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At least 217 records · Page 12

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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 NTRS1994

The physics of coronal closed-field structures

The properties of closed coronal loops are reviewed. First we discuss the main features of the static, hot loop models. In these models thermal conduction plays the dominant role in determining the temperature and density structure. Next the cool loop models and their implications for solar observations are discussed. Finally, some new theoretical results on coronal abundances are presented. It is argued that chromospheric evaporation, which is a basic feature of the hot models, can account for the observed anomalies in the coronal element abundances.

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 NTRS1998

Magnetic Roots and the Driving of Extended Coronal Heating

We report results from a continuation of a previous study, in which we found large bright coronal loops within active regions and extending from active regions that have one end rooted near an island of included magnetic polarity that is a site of enhanced coronal heating and microflares. This suggested that magnetic activity such as microflaring results in enhanced heating in both the compact core field around the island and in the large loops extending from it. We might expect that the intensity variations due to enhanced heating in the compact and extended structures would be correlated. However, although some ex- tended loops do respond to the largest events taking place in the core fields near their feet, they do not show a clear response to most smaller individual events nor to the overall envelope of coronal heating activity in the core fields at their feet as determined from longer-term observations. Thus, while it is clear that the extended loops' heating is being driven from their ends at the magnetic islands, much of this heating is apparently by some form of footpoint activity that is not strongly coupled to the heating in the footpoint core fields. One possibility is that the remote heating in the extended loops is driven by reconnection at the magnetic null over the island, and that this reconnection is driven mainly by core-field activity that produces little coronal heating within the core field itself, perhaps in the manner of the numerical simulations by Karpen, Antiochos, and DeVore.

Porter, Jason G.↗

NASA NTRS1981

Closed coronal structures. II - Generalized hydrostatic model

Numerical computations of stationary solar coronal loop atmospheres are used to extend earlier analytical work. Two classes of loops are examined, namely symmetric loops with a temperature maximum at the top but now having a length greater than the pressure scale height and loops which have a local temperature minimum at the top. For the first class, new scaling laws are found which relate the base pressure and loop length to the base heating, the heating deposition scale height, and the pressure scale height. It is found that loops for which the length is greater than about two to three times the pressure scale height do not have stable solutions unless they have a temperature minimum at the top. Computed models with a temperature inversion at the top are permitted in a wider range of heating deposition scale height values than are loops with a temperature maximum at the top. These results are discussed in relation to observations showing a dependence of prominence formation and stability on the state of evolution of magnetic structures, and a general scenario is suggested for the understanding of loop evolution from emergence in active regions through the large-scale structure phase to opening in coronal holes.

Serio, S.↗

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 NTRS1990

Multiple-wavelength analysis of energy release during a solar flare - Thermal and nonthermal electron populations

Collaborative solar investigations by Tufts University and the Observatoire de Paris have resulted in simultaneous radio observations with the Very Large Array (VLA) and the Nancay Radioheliograph (NR), comparisons of this radio data with X-ray observations, and theoretical interpretations of the dominant radiation mechanisms during a weak impulsive solar flare observed on May 28, 1988. The VLA has mapped the flaring structures at time intervals of 3.3 s, showing that the preflash and flash-phase components of the impulsive emission originate in spatially separated sources. The 20.7 cm preflash source is ascribed to thermal gyroresonance emission from coronal loops with typical magnetic field strengths of up to 270 G; this emission is associated with heating and exhibits no detectable hard X-ray radiation above 30 keV. The flash-phase 20.7 cm source and the hard X-ray emission are attributed to nonthermal electrons in the coronal and chromospheric portions of a magnetic loop. The combination of imaging observations at 20.7 and 91.6 cm excludes emission from a confined hot plasma during the flash phase.

Willson, Robert F.↗

NASA NTRS1977

Development of a complex of activity in the solar corona

Using Skylab observations of soft solar X-rays, the development of a complex of activity in the solar corona during its whole lifetime of seven solar rotations is studied. The basic components of the activity complex were determined to be permanently interconnected through sets of magnetic field lines, which suggests similar connections also below the photosphere. The visibility of individual loops in these connections, however, was greatly variable and typically shorter than one day. Each brightening of a coronal loop in X-rays seems to be related to a variation in the photospheric magnetic field near its footprint.

Howard, R.↗

NASA NTRS1992

Active region studies with coordinated SOHO, microwave, and magnetograph observations

The scientific justification for an observing campaign to study the quantitative magnetic and plasma properties of coronal loops in active regions is presented. The SOHO (Solar and Heliospheric Observatory) instruments of primary relevance are CDS (Coronal Diagnostic Spectrometer), EIT, SUMER (Solar Ultraviolet Measurement of Emitted Radiation), and MDI. The primary ground based instruments would be the VLA (Very Large Array), the Owens Valley Radio Observatory, and vector and longitudinal field magnetographs. Similar campaigns have successfully been carried out with the Solar Maximum Mission x-ray polychromator and the Soft X-ray Imaging Sounding Rocket Payload (CoMStOC '87), the Goddard Solar EUV Rocket Telescope and Spectrograph, the Lockheed Solar Plasma Diagnostics Experiment rocket payload, and the Soft X-ray Telescope in Yohkoh (CoMStoc '92). The scientific payoff from such a campaign is discussed in light of the results from these previous campaigns.

Holman, Gordon D.↗

NASA NTRS1990

Coronal temperatures of selected active cool stars as derived from low resolution Einstein observations

Mean coronal temperatures of some active G-K stars were derived from Rev1-processed Einstein-observatory's IPC-spectra. The combined X-ray and transition region emission line data are in rough agreement with static coronal loop models. Although the sample is too small to derive any statistically significant conclusions, it suggests that the mean coronal temperature depends linearly on the inverse Rossby-number, with saturation at short rotation periods.

Vilhu, Osmi↗

NASA NTRS2005

Influence of Coronal Abundance Variations

During the final year of this program we concentrated on understanding the how to constrain the models with the best available observations. Work on developing accurate temperature and density diagnostics fkom TRACE and CDS together with constrained fits of non-potential force free fields will be extremely useful in the guiding the next generation of coronal models. The program has produced three fully operation numerical codes that model multi-species of ions in coronal loops: Static models and constant flow models. The time dependent numerical models have not been completed. We have extended the steady flow investigations to study the effect these flows have on coronal structure as observed with TRACE. Coronal observations derive from heavy-ion emission; thus, we focus on the extent to which flow may modify coronal abundances by examining the heavy-ion abundance stratification within long-lived loops. We discuss the magnitudes of the physical effects modeled and compare simulated results with TRACE observations. These results can have a profound effect on the interpretation of TRACE observations.

Gurman, Joseph↗

NASA NTRS1994

A volume-limited ROSAT survey of extreme ultraviolet emission from all nondegenerate stars within 10 parsecs

We report the results of a volume-limited ROSAT Wide Field Camera (WFC) survey of all nondegenerate stars within 10 pc. Of the 220 known star systems within 10 pc, we find that 41 are positive detections in at least one of the two WFC filter bandpasses (S1 and S2), while we consider another 14 to be marginal detections. We compute X-ray luminosities for the WFC detections using Einstein Imaging Proportional Counter (IPC) data, and these IPC luminosities are discussed along with the WFC luminosities throughout the paper for purposes of comparison. Extreme ultraviolet (EUV) luminosity functions are computed for single stars of different spectral types using both S1 and S2 luminosities, and these luminosity functions are compared with X-ray luminosity functions derived by previous authors using IPC data. We also analyze the S1 and S2 luminosity functions of the binary stars within 10 pc. We find that most stars in binary systems do not emit EUV radiation at levels different from those of single stars, but there may be a few EUV-luminous multiple-star systems which emit excess EUV radiation due to some effect of binarity. In general, the ratio of X-ray luminosity to EUV luminosity increases with increasing coronal emission, suggesting that coronally active stars have higher coronal temperatures. We find that our S1, S2, and IPC luminosities are well correlated with rotational velocity, and we compare activity-rotation relations determined using these different luminosities. Late M stars are found to be significantly less luminous in the EUV than other late-type stars. The most natural explanation for this results is the concept of coronal saturation -- the idea that late-type stars can emit only a limited fraction of their total luminosity in X-ray and EUV radiation, which means stars with very low bolometric luminosities must have relatively low X-ray and EUV luminosities as well. The maximum level of coronal emission from stars with earlier spectral types is studied also. To understand the saturation levels for these stars, we have compiled a large number of IPC luminosities for stars with a wide variety of spectral types and luminosity classes. We show quantitatively that if the Sun were completely covered with X-ray-emitting coronal loops, it would be near the saturation limit implied by this compilation, supporting the idea that stars near upper limits in coronal activity are completely covered with active regions.

Wood, Brian E.↗

NASA NTRS1994

Coalescence of two current loops with a kink instability simulated by a three-dimensional electromagnetic particle code

We have studied the dynamics of a coalescence of current loops using three-dimensional electromagnetic (EM) particle simulation code. Our focus is the investigation of such kinetic processes as energy trasnfer, heating particles, and electromagnetic emissions associated with a current loop coalescence which cannot be studied by MHD simulations. First, the two loops undergo a pinching oscillation due to a pressure imbalance between the inside and outside of the current loop. During the pinching oscillation, a kinetic kink instability is excited and electrons in the loops are heated perpendicularly to an ambient magnetic field. Next, the two current loops collide and coalesce, while at the same time a helical structure grows further. Subsequently, the perturbed current, which is due to these helically bunched electrons, can drive a whistler instability. It should be noted in this case that the whistler wave is excited by the kinetic kink instability and not a beam instability. After the coalescence of two helical loops, tilting motions can be observed in the direction of left-hand rotation, and the helical structure will relax resulting in strong plasma heating mostly in the direction perpendicular to the ambient magnetic field. It is also shown that high-frequency electromagnetic waves can be emitted from the region where the two loops coalesce and propagate strongly in the direction of the electron drift velocity. These processes may be important in understanding heating mechansims for coronal loops as well as radio wave emission mechanisms from active regions of solar plasmas.

Nishikawa, K.-I.↗

NASA NTRS1986

Coronal plasmas on the sun and nearby stars

The current understanding of the quiescent, or non-flaring, microwave emission from solar active regions is summarized. The thermal radiation mechanisms that account for most of the quiescent emission is reviewed, while it is also pointed out that current-amplified magnetic fields or non-thermal radiation may be required in some instances. The 20 cm radiation of coronal loops and the thermal cyclotron lines that accurately specify their magnetic field strength are discussed. The 20 cm and X ray emission of the coronal plasma are then compared. The coronae of nearby stars is next discussed, where coherent radiation processes seem to prevail. Some thoughts toward directions for future exploration are given.

Lang, Kenneth R.↗

NASA NTRS1989

Observational goals for Max '91 to identify the causative agent for impulsive bursts

Recent studies of impulsive hard x ray and microwave bursts suggest that a propagating causative agent with a characteristic velocity of the order of 1000 km/s is responsible for these bursts. The results of these studies are summarized and observable distinguishing characteristics of the various possible agents are highlighted, with emphasis on key observational goals for the Max '91 campaigns. The most likely causative agents suggested by the evidence are shocks, thermal conduction fronts, and propagating modes of magnetic reconnection in flare plasmas. With new instrumentation planned for Max '91, high spatial resolution observations of hard x ray sources have the potential to identify the agent by revealing detailed features of source spatial evolution. Observations with the Very Large Array and other radio imaging instruments are of great importance, as well as detailed modeling of coronal loop structures to place limits on their density and temperature profiles. With the combined hard x ray and microwave imaging observations, aided by loop model results, the simplest causative agent to rule out would be the propagating modes of magnetic reconnection. To fit the observational evidence, reconnection modes would need to travel at approximately the same velocity (the Alfven velocity) in different coronal structures that vary in length by a factor of 10(exp 3). Over such a vast range in loop lengths, it is difficult to believe that the Alfven velocity is constant. Thermal conduction fronts would be suggested by sources that expand along the direction of B and exhibit relatively little particle precipitation. Particle acceleration due to shocks could produce more diverse radially expanding source geometries with precipitation at loop footprints.

Batchelor, D. A.↗

NASA NTRS1994

Loop heating by D.C. electric current and electromagnetic wave emissions simulated by 3-D EM particle zone

We have shown that a current-carrying plasma loop can be heated by magnetic pinch driven by the pressure imbalance between inside and outside the loop, using a 3-dimensional electromagnetic (EM) particle code. Both electrons and ions in the loop can be heated in the direction perpendicular to the ambient magnetic field, therefore the perpendicular temperature can be increased about 10 times compared with the parallel temperature. This temperature anisotropy produced by the magnetic pinch heating can induce a plasma instability, by which high-frequency electromagnetic waves can be excited. The plasma current which is enhanced by the magnetic pinch can also excite a kinetic kink instability, which can heat ions perpendicular to the magnetic field. The heating mechanism of ions as well as the electromagnetic emission could be important for an understanding of the coronal loop heating and the electromagnetic wave emissions from active coronal regions.

Sakai, J. I.↗

NASA NTRS2013

Coordinated Observations of X-ray and High-resolution EUV Active Region Dynamics

The recently-launched High-resolution Coronal imager (Hi-C) sounding rocket provided the highest resolution images of coronal loops and other small-scale structures in the 193 Angstrom passband to date. With just 5 minutes of observations, the instrument recorded a variety of dynamic coronal events -- including even a small B-class flare. We will present our results comparing these extreme-ultraviolet (EUV) observations with X-ray imaging from Hinode/XRT as well as EUV AIA data to identify sources of hot plasma rooted in the photosphere and track their affect on the overall topology and dynamics of the active region.

Savage, Sabrina↗

NASA NTRS2013

Coordinated Observations of X-ray and High-Resolution EUV Active Region Dynamics

Savage, Sabrina↗