R. M. MacQueen

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.

The kinematics of solar inner coronal transients

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.

The High Altitude Observatory Coronagraph/Polarimeter on the Solar Maximum Mission

The High Altitude Observatory Coronagraph/Polarimeter, to be flown on the National Aeronautics and Space Administrations 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 transients: A summary

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.

New Mauna Loa coronagraph systems.

A new set of instruments, consisting of two coronagraph systems, has been installed and is operating at the Mauna Loa Observing Station, Hawaii, operated by the High Altitude Observatory of Boulder, Colorado. The instruments are the 23-cm objective Mark III K-coronameter (K-III) system, a photoelectric instrument used to observe the inner solar corona from 1.2 R(0) to 2.2 R(0) and the 12.5-cm objective Prominence Monitor system used for the detection of H(alpha) limb activity. New features of the K-coronameter system include the use of achromatic wave plates for wide bandpass operation and linear diode array detectors. Raster scans of the coronal image are obtained in 1.5 min for a critical sampling scheme of 20-sec of arc resolution (10 x 10-sec of arc pixels) in the coronal p(B) image. This represents a 350 information gain factor for each detection channel when compared with the previous Mauna Loa K-coronameters.

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 and radio studies of the coronal transient of 14–15 September 1973

Observations of a coronal transient event were obtained in white light by the Skylab coronagraph and at metric wavelengths by the radioheliograph and spectrograph at Culgoora and the spectrograph-interferometer at Boulder. The continuum radio burst was found to originate above the outward-moving white light loop - a region of compressed material headed by a bow wave. The computed density in the region of radio emission, based on either gyro-synchrotron or harmonic plasma radiation mechanisms, was approximately 10 times the ambient coronal density; this is compatible with the density deduced from the white light observations. The magnetic energy density derived from the radio observations was greater than 10 times the thermal energy density, marginally larger than the kinetic energy density in the fastest moving portion of the transient, and considerably larger in most other regions. The ambient medium, the white light front, the compression region, the loop, and the slower, massive flow of material behind are each examined. It is found that the plasma was magnetically controlled throughout, and that magnetic forces provided the principal mechanism for acceleration of the transient material from the Sun.