Extreme ultraviolet spectrum of the solar flare of 2114 UT March 27, 1967

Scanning spectrometer measurements in the range 1310–270 Å, observed from the satellite OSO 3, are reported for the solar flare of 2114 UT March 27, 1967. This flare was a long lasting sequence of bursts with EUV spectra consisting of enhanced lines and recombination continua normally emitted from the chromosphere and chromosphere-corona transition region, with unusually small increases in lines normally emited from the corona. An EUV flare spectrum is presented and suggested as one example for interpreting broadband observations of EUV bursts. Any broadband continuum other than known recombination continua contributed less than 6 % of the meassured line and hydrogen recombination continua in the range 270–1310 Å. The ratio of photon flux of Ciii 1176 Å to that of Ciii 977 Å was 0.86, which suggests an ambient density in the region of emission greater than 10¹² cm^-3 at temperatures near 60000 K.     

On the filamentary nature of solar magnetic fields

A method is presented for obtaining information about the unresolved filamentary structure of solar magnetic fields. A comparison is made of pairs of Mount Wilson magnetograph recordings made in the two spectral lines Fei 5250 Å and Fei 5233 Å obtained on 26 different days. Due to line weakenings and saturation in the magnetic filaments, the apparent field strengths measured in the 5250 Å line are too low, while the 5233 Å line is expected to give essentially correct results. From a comparison between the apparent field strengths and fluxes and their center to limb variations, we draw the following tentative conclusions: (a) More than 90 % of the total flux seen with a 17 by 17 arc sec magnetograph aperture is channeled through narrow filaments with very high field strengths in plages and at the boundaries of supergranular cells. (b) An upper limit for the interfilamentary field strength integrated over the same aperture seems to be about 3 G. (c) The field lines in a filament are confined in a very small region in the photosphere but spread out very rapidly higher up in the atmosphere. (d) All earlier Mount Wilson magnetograph data should be multiplied by a factor that is about 1.8 at the center of the disk and decreased toward the limb in order to give the correct value of the longitudinal magnetic field averaged over the scanning aperture.Guest Investigator at the Hale Observatories, on leave from Astronomical Observatory, Lund, Sweden.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

UV Observations with the Transition Region and Coronal Explorer

The Transition Region and Coronal Explorer is a space-borne solar telescope featuring high spatial and temporal resolution. TRACE images emission from solar plasmas in three extreme-ultraviolet (EUV) wavelengths and several ultraviolet (UV) wavelengths, covering selected ion temperatures from 6000 K to 1 MK. The TRACE UV channel employs special optics to collect high-resolution solar images of the H i L line at 1216 Å, the C iv resonance doublet at 1548 and 1550 Å, the UV continuum near 1550 Å, and also a white-light image covering the spectrum from 2000–8000 Å.We present an analytical technique for creating photometrically accurate images of the C iv resonance lines from the data products collected by the TRACE UV channel. We use solar spectra from several space-borne instruments to represent a variety of solar conditions ranging from quiet Sun to active regions to derive a method, using a linear combination of filtered UV images, to generate an image of solar C iv 1550 Å emission. Systematic and statistical error estimates are also presented. This work indicates that C iv measurements will be reliable for intensities greater than 1014 photons s–1 cm–2 sr–1. This suggests that C iv 1550 Å images will be feasible with statistical error below 20% in the magnetic network, bright points, active regions, flares and other features bright in C iv. Below this intensity the derived image is dominated by systematic error and read noise from the CCD.     

Proposed Mg V,Fe XIV and forbidden Fe XVII line identifications in the EUV solar spectrum

Some line identifications in solar disk and limb spectra are proposed on the basis of recent laboratory and theoretical results reported in the literature, including allowed lines of Mgi and Fexiv in the EUV spectrum and an expected forbidden line of Fexvii near 1190 Å.     

The solar UV continuum 1440–1680 Å and its center-to-limb variation

The solar UV continuum has been derived from intensity-calibrated observations with the High-Resolution Telescope and Spectrograph - HRTS - on its second rocket flight in 1978. A database has been constructed using the spatially-resolved solar spectrum 1180–1700 Å along a slit extending from near disk center to the solar limb and crossing a sunspot and two active regions. The angular resolution is approximately 1.8″. The data consist of 1772 spectral scans in the full center-to-limb range, including both quiet and active solar regions. The distribution of solar UV intensities has been derived and the center-to-limb variations of the continuum intensities in the quiet Sun are studied. Both quantities show spectral variations, particularly across the Sii continuum edge at 1521 Å. The spectra have been fitted to curves of constant color temperature above and below the Sii edge. The derived center-to-limb variations have been compared to the values of Samain (1979) which are frequently referred to in the literature. A relatively large discrepancy may be explained by the higher spectral and angular resolution of the HRTS as compared to the rocket instrument used by Samain. Comparisons with the VAL III model calculations by Vernazza, Avrett, and Loeser (1981) show discrepancies between the observations and the model predictions, particularly with regard to the sign and amount of the intensity change across the Sii continuum edge. It is noted that some of the results presented, i.e., absolute intensities and brightness temperatures may change, pending confirmation of the SUSIM Spacelab 2 irradiance results (VanHoosieret al., 1988).     

X-ray and extreme ultraviolet (1–400 Å) spectroscopy of the sun,from OSO-III

X-ray spectra of the sun have been obtained during solar flares. New emission lines are observed in the spectral range from 1.3 Å to 3 Å, and 8 Å–20 Å, the most intense of the new emission features being tentatively attributed to optical transitions in high stages of ionization of iron (Fexxv through Fexx). Studies of the variability of these lines during flares provide new information of the development of a hot plasma in the initial stages of the flare event.     

Measurement of the X-ray emission of the solar atmosphere during a period of low activity

A large-area high-sensitivity X-ray spectrometer has been constructed and used to measure the 1.8–5.3 Å X-ray emission of the Sun under quiescent conditions. The instrument utilizes Bragg reflection from mosaic graphite crystals. The data indicate that the X-ray emission can best be accounted for by a multitemperature model of the solar atmosphere in which both the over-all corona and active regions contribute to the X-ray spectrum. Theoretical calculations of the X-ray flux of a hot, optically thin plasma have been used to estimate the solar conditions at the time when the measurements were made.     

Observations of the Mg i and ii resonance lines in an active region

Spectra from 2678-2931 Å were obtained of an active region during the 19 June 1974, flight of the University of Hawaii rocket-borne echelle spectrograph. We report behavior of the Mg i and ii resonance line cores in quiet Sun, plage, sunspot, and filament structures. Among the interesting variations in these lines we discern a strong suppression of the red Mg ii emission peaks and possible rapid changes in the Mg i core in the spatially partially resolved sunspot.     

Solar EUV emission line profiles of Si ii and Si iii and their center to limb variations

Spectral line profiles of Si ii and Si iii are presented which were observed both at solar center and near the quiet solar limb with the Naval Research Laboratory EUV spectrograph of ATM/SKYLAB. Absolute intensities and line profiles are derived from the photographic data. A brief discussion is given of their center-to-limb variations and of the optical thickness of the chromosphere in these lines. Nonthermal broadening velocities are found for the optically thin lines from their full width at half maximum intensity (FWHM).Also at High Altitude Observatory for part of this work.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Temporal trends of solar EUV and UV full-disk fluxes

Several progressions in the temporal characteristics of full-disk solar UV and EUV fluxes have been identified that raise many questions about the solar physics involved. The collective effect of numerous enhancements smaller than scaled plages contribute significantly to the solar cycle variations, especially for emissions from the cooler portions of the corona and the chromosphere. Active-region remnants are suggested to have a strong role even in solar-rotation induced variations late in an episode of major activity. Although cool coronal EUV emissions are long lasting, the persistence of the solar-rotation induced variations is even greater at photospheric UV wavelengths. Gyroresonance and possibly nonthermal radio emission at centimeter wavelengths are suggested to be particularly important during the first solar rotation of an episode of major activity.     

Solar flare line emission between 6 Å and 25 Å

A list of emission lines in the spectra of solar flares between 6 Å and 25 Å has been compiled using data obtained with a KAP crystal spectrometer on the OSO-5 satellite. The emission lines have been classified according to their sensitivity to flare activity. This classification provides a method for discriminating between iron in high stages of ionization (Fe xx-Fe xxv) and lower stages (Fe xvii- Fe xix), the lines of which are both present in the same spectral region during flares. Identifications consistent with these classifications are proposed. Anomalous intensities in the spectra of Fe xvii and Fe xx are pointed out, and implications of the observations for models of the X-ray emitting regions are discussed.     

A study of solar radio emission in the light of Sengupta's model of coronal active regions

Daily solar radio flux at six different frequencies in dm, cm and mm wavelength regions has been studied for 182 days from December 1, 1970 to May 30, 1971. It is found that the slowly varying component of the centimeter wave emission correlates well with the physical model of the coronal active regions derived by Sengupta (Sengupta, 1971b) from which, as he showed earlier, most of the solar soft X-rays of wavelength less than 20 Å comes. It is also found that the cm wave emission is consistent with the assumption that the emitting regions are optically thin in this wavelength range.Emissions in dm and mm wavelength ranges, however, show poor correlation with the physical model of the soft X-ray emitting regions.It is concluded that the preferred regions of cm wave emission are located in the same region of solar corona from where most of the soft X-rays comes, but are different from the preferred regions of mm and dm emission.     

Evidence of redshifts in the average solar line profiles of C iv and Si iv from OSO-8 observations

Measurements of the C iv 1548 Å and Si iv 1393 Å lines made with the University of Colorado Ultraviolet Spectrometer on board OSO-8 show that the mean profiles are redshifted at disk center. Assuming these lines to be optically thin, we measure an apparent average downflow of material in the 50 000 to 100 000 K temperature range which is weighted by the emission measure in these lines. The magnitude of the redshift varies from 6–17 km s^–1 with a mean of 12 km s^–1 and is persistent at least on the order of months, which is the time covered by the observations presented in this paper. Pneuman and Kopp (1978) have demonstrated that the flux of material associated with this downflow is of the same order of magnitude as the flux of material being carried upward in spicules. Thus, it is possible that material observed to be downflowing in C iv and Si iv has its origins in the upward moving spicule material.     

Further X-ray spectra of solar active regions

Measurements of the solar X-ray spectrum between 3 Å and 15 Å are reported. They were made with two slitless Bragg crystal spectrometers flown on a Sun-pointed Skylark rocket on 8 August, 1967.The use of a beryl crystal has provided higher spectral resolution than hitherto in the spectral range 12 Å and 15 Å leading, in particular, to a revised identification of the strong line at 13.71 Å. Separate components of the stronger emission lines are clearly seen from each of three coronal active regions which may be identified on radio and X-ray spectroheliograms.The absolute line fluxes are used to determine a model for each active region in terms of the differential emission measure as a function of electron temperature. Emission lines due to the transition 1s ^{2 1} S ₀ – 1s2s ³ S ₁ in several helium-like ions are identified and values of the local electron density derived from measurement of the line flux in these ions.     

The helium 10830 Å line in the undisturbed chromosphere

A study of the solar spectrum near helium 10 830 Å has shown that, where the line is very weak, the anomalous ratio of the two components is due almost certainly to faint blends. The centre-limb intensity variation over supergranule centres is in good agreement with an optically-thin law. The line is stronger over supergranule boundaries, and the ratio of the two components can be understood only if the absorbing elements have been resolved incompletely (as is probable); using centre-limb intensity variations, we have been unable to distinguish between unresolved horizontal platelets or inclined slabs.The integrated absorption in 10 830 Å over supergranule centres is double that at the boundaries. An analysis of the extension of 10 830 Å beyond the limb has failed to reveal the relative contributions from these two regions or their variations with height (though the supergranule-centre emission should be located relatively low). Line profiles in 10 830 Å, hydrogen Pa and Caii 8542 Å indicate that, out to at least 5000 km, line broadening is effectively non-thermal, with horizontal rms velocities of about 20 km s^–1.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Modeling Solar UV Variations Using Mount Wilson Observatory Indices

Understanding the magnitude and temporal structure of variations in solar ultraviolet and extreme ultraviolet irradiance is critical to understanding solar forcing of the Earth's upper and middle atmosphere and hence to assessing the relative impact of natural and anthropogenic influences on Earth's atmospheric environment. Satellite based measurements of such variations are limited to recent times, are short in duration and subject to gaps making necessary ground-based surrogates with longer and more continuous coverage. Using indices derived from synoptic solar magnetograms taken at the Mount Wilson 150-foot solar tower, we have constructed models of several UV and near EUV lines and fluxes which correlate strongly (r > 0.90) with satellite data. These lines and fluxes include the Mgii h and k core-to-wing ratio, the Lα line and the 200–205 nm flux.     

Phase Coherence Analysis of Solar Magnetic Activity

Over 24 years of synoptic data from the NSO Kitt Peak Vacuum Telescope is used to investigate the coherency and source of the 27-day (synodic) periodicity that is observed over multiple solar cycles in various solar-related time series. A strong 27.03-day period signal, recently reported by Neugebauer et al. (2000), is clearly detected in power spectra of time series from integrated full-disk measurements of the magnetic flux in the 868.8 nm Fei line and the line equivalent width in the 1083.0 nm Hei line. Using spectral analysis of synoptic maps of photospheric magnetic fields, in addition to constructing maps of the surface distribution of activity, we find that the origin of the 27.03-day signal is long-lived complexes of active regions in the northern hemisphere at a latitude of approximately 18 deg. In addition, using a new time series analysis technique which utilizes the phase variance of a signal, the coherency of the 27.03-day period signal is found to be significant for the past two decades. However, using the past 120 years of the sunspot number time series, the 27.03-day period signal is found to be a short-lived, no longer than two 11-year solar cycles, quasi-stationary signal.     

Coronal survey in X-rays of O vii and Ne ix

We report some results of a rocket experiment flown on 29 April, 1971. A survey of the solar corona was carried out with a pair of collimated Bragg spectrometers to study the resonance, intersystem and forbidden line emission from the helium-like ions O vii (22 Å) and Ne ix (13 Å). In the direction of dispersion the collimator provided a field of view of 1.7. Also, the continuum radiation near 3 Å was monitored by a collimated proportional counter within a view angle of 4.2. The observed X-ray emission came from the general corona, seven plage regions, and one dynamic feature- the late stage of a small flare. From the intensity of the O vii and Ne ix resonance lines the electron temperature and emission measure of the individual emitting regions are derived on the basis of two models, one (a) in which the region is assumed to be isothermal and another (b) in which the emission measure decreases exponentially with increasing temperature. The latter model, which is the most adequate of the two, yields for the electron temperature of the time-varying feature 2–3 × 10⁶ K, for the other active regions 1.5–2.5 × 10⁶ K, and for the general corona 1.3–1.7 × 10⁶ K. The Ne ix emitting regions are about 1.5 times as hot as the O vii regions. The emission measure ranges from 0.4–2.3 × 10⁴⁸ cm^–3 for all active regions and is about 2 × 10⁴⁹ cm^–3 for one hemisphere of the general corona above 10⁶ K. From an analysis of the ratio, R, of the forbidden and intersystem lines of O vii we conclude that none of the regions producing these lines at the time of the rocket flight had electron densities exceeding about 3 × 10⁹ cm^–3. Our data demonstrate a dependence of R upon temperature in agreement with the theory of Blumenthal et al. (1971). The wavelengths for the intersystem, the 1s ²2s ² S ^e–1s2p2s ² P ⁰ satellite, and the forbidden transition show in the case of Ne ix improved agreement with predictions. The observed strength of the satellite lines for both O vii and Ne ix agrees with the predictions of Gabriel's (1972) theory, which attributes their formation to dielectronic recombination.We are saddened to report the death of A. J. Meyerott on 13 November, 1971.     

Comparison of computed fluxes for Fex and Fexiv lines with observed values at 1980 eclipse

Fluxes have been computed for Fex (6374 Å) and Fe xiv (5303 Å) lines as a function of solar radii and at various coronal tempratures. The electron density derived from the white light corona during the total solar eclipse of 1980 were used in the computations. Fluxes in adjacent continua have also been computed. The computed ratios of line flux to the square of continuum flux at a coronal temperature of 1.6 × 10⁶ K show a good fit with the observed values for Fex line. Further, radiative excitation seems to dominate over collisional excitation beyond 1.3 solar radius.     

Variability of the quiet photospheric network

High-resolution photographs of the photospheric network taken in the Caii K 3933 Å line and at 4308 Å are analysed in order to study the variation, in latitude and over the sunspot cycle, of its density (the density is defined as the number of network elements - also called facular points - per surface unity). It appears that the density of the photospheric network is not distributed uniformly at the surface of the Sun: on September 1983, during the declining phase of the current activity cycle, it was weakened at both the low (equatorial) and high (polar) active latitudes, while it was tremendously enhanced toward the pole. The density at the equator is varying in antiphase to the sunspot number: it increases by a factor 3 or more from maximum to minimum of activity. As a quantum of magnetic flux is associated to each network element, density variations of the photospheric network express in fact variations of the quiet Sun magnetic flux. It thus results that the quiet Sun magnetic flux is not uniformly distributed in latitude and not constant over the solar cycle: it probably varies in antiphase to the flux in active regions.The variation over the solar cycle and the latitude distribution of photospheric network density are compared to those of X-ray bright points and ephemeral active regions: there are no clear correlations between these three kinds of magnetic features.