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We compare the appearance and physical parameters of the solar chromosphere, transition zone, and corona in areas of coronal holes with that of quiet areas outside the hole. Measurements of the height of emission of various ions in a coronal hole appearing at the polar limb give a quantitative indication of the increased thickness of the transition zone underlying coronal holes.
Soft X-ray observations of coronal holes are analyzed to determine the structure, temporal evolution, and rotational properties of those features as well as possible mechanisms which may account for their almost rigid rotational characteristics. It is shown that coronal holes are open features with a divergent magnetic-field configuration resulting from a particular large-scale magnetic-field topology. They are apparently formed when the successive emergence and dispersion of active-region fields produce a swath of unipolar field founded by fields of opposite polarity, and they die when large-scale field patterns emerge which significantly distort the original field configuration. Two types of holes are described (compact and elongated), and three possible rotation mechanisms are considered: a rigidly rotating subphotospheric phenomenon, a linking of high and low latitudes by closed field lines, and an interaction between moving coronal material and open field lines.
An atlas of coronal hole observations (Patrol Long and Single Frame Long Filter 3) recorded by the Skylab Apollo Telescope Mount/SO56 X-ray Telescope during the first two manned missions is presented. A total of 279 operations (excluding super long frames) were determined. Comparisons are made between coronal hole observations performed in the first manned mission and those in the second manned mission, and between a super long filter 3 image and a typical single frame (or Patrol) long image. Additional studies to enhance the S056 coronal hole observations and perhaps to extend coverage into the last manned mission are suggested. The data presented are in preliminary form.
The large-scale structure of the corona and the interplanetary medium during Carrington rotations 1601-1607 is discussed relative to recurrent high-speed solar wind streams and their coronal sources. Only streams A, C, D, and F recur on more than one rotation. Streams A and D are associated with coronal holes, while C and F originate in the high corona (20-50 solar radii) over faint X-ray emissions. The association of the streams with holes is confirmed by earlier findings that there are no large equatorial holes without an associated high-speed stream and that the area of the equatorial region of coronal holes is highly correlated with the maximum velocity observed in the associated stream near 1 AU.
Emission-line spectra of a coronal hole that coincided with the north pole of the sun are discussed which were obtained with a slit spectrograph aboard Skylab at positions within and above the solar white-light limb in the wavelength range from 1175 to 1940 A. Relative line intensities, line profiles, and full widths at half-maximum are presented for selected chromospheric and transition-zone lines observed above the present polar coronal hole. Average mass motions in the transition zone are determined as a function of electron temperature from the widths of the optically thin lines by assuming ionization equilibrium. The line intensities and profiles are compared with corresponding results deduced from spectra obtained above a quiet solar region. The coronal-hole spectra are found to imply an angular dependence for the source function as well as a radial dependence such that the source function is the smallest at the south pole and increases with decreasing solar latitude.
Interplanetary scintillation observations of the solar wind velocity during 1973 and the first part of 1974 reveal several corotating high-speed streams. These streams, of heliographic latitudes from +40 deg to -60 deg, have been mapped back to the vicinity of the sun and have been compared with coronal holes identified in wide band XUV solar images taken during the manned portions of the Skylab mission. There is some evidence that the high-speed streams are preferentially associated with coronal holes and that they can spread out from the hole boundaries up to about 20 deg in latitude. However, this association is not one to one; streams are observed which do not map back to coronal holes, and holes are observed which do not lie at the base of streams. To the extent that a statistical interpretation is possible the association is not highly significant, but individual consideration of streams and holes suggests that the statistical result is biased somewhat against a strong correlation.
Observations of coronal holes, solar-wind streams, and geomagnetic disturbances during 1973-1976 are compared in a 27-day pictorial format which shows their long-term evolution. The results leave little doubt that coronal holes are related to the high-speed streams and their associated recurrent geomagnetic disturbances. In particular, these observations strongly support the hypothesis that coronal holes are the solar origin of the high-speed streams observed in the solar wind near the ecliptic plane.
Coronal holes, regions of unusually low density and low temperature in the solar corona, are identified as Bartel's M regions, i.e., sources of high-speed wind streams that produce recurrent geomagnetic variations. Throughout the Skylab period the polar caps of the sun were coronal holes, and at lower latitudes the most persistent and recurrent holes were equatorial extensions of the polar caps. The holes rotated 'rigidly' at the equatorial synodic rate. They formed in regions of unipolar photospheric magnetic field, and their internal magnetic fields diverged rapidly with increasing distance from the sun. The geometry of the magnetic field in the inner corona seems to control both the physical properties of the holes and the global distribution of high-speed wind streams in the heliosphere. Phenomenological models for the birth and decay of coronal holes have been proposed.
Solar active regions observed during the period of Skylab observations (May 1973-February 1974) were examined for properties that varied systematically with location on the sun, particularly with respect to the location of coronal holes. Approximately 90 percent of the optical and X-ray flare activity occurred in one solar hemisphere (136-315 heliographic degrees longitude). Active regions within 20 heliographic degrees of coronal holes were below average in lifetimes, flare production, and magnetic complexity. Histograms of solar flares as a function of solar longitude were aligned with H alpha synoptic charts on which active region serial numbers and coronal hole boundaries were added.
Using soft X-ray data from the S-054 X-ray spectrographic telescope aboard Skylab, we observed temporal changes in the emission structure of the X-ray corona associated with the birth of a small coronal hole. Designated as CH6, this coronal hole was born near the equator in a time interval less than 9-1/2 hr. By constructing a light curve for a point near the center of CH6, we observed a sudden 40% decrease in X-ray emission associated with the birth of this coronal hole. On a time scale of hours, the growth of CH6 in area proceeded faster than the average rate predicted by the diffusion of solar fields. The short term decay of CH6 followed the diffusive rate to within experimental uncertainty. On a time scale of one rotation, the subsequent development of CH6 was not consistent with steady growth at the average rate predicted by diffusion.
This paper deals with general inferences about the low-density phase of interstellar gas having temperatures well above 200,000 K (the 'coronal gas') which can be drawn from O VI absorption data for 72 stars. Attention is given to the behavior of radial velocities, possible evidence for circumstellar O VI, the space distribution of the O VI gas, crowding of normal interstellar gas, and temperature distributions for the coronal gas. A model is adopted in which the coronal gas is contained within randomly distributed and nonoverlapping parcels, each with a size, pressure, and internal temperature distribution that do not vary markedly from one unit to the next. It is shown that the one-dimensional velocity dispersion for O VI regions (26 km/s) is substantially higher than the value for ordinary interstellar clouds (6.4 km/s).
Consequences are examined of the assumption that an observed coronal loop transient is a twisted rope of magnetic-field lines expanding and broadening in the background coronal plasma and magnetic field. It is shown that the expansion can be accounted for by the azimuthal component of the field; the observed broadening of the loop as it moves outward can be accounted for by the longitudinal component of the field. In order to have a net outward force and at the same time avoid a classical pinch (sausage) instability, the two components of the field must satisfy a certain inequality. It is predicted that, as the loop rises, the width (h) of its top portion should vary proportionally with distance (R) from the sun's center. This is in good agreement with measurements that show h is proportional to the 0.8 power of R. The prediction that the radius of curvature of the top portion of the loop should be proportional to R differs from the measured variation. The difference could be accounted for by a drag due to the background coronal field that flattens the loop's top.
Space-borne X-ray and XUV observations during the period 1963-74 (corresponding approximately with solar cycle 20) have been used to investigate the relationship between the occurrence and variability of coronal holes and solar activity, i.e., the solar wind. Results indicate that polar holes (prominent at solar minimum) decreased in area as solar activity increased. The equatorial holes were also small during this period and persisted for one or two solar rotations only. Solar wind streams in excess of 500 km/s were associated with coronal holes at less than 40 deg latitude, although solar coronal holes appear to have no associated wind streams at earth.
The evolution of coronal hole boundary positions on a time scale of approximately 1 day is studied on the basis of an examination of all coronal holes observed by Skylab from May to November 1973. It is found that a substantial fraction (an average of 38%) of all coronal hole boundaries shifted by at least 1 deg heliocentric in the course of a day. Most (70%) of these changes were on a relatively small scale (less than 3 times the supergranulation cell size), but a significant fraction occurred as discrete events on a much larger scale. The large-scale shifts in the boundary locations involved changes in X-ray emission from these areas of the sun. There were generally more changes in the boundaries of the most rapidly evolving holes, but no simple relationship between the amount of change and the rate of hole growth or decay.
A first-order mass- and energy-balance model is developed for steady-state EUV 'coronal rain' loops that are not associated with postflare events and are often seen over sunspot umbrae. The model disregards variations in a loop's thermodynamic and magnetic properties along magnetic-field lines and yields average properties of the resonant absorption sheath, the boundary layer, and the loop's interior. Both irreversible heating by Alfvenic surface waves and energy transport via induced boundary-layer convection are taken into account in the analysis. Results that include predictions of the temperatures of the sheath and the boundary-layer plasma, the temperature of the interior plasma, the radiative output of the loop, and the filling factor associated with this radiation are given in terms of the period and velocity amplitude of the relevant surface wave as well as various parameters that characterize a typical 'coronal rain' loop. These results are shown to be consistent with the observed minimum and maximum temperatures as well as the radiative output of typical 'coronal rain' loops.
Emission measure curves are presented for average coronal hole and quiet-sun spectra taken during the Skylab mission by Vernazza and Reeves (1978), and the curves are used to discuss the energy balance in each region. Close-coupling calculations are used for the Be sequence, assuming a 10 level ion; for B sequence ions mainly distorted wave calculations in an 11 level ion are used, but close-coupling cross sections are used for some ions; for C and Mg sequence ions, distorted wave calculations are used with 15 and 10 level ions, respectively, and close-coupling results are used for Li-like ions with two levels. Results are presented and include the following: the coronal hole spectrum shows a smaller slope in the emission measure distribution, consistent with the expected outflow effects. It is concluded that the simple constant pressure models of static coronal loops of constant cross section are basically able to match the observed emission measure distribution of the average quiet sun between 1,000,000 and 10,000,000 K. However, the cell center and network distributions are respectively steeper and shallower than predicted by the detailed cooling curve.
A search was conducted for evidence of a coronal region at the base of the winds of Epsilon Ori and Kappa Ori, by means of high signal-to-noise observations at the forbidden lines of Fe X, at 6574 A, and Fe XIV, 5303 A. Both stars have been detected as soft X-ray sources, and show anomalously strong O VI lines in their UV spectra. Large coronal emission measures were expected from the total X-ray flux and Auger-enhanced ionization, but the fact that the iron coronal lines were not detected places new limits on the emission measure if the total temperature is in the range of 700,000-3,000,000 or more than 1,000,000 for Kappa Ori and 2,000,000 for Epsilon Ori. It is suggested that at least some of the X-rays arise, not from the base corona, but from source features farther out in the wind.
A nonthermal mechanism for driving coronal hole mass loss is proposed. Three empirical results are noted, namely (1) that polar plumes with short-lived X-ray bright points (XBP) at their base are sites of matter flow sufficient to account for the total solar mass flux, (2) that solar wind densities are positively correlated with the number of XBP in coronal holes, and (3) that XBP are associated with newly emerged magnetic flux. It is noted that since the dynamical development following the onset of nonequilibrium is a hitherto unsolved problem, subsequent features in the scenario described here are relatively speculative. It is proposed that bubbles of matter ejected from magnetic reconnection sites in polar plumes drive the solar wind in coronal holes.