(Solar Phys.). The evolution of the X-ray emission from hot solar plasmas for which radiative cooling is insignificant is computed on the basis of a simple conduction model. This model considers a hot and a cool region separated by a sharp boundary. The plasma is assumed to be confined within a vertical column of constant cross-sectional area with the hot X-ray emitting region lying above the cool region, and both being in hydrostatic equilibrium. Because the electron temperature is high and thermal conductivity varies at T5/2 the hot region remains approximately isothermal during the cooling process. Cooling then proceeds by raising the temperature of the cool material at the interface thereby moving the boundary into the cool region. The emission measure, E.M. ≡ ∫N2edV, of the hot volume therefore increases as the cooling progresses. For this model, the emission measure may be expressed as a function of temperature, T, by {E.M.} = {E.M.0} (T0/T) [(T0 – Tf)/(T – Tf)]2 where E.M.0 is the emission measure when the hot region is at temperature T0, and Tf is the temperature of the cool region. This expression is shown to satisfactorily describe observations of small fast-decaying flares. Larger, slower events tend to show a decreasing emission measure during late decay phases and cannot be fitted with this model.

(Solar Phys.). Calculations of the gyro-synchrotron emission are made for conditions which might be expected in moving type IV sources in the solar corona. Two simple models for an evolving source are treated: a uniform cube and an inhomogeneous sphere. The results suggest that most moving sources have the following features: (1) A rather strong magnetic field, ≈ 10 G, is carried out within the source. This is required to achieve the high degree of circular polarization often observed. (2) Synchrotron self-absorption causes the source to be optically thick at frequencies less than about 100 MHz, thus restricting the bandwidth of the radiation. The self-absorption decreases as the source moves outward and expands. The turnover frequency, which separates the optically thick and thin spectral regimes, moves rapidly to lower frequencies, accompanied by a change from low to high circular polarization. In the case of an inhomogeneous source, the source appears to be larger at the lower frequencies. (3) Razin-Tsytovich suppression cannot be an important factor in determining the characteristics of most sources.

(Nature). Apparatus was flown aboard the French-British Concorde SST to obtain high resolution emission line intensity profiles during the 30 June 1973 total solar eclipse. A prime objective was to obtain profiles that could be used to determine the presence in the Fe xiv coronal emission line, 530.3 nm, of the 300 s periodicity such as is observed in the photosphere and chromosphere. The long totality duration of 74 min on the French Concorde 001 provided a unique opportunity for this study. At the 55000 ft cruise altitude weather influences are negligible, scattered light background reduced and seeing conditions excellent. The instrumentation included a high resolution pressure scanned Fabry-Perot interferometer and high resolution recordind. This instrumentation was perfected in earlier Los Alamos Scientific Laboratory solar eclipse flights since 1965 (Liebenberg, 1965, 1967; Hoffman et al., 1970). A 12 cm aperture window and 7.7 cm aperture f/13 telescope were designed to optimize the use of the available space as shown in Figure 1.

The Skylab Apollo Telescope Mount contains six principal instruments spanning the X-ray to visible wavelength range. These experiments include an externally occulted white light coronagraph from the High Altitude Observatory which observes the outer solar corona from 1.5 to 6.0 radii from Sun center, in broadband (3500–7000 Å) white light with approximately 10″ spatial resolution. An X-ray spectrographic telescope of the American Science and Engineering, Inc. employs six filters and an objective grating to observe a 48′ field of view in the wavelength range 3.5–6.0 Å, with approximately 3″ spatial resolution. A scanning ultraviolet polychromator, spectroheliometer of the Harvard College Observatory is capable of observing 1.2 Å spectral resolution from 300–1350 Å with a 7 detector array. The field of view of the instrument is determined by its operational mode and ranges from 5′ × 5′ to 5″ × 5″. An X-ray telescope from the Marshall Space Flight Center employs five metal filters to observe the 5–33 Å spectral region with 2.5″ spatial resolution over the 40′ field of view. The Naval Research Laboratory has supplied two instruments: the first, an XUV spectroheliograph, covers wavelength regions 150–335 Å and 321–625 Å, with somewhat better than 5″ spatial resolution and a spectral resolution of 0.13 Å (for a 10″ feature). The second instrument, a slit spectrograph, covers the spectral range 970–3940 Å in two bands, 970–1970 and 1940–3940, and has 0.5 and 0.1 Å spectral resolution respectively, with a 2″ × 60″ slit. Additionally, these principal experiments are supported by two Hα telescopes and a broadband (150–600 Å) ultraviolet monitor for astronaut use. All experiments except that of the Harvard College Observatory (which utilizes photoelectric detectors) employ film to be recovered and installed during astronaut extravehicular activity. Operating in concert in a joint observing program designed to obtain observations of certain solar phenomena, the experiments have now completed more than two months of manned operation and, in the case of the AS&E, HCO and HAO instruments, approximately two additional months of unmanned operations. Representative preliminary results are outlined from several of the experiments below.

The Naval Research Laboratory prepared three instruments for ATM and also a series of rocket payloads for purposes of calibration, under the overall direction of R. Tousey, the Principal Investigator. In this preliminary report a summary of results from operations during the first mission (SL/2) and sample results are presented.

Most of the atomic species originating in the solar atmosphere between the upper chromosphere and the corona have their strong characteristic wavelengths in the extreme ultraviolet region of the spectrum. A simple normal-incidence spectrometer system with solar blind detectors such as the Harvard instrument operating between approximately 250 Å and 1350 Å is ideally suited for observing in this most interesting range of the solar atmosphere where the temperature rises outward from 104 to 3 × 106 K. The temperature range represented by the various atomic and ionic species in the extreme ultraviolet is associated with many types of solar structure, prominences and filaments, the supergranulation cells and network, active regions and their associated loop structures and other features. Simultaneous observations in lines of different characteristic temperatures provide a three-dimensional probe of the solar atmosphere. In the instrument, the principal polychromatic position observes the Lyman continuum, Lα, C II, C III, O IV, O VI, and Mg x with seven detectors simultaneously from the same spatial image element, 5″ in size. Approximately 60 additional polychromatic positions are used routinely to carry out specific observing programs, for example, covering several lines of a given stage of ionization, observing lines or continuum from specific species of interest such as helium in prominences, comparing combinations of lines from a given ionic species such as O v where the relative intensities give a rather direct measurement of the density at a given temperature, or measuring differing positions in the Lyman continuum providing intensity measurements which can be interpreted in terms of the departure from ionization equilibrium.

Data obtained by the AS&E X-ray Telescope Experiment during the first Skylab mission have revealed a variety of temporal changes in both the form and brightness of coronal structures. Dynamical changes have been noted in active regions, in large scale coronal structures, and in coronal bright points. The coronal activity accompanying a series of Hα flares and prominence activity between 0800 and 1600 UT on 10 June 1973 in active region 137 (NOAA) at the east limb is shown in Figure 1. It is characterized by increases in the brightness and temperature of active region loops and a dramatic change in the shape and brightness of a loop structure. Figure 2 shows the reconfiguration of an apparent polar crown filament cavity between 1923 UT on 12 June 1973 and 1537 UT on 13 June 1973. A ridge of emitting material which attains a peak brightness at least four times that of the surrounding coronal structures appears within the cavity during the course of the event. Typical X-ray photographs with filters passing relatively soft X-ray wavelengths (3–32, 44–54 Å) show 90 to 100 X-ray bright points (Vaiana et al., 1973). On twelve occasions in the data from the first mission, such bright points were seen to increase in intensity by two orders of magnitude in less than 4 min. Such an event is shown in Figure 3.

The coronagraph has obtained observations, both by astronaut and ground command, spaced periodically throughout the mission on a time center of approximately six to eight hours. Programs of more frequent operation – to examine short-term temporal variations and transient activity in the corona – have also been run at various times during the mission. The initial coronagraph observations of structures near limb passage experimentally verify that there are several time scales on which visual changes in these structures occur: (a) approximately one-half rotation, presumably accompanying major reorientations of coronal magnetic fields governing large scale coronal structures, (b) hours to days wherein changes to smaller coronal features are due either to structural changes of particular coronal features or to perspective effects and (c) less than hours – during coronal transients – which caused major reorientation of coronal structures by their passage through the coronal medium. Observations of the latter phenomena have provided some of the more spectacular results from the coronagraph (see Figure 1). The particular case illustrated was one stage of a complex series of events associated with activity in region 137, National Oceanic and Atmospheric Administration numbering, near east limb passage on 10 June 1973. The limb exhibited a major system of active prominences extended from the equator to north 25° and produced surges and sprays through the early hours of the day. From approximately 0700 to 0900 UT several Hα and small X-ray flares occurred and 0815 an eruptive prominence was observed by ground-based telescopes to ascend to 1.4 R⊙. This latter event is presumably the source of the transient observed by the coronagraph between 0929 and 1001 by ground command. During this period the material front moved with the apparent projected velocity of 450 km s−1 from 3.6 to 4.8 R⊙, while the arch-like structure expanded to a diameter greater than 2 R⊙. Figure 1 shows the event at 0943 GMT; it is one photograph of the 144 obtained. Within the structure are numerous bright areas and extended structures apparently mapping the distended magnetic field configuration. The general appearance of this transient is indicative of a large magnetic loop, or bottle, expanded outward from the Sun, its leading edge compressed by interaction with the ambient corona. Although there apparently was not a metric radio burst associated with this event, other transients observed later in the mission have associated metric wavelength activity.