Collisional damping of surface waves in the solar corona
(Previously announced in STAR as N83-30213)
Gordon, B. E.↗
SEARCH · Engineering Papers
Results for “Solar corona”
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
The effect of a non-Maxwellian electron distribution on oxygen and iron ionization balances in the solar corona
Analytic expressions are derived for ionization and recombination rates in a parameterized non-Maxwellian electron velocity distribution with an enhanced high-energy tail. These expressions are then used in investigating the effect of such an enhancement in the high-energy tail of the coronal electron velocity distribution on the oxygen and iron ionization balances, O(+6) - O(+7) and Fe(+11) - Fe(+12). Relative to a Maxwellian of the same mean electron energy, the degree of ionization allowed by such a distribution is found to be either unchanged or slightly decreased for iron but often substantially increased for oxygen. The greater sensitivity of oxygen ionization balance to the high-energy distribution tail derives from the higher oxygen ionization threshold energy. It is noted that the electron temperature inferred from a measurement of the oxygen ionization ratio, O(+6)/O(+7), could indeed overestimate the actual coronal electron temperature by nearly 10 to the 6th K if the coronal electron distribution is incorrectly assumed to be Maxwellian.
Owocki, S. P.↗
Study of the solar corona using radio and space observations
The physics of coronal transients, the characteristics of radiation and accelerated particles at the time of flares, and the density/temperature structure of the transition region and corona and the coronal magnetic field are investigated.
Dulk, G. A.↗
Observations of electron density in the solar corona during sunspot minimum and global electric currents around the sun
Explore the source record for details and available documents.
Osherovich, V.↗
Current sheets in the solar corona
Coronal magnetic fields are twisted up by motion of their footpoints in the photosphere. When the twist exceeds a critical amount, kink-ballooning instabilities occur. These instabilities are studied numerically, in long, thin, axially bounded magnetic fields. Nonlinearly, the three-dimensional kinking motion compresses magnetic flux, forming a current sheet. Magnetic energy can be dissipated at a rate orders of magnitude greater than without the current sheets. The energy of footpoint motion can then go into coronal heating, via Ohmic dissipation in the current sheets.
Strauss, H. R.↗
Measurement of the 3-dimensional positions of type III bursts in the solar corona
The three-dimensional positions of type III sources in the corona are calculated on the basis of ground and spacecraft data. Simultaneous observations of the corona in visible light from Skylab make it possible to relate the apparent radio-source positions to slowly evolving coronal structures. It is found that open magnetic field lines connecting the low coronal levels to the interplanetary medium only exist in a relatively narrow region, and diverge rapidly upwards.
Poquerusse, M.↗
Electron density-sensitive line ratios for Fe XII in the solar corona
The collision strengths for electron-impact excitation of fine-structure transitions from the ground 3s23p3 configuration to the excited 3s3p4 configuration in Fe XII are presented. These are calculated by transforming the K-matrices obtained in a nonrelativistic R-matrix method to K-matrices in intermediate coupling using term-coupling coefficients which take into account the effect of breakdown of target LS coupling. Extensive configuration-interaction wave functions are used to represent the target states. The effective collision strengths are determined assuming a Maxwellian distribution of electron energies. Rydberg series of resonances converging to the excited state thresholds are explicitly included in the calculation. The relative level populations and emission-line strengths for Fe XII are calculated using the new atomic data presented in this paper and that published by Tayal, Henry, and Pradhan (1987) and Tayal and Henry (1986) over a wide range of electron densities and temperatures (1-2 x 10 to the 6th K). Present results for collision strengths and emission-line strengths are compared with the previous calculation of Flower (1977). Some significant differences are noted.
Tayal, S. S.↗
A theory for the radiation of magnetohydrodynamic surface waves and body waves into the solar corona
The Green's function for the slab coronal hole is obtained explicitly. The Fourier integral representation for the radiated field inside and outside the coronal hole waveguide is obtained. The radiated field outside the coronal hole is calculated using the method of steepest descents. It is shown that the radiated field can be written as the sum of two contributions: (1) a contribution from the integral along the steepest descent path and (2) a contribution from all the poles of the integrand between the path of the original integral and the steepest descent path. The free oscillations of the waveguide can be associated with the pole contributions in the steepest descent representation for the Green's function. These pole contributions are essentially generalized surface waves with a maximum amplitude near the interface which separates the plasma inside the coronal hole from the surrounding background corona. The path contribution to the integral is essentially the power radiated in body waves.
Davila, Joseph M.↗
The effect of ballooning modes on thermal transport and magnetic field diffusion in the solar corona
Presently favored mechanisms of coronal heating (current sheet dissipation and Alfven wave resonant heating) deposit heat in thin layers. Classical thermal conduction cannot explain how heat is transported across the magnetic field. If heating occurs in thin layers, large pressure gradients can be created which can give rise to ballooning modes. These instabilities are caused by the pressure gradient and the curvature of the magnetic field, and are stabilized by magnetic tension. The modes are broad band in wavelength and should produce turbulence. A mixing length expression for the turbulent heat transport shows that it is more than adequate to rapidly convect heat into much broader layers. Furthermore, the turbulent resistivity implies that heating occurs over most of the width of these broadened layers. The broadening also implies that much shorter time scales are required for heating. The beta values in the corona suggest that 1-10 turbulent layers are formed in typical loop or arch structures.
Strauss, H. R.↗
Corona-solar wind coupling review
Explore the source record for details and available documents.
Feynman, Joan↗
An EUV imaging spectrograph for high-resolution observations of the solar corona
An extreme ultraviolet (EUV) imaging spectrograph for the wavelength range from 235 to 450 A has been developed and used for resolution observations of the sun. The instrument incorporates a glancing incidence Wolter Type II Telescope and a near-normal incidence toroidal grating spectrograph to achieve near-stigmatic performance over this spectral range. The design of the spectrograph entrance aperture enables both stigmatic spectra with spectral resolution adequate to observe emission line profiles and spectroheliograms of restricted portions of the sun to be obtained concurrently. The design and performance of the instrument are described, and an overview of results obtained during a sounding rocket flight on May 5, 1989 is provided.
Neupert, Werner M.↗
The evolution of a sheared potential magnetic field in the solar corona
Explore the source record for details and available documents.
Karpen, J. T.↗
On neutralized currents in the solar corona
Using Ampere's integral law applied to vector magnetograph transverse field data, the current pattern in an active region associated with flaring activity is analyzed. The greater sensitivity of this integral formalism of Ampere's law may allow one to identify regions of weak return current around the actual positive current region. The effects of Faraday rotation on the inferred magnetic field measurements, and hence on the deduced current pattern, are discussed. It is concluded that the current patterns in the region consist either of a neutralized current system, well modeled by a coaxial cable, or of a potential field, with the apparent currents in the core an artifact induced by magneto-optical effects. In either case, the data do not convincingly show an unneutralized current system.
Wilkinson, Ladye K.↗
Observations of the solar corona with a high resolution EUV imaging spectrograph
An extreme ultraviolet imaging spectrograph was used to obtain coronal observations with high spectral and spatial resolution. The imaging properties of the instrument enable measurements of spectral line shapes and positions in discrete spatial elements of a region, so that the dynamic characteristics of the coronal plasma, as well as the distribution of emission measure with temperature and well known density diagnostics, can be studied for specific features. The instrumentation is described and several results of a sounding rocket flight on 5 May 1989, when the corona over NOAA Region 5464 including emission over the umbra of the region's largest sunspot and the pre-impulsive phase emission of a small flare was recorded, are summarized.
Neupert, Werner M.↗
Non-radial magnetic field structures in the solar corona
We report on the structure and geometry of coronal magnetic fields inferred from the observations of meter-decimeter type 3 and moving type 4 radio bursts, associated with an H(alpha) flare. This is the first report of type 3 radio bursts from the Nancay radioheliograph after it acquired the two dimensional multifrequency capability. Dispersion of the radio source positions with frequency suggests that open and closed field lines are considerably inclined to the radial direction which is consistent with the connectivity observed in the magnetogram. We suggest that multiple arch systems are involved in the type 4 emission. From the polarization and dispersion characteristics of the type 4 source, we infer that the emission is due to fundamental plasma emission.
Gopalswamy, N.↗
Yohkoh/SXT soft x-ray observations of sudden mass loss from the solar corona
Direct X-ray observations allow us to estimate the hot coronal mass before and after a flare or other disturbance of the type leading to a coronal mass ejection. The sudden disappearance of a large coronal structure (scale greater than 105 km) gives evidence that an ejection has occurred, if the time scales are much shorter than the conductive or radiative cooling times for such structures. A flare also typically adds large amounts of new material to the corona via evaporation resulting from the coronal energy release. This provides a competing mechanism that makes the estimation of the total mass loss somewhat difficult. We note that the X-ray observations have the advantage of covering the entire corona rather than the limb regions unlike the coronagraph observations. We have identified two examples of coronal mass disappearances. before and during long duration flare events on 21 Feb. 1992 (on the E limb) and 13 Nov. 1994 (near disk center). In latter case the total mass amounted to some 4 x 10(exp 14) g with a density of 3 x 10(exp 8)cm(exp -3) and a temperature of 2.8 MK before its disappearance. This corresponds to a radiative cooling time of some 104 S. much longer than the observed time of disappearance. We therefore suggest that these sudden mass disappearances correspond with coronal mass ejections (CMEs), and suggest that further data analysis will be able to confirm this by comparison with optical observations of specific CMEs.
Hudson, H. S.↗
The dynamic quiet solar corona: 4 days of joint observing with MDI and EIT
The analysis of a sequence of joint extreme ultraviolet imaging telescope (EIT) Fe XII and Michelson Doppler imager (MDI) magnetogram observations of the quiet sun near disk center is presented. It was found that: all the emerging flux above the threshold of approximately 10(sup 17) Mx is associated with enhanced coronal emissions; loop systems between the polarities in ephemeral regions remain visible up to separations of 10000 up to 30000 km; brightenings between approaching opposite polarity network concentrations form when the concentrations are between 5000 and 25000 km apart, and that faint connections up to 40000 km in length form as sets of concentrations of the same polarity coagulate. The coronal emission over patches of the quiet sun depends on the total flux in connected concentrations, on their distance and on the positions and strengths of neighboring concentrations.
Schrijver, C. J.↗
Insensitivity of Line-Ratio Diagnostics to Steady-State Non-Maxwellian Electron Distributions in Solar Corona
We examine the idea that the corona is heated by a population of nonthermal particles. An upper limit on the size of the nonthermal population is derived by assuming that all of the radiation and conduction losses in the corona are provided by the nonthermal tail of the particle distribution. Only a very small percentage of nonthermal particles are allowed. These particles have a negligable effect on temperature sensitive line ratios typically observed in the EUV (Extreme Ultraviolet Radiation).
Falconer, D. A.↗