Streamer Region Research Articles

Properties of Solar Polar Coronal Hole Plasmas Observed above the Limb

We determine the line-of-sight emission measure distribution and nonthermal motions as a function of height above the limb in the north and south polar coronal holes. These quantities are derived from extreme-ultraviolet (EUV) spectra obtained from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The SUMER slit was oriented along the north-south direction for all the observations, and the spatial resolution is about 1''. The spectra were obtained from a number of different types of observations in 1996. We select a group of emission lines for analysis for which, under the usual assumption of ionization equilibrium, the maximum emissivities span the temperature range from about 3 × 105 K up to about 1.1 × 106 K. We compare our results with recently published similar observations of a west limb quiet-Sun streamer region, with other coronal hole results based on SUMER spectra, and with earlier observations of the quiet Sun and coronal holes obtained from Skylab and rocket spectra. We find that the electron temperature in the polar holes increases with height above the limb, that the emission measure distribution of plasma located at line-of-sight heights less than about 60'' peaks at a temperature of about 9 × 105 K, and that nonthermal motions sometimes, but not always, increase slightly with height above the limb. When observed, these increases level off above the limb at about 120''. We speculate that the increases with height above the limb may be a manifestation of the fast solar wind. They may also be due to the reduction in transition region structures with increasing limb height. We also discuss wave heating as a cause of the line width increases.

Connecting the Sun and the solar wind: Source regions of the fast wind observed in interplanetary space

Highly sensitive radio occultation and white light measurements of path‐integrated density have shown that the solar corona comprises three distinct morphological regions, streamer, quiet Sun, and polar coronal hole, which except for the streamer region, extend radially into interplanetary space from 1.15 Rs to at least 30 Rs. In this paper we build on these results by comparing solar wind flow speeds observed at the same time as path‐integrated density. Flow speeds are inferred from the Doppler dimming of O VI lines with the ultraviolet coronagraph spectrometer on the Solar and Heliospheric Observatory, while the simultaneous polarized brightness measurements of path‐integrated density are from the High Altitude Observatory Mauna Loa Mk III K‐coronameter. The comparison of global flow speed and density observations in 1997 produces three new results. First, it shows, that the three distinct morphological regions, identified earlier in density measurements, are present in the latitudinal profile of the flow speed in the corona. In particular, the flow speed measurements provide evidence for the quiet Sun as an additional source of fast wind. Second, the comparison shows that flow speed and density are anticorrelated in the solar corona. Third, it demonstrates that the Mk III pB measurements can readily serve as a proxy for velocity distribution in the outer corona. The extensive Mk III data set that spans nearly two solar cycles therefore provides the framework of near‐Sun measurements with which connections with solar wind measurements in interplanetary space can be made. Specifically, we show that fast wind regions in the heliosphere, observed directly by Ulysses and Wind plasma measurements and remotely by Nagoya interplanetary scintillation measurements, map radially back to fast wind regions at the Sun identified by the Mk III data.

Extreme‐Ultraviolet Spectral Line Widths in Quiet‐Sun Coronal Plasmas at Distances of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $1.03\leq R_{\odot }\leq 1.45$ \end{document} along the Solar Equatorial Plane

We measure the full width at half-maximum intensity (FWHM) of extreme-ultarviolet (EUV) spectral line profiles as a function of height in the Sun's equatorial west limb streamer region using EUV spectra recorded by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The spectra were recorded on 1996 November 21 and 22, during a special roll maneuver in which the SUMER slit was oriented in the east-west direction. Because the SUMER spectrometer is stigmatic along the slit length, the spectra are spatially resolved in the east-west direction. The spectra we discuss cover an effective distance range outside the west limb from 1.03 to 1.45 R☉; the maximum spatial resolution is 1''. We select a group of emission lines for analysis that under the usual ionization equilibrium assumption have maximum fractional abundances at temperatures ranging from 3 × 105 up to 2 × 106 K. We determine nonthermal motions as a function of position in the streamer region from the FWHMs under the assumption that the ion temperature equals the electron temperature of the streamer plasma. The electron temperature was derived previously by Feldman and coworkers.

Properties of Quiet‐Sun Coronal Plasmas at Distances of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $1.03\leq R_{\odot }\leq 1.50$ \end{document} along the Solar Equatorial Plane

We determine the physical properties, i.e., electron temperature, density, line-of-sight emission measure, and element-abundance variation with height, in the Sun's equatorial west limb streamer region from extreme-ultraviolet (EUV) spectra recorded by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft. The spectra were recorded on 1996 November 21-22 during a special roll maneuver in which the SUMER slit was oriented in the east-west direction. Because the SUMER spectrometer is stigmatic along the slit length, the spectra are spatially resolved in the east-west direction. The spectra we discuss cover an effective distance range outside the west limb from 1.03 to 1.50 solar radii (R☉); the maximum spatial resolution is 1''. We select a group of emission lines for analysis that under the usual ionization equilibrium assumption span the temperature range from 3 × 105 up to 2 × 106 K. However, we note that above the limb a major fraction of a line intensity may arise at a temperature that is far from the temperature of maximum emitting efficiency in ionization equilibrium. In this paper we assume ionization equilibrium in deriving plasma parameters. Readers can redo our analysis without this assumption if desired, because the line intensities we present in this paper are simply the measured photon production rates in the lines. Assuming ionization equilibrium, we determine the electron temperature, electron density, line-of-sight emission measure, and abundance variation with height above the limb from the line intensities and line intensity ratios. The spatial resolution has allowed us to detect an apparent element-abundance variation as a function of height above the west limb that is strong evidence for gravitational settling of heavy elements.

Study of the latitudinal dependence of H I Lyman α and O VI emission in the solar corona: Evidence for the superradial geometry of the outflow in the polar coronal holes

We study the latitudinal distribution of the H I Lyman α and O VI (103.2 nm and 103.7 nm) line emission during the period of the Whole Sun Month campaign (August 10 to September 8, 1996) when the Sun was close to the minimum of its activity. The H I Lyman α and O VI line intensities appeared to be almost constant with latitude within the polar coronal holes and have abrupt increases toward the streamer region. We found that both north and south polar coronal holes had similar line intensities and line‐of‐sight velocities, as well as kinetic temperatures of H0 and O5+. The dependence of these parameters on latitude and radius is provided. We derived boundaries of the polar coronal holes based on the H I Lyman α and O VI line intensity distributions for several days during the Whole Sun Month campaign. We found that the polar coronal hole boundaries clearly have a superradial geometry with diverging factor ƒmax ranging from 6.0 to 7.5, and they are consistent with boundaries previously derived from the electron density distributions. We also found that, in general, they are not symmetric with respect to the heliographic poles, and their size and geometry change over periods of days. The H I Lyman α, O VI (103.2 nm), and the O VI (103.7 nm) line intensities showed similar boundaries within the uncertainties of our data. We modeled the latitudinal distribution of the H I Lyman α and O VI (103.2 nm and 103.7 nm) line intensities in the south polar coronal hole on August 17, 1996, assuming the coronal plasma outflow along either purely radial or nonradial flux tubes. A comparison of model predictions with the observed distributions shows evidence that the outflow velocity vectors follow nonradial intensity pattern.

Microstreamer dynamics during plasma remediation of NO using atmospheric pressure dielectric barrier discharges

Environmental concerns have resulted in the need for alternative energy efficient methods for the removal of oxides of nitrogen (NxOy) from atmospheric gas streams and among those techniques is plasma remediation. The devices typically used as plasma sources (corona and dielectric barrier discharges) operate in a filamentary mode which produces radicals and gas heating in narrow channels. Transport of radicals and products is therefore an important consideration in optimizing their efficiency. In this article we describe and present results for a radially dependent model of NxOy remediation in dielectric barrier discharges. We examine the consequences of localized energy deposition, and diffusive and advective radial transport, and determine how these spatial dependencies affect the energy efficiencies for NxOy removal. Local energy deposition in the streamer can produce high temperatures initiating advection and facilitating production of NO through N+O2→NO+O. Diffusion of NO into the streamer region from the bulk gas and advective transport of N outward into the bulk gas increase remediation of NxOy by channeling N into remediation, NO+N→N2+O, rather than radical reassociation, N+N+M→N2+M. Remediation efficiency generally decreases with increasing energy deposition due to the reassociation reaction and formation of NO. Transport of N out of and NO into the streamer region reduces this effect.

Propagation of the Shock Wave through the Magnetic Helmet-Type Region.

This paper presents the numerical simulation of the evolution of shock wave propagating through the magnetic helmet streamer region and further out along the current sheet. It was obtained by using a set of complete projected-characteristic boundary conditions at the inner (1.1 Rs) and outer (62 Rs) boundaries in the solarmeridian. Mainresults reveal that (1) the convection effect of the background wind speed on the propagation of shock wave is significant, especially in the range of ≤60 Rs; (2) a dip of flow speed is formed in the magnetic helmet region and after that, it is weakened in the current sheet region; (3) two high-speed belts appear near the two sides of the magnetic helmet when this is stretched outward; (4) a high-density ring displaces behind the shock surface and could be torn when it moves from the magnetic helmet into the current sheet region.

Numerical modeling of coronal mass ejections based on various pre-event model atmospheres

We examine how the initial state (pre-event corona) affects the numerical MHD simulation for a coronal mass ejection (CME). Earlier simulations based on a pre-event corona with a homogeneous density and temperature distribution at the lower boundary (i.e., solar surface) have been used to analyze the role of streamer properties in determining the characteristics of loop-like transients. The present paper extends these studies to show how a broader class of global coronal properties leads not only to different types of CMEs, but also modifies the adjacent quiet corona and/or coronal holes.We consider four pre-event coronal cases: (1) constant boundary conditions and a polytropic gas with γ = 1.05; (2) non-constant (latitude dependent) boundary conditions and a polytropic gas with γ = 1.05; (3) constant boundary conditions with a volumetric energy source and γ = 1.67; (4) non-constant (latitude dependent) boundary conditions with a volumetric energy source and γ = 1.67. In all models, the pre-event magnetic fields separate the corona into closed field regions (streamers) and open field regions. The CME's initiation is simulated by introducing at the base of the corona, within the streamer region, a standard pressure pulse and velocity change. Boundary values are determined using MHD characteristic theory.The simulations show how different CMEs, including loop-like transients, clouds and bright rays, might occur. There are significant new features in comparison to published results. We conclude that the pre-event corona is a crucial factor in dictating CMEs properties.

Dynamical evolution of a coronal streamer-bubble system

We present a self-consistent, time-dependent magnetohydrodynamic (MHD) description of a dipolar configuration helmet streamer in which a magnetic bubble structure has been introduced into the closed field region of the helmet streamer. A numerical model is used to study the magnetohydrodynamic evolution of this streamer-bubble system. We find two distinct behaviors characterized by the size of the magnetic bubble: (i) if the bubble's radius is less than or equal to 0.15R c (whereR c is the characteristic length of the helmet streamer), the numerical solution describes a quasi-equilibrium state, and this coronal streamer-bubble structure is stable; and (ii) if the bubble's radius is equal to or larger than 0.25R c the numerical solution describes a non-equilibrium state and, after a period of slow evolution, this coronal streamer bubble structure becomes unstable and breaks up dynamically. In the case of the dipolar configuration helmet streamer,R c is the solar radius. We identify the non-equilibrium solution as a loss of equilibrium of this coronal magnetic structure. Its subsequent dynamic development may be a candidate for some of the commonly observed coronal mass ejections (CMEs).

Spartan 201 white light coronagraph experiment

A white light coronagraph was launched into orbit aboard the space shuttle OV 103 (Discovery) on 7 April 1993. This device was one of two instruments included in the SPARTAN 201-1 payload, a completely autonomous sub-satellite deployed from the shuttle for a period of about 47 hours. The WLC system is an externally occulted coronagraph system which incorporates a rotating half-wave plate polarimeter, and the image data is used to infer the brightness, the polarized brightness and the degree of polarization of the white light emission from the solar corona. These data are in turn used to infer estimates of the K- and F-coronal brightnesses and density distributions. We shall present preliminary results of the electron density estimate in the coronal streamer and hole region and describe the methods employed.

Influence of humidity on the breakdown mechanism of medium length rod-plane gaps stressed by positive impulse voltages

To better understand the influence of atmospheric humidity on the breakdown mechanism of medium length rod-plane gaps when stressed by positive impulse voltages, emphasis was given to the initial prebreakdown phenomena (the first and second coronas) as well as the dark period. The salient events have been measured and assessed, especially the variation of their statistical distribution. Results were also taken and assessed to determine the influence of humidity on the breakdown probability curves, on the 50% breakdown voltages and on the distribution of the times to breakdown. During this investigation the main parameters, in addition to humidity, were the gap length, the geometry of the end of the rod and the front duration of the impulse voltage. Humidity was found to have a great influence on the breakdown mechanism, especially its streamer region. It influences not only the 50% breakdown voltage but also its standard deviation. It was also found to cause changes in the shape of the U-curves. The necessary correction for humidity was found to vary considerably with the front duration of the applied impulse.

Influence of gas mixture and primary ionization on the performance of limited streamer mode tubes

We report a study of the dependence of limited streamer mode operation on gas composition. Results are given for the plateau onset voltage, plateau length, charge versus voltage, charge spectra and pulse width for various fractions of (Ar, CO 2, pentane) and (Ar, isobutane). In addition, a series of argon-free strong quenching gas mixtures has been studied which have very attractive characteristics. Chamber lifetime tests for these are also reported. As part of a study of the nature of the limited streamer mode mechanism, the response to X-rays and minimum ionizing particles are compared and differences noted. The character of the primary ionization is found to have a clear effect on the chamber response even in the streamer region.

On the Interpretation of Weak Sources of Enhanced 80 MHz Emission in the Solar Corona

Recent two-dimensional observations of the quiet sun at 80 MHz by Sheridan showed a persistent low-intensity source (radio enhancement) whose daily motion was consistent with a source at radius 0.95 R® rotating with a synodic period of 27.3 d. In this paper we shall attempt to interpret this result in terms of the effects of refraction and opacity in model coronas. Comprehensive studies of the Sun at 169 MHz show that similar radio enhancements are associated with coronal streamers and that reasonable values for electron density and temperature within the coronal streamer allow the observations to be explained in terms of thermal radiation. Hence we shall consider only coronal models that possess streamer structure. We shall show that temperature enhancements within the streamer (‘thermal model’) do not lead to an apparent radius as small as that observed by Sheridan and that his observation may be better understood in terms of a non-thermal model for radiation from the streamer region.

A concentric ellipse multiple-arch system in the solar corona

A typical concentric ellipse multiple-arch system was observed in the solar corona during the February 4, 1962 eclipse in New Guinea. The following results have been obtained from analysis of a white-light photograph taken by N. Owaki (see Owaki and Saito, 1967a). (1) The arches are composed of four equidistant components, elliptical in shape, and almost concentric with a prominence at the common center of the ellipses. (2) The prominence and arch system appears to be the lower region of a helmet-shaped streamer. (3) The widths of the arches are observed to increase with height. (4) Analysis was made in the light of three models for the coronal structures that could lead to the observed arches: (a) rod-like concentrations of electrons; (b) tunnel-shaped elliptical shells of electrons; and (c) dome-like ellipsoidal shells of electrons. Electron densities are derived for the models, and the dome-like model is excluded as a possibility for arch systems exhibiting a coronal cavity. (5) The scale height in the arch-streamer region is found to be almost the same as that of the K-corona, suggesting equal temperatures, density distributions, etc. in each region. (6) There is a dark space (a coronal cavity) between the innermost arch and the prominence. The brightness of this cavity is 1/5 that of the adjacent arch. It is 3% brighter than the background corona of the arch-streamer system. (7) A comparison is made between the deficiency of electrons in the coronal cavity and the excess of electrons in the prominence. It is found that the ratio of the excess to the deficiency lies between 0.9 and 40. (8) A comparison between the electron efflux from the ‘leaky magnetic bottle’ possibly formed by rod-shaped coronal arches and the electron influx into those arches from the chromosphere leads us to the conclusion that the rod model is probably valid and that spicules appear to be an adequate supply for the electrons observed in the arches. The tunnel model may be valid, but in that case spicules are probably not the sources of the electrons observed in coronal arches.

Dynamical Properties of Stellar Coronas and Stellar Winds. III. The Dynamics of Coronal Streamers.

view Abstract Citations (86) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Dynamical Properties of Stellar Coronas and Stellar Winds. III. The Dynamics of Coronal Streamers. Parker, E. N. Abstract Observations suggest that the solar corona may perhaps consist of thin radial filaments and streamers, rather than bring an amoiphous atmosphere. The filaments and streamers presumably are confined by the magnetic field, along whose lines 9f force they lie. To investigate the consequences of a filamentary corona for the solar wind, the dynamics of an idealized cona composed of gaseous streamers, with vacuum between the streamers, is worked out neglecting solar rotation, etc. The pnndpal conclusions are that the streamers occupy most of space at large radial distance from the Sun, no matter how small a fraction they may occupy at the Sun; the velocity of the solar wind is not greatly affected by filamentary structure for a given coronal temperature distribution; the mean-wind density is not affected by more than a factor of 2. Mtogether, then, except for the presence of some thin interstreamer regions, there is no great effect of a filamentary structure on the mean values of the solar-wind velocity and density at large distance from the Sun. The possibility of instability of coronal streamers is looked into, and it is shown that, if the corona is composed principally of streamers, the Helmholtz instability may be expected to disorder and mix up the streamer and interstreamer regions beyond some distance 0 (1 a.u.) from the Sun, which might perhaps be the origin of some of the disorder observed by Mariner. More quantitative observational information of the solar corona and/or of the structure of the solar wind close to the Sun will have to be available before quantitative statements can be made concerning the role of streamers in the solar wind. Publication: The Astrophysical Journal Pub Date: February 1964 DOI: 10.1086/147795 Bibcode: 1964ApJ...139..690P full text sources ADS | Related Materials (4) Part 1: 1964ApJ...139...72P Part 2: 1964ApJ...139...93P Part 4: 1965ApJ...141.1463P Part 5: 1966ApJ...143...32P