The reduction of the solar velocity field into its oscillatory and slowly-varying components
Spectroheliogram Doppler movies of solar velocity fields showing oscillatory and slowly varying components
Bhatnagar, A.↗
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Measurements of the oscillatory and slowly-varying components of the solar velocity field
Solar velocity field decomposition into oscillatory and slowly varying components by spectroheliograms with high spatial resolution
Bhatnagar, A.↗
Two-dimensional observations of the velocity field in and around sunspots
Two dimensional velocity field observations in and around sunspots from Doppler spectroheliograms
Bhatnagar, A.↗
Observed heights of EUV lines formed in the transition zone and corona.
The heights of formation of a number of extreme ultraviolet lines in active regions have been measured from OSO-IV spectroheliograms. Using the Lyman continuum at 2000 km above the white light limb as a reference, we find heights for He I, He II, C III, N III, O IV, O VI, Ne VIII, Mg X, Si XII, Fe XV and Fe XVI that are in approximate agreement with models based on analysis of EUV emission intensities. The height of C II is anomalously high. The accuracy of measurement is typically about 2000 km. The data suggest that the transition zone is less steep than calculated from EUV emission intensities; however, higher resolution observations are necessary to resolve the discrepancy.
Simon, G. W.↗
Extreme-ultraviolet emission from solar prominences.
Spectra and spectroheliograms of prominences have been obtained at wavelengths of 300 to 1400 A from instruments aboard the OSO 4 and OSO 6 spacecraft. Quiescent prominences appear in emission above the limb for all strong lines formed at temperatures below 300,000 K, but not at higher temperatures. The gas pressure in the 100,000-K transition zone around prominences is approximately equal to that in the cooler (6500 K) central regions. The temperature and the hydrogen ground-state departure coefficient in the central regions are determined from the Lyman-continuum spectrum. Prominences on the disk (filaments) are visible in absorption in many lines, especially those at wavelengths below the hydrogen Lyman limit at 912 A.
Noyes, R. W.↗
Outflow of chromospheric emission features from the rim of a sunspot
In viewing a 16 mm movie made from a time sequence of spectroheliograms, some of these emission features are found to move outward from the rim of the sunspot until they are eventually lost in the small plage. There are two interpretations for the streaming of the magnetic features. It is possible that kinks in the line of force propagate along a horizontal extension of the penumbral magnetic field. Alternatively, fragments of the sunspot magnetic field are carried away by the photospheric velocity field.
Liu, S.-Y.↗
Solar rotation in the chromosphere and corona
Extreme ultraviolet spectroheliograms in Mg X (625 A) and the Lyman continuum (897 A) obtained from OSO-6 are used to determine the differential rotation rate in the solar chromosphere and corona. The equatorial rotation rate agrees with spectroscopic measurements of the photospheric plasma velocity; the variation of rate with latitude is less pronounced than in most other determinations. We cannot discern a variation in the rotation rate between the chromosphere and corona.
Henze, W., Jr.↗
On spatial variations in the intensity of chromospheric H-alpha
Investigation of the formation of patterns in H-alpha spectroheliograms and filtergrams. Introducing a source-sink-control diagram, it is concluded that the H-alpha line source function in the quiet solar chromosphere is indirectly controlled by the photospheric radiation fields in the Balmer and Paschen continua. It is demonstrated that in producing the observed patterns, horizontal spatial variations in the shape of the absorption profile are extremely effective compared to changes in the source and sink terms. Applying this mechanism, asymptotic values are computed for the contrasts and visibilities in chromospheric H-alpha.
Gebbie, K. B.↗
X-ray bursts from solar flares behind the limb
X-ray bursts are identified from the UCSD OSO-7 X-ray experiment data. X-ray spectroheliograms of OSO-5, H alpha activity at the limb, and the emergence and disappearance of sunspot groups at the limb were studied and 17 active centers were found as likely seats of the X-ray bursts beyond the limb. The analysis of 37 X-ray bursts and their physical parameters is presented. Results show that (1) the distributions of maximum temperature, maximum emission measure, and characteristic cooling time of the over-the-limb events do not significantly differ from those of disk events; (2) that radiation is the dominant cooling mechanism for the hot flare plasma; and (3) that the scale height for X-ray emission in the 5-10 keV range is large. Observations show that the fraction of soft X-ray bursts which have a nonthermal component is the same on and off of the disk. Hard X-ray emission over extended regions is indicated.
Roy, J. R.↗
The photoelectric spectroheliometer on ATM
The Harvard College Observatory instrument on the Skylab Apollo Telescope Mount is a photoelectric spectroheliometer designed to obtain up to seven simultaneous spectroheliograms in the range between 280 and 1340 A with a spatial resolution of 5 arc sec, as well as spectral scans with a resolution of 1.6 A over the same range of wavelengths. The optical schematic of the instrument is illustrated and discussed. Because of its large size, the instrument has a sensitivity far greater than that of any other EUV spectroheliometer flown to date. The instrument has operated correctly in orbit since May 1973. Valuable data have been obtained during this period on a variety of solar phenomena, on the comet Kohoutek, and on the atmospheres of the earth and Mercury. The instrument was successfully recalibrated in orbit by comparing the response with that of recently calibrated spectroheliometers flown on sounding rockets.
Reeves, E. M.↗
XUV observations of coronal magnetic fields
Spectroheliograms obtained with the Naval Research Laboratory's 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.
Sheeley, N. R., Jr.↗
The reconnection of magnetic field lines in the solar corona
Skylab XUV coronal spectroheliograms and photospheric magnetograms are compared. This comparison shows that, as new bipolar magnetic fields emerge through the solar surface into the corona, the new coronal fields interact with the old ones in a manner that suggests the reconnection of the field lines.
Sheeley, N. R., Jr.↗
A newly observed solar feature - Macrospicules in He II 304 A
He II 304 A spectroheliograms, obtained with the NRL extreme-ultraviolet slitless spectrograph during the Skylab mission, show spikelike structures at the sun's polar limb which resemble the familiar H-alpha spicules. However, the relatively large size and long life of these He II features has led to distinguishing them by the name 'macrospicules'. The macrospicules appear as protuberances or jets, ranging from 5 to over 60 sec in length, from 5 to 30 sec in width, and from 5 to over 40 minutes in lifetime. Perhaps the most radical departure from H-alpha spicules is that macrospicules occur only within the chromospheric boundaries of coronal holes. Thus macrospicules are most easily visible over the solar poles due to the coronal holes normally present there, and much less frequently at lower latitudes during limb passage of relatively rare, low-latitude coronal holes.
Bohlin, J. D.↗
EUV observations of the active sun from the Harvard experiment on ATM
Some extreme UV observations of solar active regions made with a scanning spectroheliometer are described. Spectroheliograms constructed from digital data using a computer-driven cathode-ray tube display show clearly how the appearance of an active region changes as a function of temperature. Flare studies indicate that the impulsive rise in EUV emission occurs essentially simultaneously at all levels from the transition zone to the corona. Observations of sunspots reveal a very intense emission in transition zone lines. A matrix of Mg x rasters covering the entire sun reveals several hundred bright points having dimensions of 30 arc seconds or less. Other observations include coronal holes and prominences.
Noyes, R. W.↗
Solar activity observed in X-rays and the EUV from OSO 7
Spectroheliograms obtained with the EUV spectroheliograph aboard OSO 7 between 1.7 and 400 A with a spatial resolution of at least 20 x 20 arc seconds are described, and results of an initial analysis of the Goddard X-ray polarimeter measurements are given and discussed in the light of current flare theories. The EUV observations are consistent with a picture in which coronal enhancement overlying an active region is made up of a nested set of arches or loops, with foot-points in regions of opposite magnetic polarity. The outermost arches are coolest, while those nearer the center are hotter. Along a given field line there is a temperature gradient such that the highest temperature occurs at the highest point of the magnetic arch. Polarimeter data provide evidence for polarization of about 20% in a number of X-ray bursts.
Thomas, R. J.↗
EUV analysis of an active region
A sequence of extreme-ultraviolet (EUV) spectroheliograms of McMath region No. 10283 were obtained by OSO-6. The lines O VI (1032 A) Mg X (625 A), Si XII (499 A), and Fe XVI (335 A) were used to determine coronal temperatures and densities above the active region. A comparison of theoretical and observed line ratios yielded coronal temperatures of 2.2 to 2.3 million K above the active region and 2.0 to 2.1 million K in the surrounding area. The temperatures derived from ratios involving the O VI intensities are systematically higher than the others. This is attributed to an error in the theoretical O VI intensities. The intensities observed above the limb are compared with intensities predicted by a simple model based on cylindrical geometry. The overall agreement shows that the assumption of an isothermal corona in hydrostatic equilibrium above the active region is a resonable working hypothesis and that the adopted geometrical model for the electron density distribution is adequate.
Raghavan, N.↗
Coronal changes associated with a disappearing filament
Skylab/ATM observations of a disappearing filament near the center of the solar disk are described using XUV and H-alpha spectroheliograms, X-ray filterheliograms, and photospheric magnetograms. The temperature of the coronal plasma as the filament disappeared is estimated to have been in excess of 6 million K, and it is noted that the time history of the soft X-ray and microwave fluxes displayed the gradual-rise-and-fall (GRF) signature, suggesting that the present event may have properties that are characteristic of a wide class of long-duration X-ray and radio events. A comparison with other spatially resolved long-duration X-ray events indicates that all such long-lived bursts involve transients in the outer corona and that some two-thirds of them involve either the eruption or the major activation of a prominence. It is also found that long-lived events are characterized by the appearance of new emission loops in the lower corona during the declining phase of the X-ray emission and that these loops sometimes disappear after the X-ray events and sometimes remain indefinitely.
Sheeley, N. R., Jr.↗
Plasma diagnostic techniques in the ultraviolet - The C III density-sensitive lines in the sun
Spectra and spectroheliograms of the C III transitions at 977 and 1176 A are obtained with the Harvard extreme-ultraviolet spectrometer on Skylab. Analysis of the intensities of these lines, and of their density-sensitive ratio, indicates a wide range of temperature gradients and electron densities in the transition region of various solar features. From values of the observed ratio, we suggest necessary revisions to the excitation rates, and propose a relationship between the ratio and density. The significantly higher ratio found in active regions indicates a density increase of about a factor 2 relative to the network. In the quiet sun, there is no significant difference in density between network and cell interiors, but the uncertainty is as large as a factor 3. The very central 10% of the areas of cell interiors shows a significantly higher density than the mean value for cell interiors.
Dupree, A. K.↗