R. H. Munro

The association of coronal mass ejection transients with other forms of solar 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 speeds of coronal mass ejection events

The outward speeds of mass ejection events observed with the white light coronagraph experiment on Skylab varied over a range extending from less than 100 km s−1 to greater than 1200 km s−1. For all events the average speed within the field of view of the experiment (1.75 to 6 solar radii) was 470 km s−1. Typically, flare associated events (Importance 1 or greater) traveled faster (775 km s−1) than events associated with eruptive prominences (330 km s−1); no flare associated event had a speed less than 360 km s−1, and only one eruptive prominence associated event had a speed greater than 600 km s−1. Speeds versus height profiles for a limited number of events indicate that the leading edges of the ejecta move outward with constant or increasing speeds.Metric wavelength type II and IV radio bursts are associated only with events moving faster than about 400 km s−1; all but two events moving faster than 500 km −1 produced either a type II or IV radio burst or both. This suggests that the characteristic speed with which MHD signals propagate in the lower (1.1 to 3 solar radii) corona, where metric wavelength bursts are generated, is about 400 to 500 km s−1. The fact that the fastest mass ejection events are almost always associated with flares and with metric wavelength type II and IV radio bursts explains why major shock wave disturbances in the solar wind at 1 AU are most often associated with these forms of solar activity rather than with eruptive prominences.

Frequency of coronal transients and solar activity

The High Altitude Observatorys white light coronagraph aboard Skylab observed some 110 coronal transients - rapid changes in appearance of the corona - during its 227 days of operation. The longitudes of the origins of these transients were not distributed uniformly around the solar surface (51 of the 100 events observed in seven solar rotations arose from a single quadrant of longitude). Further, the frequency of transient production from each segment of the solar surface was well correlated with the sunspot number and Ca ii plage (area × brightness) index in the segment, rotation by rotation. This correlation implies that transients occur more often above strong photospheric and chromospheric magnetic fields, that is, in regions where the coronal magnetic field is stronger and, perhaps, more variable. This pattern of occurrence is consistent with our belief that the forces propelling transient material outward are, primarily, magnetic. A quantitative relation between transient production from an area and the Zürich sunspot number appropriate to that area is derived, and we speculate that the relation is independent of phase in the solar activity cycle. If true, the Sun may give rise to as many as 100 white light coronal transients per month at solar cycle maximum.

Direct observations of a flare related coronal and solar wind disturbance

Numerous mass ejections from the Sun have been detected with orbiting coronagraphs. Here for the first time we document and discuss the direct association of a coronagraph observed mass ejection, which followed a 2B flare, with a large interplanetary shock wave disturbance observed at 1 AU. Estimates of the mass (2.4 × 1016 g) and energy content (1.1 × 1032 erg) of the coronal disturbance are in reasonably good agreement with estimates of the mass and energy content of the solar wind disturbance at 1 AU. The energy estimates as well as the transit time of the disturbance are also in good agreement with numerical models of shock wave propagation in the solar wind.

The large coronal transient of 10 June 1973

During the 8.5 month flight of the High Altitude Observatorys white light coronagraph on board Skylab, over 100 coronal transients were observed. In this paper we present a description of one well observed loop transient, that of 10 June 1973. The transient apparently resulted from the eruption of a quiescent prominence on the limb; the emergence of a new, bipolar active region near the prominence may have caused the eruptior. The transients leading edge rose from 3.6 to 5.0 solar radii (R⊙) from Sun center at approximately 500 km s−1 during the 32 min of coronagraph observations. Material in a pre-existing streamer was swept away by the transient, causing the streamer to disappear. The mass ejected into the corona above a projected height of 2 R⊙ was ≈ 5.4 × 1015 g, the potential energy associated with the ejected transient material was ⩾7.0 × 1030 erg, and the kinetic energy of the ejected material is estimated as 1.7 × 1030 erg. The 10 June 1973 transient was, in most respects, typical of other loop transients observed by Skylab.

White-light coronal transients observed from Skylab May 1973 to February 1974: a classification by apparent morphology

The apparent morphologies of the major coronal mass ejection transients observed during the Skylab mission with the High Altitude Observatorys white-light coronagraph are described and illustrated. The 77 major events are grouped into classes referred to as Loop, Filled Bottle, Material Injected into Streamer, Ray, Cloud, and Streamer Separation events, with 14 being Unclassifiable because of incomplete observations. One example of each class is shown, with references to others described in the literature. A chronological listing of all the events is given.

The High Altitude Observatory White Light Coronagraph

Most of the instruments of the Apollo Telescope Mount are satellite-borne because they observe in regions of the electromag-netic spectrum where the telluric atmosphere is opaque. For the coronagraph of the High Altitude Observatory, observing in visible light, this is not so. The structure of the solar corona is obscured from ground-based observations by scattered light in the earths atmosphere, and observations from space are required to reduce this scattered light to a level which is negligible with respect to the brightness of the outer solar corona.

Radiance calibration of the High Altitude Observatory white-light coronagraph on Skylab.

The High Altitude Observatory coronagraph produced over 35,000 photographs of the solar corona over 9 mo. Images were obtained on Kodak special film type 026-02, a 3400 (Pan-X) emulsion modified by Kodak for minimal reciprocity losses at the exposure times (3 sec, 9 sec, 27 sec) used on the coronagraph. The film was processed in a specially developed chemistry to obtain the best compromise between speed, low fog, and modulation transfer function. The calibration of a nineteen-step wedge within the coronagraph is based on previously calibrated glass opal filters. The step wedge, illuminated by attenuated sunlight, is imaged on each photograph made by the instrument. Data reduction procedures employ average characteristic curves for each data set (approximately 4000 frames). It is found that the effects of radiation fog and latent image loss are negligible within these sets. The relative coronal radiance error, determined by measuring the coronal plus stray light radiance over the solar pole, is found to be le s than 8%. Based on an estimated error of 15% in the absolute calibration of the step wedge, the net absolute accuracy of a given radiance measurement is estimated to be 20%.

Initial Results from the High Altitude Observatory White Light Coronagraph on Skylab - A Progress Report

The frequent, periodic observations by the white light coronagraph allow an examination of coronal variations over a broad range of temporal scales. Examples of the slowest and most rapid variations are presented. An example of extremely slow coronal variations is the gradual evolution-to a large equatorial stream er-in association with a marked decrease in solar activity, as the total magnetic flux in one hemisphere decreased. Another example is given of a long-lived quasi-stable coronal streamer, apparently associated with a stable filament channel; comparison of this streamer with coronal potential magnetic field computations show little correlation. The remainder of the paper summarizes some results on coronal transients - the most rapid variations observed. Characteristic mass and energies involved in mass ejection transients, their temporal and spatial distributions, their associations with surface phenomena and possible interplanetary signatures, and finally their role in coronal evolution are briefly noted.

Mass flow and the validity of ionization equilibrium on the Sun

Ionization equilibrium is a useful assumption which allows temperatures and other plasma properties to be deduced from spectral observations. Inherent to this assumption is the premise that the ion stage densities are determined solely by atomic processes which are local functions of the plasma temperature and electron density. However, if the time scale of plasma flow through a temperature gradient is less than the characteristic time scale for an important atomic process, deviations from the ionization stage densities expected for equilibrium will occur which could introduce serious errors into subsequent analyses. In the past few years, significant flow velocities in the upper solar atmosphere have been inferred from observations of emission lines originaing in the transition region (about 104–106 K) and corona. In this paper, three models of the solar atmosphere (quiet Sun, coronal hole, and a network model) are examined to determine if the emission expected from these model atmospheres could be produced from equilibrium ion populations when steady flows of several kilometers per second are assumed. If the flows are quasi-periodic instead of steady, spatial and temporal averaging inherent in the observations may allow for the construction of satisfactory models based on the assumption of ionization equilibrium. Representative emission lines are analysed for the following ions: C iii, iv, O iv, v, vi, Ne vii, viii, Mg ix, x, Si xii, and Fe ix–xiv. Two principle conclusions are drawn. First, only the iron ions are generally in equilibrium for steady flows of 20 km s−1. For carbon and oxygen, ionization equilibrium is not a valid assumption for steady flows as small as 1 km s−1. Second, the three models representing different solar conditions behave in a qualitatively similar manner, implying that these results are not particularly model dependent over the range of temperature gradients and electron densities thus far inferred for the Sun. In view of the flow velocities which have been reported for the Sun, our results strongly suggest caution in using the assumption of ionization equilibrium for interpreting spectral lines produced in the transition region.