S xi Emission Lines in Active Region Spectra Obtained with the Solar euv Rocket Telescope and Spectrograph (Serts)

Theoretical electron density sensitive emission line ratios involving a total of eleven 2s ²2p ²–2s2p ³ transitions in Sxi between 187 and 292 Å are presented. A comparison of these with solar active region observations obtained during rocket flights by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals generally good agreement between theory and experiment. However, the 186.87 Å line is masked by fairly strong Fexii emission at the same wavelength, while 239.83 Å is blended with an unknown feature, and 285.58 Å is blended with possibly Niv 285.56 Å. In addition, the 191.23 Å line appears to be more seriously blended with an Fexiii feature than previously believed. The presence of several new Sxi lines is confirmed in the SERTS spectra, at wavelengths of 188.66, 247.14 and 291.59 Å, in excellent agreement with laboratory measurements. In particular, the detection of the 2s ²2p ^{2 3} P ₁ –2s2p ^{3 3} P _0,1 transitions at 242.91 Å is the first time (to our knowledge) that this feature has been identified in the solar spectrum. The potential usefulness of the Sxi line ratios as electron density diagnostics for the solar transition region and corona is briefly discussed.     

Emission line ratios for C III in the sun

Theoretical electron-density-sensitive C III emission line ratios are presented forR ₁ =I(2s2p ³ P – 2p ^{2 3} P)/I(2s2p ¹ P – 2p ^{2 1} S) =I(1176 Å)/I(1247 Å),R ₂ =I(2s2p ³ P – 2p ^{2 3} P)/I(2s ^{2 1} S – 2s2p ³ P ₁) =I(1176 Å)/I(1908 Å), andR ₃ =I(2s2p ¹ P – 2p ^{2 1} S)/I(2s ^{2 1} S – 2s2p ³ P ₁) =I(1247 Å)/I(1908 Å). These are significantly different from those deduced previously, principally due to the adoption of improved electron impact excitation rates in the present analysis. Electron densities deduced from the present theoretical line ratios, in conjunction with observed values ofR ₁,R ₂, andR ₃ measured from solar spectra obtained by the Naval Research Laboratory's S082B instrument on boardSkylab, are found to be generally compatible. In contrast, previous diagnostic calculations imply electron densities fromR ₁,R ₂, andR ₃ that differ by up to two orders of magnitude. These results provide observational support for the accuracy of the atomic physics adopted in the present calculations, and the methods employed in the derivation of the theoretical line ratios.     

Theoretical Siiv line ratios compared to extreme ultraviolet solar observations

New theoretical electron temperature sensitive emission line ratios in Siiv involving the 3d ² D – 3p ² P and 4s ² S – 3p ² P multiplets at 1125 and 816 Å, respectively, are derived using recent R-matrix electron excitation rate calculations. A comparison of these with observational data for a solar active region at the limb obtained with the Harvard S-055 spectrometer on board Skylab reveals that there is good agreement between theory and observation for ratios that include the ² D _{3/2, 5/2} – ² P _3/2 transition at 1128.3 Å. This is in contrast to the findings of Keenan, Dufton, and Kingston (1986) and provides support for the atomic data adopted in the calculations. However, the ² D _3/2 – ² P _1/2 line at 1122.5 Å appears to be severely blended, as suggested previously by Burton and Ridgeley (1970) and Feldman and Doschek (1977), as it leads to electron temperature estimates that differ significantly from that expected in ionisation equilibrium. The fact that the I(1122.5 Å)/I(1128.3 Å) intensity ratios determined from several flare spectra are closer to theory than that for the active region indicates that the blending is probably due to species with relatively low ionization potentials, as noted by Flower and Nussbaumer (1975). Electron temperatures deduced for a sunspot are much lower than that predicted from ionisation balance calculations, in agreement with earlier results, and imply that a cooling flow may be present.     

A Comparison of Theoretical mgvi Emission Line Strengths with Active-Region Observations From Serts

R-matrix calculations of electron impact excitation rates in N-like Mgvi are used to derive theoretical electron-density-sensitive emission line ratios involving 2s ²2p ³–2s2p ⁴transitions in the 269–403 Å wavelength range. A comparison of these with observations of a solar active region, obtained during the 1989 flight of the Solar EUV Rocket Telescope and Spectrograph (SERTS), reveals good agreement between theory and observation for the 2s ²2p ^{3 4} S–2s2p ^{4 4} Ptransitions at 399.28, 400.67, and 403.30 Å, and the 2s ²2p ^{3 2} P–2s2p ^{4 2} Dlines at 387.77 and 387.97 Å. However, intensities for the other lines attributed to Mgvi in this spectrum by various authors do not match the present theoretical predictions. We argue that these discrepancies are not due to errors in the adopted atomic data, as previously suggested, but rather to observational uncertainties or mis-identifications. Some of the features previously identified as Mgvi lines in the SERTS spectrum, such as 291.36 and 293.15 Å, are judged to be noise, while others (including 349.16 Å) appear to be blended.     

Theoretical emission line strengths for the beryllium-like ion Sxiii compared to XUV observations of solar flares

A comparison of Skylab S082A observations for several solar flares with calculations of the electron temperature sensitive emission line ratio R ₁ = I(2s2p ¹ P – 2s ^{2 1} S)/I(2s2p ³ P ₁ - 2s ^{2 1} S) = = I(256.68 Å)/I(491.45 Å) in Be-like SXIII reveals good agreement between theory and experiment, which provides observational support for the accuracy of the adopted atomic data. However, observed values of the electron density sensitive ratio R ₂ = I(2s2p ¹ P – 2s ^{2 1} S)/I(2p ^{2 3} P ₂ - 2s2p ³ P ₂) = = I(256.68 Å)/I(308.96 Å) all lie below the theoretical high density limit, which is probably due to blending in the 308.96 Å line.     

Extreme Ultraviolet Emission Lines of ca xv in Solar and Laboratory Spectra

New R-matrix calculations of electron impact excitation rates in Caxv are used to derive theoretical electron density diagnostic emission line intensity ratios involving 2s ²2p ²–2s2p ³ transitions, specifically R ₁=I(208.70 Å)/I(200.98 Å), R ₂=I(181.91 Å)/I(200.98 Å), and R ₃=I(215.38 Å)/I(200.98 Å), for a range of electron temperatures (T _e=10^6.4–10^6.8 K) and densities (N _e=10⁹–10¹³ cm^–3) appropriate to solar coronal plasmas. Electron densities deduced from the observed values of R ₁, R ₂, and R ₃ for several solar flares, measured from spectra obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab, are found to be consistent. In addition, the derived electron densities are in excellent agreement with those determined from line ratios in Caxvi, which is formed at a similar electron temperature to Caxv. These results provide some experimental verification for the accuracy of the line ratio calculations, and hence the atomic data on which they are based. A set of eight theoretical Caxv line ratios involving 2s ²2p ²–2s2p ³ transitions in the wavelength range 140–216 Å are also found to be in good agreement with those measured from spectra of the TEXT tokamak plasma, for which the electron temperature and density have been independently determined. This provides additional support for the accuracy of the theoretical line ratios and atomic data.     

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.     

Transitions between the n = 2 and n = 3 levels of Ne VII in the solar spectrum

Theoretical populations of the 2s3l levels of Ne vii are presented for electron temperatures from 2.5 × 10⁵ K to 4 × 10⁶ K and electron densities from 10⁸ cm^–3 to 10¹² cm^–3. These, in conjunction with intensities of previously observed solar Ne vii lines and wavelengths and intensities observed in the laboratory, are used to identify further Ne vii lines in the solar spectrum. The dependence on temperature of intensity ratios such as I(2s2p ¹ P – 2s3d ¹ D)/I(2s2p ³ P – 2s3d ³ D) is demonstrated and the advantages of the small wavelength separation of such lines for solar electron temperature diagnostics are discussed.     

The identification of Feix and Nixi in the solar corona

The levels of the configuration 3s ²3 _p ⁵3d of Fe ix have been experimentally determined from their combinations with 3s3 _p ⁶3d ³ D in the region 300–400 Å. Wavelengths can now be accurately predicted for all transitions within 3s ²3 _p 53^d, and eight of these can be identified with coronal lines from 2042 to 4585 Å. Also, identifications of solar lines from 171 to 245 Å with electric-dipole and magnetic-quadrupole transitions to the ground state, 3s ²3p ^{6 1} S, are confirmed and extended. Solar identifications with corresponding transitions in Ni xi, both within 3s ²3 _p ⁵3d and to the ground state, are proposed on the basis of a short extrapolation.     

EMISSION LINES OF Ni XVIII IN THE SOLAR EUV SPECTRUM

Using electron excitation rates calculated with the R-matrix code, theoretical Nixviii electron-temperature-sensitive emission line ratios are presented for R ₁ = I(220.41 Å)/I(320.56 Å) , R ₂ = I(233.79 Å)/I(320.56 Å) , and R ₃ = I(220.41 Å)/I(292.00 Å) . A comparison of these with observational data for two solar flares, obtained by the Naval Research Laboratory's S082A slitless spectrograph on board Skylab, reveals good agreement between theory and observation for R ₁ and R ₂ in two spectra, which provides limited support for the accuracy of the atomic data adopted in the analysis. However, several of the measured ratios are much larger than theory predicts, which is probably due mainly to saturation of the strong 292.00 and 320.56 Å lines on the photographic film used to record the S082A data. A comparison of our line ratio calculations with active region observations made by the Solar EUV Rocket Telescope and Spectrograph (SERTS) indicate that a feature at 236.335 Å, identified as the Nixviii 3p ² P _3/2 - 3d ² D _3/2 transition in the SERTS data, is actually the Arxiii 2s ²2p ^{2 3} P ₀ - 2s2p ^{3 3} D ₁ line. The potential usefulness of the Nixviii line ratios as electron temperature diagnostics for the solar corona is briefy discussed.     

Al xii line ratios in the Sun

Recent calculations of electron impact excitation rates in He-like Alxii are used to derive the theoretical electron temperature and density sensitive emission line ratios G ( = (f + i)/r and R ( = f/i, where f, i, and r are the forbidden 1s ^{2 1} S – 1s2s ³ S, intercombination 1s ^{2 1} S – 1s2p ³ P and resonance 1s ^{2 1} S – 1s2p ¹ P transitions, respectively. These ratios are found to be significantly different from earlier calculations, and are in much better agreement with X-ray spectral data for two solar flares obtained with the SMM and P78-1 satellites.     

Thallium in the solar atmosphere

Umbral spectra are shown to contain an absorption feature attributable to the Tl i transition 6p ² P°_3/2–7s ² S _1/2 at 5350 Å. Analysis of the umbral spectrum suggests a solar abundance in the 0.72< log N(Tl)T<1.10 on the standard scale log N(H) = 12.00. Unidentified blends limit the accuracy of the abundance determination.     

EUV line ratios from MgVII and SiIX

Following the work of Burgess et al. (1991), the EUV line ratios from MgVII and SiIX ions have been computed using most appropriate atomic data. The density dependent line ratio R₁ = (2s2p^{3 1}D°–2s²2p^{2 1}D)/ (2s2p^{3 3}S°–2s²2p^{2 3}P₂) and R₂ = (2s2p^{3 1}p°–2s²– –2p^{2 1}D)/(2s2p^{3 3}S°–2s²2p^{2 3}P₂) for MgVII and SiIX are compared with the earlier results of Mason and Bhatia (1978) and Keenan et al. (1986). Electron densities derived using observed R₁ and R₂ ratios from skylab NRL EUV spectra of solar flares and active regions are tabulated along with the values obtained by Keenan et al. (1986).     

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.     

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.     

Investigation of the Mg XII 8.42 Å doublet in solar flare spectra

The intensity ratio of the components of the Mg xii 8.42 Å (1s ² S _1/2 – 2p ² P _{1/2, 3/2}) doublet in solar flare spectra has been investigated using observations recorded from the Intercosmos 7 satellite. The observed values of the ratio fall within the interval 0.38–0.66 and have been compared with recent theoretical predictions based on an optically thin collisional-radiative model. It has been found that for the flare plasma the low values of the ratio cannot be explained since they fall below the smallest theoretical value. The highest values on the other hand require that an unacceptably high electron density be postulated. It is suggested that both high and low values may be caused by the resonance line scattering of the Mg xii quanta in the flare volume, provided that the volume is elongated and not spherical.The intensity of the nearby satellite lines is also investigated. Good agreement between the theoretical and observed intensities is found.     

Identification of forbidden coronal lines of Fe x and Ni xii

The transitions 3s ²3p ⁴3d ⁴ D, ⁴ F–3s3p ⁵3d ⁴ F have been studied in the Cl i isoelectronic sequence from Ca iv to Fe x. The determination of the 3s ²3p ⁴3d quartet intervals in Fe x has led to the identification of eleven coronal lines in the region from 1463 Å to 5539 Å as forbidden transitions within this configuration. By extrapolation, an additional coronal line is identified with a similar transition in Ni xii.     

A comparison of theoreticalCIV emission line strengths with active region observations obtained with the Solar EUV Rocket Telescope and Spectrograph (SERTS)

Theoretical line ratios involving 2s ² S - 3p ² P, 2p ² P - 3s ² S, and 2p ² S - 3d ² D transitions inCiv between 312 and 420 Å are presented. A comparison of these with solar active region observational data obtained during a rocket flight by the Solar EUV Rocket Telescope and Spectrograph (SERTS) reveals good agreement between theory and experiment, with discrepancies that average only 22%. This provides experimental support for the accuracy of the atomic data adopted in the line ratio calculations, and also resolves discrepancies found previously when the theoretical results were compared with solar data from the S082A instrument on boardSkylab. The potential usefulness of theCIV line ratios as electron temperature diagnostics for the solar transition region is briefly discussed.     

The impulsive X-ray burst of October 10, 1970

An impulsive burst of 100–400 keV solar X-rays associated with a small solar flare was observed on October 10, 1970 with a large area scintillator aboard a balloon floating at an altitude of 4.2 g cm^-2 above the Earth's surface. The X-ray burst was also observed simultaneously in 10–80 keV range by the OGO-5 satellite and in 8–20 Å range by the SOLRAD-9 satellite. The impulsive X-ray emission reached its maximum at 1643 UT at which time the differential photon spectrum in 20–80 keV range was of the form 2.3 × 10⁴ E ^-3.2 photons cm^-2 s^-1 keV^-1 at 1 AU. The event is attributed to a H-subflare located approximately at S13, E88 on the solar disc. The spectral characteristics of this event are examined in the light of the earlier X-ray observations of small solar flares.     

Some requirements of a colliding comet source of gamma ray bursts

Colliding comets in the Solar System may be an important source of gamma ray bursts. The spherical gamma ray comet cloud required by the results of the Venera Satellites (Mazets and Golenetskii, 1987) and the BATSE detector on the Compton Satellite (Meeganet al., 1992a, b) is neither the Oort Cloud nor the Kuiper Belt. To satisfy observations ofN(>P _max) vsP _max for the maximum gamma ray fluxes,P _max > 10^–5 erg cm^–2 s^–1 (about 30 bursts yr^–1), the comet density,n, should increase asn a ¹ from about 40 to 100 AU wherea is the comet heliocentric distance. The turnover above 100 AU requiresn a ^–1/2 to 200 AU to fit the Venera results andn a ^1/4 to 400 AU to fit the BATSE data. Then the masses of comets in the 3 regions are from: 40–100 AU, about 9 earth masses,m _E; 100–200 AU about 25m _E; and 100–400 AU, about 900m _E. The flux of 10^–5 erg cm^–2 s^–1 corresponds to a luminosity at 100 AU of 3 × 10²⁶ erg s^–1. Two colliding spherical comets at a distance of 100 AU, each with nucleus of radiusR of 5 km, density of 0.5 g cm^–3 and Keplerian velocity 3 km s^–1 have a combined kinetic energy of 3 × 10²⁸ erg, a factor of about 100 greater than required by the burst maximum fluxes that last for one second. Betatron acceleration in the compressed magnetic fields between the colliding comets could accelerate electrons to energies sufficient to produce the observed high energy gamma rays. Many of the additional observed features of gamma ray bursts can be explained by the solar comet collision source.