acceleration mechanisms in the solar wind at polar latitudes

Observations of solar Lyman alpha have been interpreted as indicating that the polar mass flux density is lower than the equatorial average. This led Lallement et al (1986) to make a parametric study of solar wind acceleration, along the lines of earlier the Munro-Jackson study (1977), in which they concluded that uncertainties in the polar mass flux were large enough to be consistent with two extreme opposites: (1) a substantial energy supply beyond classical thermal conduction is required; or (2) classical thermal conduction is adequate to drive the flow. This ambiguity has been clarified by Ulysses observations of the polar outflow (Phillips et al, 1994). The polar mass flux density lies in the middle of the range studies by Lallement et al (1986), which suggests that extended heating is going on out to at least approximately 5 AU. Independent, purely energetic arguments can be made to estimate the required coronal source (electron) temperature that would be required to account for the observed energy flux density. An electron temperature of at least 2 x 10(exp 6) K would be required for the classical conduction flux density to be comparable to the total energy flux density; such a high temperature is thought to be unlikely in a coronal hole. These arguments strongly suggest that some extended heating or momentum transfer mechanism is required to drive the solar wind from the polar coronal hole. A number of mechanisms are discussed.

Barnes, A.↗