Explosive events in the solar transition zone

The properties of explosive events in the solar transition zone are presented by means of detailed examples and statistical analyses. These events are observed as regions of exceptionally high velocity ( 100 km s^–1) in profiles of Civ, formed at 10⁵ K, observed with the High Resolution Telescope and Spectrograph (HRTS). The following average properties have been determined from observations obtained during the third rocket flight of the HRTS: full width at half maximum extent along the slit - 1.6 × 10³ km; maximum velocity - 110 km s^–1; peak emission measure - 4 × 10⁴¹ cm^–3; lifetime - 60 s; birthrate - 4 × 10^–21 cm^–2 s^–1 in a coronal hole and 1 × 10^–20 cm^–2 s^–1 in the quiet Sun; mass - 6 × 10⁸ g; and, kinetic energy - 6 × 10²² erg. The 6 examples show that there are considerable variations from these average parameters in individual events. Although small, the events show considerable spatial structure and are not point-like objects. A spatial separation is often detected between the positions of the red and blue shifted components and consequently the profile cannot be explained by turbulence alone. Mass motions in the events appear to be isotropic because the maximum observed velocity does not show any correlation with heliographic latitude. Apparent motions of the 100 km s^–1 plasmas during their 60 s lifetime should be detected but none are seen. The spatial frequency of occurrence shows a maximum near latitudes of 40–50°, but otherwise their sites seem to be randomly distributed. There is enough mass in the explosive events that they could make a substantial contribution to the solar wind. It is hard to explain the heating of typical quiet structures by the release of energy in explosive events.     

Extreme ultraviolet spectra of solar active regions and their analysis

Extreme ultraviolet spectra of several active regions are presented and analyzed. Spectral intensities of 3 active regions observed with the NRL Skylab XUV spectroheliograph (170–630 Å) are derived. From this data density sensitive line ratios of Mg viii, Si x, S xii, Fe ix, Fe x, Fe xi, Fe xii, Fe xiii, Fe xiv, and Fe xv are examined and typically yield, to within a factor of 2, electron pressures of 1 dyne cm^–2 (n _e T = 6 × 10¹⁵ cm^–3 K). The differential emission measure of the brightest 35 × 35 portion of an active region is obtained between 1.4 × 10⁴ K and 5 × 10⁶ K from HCO OSO-VI XUV (280–1370 Å) spectra published by Dupree et al. (1973). Stigmatic EUV spectra (1170–1710 Å) obtained by the NRL High Resolution Telescope and Spectrograph (HRTS) are also presented. Doppler velocities as a function of position along the slit are derived in an active region plage and sunspot. The velocities are based on an absolute wavelength scale derived from neutral chromospheric lines and are accurate to ±2 km s^–1. Downflows at 10⁵ K are found throughout the plage with typical velocities of 10 km s^–1. In the sunspot, downflows are typically 5 to 20 km s^–1 over the umbra and zero over the penumbra. In addition localized 90 and 150 km s^–1 downflows are found in the umbra in the same 1 × 1 resolution elements which contain the lower velocity downflows. Spectral intensities and velocities in a typical plage 1 resolution element are derived. The velocities are greatest ( 10 km s^–1) at 10⁵ K with lower velocities at higher and lower temperatures. The differential emission measure between 1.3 × 10⁴ K and 2 × 10⁶ K is derived and is found to be comparable to that derived from the OSO-VI data. An electron pressure of 1.4 dynes cm^–2 (n _e T = 1.0 × 10¹⁶ cm^–3 K) is determined from pressure sensitive line ratios of Si iii, O iv, and N iv. From the data presented it is shown that convection plays a major role in determining the structure and dynamics of the active region transition zone and corona.     

Solar X-ray spectra observed from the ‘Intercosmos-4’ satellite and the ‘Vertical-2’ rocket

Results are given of the detailed analysis of fourteen Fe xxv-xxiii lines ( = 1.850–1.870 Å) in the spectra of a solar flare on 16 Nov. 1970. The spectra were obtained with a resolution of about 4 × 10^–4 Å, which revealed lines not previously observed and allowed the measurement of line profiles. The measured values of the wavelengths and emission fluxes are presented and compared with theoretical calculations. The analysis of the contour of the Fe xxv line ( = 1.850 Å) leads to the conclusion that there is unidirectional macroscopic gas motion in the flare region with the velocity (projection on the line of sight) ± 90 km s^–1.Measurements of the 8.42 Å Mg xii and 9.16 Å Mg xi lines in the absence of solar flares indicate prolonged existence of active regions on the solar disk with T _e = 4–6 × 10⁶K and emission measure ME 10⁴⁸ cm^–3. The profile of the Mg xii line indicates a macroscopic ion motion with a velocity up to 100 km s^–1.     

Analysis of the emission line spectra of a solar flare observed from Skylab

The EUV emission spectra in the wavelength range 110–1900 Å of the 5 September 1973 flare observed with the NRL slit spectrograph on Skylab are studied. The results are: (1) The chromospheric and transition-zone lines are greatly enhanced during the flare. In particular, the allowed lines are enhanced more than the intersystem lines. The Ni ii and P ii lines show the greatest enhancement with a factor of 800 increase in intensity. Other lines such as O i, C i, Si iii, S iii, S iv, O iv, O v, and N v show increases in intensity 10–100 times during the flare. (2) The chromospheric lines, although greatly enhanced during the flare, maintain their sharp and gaussian profiles and are not appreciably broadened. The transition zone lines, on the other hand, show a red-shifted component during the initial phase of the flare. The deduced downward velocity in the transition zone is 50 km s^–1. In addition, there are large turbulent mass motions. The downward mass motion is probably caused by the pressure imbalance between the flare hot plasma at 13 × 10⁶ K and the cooler plasma at 10⁵ K. (3) The density of the 10⁵ K flare plasma, as deduced from density-sensitive lines, is greater than 10¹² cm^-3. The depth of the 10⁵ K plasma in the flare transition zone is only of the order of 0.1 km, giving a steep temperature gradient. Consideration of the energy balance between the conductive flux and the radiative energy losses shows that, indeed, the high density in the transition zone requires that its thickness be very small. This is a consequence of the maximum radiative efficiency at the temperature around 10⁵ K in the solar plasma.Ball Brothers Research Corporation.     

Densities and mass motions in transition-zone plasmas in solar flares observed from Skylab

We studied the EUV line spectra of three flare observed with the NRL slit spectrograph on Skylab. The electron densities in the flare transition-zone plasmas are determined from density-sensitive lines of Si iii and O iv. The electron densities in all three flares studied were greatest during the flare maximum with values of the order of 10¹² cm^–3. The density decreases by a factor of 2 to 3 in the decay phase of the flares. The intensities of EUV lines from the flare chromospheric and transition-zone plasmas all are greatly enhanced. In contrast to lines for Oi, Ci, Feii and other chromospheric ions, the lines of Oiv and Nv and other transition-zone lines are not only enhanced but also very much broadened.Fitting of the N v 1242 Å line with a two-gaussian model shows that for two of the flares studied, there is a red-shifted component in addition to an unshifted component. The shifted component in the N v line profiles is interpreted as due to a dynamic and moving plasma with a bulk motion velocity of 12 km s^–1 for one flare and more than 70 km s^–1 for the other. The broadened line profiles indicate that there are large turbulent mass motions with random velocities ranging from 30 to 80 km s^–1.Ball Corporation. Now with NASA/Marshall Space Flight Center.     

The far UV spectrum of the Be star AX Monocerotis

In this paper we study the main features of the far UV spectrum of the binary star AX Mon, observed with the IUE satellite at phase 0.568.Ions indicating a large range of ionization stages, going fromCi,Oi,Ni toSiv,Civ,Nv are present.The spectrum is dominated by shell absorption lines of Feii, Feiii, Siiii,Cii, Alii, Mgii and Niii.Two satellite components are clearly indicated in all these lines except for Niii which presents only one. Their mean velocities are +10±5 km s^–1, –75±10 km s^–1, and –260±15 km s^–1.Red emission wings are observed in the Mgii resonant doublet at 2800 Å, which shows a P Cygni profile. Each of the absorption lines of the Mgii doublet is formed by a narrow component, which is blended with the Mgii interstellar line and a broad component, which shows a complex structure.Broad and asymmetrical profiles are observed for the Siiv,Civ, andNv resonance lines with blue edge velocities about –700±30 km s^–1.     

Absolute redshifts in the C iv 1548 Å line in the transition region of the quiet Sun

Observations with the UVSP instrument on the SMM spacecraft were made at the polar limb and disk center for the accurate determination of Doppler shifts of the Civ 1548 Å emission line formed at 10⁵ K in the transition region of the quiet Sun. Individual data points representing 3 arc sec square pixels yield both redshifts and blueshifts, but the mean values from four different days of observations are toward the red. The mean redshifts are in the range 4–8 km s^-1 and are produced by nearly vertically directed flows; the uncertainty associated with the mean values correspond to ±0.5 km s^-1. The redshift increases with brightness of the Civ line.     

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.     

The formation of Mg i 4571 Å in the solar atmosphere

An analysis of the 4571 Å line of neutral magnesium is presented in which one-dimensional macroscopic velocity fields are included. It is shown that gradients over restricted heights in the vertical and horizontal components of the velocity field of order -0.005 s^–1 and -0.004 s^–1 (such that velocity towards the observer decreases as height increases), respectively, result in asymmetries in the computed line profile similar to those observed. The heights in the solar atmosphere at which these velocity gradients exist are shown to be very critical in reproducing the observations. It was found that the best results were obtained when the gradients existed in the height range from 200 km to 300 km below the temperature minimum. The results indicate that for the Mg i 4571 Å line model calculations that do not include one-dimensional flow velocities may safely be compared with frequency-averaged observations.     

Relation between VLF phase deviations and solar X-ray fluxes during solar flares

Sudden phase anomalies (SPA's) observed in the phase of GBR 16 kHz VLF signals during the years 1977 to 1983 have been analysed in the light of their associated solar X-ray fluxes in the 0.5–4 Å and 1–8 Å bands. An attempt has been made to investigate the solar zenith angle () dependence of the integrated solar X-ray flux for producing SPA's. It is deduced from the observations for < 81° that the phase deviation increases linearly as a whole with increasing solar X-ray fluxes in these two bands. The threshold X-ray flux needed to produce a detectable SPA effect has been estimated to be 1.6 × 10^–4 ergcm^–2 s^–1 and 1.8 × 10^–3 ergcm^–2 s^–1 in the 0.5–4 Å and 1–8 Å bands, respectively. For both bands the average cross section for all atmospheric constituents at a height of 70 km is almost equal to the absorption cross section for the 3 Å X-ray emission.     

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.     

EUV observations of quiescent prominences from Skylab

We report measurements of line intensities and line widths for three quiescent prominences observed with the Naval Research Laboratory slit spectrograph on ATM/Skylab. The wavelengths of the observed lines cover the range 1175 Å to 1960 Å. The measured intensities have been calibrated to within approximately a factor 2 and are average intensities over a 2 arc sec by 60 arc sec slit. We derive nonthermal velocities from the measured line widths. The nonthermal velocity is found to increase with temperature in the prominence transition zone. Electron densities and pressures are derived from density sensitive line ratios. Electron pressures for two of the prominences are found to lie in the range 0.04–0.08 dyn cm^–2, while values for the third and most intense and active of the three prominences are in the range 0.07–0.22 dyn cm^–2.     

FLOWS AND DYNAMICS IN THE CORONA OBSERVED WITH THE CORONAL DIAGNOSTIC SPECTROMETER (CDS)

EUV spectra obtained with the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric Observatory (SOHO) show significant flows of plasma in active region loops, both at coronal and transition region temperatures. Wavelength shifts in the coronal lines Mgix 368 Å and Mgx 624 Å corresponding to upflows in the plasma reaching velocities of 50 km s^-1 have been observed in an active region. Smaller velocities are detected in the coronal lines Fexvi 360 Å and Sixii 520 Å. Flows reaching 100 km s^-1 are observed in spectral lines formed at transition region temperatures, i.e., Ov 629 Å and Oiii 599 Å, demonstrating that both the transition region and the corona are clearly dynamic in nature. Some high velocity events show even higher velocities with line profiles corresponding to a velocity dispersion of 300–400 km s^-1. Even in the quiet Sun there are velocity fluctuations of 20 km s^-1 in transition region lines. Velocities of the magnitude presented in this paper have never previously been observed in coronal lines except in explosive events and flares. Thus, the preliminary results from the CDS spectrometer promise to put constraints on existing models of the flows and energy balance in the solar atmosphere. The present results are compared to previous attempts to observe flows in the corona.     

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.     

Fabry-Pérot line profiles in the λ5303 å and λ6374 å coronal lines obtained during the 1983 Indonesian eclipse

During the total solar eclipse of 11 June, 1983, an imaging dual-channel Fabry-Pérot interferometer was used to obtain line profiles simultaneously in the green 5303 Å [Fe xiv] and the red 6374 Å [Fe x] coronal lines at various positions in the corona. Extensive microdensitometry followed by multi-Gaussian curve-fitting analysis has resulted in the determination of coronal temperatures and velocity separations between different pockets of coronal gas in the line of sight over a large extent of the corona. Fewer high temperature zones are to be found in the corona of 1983 compared with our similar green-line measurements of the solar maximum corona of 1980. The data are consistent with a temperature maximum occurring at 1.2 R , as found at the 1980 eclipse, but our new data are insufficient to observe farther out than this radius and so determine the position of a maximum. The velocity field in the corona at the 1983 eclipse is less structured compared with that at the 1980 eclipse and is mainly confined to the zone 20–30km s^–1.     

A comparison of theoretical line strengths for the 2s 22p 2 − 2s2p 3 transitions in Ne v with solar data

Results are presented for several theoretical line ratios in Nev involving transitions between multiplets in the 2s ²2p ² and 2s2p ³ configurations. A comparison of these with solar data from the S082A and S-055 instruments on board Skylab reveals generally good agreement between theory and experiment, especially in the case of the high-resolution (S082A) observations. However the 2s ²2p ^{2 1} D – 2s2p ^{3 1} P (365.6 Å) and 2s ²2p ^{2 3}P – 2s2p ^{3 3} S (359 Å) lines appear to be blended, possibly with transitions in Fex and Fexi/Fexiii, respectively. We note that the intensity ratio I(365.6 Å)/I(416.2 Å) should be a valuable calibration check for a high-resolution extreme ultraviolet instrument in the spectral range 360–420 Å.     

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.     

The coronal disturbance (W 90 °, N 25 °) in the eclipse spectra of July 31, 1981

The physical properties in the coronal disturbance (CD) (W90, N25°) associated with an active prominence are investigated on the basis of the intensities and profiles of 5694 Å Caxv and 6702 Å Nixv lines and continuum measured in the eclipse coronal spectra of 31 July, 1981. The spectrograms have been taken with a dispersion of between 7 to 10 Å mm^-1 and a solar image of 15 mm in diameter. The following characteristics of the CD have been deduced. The CD occurred cospatially with an active prominence and consisted of two discrete regions with different temperatures penetrating each other. (1) Caxv region: T _e= 3.8 × 10⁶ K, the length along the slit of the spectrograph Z 65000 km, the effective line-of-sight length L 20000 km, the average electron density , nonthermal velocities V _t= (20–32) km s^-1. (2)Nixv-Caxiii region: T _e= 2.3 × 10⁶ K, Z 37000 km, L 35000 km, n _e 1 × 10⁹ cm^-3, V _t= (23–30) km s^-1. A macroscopic mass motion has been discovered within the Nixv region of the CD from the Doppler shifts of the 6702 Å Nixv line: V _r= + 27 km s^-1 on the lower and V _r= - 12 km s^-1 on the upper border of the CD. The average height of the CD was H 0.08 R . The radial velocities in the prominence found from the emission line tilts are + 12 and - 8 km s^-1 on its lower and upper borders. A similar picture of the mass motion in the CD and the prominence speaks in favour of an intimate relation between them.     

Evolution of the high-temperature plasma in the 15 June 1973 flare

The analysis of the high temperature plasma in Fe xxiii–xxiv in the 15 June 1973 flare is presented. The observations were obtained with the NRLXUV spectroheliograph on Skylab. The results are: (1) There was preheating of the active region in which the flare occurred. In particular, a large loop in the vicinity of the flaring region showed enhanced brightness for many hours before the flare. The loop disappeared when the flare occurred, and returned in the postflare phase, as if the energy flux which had been heating the large loop was blocked during the flare and restored after the flare was gone. The large magnetic fields did not change significantly. (2) The flare occurred in low-lying loop or loops. The spatial distribution of flare emission shows that there was a temperature gradient along the loop. (3) The high temperature plasma emitting Fe xxiii and xxiv had an initial upward motion with a velocity of about 80 km s^–1. (4) There was large turbulent mass motion in the high temperature plasma with a random velocity of 100 to 160 km s^–1. (5) The peak temperature of the hot plasma, determined from the Fe xxiii and xxiv intensity ratio, was 14 × 10⁶ K. It decreased slightly and then, for a period of 4 min, remained at 12.6 × 10⁶ K before dropping sharply to below 10 × 10⁶ K. The density of the central core of the hot plasma, determined from absolute intensity of Fe xxiv 255 Å line, was of the order of 10¹¹ cm^–3.The persistence of the high level of turbulence and of the high temperature plateau in the decaying phase of the flare indicates the presence of secondary energy release. From the energy balance equation the required energy source is calculated to be about 3 to 7 ergs cm^–3 s^–1.Ball Brothers Research Corporation.     

Solar flare X-ray spectra

Results are presented of an investigation of solar flare X-ray spectra in the region 1.70–1.95 Å, obtained aboard the Intercosmos-4 satellite during the maximum of solar activity (October–November, 1970). With the use of 6 high resolution spectra in the region 1.85–1.87 Å the identification of lines due to 18 transitions of 2p 1s type, consisting of the resonance, intercombination and forbidden Fe xxv ion lines and the satellite Fe xxiv lines has been performed. With the use of the recent laboratory data the averaged wavelengths of the lines were obtained confirming the theoretically calculated ones with an accuracy about ± 0.0004 Å. A variable Doppler shift of the Fe xxv resonance lines was observed for the flare of November 16, 1970, which points to hot plasma motions with velocities up to 400 km s^-1.