Coronal Hole Regions Research Articles

Effects of High Ion Temperatures on Spectral Line Diagnostics in the Source Region of the High‐Speed Solar Wind

Theoretical work on solar wind acceleration has suggested that the proton and electron temperatures at the base of the solar coronal hole region are not thermally equilibrated but that instead the proton temperature may exceed the electron temperature significantly. Recently, Kohl, Strachan, & Gardner have measured both broad and narrow components of the H Lyα profile and have suggested that the broad component may be the signature of high proton temperatures, ~4-6 × 106 K or more. Since proton impact excitation can contribute to the emission line excitation processes, high proton temperatures have important implications for some spectral line emissivities that are useful for electron temperature and density determinations. The diagnostics most affected are those which are sensitive to the distribution of population within the fine structure of the ground state configuration. We discuss selected case studies. For modest proton temperatures (Tp = 3Te) we have found effects on temperature diagnostics to be less than 30% but effects on some density diagnostics may be as large as 1 order of magnitude.

The Structure and Properties of Solar Active Regions and Quiet‐Sun Areas Observed in Soft X‐Rays withYohkoh/SXT and in the Extreme‐Ultraviolet with SERTS

We observed two solar active regions (NOAA regions 7563 and 7565), quiet-Sun areas, and a coronal hole region simultaneously with Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph (SERTS) and with the Yohkoh Soft X-ray Telescope (SXT) on 1993 August 17. SERTS provided spatially resolved active region and quiet-Sun slit spectra in the 280 to 420 A wavelength range, and images in the lines of He II λ303.8, Mg IX λ368.1, Fe XV λ284.1, and Fe XVI λλ335.4 and 360.8 SXT provided images through multiple broadband filters in both the full-frame imaging mode and the partial-frame imaging mode. The SERTS images in Fe XV (log Tmax = 6.33, where Tmax is the temperature which maximizes the fractional ion abundance in the available ionization equilibrium calculations, i.e., the formation temperature) and Fe XVI (log Tmax = 6.43) exhibit remarkable morphological similarity to the SXT images. Whereas the Fe XV and XVI images outline the loop structures seen with SXT, the cooler He II (log Tmax = 4.67) and Mg IX (log Tmax = 5.98) images outline loop footpoints. In addition, the Mg IX emission outlines other structures not necessarily associated with the hot loops; these may be cool (T 1 × 106 K) loops. From the spatially resolved slit spectra, we obtained emission-line profiles for lines of He II λ303.8, Mg IX λ368.1, Fe XIII λ348.2, Si XI λ303.3, Fe XIV λ334.2, Fe XV λ284.1, and Fe XVI λ335.4 for each spatial position. Based upon the spatial variations of the line intensities, active region 7563 systematically narrows when viewed with successively hotter lines, and appears narrowest in the broadband soft X-ray emission. The active region width (full width at half-maximum intensity) diminishes linearly with log Tmax; the linear fit yields an extrapolated effective log Tmax of 6.51 ± 0.01 for the X-ray emission. The most intense, central core straddles the magnetic neutral line. Active region and quiet-Sun one-dimensional temperature scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines, and from coregistered SXT filter ratios. The highest plasma temperatures were measured in the most intense, central core of region 7563. The temperatures derived from Fe XVI λ335.4/Fe XV λ284.1 and Fe XVI λ335.4/Fe XIV λ334.2 vary significantly (based upon the measurement uncertainties) but not greatly (factors of less than 1.5) across the slit. The average log T values derived from the above two ratios for region 7563 are 6.39 ± 0.04 and 6.32 ± 0.02, respectively. Somewhat larger systematic variations were obtained from all available SXT filter ratios. The average active region log T values derived from the SXT AlMgMn/thin Al, thick Al/thin Al, and thick Al/AlMgMn filter ratios are 6.33 ± 0.03, 6.45 ± 0.02, and 6.49 ± 0.03, respectively. Active region and quiet-Sun one-dimensional density scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines of Fe XIII and Fe XIV. The derived densities show neither systematic nor significant variations along the slit in either the active region or the quiet-Sun, despite the fact that the intensities themselves vary substantially. This indicates that the product of the volume filling factor and the path length (fΔl) must be greater by factors of 3-5 in the active region core than in the outskirts. Furthermore, the derived active region densities are ~2 times the quiet-Sun densities. This density difference is adequate to explain the factor of ~4 intensity difference in Fe XII and Fe XIII between the active and quiet areas, but it is not adequate to explain the factor of ~8 intensity difference in Fe XIV between the active and quiet areas. We attribute the latter to a greater fΔl in the active regions. Statistically significant Doppler shifts are not detected in region 7563 or in the quiet-Sun with any of the EUV lines.

First results of the SUMER telescope and spectrometer on SOHO: II. Imagery and data management

SUMER – Solar Ultraviolet Measurements of Emitted Radiation – is not only an extreme ultraviolet (EUV) spectrometer capable of obtaining detailed spectra in the range from 500 to 1610 A, but, using the telescope mechanisms, it also provides monochromatic images over the full solar disk and beyond, into the corona, with high spatial resolution. We report on some aspects of the observation programmes that have already led us to a new view of many aspects of the Sun, including quiet Sun, chromospheric and transition region network, coronal hole, polar plume, prominence and active region studies. After an introduction, where we compare the SUMER imaging capabilities to previous experiments in our wavelength range, we describe the results of tests performed in order to characterize and optimize the telescope under operational conditions. We find the spatial resolution to be 1.2 arc sec across the slit and 2 arc sec (2 detector pixels) along the slit. Resolution and sensitivity are adequate to provide details on the structure, physical properties, and evolution of several solar features which we then present. Finally some information is given on the data availability and the data management system.

White-Light Polarization and Large-Scale Coronal Structures

The results of the white-light polarization measurements performed during three solar eclipses (1973, 1980, 1991) are presented. The eclipse images were processed and analysed by the same technique and method and, consequently, the distributions of the polarization and coronal intensity around the Sun were obtained in unified form for all three solar eclipses. The mutual comparisons of our results, and their comparison with the distributions found by other authors, allowed the real accuracy of the current measurements of the white-light corona polarization, which is not worse than ±5%, to be estimated. We have investigated the behaviour of the polarization in dependence on heliocentric distance in helmet streamers and coronal holes. Simultaneous interpretation of the data on polarization and intensity in white-light helmet streamers is only possible if a considerable concentration of coronal matter (plasma) towards the plane of the sky is assumed. The values obtained for the coronal hole regions can be understood within the framework of a spherically symmetrical model of the low density solar atmosphere. A tendency towards increasing polarization in coronal holes, connected with the decrease of the hole's size and with the transition from the minimum to the maximum of the solar cycle, was noticed. The problem of how the peculiarities of the large-scale coronal structures are related to the orientation of the global (dipole) solar magnetic field and to the degree of the goffer character of the coronal and interplanetary current sheet is discussed briefly.

A high-temperature component in coronal holes observed with Yohkoh SXT

Temperatures of coronal holes are estimated from several sets of soft X-ray images taken through various broad-band filters with the Soft X-ray Telescope (SXT) aboard Yohkoh. The effect of scattered X-rays from bright regions surrounding a temperature determination area, especially those from nearby active regions, is carefully removed with the point spread function derived from the post-launch data. An isothermal approximation is applied to thus corrected data. The temperatures of coronal holes near the disk center are found to be 1.8 – 2.4 × 10 6 K, which is almost the same as those derived for quiet regions. The emission measures in coronal holes are estimated to be 10 25.5–26.2 cm −5, about ten times smaller than in quiet regions. We conclude that temperatures in coronal holes do not differ from those in quiet regions, and that the depression in soft X-ray intensity of coronal hole regions results from a lower density by a factor of 3 than quiet regions. We propose that the coronal hole component observed with the SXT is not the same one which is observed with the Skylab EUV instrument. An X-ray intensity from a coronal hole is independently confirmed by the eclipse observation on 1993 November 13, and consistent with intensities derived from the scattering correction.

Alfvén wave heating of coronal holes and the relation to the high‐speed solar wind

Coronal hole regions are well‐known sources of high‐speed solar wind; however, to account for the observed properties of the solar wind, a source of energy must be included in addition to heat conduction. Alfvén waves were suggested as the possible source of heating that accelerates the solar wind. We investigate the heating and propagation of the fast and shear Alfvén waves in coronal holes via numerical solution of the time‐dependent, linearized, resistive, low‐β, two‐dimensional MHD equations in slab geometry. The waves are driven at the lower boundary of the coronal hole and propagate into the corona. We find that fast waves are partially reflected at the coronal hole boundary and significant part of the wave energy leaks out of the coronal hole. We compare the calculated wavelengths and the attenuation rate of the fast waves in the leaky waveguide formed by the coronal hole with the analytical ideal MHD solutions for ky = 0, where ky is the perpendicular wavenumber, and find an excellent agreement. When ky ≠ 0 the fast waves couple to the shear Alfvén waves and transfer energy across field lines. Resonance heating layers are found to occur when shear Alfvén waves are driven and a continuous density profile is assumed for the coronal hole. When resonance absorption is considered, the leakage is small compared to the heating rate. The heating is enhanced by phase mixing when coronal hole inhomogeneities (i.e., plumes) are included. We investigate the dependence of the heating rate on the driver frequency and the Lundquist number S and find a good agreement with the analytical S1/3 scaling of the dissipation length. We find that when S = 104 the low‐frequency Alfvén waves can be a significant source of heating of coronal holes at several solar radii. At larger values of S, nonlinear effects might reduce the effective dissipation length. We discuss the relation of our results to the observed properties of high‐speed solar wind and coronal holes.

Physical Properties of Polar Coronal Rays and Holes as Observed with the Spartan 201-01 Coronagraph

Physical conditions and characteristics of polar coronal rays and polar coronal holes are derived from white-light coronal observations aboard the Spartan 201-01 spacecraft and the ground-based K-coronameter in Mauna Loa, Hawaii, on 1993 April 11-12. An array of polar rays extending from 1.16 to 5.0 R☉ was observed in both the north and south polar coronal hole regions. They appear as coherent structures at much higher altitudes than previously observed. Densities and scale height temperatures are estimated as a function of radial height for the holes and the rays. These profiles suggest that there is extended heating up to heights of 1.4-2.6 R☉.

On the occurrence rate of high-speed solar wind events

We have investigated the rate of occurrence of solar wind phenomena observed between 1972–1984 using power-spectrum analysis. The data have been taken from the high-speed solar wind (HSSW) stream catalogue published by Mavromichalaki, Vassilaki, and Marmatsouri (1988). The power-spectrum analysis of HSSW events indicates that HSSW stream events have a periodicity of 9 days. This periodicity of HSSW events is\(\frac{1}{3}\) of the 27-day period of coronal holes, which are major sources of solar wind events. In our opinion, the 9-day period may be the energy build-up time for coronal hole regions to produce the HSSW stream events.

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.

Elemental abundances in the upper solar atmosphere of quiet and coronal hole regions (Te is approximately equal to 4.3 X 10 exp 5 K)

view Abstract Citations (63) References (34) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Elemental Abundances in the Upper Solar Atmosphere of Quiet and Coronal Hole Regions (T E 4.3 X 10 5 K) Feldman, U. ; Widing, K. G. Abstract Detailed examination of the sun's upper atmosphere reveals that elemental abundances vary between different solar regions. Some solar regions exhibit the well-established photospheric abundances, while in other regions the abundances of some elements diverge from photospheric by an order of magnitude, and perhaps even more. Elemental abundances in the solar wind (SW) are also different from elemental abundances in the photosphere. It is postulated that elemental abundances in the SW reflect the elemental abundances of their place of origin near the solar surface. The solar surface is covered, predominantly, by quiet and coronal hole regions. In this paper we measure the intensities of limb-brightening rings over quiet and coronal hole regions in lines of Mg VI and Ne VI (Te is approximately equal to 4.3 x 10 exp 5 K) and determine the relative elemental abundances of the features producing the rings. The rings are formed by a plasma component occupying only a small fraction of the immediate volume above the solar surface. Publication: The Astrophysical Journal Pub Date: September 1993 DOI: 10.1086/173084 Bibcode: 1993ApJ...414..381F Keywords: Abundance; Coronal Holes; Solar Atmosphere; Solar Corona; Solar Wind; Energetic Particles; Ionization Potentials; Limb Brightening; Photosphere; Solar Physics; SUN: ABUNDANCES; SUN: CORONA; SUN: SOLAR WIND full text sources ADS |

The source regions of solar coronal mass ejections

Knowledge of the origin of the solar coronal mass ejection (CME) may be crucial to our understanding of several active solar phenomena, such as flares, as well as to the structure and stability of the corona and the prediction of interplanetary disturbances. In recent years, two camps of opinion have emerged, based on the belief that CMEs either commonly originate from structures intimately linked to active regions or they originate from coronal hole regions. This present study investigates the locations of 95 CME events observed during 1984–1986 relative to coronal hole and active region features. We find no evidence to support the coronal hole hypothesis and many indications that active regions are indeed associated with the source regions of CMEs.

Spectral characteristics of HeI 10830 A line in the quiet chromosphere

We describe preliminary results of the observation of the HeI 10830 AD line in the quiet chromosphere in 1986 and 1987 with the newly installed Reticon system on the solar spectrograph of Purple Mountain Observatory. We found: (1) The HeI 10830 A line has an emission maximum outside the solar limb. The maximum intensity in 1987 was definitely greater than the maximum in 1986. (2) The maximum intensity was roughly proportional to the line half-width. (3) The line half-width in coronal hole regions wasclearly smaller than in non-hole regions and was independent of the emission intensity. (4) The line half-width before the emission maximum is basically a constant, mostly with a tendency of increasing outward, the height at which the change begins depends on the optical thickness. (6) The emission maximum only correlates positively with the monthly smoothed sunspot number and there is no trace of it correlating with the 1–8 A X-ray flux or the intensity of the coronal green line above the observed point. We also give preliminary results of fitting our calculated theoretical profiles to the observed profile. The uniform model cannot give a good fit; a simple two-layer model not only greatly improved the fit, it also gave a reasonable absorption profile for solar disk observation. There seems no need for assuming a 20 km/s horizontal turbulence field, as many authors have done.

Population mechanisms for the He+n = 3 levels determined from measurements of the solar 1640 Å emission

The 1640 A line profile is analyzed using solar data obtained by the NRL slit spectrograph on Skylab. Data from coronal hole regions, quiet sun regions, active regions, prominence regions, and flare regions are presented. The relative densities of 3s, 3p, and 3d levels are determined from the data, and the dominant population mechanisms for these levels are inferred using a rate equation model for the He(+) ion. The relative importance of collisional excitation, radiative recombination, and photoexcitation is determined for each of the solar regions.

Temporal evolution of the solar wind and the formation of a standing shock

The temporal evolution of the solar wind from one steady state to another is explored when momentum deposition produces multiple critical points in the flow. We show that the wind always evolves in time to a new steady state compatible with the solution of the steady state equation of motion. However, for the same initial state and identical asymptotic momentum deposition rate the temporal evolution pattern of the wind depends on the detailed time history of momentum addition and is therefore not unique. This feature plays an important role in the particular case when multiple (three in this study) steady states exist for identical boundary conditions; each one of these solutions is thus shown to be physically accessible. The details of the temporal evolution pattern of the wind reveal the formation of a shock discontinuity whenever the flow becomes supersonic at a critical point upstream from the initial critical point. If the flow remains supersonic at that inner critical point, the shock can become a standing one, depending on the strength and the temporal history of momentum addition. The results of this study indicate that the time scale required for the solar wind to evolve between steady states is of the order of 30–60 hours. Furthermore, the results also reveal the interesting and novel phenomenon that a standing shock is likely to develop in the inner solar wind flow within this time frame, in particular, in coronal hole regions with rapidly diverging geometries.

HRTS observations of spicular emission at transition region temperatures above the solar limb

Slit spectra and spectroheliograph observations were obtained during the fourth rocket flight of the High Resolution Telescope and Spectrograph (HRTS) on 7 March 1983. A curved slit 900 arc sec in length was placed at the solar limb on the western edge of the south polar coronal hole, giving both coronal hole and quiet region coverage. In addition, spectroheliograph images tuned to cover a passband around 1550 Å (primarily C IV at 10 5 K) were taken over an 8 × 15 arc min field. Simultaneous Hα images were obtained at Sac Peak Observatory. The C IV spectroheliograms show general spiked emission above the limb, and also several small loo- or prominence-like events. Slit spectra along the tops of several of these structures show tilted features which could be interpreted as rotational velocities of approximately 50 km s −1.

Fast-mode magnetohydrodynamic waves in coronal holes and the solar wind

Fast-mode MHD waves in the solar corona can propagate in any direction relative to the background magnetic field. In coronal holes, they refract into regions of low Alfven speed and are relatively difficult to damp. These characteristics lead to the possibility that fast-mode waves transport energy from magnetically closed coronal regions into coronal holes, that they are refracted into the central regions of coronal holes, and that they deposit most of their energy in the region of supersonic flow of high-speed solar wind streams emanating from coronal holes. To investigate whether this possibility might be realized and fast-mode waves might play a significant role in driving high-speed streams, a parameter study is carried out to examine the propagation and damping of fast-mode waves in various coronal hole models. This study indicates a broad range of coronal hole parameters for which fast-mode waves can play such a role and emphasizes the need for an improved knowledge of large-scale coronal magnetic structure, which is required before any firm conclusions can be drawn.

Why are spicules absent over plages and long under coronal holes?

One-dimensional hydrodynamic simulations are performed in order to examine the influence of initial atmospheric structures on the dynamics of spicules. This is an extended version of our previous spicule theory: spicules are produced by the shock wave (MHD slow mode shock) which originates from a bright point appearance (sudden pressure increase) at the network in the photosphere or in the low chromosphere. Simulation results well reproduce the observational facts that spicules are absent over plages and long under coronal holes. The physical reason is that the growth of a shock wave during its propagation through the chromosphere is small in plage regions and large in coronal hole regions, since the growth of a shock is determined by the density ratio (ϱ h 0/ϱ c ) between the bright point and the corona. An empirical formula ΔH max ∼ (ϱ h 0/ϱ c )0.46 is obtained, where ΔH max is the maximum height of spicules above the transition region. The cross-section of the vertical magnetic flux tube is assumed to be constant in the numerical simulations.

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.

The energy budget in and out of coronal holes

Energy is transmitted into, out of and across the solar atmosphere by such physical processes as radiation, thermal conduction, waves and so forth. Estimates can be made of the magnitudes of some of these and hence an energy budget drawn up for different regions of the atmosphere. In particular it appears that the energy requirements of coronal hole regions of the solar surface are substantially greater than those of other regions. The main factor determining this conclusion is an estimate of the energy required to accelerate the solar wind in regions above coronal holes. Of two possible mechanisms for the acceleration of the solar wind, namely thermal plasma pressure and Alfvén waves, it may be shown that the former places even more severe demands on the energy budget of these regions, whereas the latter is more easily accommodated.

Emission-line spectra of two active regions on the solar limb - 1175-1940 A

Skylab emission-line spectra (1175 to 1940 A) of two active regions on the solar limb are examined. Electron densities for the active regions are derived using the intensity ratios of selected intersystem and allowed lines. The intensity ratios in the active regions are compared with those observed previously in quiet-sun and coronal-hole regions. The behavior of coronal forbidden lines relative to transition-zone lines is discussed along with the solar continuum intensities near and above the limb. These continuum intensities are then compared with those at similar slit positions for the quiet sun and the previously studied coronal hole. The active-region data are found to be consistent with either (1) multithermal loops with the high-temperature plasma occupying the tops of the loops or (2) nearly isothermal loops with the hotter loops extending to higher altitudes.