In this paper we review some preliminary results from the Harvard College Observatory Extreme Ultraviolet Spectroheliometer on ATM that pertain to solar activity. The results reviewed here are described in more detail in other papers referred to in the text. In the following paragraphs we first describe the instrument and its capabilities, and then turn to results on active regions, sunspots, flares, EUV bright points, coronal holes, and prominences.

Babcock (1961) outlined the sequence of events which takes place in the Sun's atmosphere during a solar cycle. Magnetic field loops, having preferred directions, emerge from the solar surface and thereafter merge with neighbouring loops to produce more extended structures. Although flux tubes emerge with a strong E-W field component, having the field direction reversed from one side of the equator to the other, there is a tendency for the longer loops produced by merging to have a significant N-S alignment (Hansen et al., 1972).

An example of the overall view of the X-ray corona (nominal filter passband 2–32 Å and 44–54 Å) showing a coronal hole, filament activity, bright points and the large scale-scale loop structures, is shown in Figure 1. This is one of the 32000 X-ray images obtained with the AS & E X-ray telescope on Skylab. A comprehensive review describing the characteristics of the various features and their implications regarding the high velocity solar streams, evolution of magnetic fields in active regions, and sources of soft X-ray emission has been given by Vaiana et al. (1975). In the present summary we will only be concerned with the bright points. Studies of solar X-ray bright points, show that these features represent a distinct class of solar activity. Bright points appear first as a diffuse cloud of soft X-ray emission typically growing to 30″ in diameter, with growth rates of ∼1 km s−1. Several hours after the point first becomes visible a bright compact core forms, growing to 10″. The lifetime distribution of bright points follows a Poisson distribution with a mean of eight hours (see references). The points are distributed uniformly over the entire solar surface, with approximately 500 on the Sun at any time. Their occurrrence appears to be independent of major active regions, except for a visibility factor near high loop structures or a possible decrease in number in active region latitudes.

Since 1971 the Goddard X-ray and EUV spectroheliograph aboard OSO 7 has been measuring the spatial distribution and time-variations of localized temperature and density features in solar active centers and flares. In some cases the sizes, shapes, orientations and locations of emitting plasmas at temperatures ranging from 104 K (Hα) to as high as 2 × 107 K (Fe xxv) have now been measured simultaneously. Our observations of active regions are consistent with the coronal structure being made up of nested systems of arches with footpoints in areas of opposite magnetic polarity. Temperatures seem to increase for arches nearer the center and also towards the top of any given magnetic arch, the innermost loops having the highest temperature gradients. There is also some evidence for electric current flow along such loops. Radiative cooling is significant for the region's hot central core which therefore must be maintained by a more or less continuous injection of energy.

A nested arch structure is also indicated for XUV flares of the two-component type, which likewise may require continuous energy input since conduction cooling should otherwise be very rapid. Multiple spikes during the impulsive phase seem to represent the consecutive triggering of different sources within the region and may occur outside of any detectable pre-existing coronal feature. Comparison of spatial distributions at several wavelengths during various stages of flare events provides information on interactions between the wide range of atmospheric levels involved. We have evidence for polarization of about 20% in a number of X-ray bursts, continuing throughout the decay phase. At least for some flares, our measurements seem to contradict the model of electron beams being radially injected into the chromosphere.

The last few years have seen great progress in our understanding of X-ray spectra of solar active regions. This paper demonstrates both the usefulness and the limitations of the techniques, both scientific and instrumental, that have recently become available. Improvements in spectral resolution led to the discovery of weak satellite lines to helium-like ions; the quantitative theory for these lines is also discussed. The observed intensities of the Fe XVII lines are also investigated and found to be in agreement with calculations that allow for cascading processes.

This paper describes the analysis of soft X-ray spectra of solar active regions observed on a Skylark sounding rocket flight. The experiment was launched from Woomera, South Australia, at 0535 UT on 26th November 1973. The payload consisted of 3 plane scanning Bragg crystal spectrometers, covering the wavelength range 4 to 23 Å, and collimated to 3 arc min FWHM. The instrument was the same as that used by Parkinson (1972, 1973). The launch was part of a collaborative observing program with American Science & Engineering who obtained X-ray photographs of the Sun with the S0-54 ATM X-ray telescope, simultaneously with the Leicester observations. We report preliminary results for McMath region 12624 (S10 W28). This region was observed about 60 min after the peak of an importance – N, class CO flare.

X-ray spectra from a number of coronal active regions were obtained during ATM support rocket flights carried out by the Lockheed group on June 11 and December 19, 1973. Multi-grid collimators were used to provide fields of view of 40″ diameter and 90″ diameter for a number of scanning crystal spectrometers and a bent crystal spectrometer which employed a position sensitive proportional counter to register the diffracted spectrum. A solar image was produced on film and on a TV camera on board the rocket with the aid of a 1 Å Hα filter. A small part of the X-ray collimator was used to generate a multiple spot diffraction pattern which was superimposed on the Hα image and the composite picture was transmitted to the ground. Pre-launch calibrations allowed the spot corresponding to the X-ray collimator axis to be identified and so the collimator pointing direction on the solar disc was controlled from the ground by means of commands sent to the rocket.

This summary reports new emission lines of Fe XVII observed in two coronal active regions (McMath regions 12624, 12628). The lines were recorded by a Bragg plane crystal spectrometer collimated to 3′ FWHM, with a gypsum crystal scanning the wavelength range 8–14 Å. This spectrometer, part of a three spectrometer package previously used by Parkinson (1972, 1973), was mounted aboard a Skylark rocket (SL 1206) launched from Woomera at 0535 UT on 26th November 1973. One of the active regions studied (12624) was observed about 60 minutes after the peak of an importance – N class CO flare.

With the accumulation of numerous identifications of the solar X and XUV spectral lines, the need is felt for systemizing such identification. The results are presented below.

This paper presents a review of recent analytical studies of the coronal X-ray spectrum below 25 Å. The techniques used to compute the theoretical coronal spectrum, and the currently available atomic rate constant data are reviewed first. Spectroscopic techniques which have been proposed for the determination of coronal temperature and density structure, and the results derived from their application to coronal spectra are also reviewed.

A number of coronal models based on X-ray observations have been developed recently, and the coronal temperature structure and composition predicted by these models is discussed, and compared with models of the corona and transition region derived from studies of the solar EUV spectrum.

The MSSL/Leicester University package on OSO 5 contained proportional counters having fields of view small compared with the area of the solar disk (Herring et al., 1971). Results discussed here were obtained with a detector sensitive in the band 0.3–0.9 nm. This had an entrance window collimated to examine a strip of angular width 2′ lying across the Sun.

The AS&E X-ray telescope experiment on Skylab has obtained images of the solar X-ray corona with a variety of time resolutions ranging from 21/2 s to the regular 12 ± 2 h synoptic observation rate. The form and brightness of coronal active region structures are seen to vary on time scales ranging from seconds, for flare associated changes, to several solar rotations for long term evolution of the regions. The extrapolation of photospheric magnetic fields into the corona, using the potential field approximation, results in a good morphological agreement between the form of the computed coronal field lines and the structure of many of the active regions observed. Thus, in general, the coronal active region structures follow potential field lines and the long term evolutionary changes can be explained on the basis of the spreading of the fields. Short term changes in active region structure frequently take the form of selective brightening or dimming of pre-existing loops due to changes in the pressure of the emitting coronal plasma. In these cases, variations in the non-potential component of the coronal fields supporting and containing the plasma are implied.

Some characteristic features of UV spectra which can be seen in flares (Brueckner, this Symposium, p. 135) can also be detected in active regions without the presence of typical other flare phenomena like X-ray enhancement. Another distinct difference between these ‘flare-like’ spectra and flare spectra is the absence of very high temperature ions like Fe XXI in the ‘flare-like’ spectra. They occur in small areas and can be detected as ‘fluctuating Hα bright points’ in broad band Hα. Simultaneously, a strong UV brightening can be seen. As reported by the Skylab crews, these brightenings occur more frequently and quasi-periodic prior to flares. Their spectra show the very broad transition zone lines and often strong line shifts toward the blue or red. Figure 1 shows a selection of typical spectra. One notices a very asymmetric Lα profile in one case. Spectra prior to, during, and after a flare are reproduced in Figure 2. One recognizes that the transition zone instability started intermittently prior to the flare and could be seen during short time intervals long after the flare had ceased.

The structure of solar active regions derived from EUV and soft X-ray observations is reviewed. The methods by which the emission measure as a function of temperature can be interpreted are discussed. The models of density and temperature which can be made from a variety of combinations of the emission measure with information on the spatial distribution of material, are broadly consistent. They show that the plasma at low heights over the central parts of an active region is hotter and denser than that which extends to greater heights. It appears that much of the emitting material exists in the form of loop structures, presumably magnetically controlled flux tubes. Analytical relationships between the physically important parameters describing the properties of the active region at Te > 2 × 105 K are developed and discussed.

A preliminary description of ultraviolet spectra of active regions and flares, photographed from Skylab by the Naval Research Laboratory's UV Spectrograph is given. The findings can be summarized as follows: (1) Line profiles of medium ionized lines (transition zone lines) show the most pronounced broadenings and shifts in flares and flare like events. (2) Typical full width at half maximum of these lines correspond to Doppler-velocities of 70 km s-1. (3) Shifts of the same magnitude can be observed. (4) Intersystem lines are not broadened nor shifted. (5) Forbidden coronal lines and intersystem lines become enhanced in the flare spectrum at the moment, when the turbulence seen in the allowed transitions, disappears. (6) A very broad line at 1354.2 Å which appears only in flare spectra, seems to be the forbidden transition 3P1 – 3P0 of Fe XXI.

During the Skylab Mission, the NRL slitless spectrograph photographed a number of flares in the 170–600 Å region with a spatial resolution approaching 2″. At flare maximum the 2s2S1/2 — — 2p2P1/2, 3/2 transitions of Fe XXIV are present, and show the location of the (approx.) 20 × 106 deg plasma with respect to the surface magnetic field and chromospheric (He iiemissions. Three examples are discussed (two only briefly).

In the small, intense disk flare of 1973, August 9 the high temperature region appears at the foot of a low altitude arch. The estimated electron density is 5 × 1011 cm–3.

In the limb flare of 1974, January 15 the hot X-ray emitting component is at a very low altitude compared to the flare loops.

In the impulsive double ribbon flare of 1973, June 15 the Fe XXIV emission is centered over the neutral line, forming a bridge-like structure between magnetic regions of opposite polarity. The estimated electron density is 5 × 1010 cm–3.

The Fe XXIV emission was visible 8 to 10 min as compared with a calculated cooling time by conduction of only 5 min. The lengthened life of the emission may be associated with the observed ‘turbulence’, which inhibits the heat conduction, or alternatively, with a slower energy release prolonged beyond the end of the burst phase.

Recent laboratory work relevant to solar flares on the spectroscopy of highly ionized atoms is reviewed. Much of this work has concerned the X-ray and EUV spectrum of iron ions, Fe XVIII–Fe XXIV, which produce prominent emission lines in the spectra of solar flares. Also discussed are recently obtained laboratory X-ray spectra of emission lines of hydrogen-like and helium-like ions, and associated satellite lines due to transitions of the type, 1s2l—2p2l, 1s22l—1s2p2l, and 1s22l— —1s2l3p. Satellite lines have also been identified in spectra of solar flares, and can be used to determine the electron temperature of the plasma. The laboratory work is important in the planning of future experiments in solar flare X-ray and EUV spectroscopy.

It is known that the flare component of solar X-ray emission is sensitive to the level of solar activity. The location of the X-ray flaring region is not directly known but can be inferred from the location of the associated Hα flare. A detailed study of more than 4000 solar X-ray flares recorded by UI and the NRL detectors during past eight years has shown that 85% of these flares definitely occurred in the active regions. For the rest 15% no definite conclusion was possible because either Hα flare data was not available or no Hα flare was reported within ± 15 min of these X-ray flares.