J. D. Purcell

Coronal changes associated with a disappearing filament

This paper describes Skylab/ATM observations of the events associated with a disappearing filament near the center of the solar disk on January 18, 1974. As the filament disappeared, the nearby coronal plasma was heated to a temperature in excess of 6 × 106K. A change in the pattern of coronal emission occurred during the 11/3 hr period that the soft X-ray flux was increasing. This change seemed to consist of the formation and apparent expansion of a loop-like coronal structure which remained visible until its passage around the west limb several days later. The time history of the X-ray and microwave radio flux displayed the well-known gradual-rise-and-fall (GRF) signature, suggesting that this January 18 event may have properties characteristic of a wide class of X-ray and radio events.In pursuit of this idea, we examined other spatially-resolved Skylab/ATM observations of long-duration X-ray events to see what characteristics they may have in common. Nineteen similar long-lived SOLRAD X-ray events having either the GRF or ‘post-burst’ radio classification occurred during the nine-month Skylab mission. Sixteen of these occurred during HAO/ATM coronagraph observations, and 7 of these 16 events occurred during observations with both the NRL/ATM slitless spectrograph and the MSFC-A/ATM X-ray telescope. The tabulation of these events suggests that all long-lived SOLRAD X-ray bursts involve transients in the outer corona and that at least two-thirds of the bursts involve either the eruption or major activation of a prominence. Also, these observations indicate that long-lived SOLARD events are characterized by the appearance of new loops of emission in the lower corona during the declining phase of the X-ray emission. However, sometimes these loops disappear after the X-ray event (like the post-flare loops associated with a ‘sporadic coronal condensation’), and sometimes the loops remain indefinitely (like the emission from a ‘permanent coronal condensation’).

Extreme ultraviolet spectroheliograph ATM experiment S082A.

The XUV spectroheliograph, Apollo Telescope Mount experiment S082A, is described. The instrument was a slitless Wadsworth grating spectrograph that employed photographic recording. The grating was of 4-m radius, ruled with 3600 grooves/mm. By rotating the grating to positions where the normal was at 255 A or 400 A, the spectral ranges 175-335 A and 320-480 A, respectively, were covered with 2-sec of arc spatial resolution. Close to the normal the resolution reached 2 sec of arc, but at the extreme limit, 630 A, it was 10 sec of arc or worse. The aberrations of the instrument are discussed in detail as are the provisions necessary to maintain optimum imagery and reliability in a space environment. During the mission about 1020 exposures were made covering 171-335 A or 320-630 A.

Extreme ultraviolet spectrograph ATM experiment S082B

The extreme ultraviolet, double-dispersion, photographic spectrograph, Apollo Telescope Mount (ATM) Experiment S082B on Skylab is described. Novel features were the use of a predisperser grating with a ruling whose spacing varied approximately linearly with distance for the purpose of increasing the instrument speed by reducing the astigmatism and a photoelectric servo-system to stabilize to 1 sec of arc the solar image at various near-limb positions. The 970-3940-A range was covered in two sections with effective lambda/Deltalambda congruent with 30,000 from 1100 A to 1970 A. The spatial resolution was 2 x 60 solar sec of arc. During the Skylab mission 6400 exposures were made with the instrument pointed by an astronaut at selected and recordedsolar positions.

XUV observations of coronal magnetic fields

Spectroheliograms obtained with the Naval Research Laboratorys Extreme Ultraviolet Spectrograph (S082A) on Skylab are compared with Kitt Peak National Observatory magnetograms. A principal result is the characteristic reconnection of flux from an emerging bipolar magnetic region to previously existing flux in its vicinity. Examples of the disappearance of magnetic flux from the solar atmosphere are also shown. The results of a particularly simple, potential field calculation are shown for comparison with the Skylab observations.

THE UV CONTINUUM 1450--2100 A AND THE PROBLEM OF THE SOLAR TEMPERATURE MINIMUM.

This study is based on a set of ten solar rocket spectra well exposed for photometry photographed on July 27, 1966 by Purcell, Snider, and Tousey.The photometry of the far UV continuum illustrates the transition of the solar temperature minimum at 1700 Å in the solar spectrum - (a) the continuum intensity decreases by 30–50% between 1700 Å and the1D limit of silicon at 1682 Å, and (b) the equivalent brightness temperature shows minimum values throughout the spectral range 1540–1682 Å, which average just under 4700 ± 100K.The minimum UV brightness temperature is 300K higher than the far infrared measurement of the solar minimum temperature, and possible reasons for this are discussed.Brightness temperatures measured in prominent CO band heads and in the aluminum 1937 Å auto-ionization line also are given.

Forbidden lines of highly ionized iron in solar flare spectra

Forbidden lines of Fe xvm, Fe xix, and Fe xxi are identified at 974, 1118, and 1354 Ä, respectively, in NRL ATM solar flare spectra. These lines are due to magnetic dipole transitions between levels of the ground configurations. The widths of the Fe xix and Fe xxi lines are ^0.5 Ä, which is substantially greater than expected in ionization equilibrium. The intensity-time behavior and the widths of these lines are discussed for the 1973 June 15 flare. Subject headings: flares, solar — forbidden lines — spectra, solar — ultraviolet Spectra of solar flares have been obtained in the wavelength region from ~970 Â to 4000 A from the NRL normal-incidence spectrograph in the Apollo Telescope Mount on Skylab. These spectra reveal a transition zone that has been greatly affected by the energy released in the flare. Lines from ions such as C iv and O v are enhanced and broadened in flares, and anisotropic mass motions are also observed (Brueckner 1974a). Although most of the lines observed in the 1000 Â region are transition-zone and chromospheric lines, we have recently identified forbidden lines of Fe xvm, Fe xix, and Fe xxi in flare spectra, which are ions found in the soft X-ray emitting coronal plasmas that overlie the enhanced chromospheric and transitionzone regions. These forbidden lines are similar in type of transition to the well-known forbidden lines emitted in the visible spectral region by the nonflaring corona. Iron ions such as Fe xix, however, exist only in flaring coronal plasmas at electron temperatures on the order of 10° K (Doschek 1972). The temperatures of maximum emitting efficiency of Fe xvm, Fe xix, and Fe xxi are approximately 7 X 10 °, 9 X 10 °, and 11 X 10 0 K, respectively (Jordan 1970). The identification of the forbidden lines was made on the basis of predicted wavelengths derived from laboratory spectra of iron lines emitted near 100 Á due to transitions of the type, 2s2p-2s2p (Feldman et al. 1973, 1975; Doschek et al. 1974). These lines were observed in spectra obtained from laser-produced plasmas. The predicted wavelengths of some of the forbidden lines can also be derived with a fair degree of accuracy by extrapolating the data for the lighter ions of the isoelectronic sequence given in Atomic Energy Levels (Moore 1949), and from extrapolations of semi-empirical calculations of Edlén (1969, 1972). Table 1 gives the predicted and measured wavelengths of the strongest forbidden lines of highly ionized iron expected in flare spectra. Fe xvu, Fe xx, Fe xxm, and Fe xxiv do not have forbidden lines that are likely to be intense because the ground terms are not split. We * On leave from Tel-Aviv University, Tel Aviv, Israel. Work supported in part by a grant from NASA. TABLE 1 Predicted and Observed Forbidden Lines of Highly Ionized Iron Ion Transition Yred (Y Nbs (a) Fe xvm. Fe xix. . Fe xxi. . Fe xxi. . Fe xxii. . Fe xxm. 2S*2PKP3I2-P1,2) 2snp\P2Pi) 2s2p‘(pP \-P 2) 2s2p(*P {rP\) 2s2p(Pll2Ptl2) 2s2p(*Pl-*P2) 974.2± I 1118.4±2 2304 ±6 1355 ±2 841 1093 974.8 1118.1 ±0. l 1354.1 + 0.l 845.1 + 0.2 a Laser-plasma laboratory data. b NRL Skylab data. c Ab initio calculations (R. D. Cowan). d Harvard OSO-6 data. e Isoelectronic extrapolation. were fortunate to be able to observe the forbidden lines of Fe xvm (974 Â) and Fe xix (1118 A) because their wavelengths fall at the short-wavelength end of the spectrograph where the instrument sensitivity is falling rapidly (Bartoe and Brueckner 1974). In fact, the Fe xvm line has been found on only one plate out of the entire Skylab series, and the Fe xix line only appears on the longest exposure spectra (80 s) of the most intense flares. The Fe xxii line (845 Â) in table 1 was previously observed by the Harvard group in a flare spectrum obtained from OSO-6 (e.g., Noyes 1973); and the Fe xxi line at 2304 Á, although expected in flare spectra, is lost in the strong solar continuum at these wavelengths. The Fe xxm line given in table 1 was not observed, even though it might be expected in flare spectra. The three forbidden lines found in the NRL flare spectra appear to be produced by two main processes: (a) direct electronand proton-impact excitation from the ground state into the upper levels of the transitions (Seaton 1964), and (b) excitation from the ground state to levels of the 2s2p excited configurations followed by cascade into the upper levels of the forbidden transitions. Photoexcitation from the continuum is negligible.

An echelle spectrograph for middle ultraviolet solar spectroscopy from rockets.

An echelle grating spectrograph is ideal for use in a rocket when high resolution is required becaus itoccupies a minimum of space. The instrument described covers the range 4000-2000 A with a resolution of 0.03 A. It was designed to fit into the solar biaxial pointing-control section of an Aerobee-150 rocket. The characteristics of the spectrograph are illustrated with laboratory spectra of iron and carbon are sources and with solar spectra obtained during rocket flights in 1961 and 1964. Problems encountered in analyzing the spectra are discussed. The most difficult design problem was the elimination of stray light when used with the sun. Of the several methods investigated, the most effective was a predispersing system in the form of a zero-dispersion double monochromator. This was made compact by folding the beam four times.

A preliminary study of the Extreme Ultraviolet spectroheliograms from Skylab

Some of the first observations obtained with the Naval Research Laboratorys Extreme Ultraviolet Spectrograph (S082A) during the first Skylab mission are presented and compared with magnetograms and other ground-based data. The instrument is a slitless objective-type grating spectrograph covering 170–630 Å and described in Solar Phys.27, 251 (1972). Chromospheric network, loop prominences, active regions, a flare, limb brightening, XUV bright points, and ‘coronal holes’ are among the phenomena shown and discussed.

The extreme Ultraviolet emission from the Sun between the Lyman-alpha lines of H I and C VI

As a result of research carried out with rocket-borne grating spectrographs, the nature of the extreme ultraviolet spectrum of the Sun is now known to a short wavelength limit of 33.7 A, the Lyman-alpha line of C VI. Most of the emission lines of wavelengths greater than 400 A have been identified, as have those from 80 A to 33.7 A. Between 149 A and 400 A, however there are many intense emission lines whose identity has not as yet been established. Twenty or more have been proved to be from iron, since they appear in spectra obtained from high temperature plasmas into which iron has been introduced, but the stages of ionization have not yet been established. Lines from the elements most abundant in the Sun, H, He, O, N, O, Ne, Mg, Al, Si, S and Fe, in most of the stages of ionization requiring 500 eV or less for production have been found. The outstanding exceptions are the lines in the fluorine and neon sequences. Spectroheliograms, photographed with normal incidence spectrographs, show that the emission lines Fe XV 284 A, Fe XVI 335, 361 A, originate principally from active regions, in contrast to He II 304 A, which is emitted with great intensity from the disc also. Continuum emission, in the wavelength range 170–300 A, has been recorded from intense centers of activity.

Evaluation of Pinholes in Unbacked Metal Film Filters to be Used in Rocket- and Satellite-Borne XUV Spectroheliographs

Extreme ultraviolet (XUV) spectroheliographs require thin metal film filters that transmit the XUV radiation and eliminate scattered visible and near-uv radiation that would fog the photographic film on which the XUV images are recorded. Pinholes in the filters cause local fogging of the film during exposures in flight. It will be shown that the best way for preflight evaluation of pinhole effects is by using the filter in the flight instrument and photographing the sun from the earths surface. An alternative method that appears to be as good, and is more convenient. is to test the filters in a simulated flight instrument. The results of evaluations using both the flight instrument and a simulated flight instrument will be shown.