We have investigated the three-dimensional distribution of the polarization-brightness product (pB) and then quantitatively determined the electron density distribution relative to the inferred heliographic current sheet during the declining phase of solar cycle 20 (1973-1976). The current sheet is taken as the center of the bright, dense structures from combined synoptic pB data from ground-based K-coronameter and the white-light coronagraph aboard Skylab. Analyses of pB scans as a function of minimum distance from the current sheet (theta(min)) over the radial distance range 1.13 to 5.0 R. (from Sun center) led to the following new results: (1) a quantitative description of pB obtained around the inferred neutral line is given by the following equation:pB(rho, theta(min)) = pB(p)(rho) + [pB(cs)(rho) - pB(p)(rho)]e(-theta min2/w2(r)),where rho is the shortest distance to the line of sight from the Sun center, pB(cs)(rho) and pB(p)(rho) are the observed polarized brightness at the current sheet and the poles, respectively, and w(r) is the half-width of the distribution; (2) the electron density obtained by inverting the pB data is given byN(r, theta(mg)) = N-p(r) + [N-cs(r) - N-p(r)]e(-theta mg2/w2(r)),where N(r, theta(mg)) is the number of free electrons per cm(3), N-cs(r) and N-p(r) are the electron densities at the current sheet and the poles, respectively, and theta(mg) is the magnetic latitude. Here theta(mg) is given bytheta mg = sin(-1) [-cos theta sin alpha sin (phi -phi(o)) + sin theta cis alpha],where theta and phi are heliographic latitude and longitude, alpha is the tilt angle of the dipole axis with the rotation axis, and phi(o) is the intersecion of the heliomagnetic and heliographic equators; (3) during the period studied (the last third of the solar cycle), the mean pB at the current sheet and above the polar holes is approximately independent of the phase of the solar cycle; and (4) the organization of pB data about the neutral line allows inference of the boundary of the polar coronal holes.The usefulness of one-dimensional white-light density constraint in solar wind modeling has already been demonstrated by Habbal et al. The present three-dimensional model should prove very useful in better understanding of the global hydromagnetic structure of the corona and the solar wind, relating as it does to the magnetic structure of the corona, as opposed to heliocentric coordinates. For example, the density model could provide constraints on coronal temperature, flow velocity, and magnetic structure subject to a suitable analysis of geometric effects, which in turn would provide constraints on energy balance in the coronal expansion.

The coronal transient event of 20–21 November is unusual in that its appearance is distinctly non-loop-like; rather, the transient resembles a confined ray or fan-like volume. Studies of the distribution of the coronal material with time indicate that this is a mass ejection event, involving about 1 × 1015 g of material from the lower corona. Analysis of the polarization signal of the event suggests that the event is associated with chromospheric activity in a region near longitude E68. The observed properties (distributions in brightness and polarization) of the transient are compared with the properties of a well-studied event of typical loop-like appearance, but rotated to simulate an ‘edge-on’ appearance; the differences suggest that the 20–21 November event is not such an edge-on, loop-like transient, but rather is most simply described as an axisymmetric-cylindrical or conical volume, the boundaries of which remain constant over the events' lifetime. On this basis, the variation of the transient spatial density with height and the variation of density with time can be specified rather more certainly than for previously-studied coronal mass ejection events. Densities are found to range from 3 × 10−16 g cm−3 at 2.1 R⊙ heliocentric height early in the event to 1 × 10−18 g cm−3 at 4.0 R⊙ late in the event. Typical temporal variations of the ejected material (at a given heliocentric height) are found to be on the order of 10−18 g cm−3 s−1. The mass and momentum balance in the event have been estimated from the observed parameters, employing a multiparameter approach. We find that a model with modest mass flux typified by material speed u0 ≲ 50 km s−1 and a near balance between the event's pressure gradient force and gravity — with possibly a small hydromagnetic wave contribution to the total pressure — is consistent with the observations. The kinetic energy of the event, determined from the motion of the center of mass of the ejected material, is only about 1026 ergs, and thus is the smallest for any solar mass ejection studied to date.

Some recent progress in understanding the nature of the evolution of the global coronal magnetic field is reviewed. Particularly, the efforts of Hoeksema (1984) in defining the character of the evolving modes of the potential coronal field are compared with the currently-known evidence for evolution of coronal white light structures. Recent work in examining the soft x-ray intensity of coronal holes over a major portion of the solar cycle is noted, as are two new studies investigating the relation of coronal mass ejections to the ambient global coronal magnetic field.

Measurements of the direction of propagation of 29 coronal mass ejection events observed during the Skylab epoch (1973–1974) and 19 events observed during the SMM epoch (1980) reveal that the former undergo an average 2.2° equatorward deflection, while the latter do not deviate significantly from radial motion. No differences between eruptive prominence‐associated or flare‐associated events can be detected for either epoch. The results suggest that coronal mass ejection events are influenced by the background coronal magnetic and flow patterns; the nonradial forces affecting the Skylab epoch mass ejections arise from the large‐scale dipolar magnetic field and flow configuration present at that time.

Journal of Geophysical Research: Space PhysicsVolume 90, Issue A1 p. 559-561 CommentariesFree Access Comments on “Density distribution in looplike coronal transients: A comparison of observations and a theoretical model” by D. G. Sime, R. M. MacQueen, and A. J. Hundhausen M. Dryer, M. DryerSearch for more papers by this authorS. T. Wu, S. T. WuSearch for more papers by this author M. Dryer, M. DryerSearch for more papers by this authorS. T. Wu, S. T. WuSearch for more papers by this author First published: 1 January 1985 https://doi.org/10.1029/JA090iA01p00559Citations: 16AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article. References Bird, M. K., H. Volland, B. L. Seidel, C. T. Stelzried, The crosssectional magnetic profile of a coronal transient, Solar and Interplanetary Dynamics, IAU Symp., 91 M. Dryer, E. Tandberg-Hanssen, 475– 482, D. Reidel, Hingham, Mass., 1980. Dryer, M., Coronal transient phenomena, Space Sci. Rev., 33, 233, 1982. Dryer, M., A. Maxwell, Radio data and a theoretical model for the fast-mode MHD shock wave generated by the solar flare of 1973 September 5, 18: 26 UT, Astrophys. J., 231, 945, 1979. Dryer, M., S. T. Wu, R. S. Steinolfson, R. M. Wilson, Magnetohydrodynamic models of coronal transients in the meridional plane, II, Simulation of the coronal transient of 1973 August 21, Astrophys. J., 227, 1059, 1979. Howard, R. A., D. J. Michels, N. R. Sheeley Jr., M. J. Koomen, The observation of a coronal transient directed at earth, Astrophys. J., 263, L101, 1982. Howard, R. A., N. R. Sheeley Jr., M. J. Koomen, D. J. Michels, Coronal mass ejections: 1979–1981, J. Geophys. Res., 1984. MacQueen, R. M., R. Fisher, The kinematics of solar inner coronal transients, Sol. Phys., 89, 89, 1983. Michels, D. J., R. A. Howard, N. R. Sheeley Jr., M. J. Koomen, Evidence for directivity of coronal transients, Proceedings of STIP Workshop on Solar/Interplanetary Intervals M. A. Shea, D. F. Smart, S. M. P. McKenna-Lawlor, 319– 330, BookCrafters, Chelsea, Mich., 1984. Mouschovias, T. C., A. I. Pol, Expansion and broadening of coronal loop transients: A theoretical explanation, Astrophys. J., 220, 675, 1978. Nakagawa, Y., S. T. Wu, S. M. Han, Magnetohydrodynamics of atmospheric transients, I, Basic results of two-dimensional plane analyses, Astrophys. J., 219, 314, 1978. Sheeley Jr., N. R., R. A. Howard, M. J. Koomen, R. Schwenn, K. H. Muhlhauser, H. Rosenbauer, Associations Between coronal mass ejections and interplanetary shocks, Solar Wind Five, NASA Conf Publ., 2280, 693– 702, 1983. Sheeley Jr., N. R., R. A. Howard, M. J. Koomen, D. J. Michels, R. Schwenn, K. H. Muhlhauser, H. Rosenbauer, Cororial mass ejections and interplanetary shocks, J. Geophys. Res., 1985. Sime, D. G., R. M. MacQueen, A. J. Hundhausen, Density distribution in looplike coronal transients: A comparison of observations and a theoretical model, J. Geophys. Res., 89, 2113, 1984. Steinolfson, R. S., Coronal loop transients in streamer configurations, Astron. Astrophys., 155, 39, 1982. Wu, S. T., M. Dryer, Y. Nakagawa, S. M. Han, Magnetohydrodynamics of atmospheric transients, II, Two-dimensional numerical results for a model solar corona, Astrophys. J., 219, 324, 1978. Wu, S. T., Y. Nakagawa, S. M. Han, M. Dryer, Magnetohydrodynamics of atmospheric transients, IV, Nonplane two-dimensional analysis of energy conversion and magnetic field evolution, Astrophys. J., 262, 369, 1982. Wu, S. T., S. Wang, M. Dryer, A. I. Poland, D. G. Sime, C. J. Wolfson, L. E. Orwig, A. Maxwell, Magnetohydrodynamic simulation of the coronal transient associated with the solar limb flare of 1980, June 19, 18:21 UT, Sol. Phys., 85, 351, 1983. Citing Literature Volume90, IssueA11 January 1985Pages 559-561 ReferencesRelatedInformation

view Abstract Citations (8) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Broadening of looplike solar coronal transients MacQueen, R. M. ; Cole, D. M. Abstract The broadening of the tops of nine outwardly expanding looplike coronal mass-ejection transients observed during 1973-1974 and 1980 has been examined. Five of the nine transients exhibit increased breadth with time (height); although the rates vary widely, no event shows an increase which can be fitted by a power-law form with a greater-than-linear increase with height. The breadth of four events decreases with height. When all nine events are considered together, the transient breadth with height is fitted with an expression h varies as R exp 0.48 with large scatter. In addition, measurements of the total observed loop length of the nine transients show that this length L varies as R exp 1.30. Both of these results are contrary to the predictions of one model of loop expulsion as a result of an assumed azimuthal magnetic field gradient. Finally, when combined with the facts that looplike mass ejections are usually associated with the occurrence of eruptive prominences, and that such mass ejections are generally accelerated through the corona, it is found that the observed variation of transient breadth with height contradicts some characteristics of self-similar fluid flow which have been ascribed to mass-ejection transients. Publication: The Astrophysical Journal Pub Date: December 1985 DOI: 10.1086/163720 Bibcode: 1985ApJ...299..526M Keywords: Coronal Loops; Solar Corona; Stellar Mass Ejection; Transient Response; Compression Waves; Magnetohydrodynamic Flow; Skylab Program; Solar Magnetic Field; Solar Maximum Mission; Solar Physics full text sources ADS |

The fact that eruptive-prominence associated coronal mass ejection events may be accelerated over significant heights and times in the corona complicates the determination of possible surface or low coronal associations. A specific example of one such eruptive-prominence associated event, that observed in both the inner and outer solar corona on August 5, 1980, is used to illustrate the magnitude of the uncertainty of determining an onset time of the ejection. It is noted that such uncertainties may influence statistically-determined associations.

Examination of the intensity changes in five outer coronal “looplike” transients observed by the Skylab coronagraph shows general tendencies for (1) greatest concentration of material at the flanks rather than at the tops of the bright loops that characterize these transients, (2) presence of a large region of depleted density within these loops, and (3) development of bright legs that contain most of the material in the transient and that display very little lateral motion as the top of the bright loop moves radially outward through the outer corona. These properties of looplike coronal transients provide useful constraints on theoretical models of this phenomenon. In particular, direct comparison of the observed density distributions with those predicted by models of compressional waves initiated by an impulsive energy release in the low corona are a necessary test of these models. These models predict a maximum enhancement at the top of the loop, rather than at the flanks and “legs” that move laterally with a significant fraction of the propagation speed of the loop top, in conflict with the observed tendencies. If the observed loops are taken to have the geometry assumed in the compressive wave models (“toroidal symmetry” about a rotation axis passing through the center of the sun), the predicted density enhancements are several times larger than those inferred from the observations. Agreement cannot be achieved without use of a geometry which conflicts with that used for the model calculations.

The kinematic properties of a dozen ‘loop-like’ coronal transients have been examined over the range 1.2–2.4 R⊙ from Sun center. Values and trends of transient geometry, including radial height, lateral width at maximum extent, distance from loop top to height of maximum width, and lateral width at a fixed height above the instrument occulting disk at 1.2 R⊙, are given. Radial and lateral speeds of expansion are tabulated, and range from 60–900 km s-1, and 10–500 km s-1, respectively. Flare-associated events are found to exhibit the highest speeds, and show little acceleration with height; on the other hand, eruptiveassociated events exhibit large accelerations (some in excess of 50 m s-2). This clear discrimination between flare and eruptive-associated events suggests that two different physical processes are present; it is suggested that flare-associated events result from an impulsive, localized input to the corona. On the other hand, accelerated, eruptive-associated events are subjected to appreciable net forces over radial heights of one solar radius (or more) above the solar limb. It is conjectured that the pressure gradient forces responsible for the generation of the solar wind may play an important role in accelerating these latter events.

Observations of the outer solar corona obtained by the High Altitude Observatory's coronagraph aboard Skylab reveal the presence of dark, ray-like structures in the corona. A systematic identification of these voids, which exist for periods of about 24 hr, is presented and their existence as a coronal phenomenon, as opposed to a subtle photographic effect, verified. Photometric analysis indicates that these features represent reductions in the coronal radiance on the order of 5% - or about 2–3 × 10−10B⊙ at 3 R⊙. The use of a previously determined model of the electron component of the corona permits specification of the electron density in the voids over the range 2.5–4.5 R⊙. In spite of the inevitable uncertainties regarding their longitudinal extent, we estimate that their electron density is comparable to, or less than, that in coronal holes at similar heights. Projection of the phenomena onto synoptic surface maps indicates a close relationship with filaments and neutral lines; a potentially significant temporal correlation between the void formation and that of the underlying prominence is noted. The spatial and temporal resolution of the data set places stringent restrictions on any model which may be used to infer the physical processes of formation or decay of voids; several possibilities are suggested which involve either changes in the coronal base temperature or the magnetic flux.

The Harvard-Smithsonian Center for Astrophysics and the High Altitude Observatory have defined a joint coronagraphs experiment for a future Spacelab mission. The instrumentation package would include an ultraviolet light coronagraph to measure the intensity and profiles of spectral lines formed between 1.2 and 8 solar radii from Sun center and a white light coronagraph to measure the intensity and polarization of visible light. The overall goals of the joint program are to use new coronal plasma diagnostic techniques to understand the physical processes and mechanisms operating in the solar corona, to understand the acceleration of high-speed and low-speed solar wind streams and to extrapolate this knowledge to other stars in order to help understand the physics of stellar coronae and stellar mass loss.

view Abstract Citations (49) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Measurements of coronal kinetic temperatures from 1.5 to 3 solar radii Kohl, J. L. ; Weiser, H. ; Withbroe, G. L. ; Noyes, R. W. ; Parkinson, W. H. ; Reeves, E. M. ; Munro, R. H. ; MacQueen, R. M. Abstract A rocket-borne Lyman-alpha coronagraph has been used to make the first measurements of the spectral line profile of resonantly scattered hydrogen Lyman-alpha coronal radiation between 1.5 and 3 solar radii. These data provide, for the first time, direct measurements of coronal temperatures above 1.5 solar radii. Data were obtained in a coronal hole, quiet region, and streamer. The widths of the profiles from the quiet region decrease with height and correspond to a steady decrease in hydrogen kinetic temperature, with increasing radius, from about 2.5 million K at r = 2 solar radii to about 1 million K at r = 9 solar radii. In the coronal hole the measured line widths indicate a kinetic temperature of 1.8 million K at r = 2.5 solar radii. Publication: The Astrophysical Journal Pub Date: October 1980 DOI: 10.1086/183373 Bibcode: 1980ApJ...241L.117K Keywords: Coronal Holes; Solar Corona; Solar Temperature; Solar Wind; Coronagraphs; Lyman Alpha Radiation; Rocket-Borne Instruments; Solar Spectra; Spectral Line Width; Solar Physics full text sources ADS |

Observations with orbiting coronagraphs have illuminated the role of coronal mass ejections in solar activity, and raised a number of questions concerning their origin, the nature of the forces driving the coronal material, and their signature in interplanetary space. Current models of the ejection process - including propagation of loops as a result of azimuthal field gradients, ring currents or a build-up of magnetic pressure from below - are summarized, as are magnetohydrodynamic codes intended to stimulate transient conditions. Metric radio observations, can, in principle, distinguish the relative role of the magnetic field in the ejection process; observations to date are surveyed. It is concluded that at present, no compelling evidence is available to distinguish between transient driving mechanisms, but future observations of the corona and interplanetary medium may resolve the present ambiguity.

The High Altitude Observatory Coronagraph/Polarimeter, to be flown on the National Aeronautics and Space Administration's Solar Maximum Mission satellite, is designed to produce images of the solar corona in seven wavelength bands in the visible spectral range. The spectral bands have been chosen to specifically exclude or include ‘chromospheric’ spectral lines, so as to allow discrimination between ejecta at high (coronal) and low (chromospheric) temperatures, respectively. In addition, the instrument features spectral filters designed to permit an accurate color separation of the F and K coronal components, and a narrow band (5.5 Å) filter to observe the radiance and polarization of the Fe xiv 5303 Å line. The effective system resolution is better than 10 arc sec and the instrument images a selected quadrant (or smaller field) on an SEC vidicon detector. The total height range that may be recorded encompasses 1.6 to more than 6.0R⊙ (from Sun center). The instrument is pointed independently of the SMM spacecraft, and its functions are controlled through the use of a program resident within the onboard spacecraft computer. Major experimental goals include: (a) Observation of the role of the corona in the flare process and of the ejecta from the flare site and the overlying corona; (b) the study of the direction of magnetic fields in stable coronal forms, and, perhaps, ejecta; and (c) examination of the evolution of the solar corona near the period of solar maximum activity.

Coronal mass ejection transients observed with the white light coronagraph on Skylab are found to be associated with several other forms of solar activity. There is a strong correlation between such mass ejection transients and chromospheric Hα activity, with three-quarters of the transients apparently originating in or near active regions. We infer that 40% of transients are associated with flares, 50% are associated with eruptive prominences solely (without flares), and more than 70% are associated with eruptive prominences or filament disappearances (with or without flares). Nine of ten flares which displayed apparent mass ejections of Hα-emitting material from the flare site could be associated with coronal transients. Within each class of activity, the more energetic events are more likely to be associated with an observable mass ejection.

The coronal structure overlying, and presumably associated with, an isolated active region, McMath 12686, is identified during its west limb passage 15–17 January 1974. The region had a ‘flaring site’ on its border close to a plage filament and exhibited various forms of activity, including type III burst production, during the three days of study. Although the coronal structure overlying the region was of small scale, its estimated electron density was ~10 × that of the background corona, and it varied in density by a factor of two over a time scale of hours. Some implications of such a structure on the interpretation of type III emission are considered.