J. L. R. Saba

Fe XVII Soft X-Ray Lines: Theory and Data Comparisons

Fe XVII soft X-ray spectral emission lines are examined using data from the Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission satellite. Results are compared with theoretical calculations and with other recent observational results. Disparate findings from different studies on the inferred opacity of the bright resonance line at 15.01 A and on its center-to-limb behavior are reviewed. Present limitations on the use of resonance scattering to infer coronal plasma densities and absolute elemental abundances are discussed. An analysis is made of the temperature-insensitive ratio of the 15.01 A line of Fe XVII to the optically thin 16.78 A line. This analysis shows that approximately half of the photons expected in the 15.01 A line are missing from the bright emission cores of quiescent active regions on the solar disk; the missing fraction increases at most by 50% near the solar limb. If the missing flux has been resonantly scattered out of the line of sight, then the equivalent optical depth at line center of the 15.01 A line is τ0~2 on the disk, based on a simple escape probability treatment for a slab geometry. This suggests that the effects of resonance scattering for other FCS lines, with the possible exception of the O VIII doublet at 18.97 A, should be negligible for quiescent active region conditions. This is consistent with the lack of systematic center-to-limb dependence found previously for FCS lines other than Fe XVII at 15.01 A. Yohkoh Soft X-ray Telescope measurements of the expected lines of sight for active regions as a function of location on the solar disk, and resonance scattering results from other soft X-ray active region data sets all support a trend of increased opacity at the limb compared to disk center.

The Structure and Properties of Solar Active Regions and Quiet-Sun Areas Observed in Soft X-Rays with Yohkoh/SXT and in the Extreme-Ultraviolet with SERTS

We observed two solar active regions (NOAA regions 7563 and 7565), quiet-Sun areas, and a coronal hole region simultaneously with Goddard Space Flight Centers Solar EUV Rocket Telescope and Spectrograph (SERTS) and with the Yohkoh Soft X-ray Telescope (SXT) on 1993 August 17. SERTS provided spatially resolved active region and quiet-Sun slit spectra in the 280 to 420 A wavelength range, and images in the lines of He II λ303.8, Mg IX λ368.1, Fe XV λ284.1, and Fe XVI λλ335.4 and 360.8 SXT provided images through multiple broadband filters in both the full-frame imaging mode and the partial-frame imaging mode. The SERTS images in Fe XV (log Tmax = 6.33, where Tmax is the temperature which maximizes the fractional ion abundance in the available ionization equilibrium calculations, i.e., the formation temperature) and Fe XVI (log Tmax = 6.43) exhibit remarkable morphological similarity to the SXT images. Whereas the Fe XV and XVI images outline the loop structures seen with SXT, the cooler He II (log Tmax = 4.67) and Mg IX (log Tmax = 5.98) images outline loop footpoints. In addition, the Mg IX emission outlines other structures not necessarily associated with the hot loops; these may be cool (T 1 × 106 K) loops. From the spatially resolved slit spectra, we obtained emission-line profiles for lines of He II λ303.8, Mg IX λ368.1, Fe XIII λ348.2, Si XI λ303.3, Fe XIV λ334.2, Fe XV λ284.1, and Fe XVI λ335.4 for each spatial position. Based upon the spatial variations of the line intensities, active region 7563 systematically narrows when viewed with successively hotter lines, and appears narrowest in the broadband soft X-ray emission. The active region width (full width at half-maximum intensity) diminishes linearly with log Tmax; the linear fit yields an extrapolated effective log Tmax of 6.51 ± 0.01 for the X-ray emission. The most intense, central core straddles the magnetic neutral line. Active region and quiet-Sun one-dimensional temperature scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines, and from coregistered SXT filter ratios. The highest plasma temperatures were measured in the most intense, central core of region 7563. The temperatures derived from Fe XVI λ335.4/Fe XV λ284.1 and Fe XVI λ335.4/Fe XIV λ334.2 vary significantly (based upon the measurement uncertainties) but not greatly (factors of less than 1.5) across the slit. The average log T values derived from the above two ratios for region 7563 are 6.39 ± 0.04 and 6.32 ± 0.02, respectively. Somewhat larger systematic variations were obtained from all available SXT filter ratios. The average active region log T values derived from the SXT AlMgMn/thin Al, thick Al/thin Al, and thick Al/AlMgMn filter ratios are 6.33 ± 0.03, 6.45 ± 0.02, and 6.49 ± 0.03, respectively. Active region and quiet-Sun one-dimensional density scans were derived from intensity ratios of spatially resolved SERTS slit spectral lines of Fe XIII and Fe XIV. The derived densities show neither systematic nor significant variations along the slit in either the active region or the quiet-Sun, despite the fact that the intensities themselves vary substantially. This indicates that the product of the volume filling factor and the path length (fΔl) must be greater by factors of 3-5 in the active region core than in the outskirts. Furthermore, the derived active region densities are ~2 times the quiet-Sun densities. This density difference is adequate to explain the factor of ~4 intensity difference in Fe XII and Fe XIII between the active and quiet areas, but it is not adequate to explain the factor of ~8 intensity difference in Fe XIV between the active and quiet areas. We attribute the latter to a greater fΔl in the active regions. Statistically significant Doppler shifts are not detected in region 7563 or in the quiet-Sun with any of the EUV lines.

Coronal magnetic structures observing campaign. I - Simultaneous microwave and soft X-ray observations of active regions at the solar limb

Using simultaneous microwave and soft X-ray measurements made with the VLA at 6 and 20 cm and the X-ray Polychromator (XRP) aboard the Solar Maximum Mission, two active regions near the solar limb have been studied. The images in soft X-rays and at 20 cm wavelength are similar : both show peaks above the active regions and extended bridge of emission 200000 km long connecting the two regions. The brightness temperature of the 20 cm emission is lower than that predicted from the X-ray emitting material, however; it can be attributed to free-free emission in cooler plasma not visible to XRP, with an optical depth ∼ 1.

Emission Measure Distribution for an Active Region Using Coordinated SERTS and Yohkoh SXT Observations

Often the derived temperature of an active region re—ects the method and the nature of the instrument used in its measurement. The emission measure (i.e., the amount of emitting material) derived from spec- troscopic observations usually depends on assumptions about the absolute elemental abundances and ionization fractions of the emitting ions. Yet establishing the distribution of emission measure with tem- perature is the —rst step needed to proceed with most of the interesting physics of active regions¨ including heating processes, cooling timescales, and loop stability. Accurately characterizing the thermal distribution of the coronal plasma requires data which can resolve multithermal features and constrain both low- and high-temperature emission. To model the temperature distribution of NOAA Active Region 7563, we have combined broadband —lter data from the Yohkoh Soft X-Ray Telescope (SXT) with simultaneous spectral line data from the Goddard Solar EUV Rocket Telescope and Spectrograph (SERTS) taken during its —ight on 1993 August 17. We have used a forward-folding technique to deter- mine the emission measure distribution of the active region loops. We have found that (1) the SXT response functions are sensitive to both the elemental abundances and the ionization fractions assumed to compute the solar spectrum that is folded through the instrument eUective area; (2) the relative cali- bration between the SERTS and the SXT instruments must be adjusted by a factor of 2 (a value consis- tent with the absolute measurement uncertainty of the 1993 SERTS —ight) no matter which abundances or iron ionization fractions are used; (3) the two-peaked diUerential emission measure previously deter- mined using SERTS data alone is not consistent with the SXT data: including the SXT data as a high- temperature constraint in the analysis requires that the emission above about 3 MK drop oU steeply rather than extending out to 6 MK. The sensitivity of the SXT —lter response functions to elemental abundance and iron ionization fraction could have a major impact on many routine analyses of SXT data. The emission measures can be greatly aUected (up to a factor of 7) and temperatures derived from —lter ratios can be signi—cantly altered (up to at least 40%) by adopting diUerent sets of commonly used elemental and ionic abundances. The results of our multithermal analysis imply that using broadband SXT data or a comparable high-temperature constraint in conjunction with high-resolution spectra covering a wide lower temperature range to study solar active regions can signi—cantly improve the information derived from either data set alone. In this study, the revised multithermal distribution reduces the thermal energy content of the region by about a factor of 2 and the required heating by about a factor of 5, which in turn relaxes some constraints on possible heating models. Subject headings: Sun: activitySun: X-rays, gamma rays

Evidence for coronal turbulence in a quiescent active region

The first evidence for nonthermal broadening of X-ray lines in a quiescent active region was based on a single observation of a limb active region by the Flat Crystal Spectrometer (FCS) on the Solar Maximum Mission (SMM) satellite /1/. With the renewal of SMM operations, the FCS has been used to further investigate this phenomenon. On 28 April 1984, a map of Mg XI resonance line profiles was made for a bright area in NOAA Active Region 4474 during a nonflaring period. The narrowest line profiles are consistent with the nominal instrumental width plus a thermal width equivalent to about 3 × 106 K, the temperature derived from line ratios of O VIII, Ne IX, and Mg XI. The broadest line profiles are consistent with the instrumental width plus a thermal width equivalent to about 7 × 106 K, but a substantial amount of plasma at this temperature would result in much greater flux in the FCS higher-temperature channels than was seen. If the excess width is attributed solely to plasma turbulence, the corresponding velocity would be about 40±10 km s−1. If associated with energy dissipation, such turbulent motions could have important consequences for coronal heating. Correlations of line widths and intensities with magnetic and Hα structures are investigated.

Coronal abundances of O, Ne, Mg, and Fe in solar active regions

High resolution soft X-ray spectra acquired with the Flat Crystal Spectrometer on Solar Maximum Mission provide an excellent data base for studying abundances in solar active regions, and testing current ideas on elemental selection by first ionization potential (FIP). The temperature-diagnostic line ratio Fe XVIII/Fe XVII allows removal of the weak temperature dependence of line ratios involving O VIII, Ne IX, Mg XI, and Fe XVII, to determine the relative abundances among the “high-FIP” elements O and Ne, and the “low-FIP” elements Mg and Fe. Significant variability is found for all combinations of relative abundance. The low-FIP/high-FIP Fe:Ne, Mg:Ne, and Mg:O vary by at least a factor of 5; the low-FIP/high-FIP Fe:O, the low-FIP/low-FIP Mg:Fe and the high-FIP/high-FIP Ne:O vary by a factor of 3. The extreme values of the relative abundances depend on the line emissivity calculations used, but the magnitude of variation is about the same for any simple curve through the data. No single set of atomic parameters and ion fractions can account for the observed variations.

Looking for the FIP Effect in EUV Spectra: Examining the Solar Case

Systematic differences between elemental abundances in the corona and in the photosphere have been found in the Sun. The abundance anomalies are correlated with the first ionization potentials (FIP) of the elements. The overall pattern is that low-FIP elements are preferentially enhanced relative to high-FIP elements by about a factor of four; the transition occurs at about 10 eV. This phenomenon has been measured in the solar wind and solar energetic particle composition, and in EUV and X-ray spectra of the corona and flares. The FIP effect should eventually offer valuable clues into the process of heating, ionization and injection of material into coronal and flaring loops for the Sun and other stars. The situation for the Sun is remarkably complex: substantial abundance differences occur between different types of coronal structures, and variations occur over time in the same region and from flare to flare. Anomalies such as enhanced Ne/O ratios, distinctly at odds with the basic FIP pattern, have been reported for some flares. Are the high-FIP elements underabundant or the low-FIP elements overabundant with respect to hydrogen? This issue, which has a significant impact in physical interpretation of coronal spectra, is still a subject of controversy and an area of vigorous research.

A study of solar preflare activity using two-dimensional radio and SMM-XRP observations

We present a study of type III activity at meter- decameter wavelengths in the preflare phase of the 1986 February 3 flare using data obtained with the Clark Lake Multifrequency Radioheliograph. We compare this activity with similar type III burst activity during the impulsive phase and find that there is a displacement of burst sources between the onset and end times of the activity. A comparison of this displacement at three frequencies suggests that the type III emitting electrons gain access progressively to diverging and different field lines relative to the initial field lines. The energetics of the type III emitting electrons are inferred from observations and compared with those of the associated hard X-ray emitting electrons. The soft X-ray data from SMM-XRP shows enhanced emission measure, density and temperature in the region associated with the preflare type III activity.

Preliminary results from the Coronal Magnetic Structures Observing Campaign (CoMStOC)

Abstract The object of the Coronal Magnetic Structures Observing Campaign (CoMStOC) was to measure the electron density and the magnetic field strength in coronal loops, quantities which are poorly known but essential to the understanding of the solar corona. Simultaneous soft X-ray and microwave observations were taken by the Solar Maximum Missions (SMM) X-Ray Polychromator (XRP) and the Very Large Array (VLA) on four days in the campaign period (25 Nov to 21 Dec 1987). Supporting multi-waveband observations were used to choose target regions, understand morphology, track evolution, and co-register images. XRP maps in soft X-ray resonance lines formed at different coronal temperatures provide accurate temperature and emission measure diagnostics. VLA maps at several frequencies in the 20 cm and 6 cm bands and Owens Valley spectra yield information on microwave structure, spectrum and polarization. The combined data set separates contributions from the two dominant microwave emission mechanisms, thermal bremsstrahlung and gyroresonance. Where gyroresonance is important, the coronal magnetic field strength can be determined with the aid of theoretical modeling. CoMStOC has provided an unprecedented set of coordinated multi-waveband observations of five new cycle active regions, offering a varied sample of intensity, activity, complexity, and projection angle. Whatever the final scientific return from the detailed studies now in progress, CoMStOC has already provided a wealth of experience in obtaining coordinated, multi-waveband observations of solar active regions.

NeO, MgO and FeO abundances derived from spectroscopic and sep analysis

The relative abundances NeO, MgO, and FeO derived from 33 Solar Maximum Mission Flat Crystal Spectrometer spectra are compared with those derived from the in situ Solar Energetic Particle analysis of Reames (1995). The spectra were obtained from non-flaring, quasi-stable active regions and include lines of the ions Mg XI (9.17 A), Ne IX (13.45 A), Fe XVIII (14.22 A), Fe XVII (15.25 A), Fe XVII (16.78 A), and O VIII (18.97 A) which were used in this study. With a characteristic temperature determined from the ratio of the Fe XVIII to Fe XVII (16.78 A) line fluxes, the abundance ratios are obtained using the fluxes and emissivity functions of lines from these different elements.