Solar Extreme-Ultraviolet Research Telescope Research Articles

DETERMINATION OF NON-THERMAL VELOCITY DISTRIBUTIONS FROM SERTS LINEWIDTH OBSERVATIONS

Non-thermal velocities obtained from the measurement of coronal Extreme Ultraviolet (EUV) linewidths have been consistently observed in solar EUV spectral observations and have been theorized to result fro m many plausible scenarios including wave motions, turbulence, or magnetic reconnection. Constraining these velocities can provide a physical limit for the available energy resulting from unresolved motions in the corona. We statistically determine a series of non-thermal velocity distributions from linewidth measurements of 390 emission lines from a wide array of elements and ionization states observed during the Solar Extreme Ultraviolet Research Telescope and Spectrograph 1991-1997 flights covering the spectral range 174-418 A and a temperature range from 80,000 K to 12.6 MK. This sample includes 248 lines from active regions, 101 lines from quiet-Sun regions, and 41 lines were observed from plasma off the solar limb. We find a strongly peaked distribution corresponding to a non-thermal velocity of 19-22 km/s in all three of the quiet-Sun, active region, and off-limb distributions. For the possibility of Alfven wave resonance heating, we find thai velocities in the core of these distributions do not provide sufficient energy, given typical densities and magnetic field strengths for the coronal plasma, to overcome the estimated coronal energy losses required to maintain the corona at the typical temperatures working as the sole mechanism. We find that at perfect efficiency 50%-60% of the needed energy flux can be produced from the non-thermal velocities measured.

Emission lines of Fe xi in the 257-407 Å wavelength region observed in solar spectra from EIS/Hinodeand SERTS

Theoretical emission-line ratios involving Fe xi transitions in the 257–407 Å wavelength range are derived using fully relativistic calculations of radiative rates and electron impact excitation cross-sections. These are subsequently compared with both long wavelength channel Extreme-Ultraviolet Imaging Spectrometer (EIS) spectra from the Hinode satellite (covering 245–291 Å) and first-order observations (∼235–449 Å) obtained by the Solar Extreme-ultraviolet Research Telescope and Spectrograph (SERTS). The 266.39, 266.60 and 276.36 Å lines of Fe xi are detected in two EIS spectra, confirming earlier identifications of these features, and 276.36 Å is found to provide an electron density (Ne) diagnostic when ratioed against the 257.55 Å transition. Agreement between theory and observation is found to be generally good for the SERTS data sets, with discrepancies normally being due to known line blends, while the 257.55 Å feature is detected for the first time in SERTS spectra. The most useful Fe xi electron density diagnostic is found to be the 308.54/352.67 intensity ratio, which varies by a factor of 8.4 between Ne= 108 and 1011 cm−3, while showing little temperature sensitivity. However, the 349.04/352.67 ratio potentially provides a superior diagnostic, as it involves lines which are closer in wavelength, and varies by a factor of 14.7 between Ne= 108 and 1011 cm−3. Unfortunately, the 349.04 Å line is relatively weak, and also blended with the second-order Fe x 174.52 Å feature, unless the first-order instrument response is enhanced.

Emission lines of Fe x in active region spectra obtained with the Solar Extreme-ultraviolet Research Telescope and Spectrograph

Fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe x are used to derive theoretical emission-line ratios involving transitions in the 174–366 Å wavelength range. A comparison of these with solar active region observations obtained during the 1989 and 1995 flights of the Solar Extreme-ultraviolet Research Telescope and Spectrograph (SERTS) reveals generally very good agreement between theory and experiment. Several Fe x emission features are detected for the first time in SERTS spectra, while the 3s23p52P3/2--3s23p4(1S)3d 2D3/2 transition at 195.32 Å is identified for the first time (to our knowledge) in an astronomical source. The most useful Fe x electron density (Ne) diagnostic line ratios are assessed to be 175.27/174.53 and 175.27/177.24, which both involve lines close in wavelength and free from blends, vary by factors of 13 between Ne= 108 and 1011cm−3, and yet show little temperature sensitivity. Should these lines not be available, then the 257.25/345.74 ratio may be employed to determine Ne, although this requires an accurate evaluation of the instrument intensity calibration over a relatively large wavelength range. However, if the weak 324.73 Å line of Fe x is reliably detected, the use of 324.73/345.74 or 257.25/324.73 is recommended over 257.25/345.74. Electron densities deduced from 175.27/174.53 and 175.27/177.24 for the stars Procyon and α Cen, using observations from the Extreme-Ultraviolet Explorer (EUVE) satellite, are found to be consistent and in agreement with the values of Ne determined from other diagnostic ratios in the EUVE spectra. A comparison of several theoretical extreme-ultraviolet Fe x line ratios with experimental values for a θ-pinch, for which the plasma parameters have been independently determined, reveals reasonable agreement between theory and observation, providing some independent support for the accuracy of the adopted atomic data.

An investigation of Fe XVI emission lines in solar and stellar extreme-ultraviolet and soft X-ray spectra

New fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe XVΙ are used to determine theoretical emission-line ratios applicable to the 251-361 and 32-77 A portions of the extreme-ultraviolet (EUV) and soft X-ray spectral regions, respectively. A comparison of the EUV results with observations from the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS) reveals excellent agreement between theory and experiment. However, for emission lines in the 32A 9 A portion of the soft X-ray spectral region, there are large discrepancies between theory and measurement for both a solar flare spectrum obtained with the X-Ray Spectrometer/Spectrograph Telescope (XSST) and for observations of Capella from the Low-Energy Transmission Grating Spectrometer (LETGS) on the Chandra X-ray Observatory. These are probably due to blending in the solar flare and Capella data from both first-order lines and from shorter wavelength transitions detected in second and third order. By contrast, there is very good agreement between our theoretical results and the XSST and LETGS observations in the 50-77 A wavelength range, contrary to previous results. In particular, there is no evidence that the Fe XVI emission from the XSST flare arises from plasma at a much higher temperature than that expected for Fe xvi in ionization equilibrium, as suggested by earlier work.

Fe xiii emission lines in active region spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph

Abstract Recent fully relativistic calculations of radiative rates and electron impact excitation cross-sections for Fe xiii are used to generate emission-line ratios involving 3s23p2–3s3p3 and 3s23p2–3s23p3d transitions in the 170–225 and 235–450 Å wavelength ranges covered by the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS). A comparison of these line ratios with SERTS active region observations from rocket flights in 1989 and 1995 reveals generally very good agreement between theory and experiment. Several new Fe xiii emission features are identified, at wavelengths of 203.79, 259.94, 288.56 and 290.81 Å. However, major discrepancies between theory and observation remain for several Fe xiii transitions, as previously found by Landi and others, which cannot be explained by blending. Errors in the adopted atomic data appear to be the most likely explanation, in particular for transitions which have 3s23p3d 1D2 as their upper level. The most useful Fe xiii electron-density diagnostics in the SERTS spectral regions are assessed, in terms of the line pairs involved being (i) apparently free of atomic physics problems and blends, (ii) close in wavelength to reduce the effects of possible errors in the instrumental intensity calibration, and (iii) very sensitive to changes in Ne over the range 108–1011 cm−3. It is concluded that the ratios which best satisfy these conditions are 200.03/202.04 and 203.17/202.04 for the 170–225 Å wavelength region, and 348.18/320.80, 348.18/368.16, 359.64/348.18 and 359.83/368.16 for 235–450 Å.

FexiEmission Lines in a High‐Resolution Extreme‐Ultraviolet Active Region Spectrum Obtained by the Solar Extreme Ultraviolet Research Telescope and Spectrograph

New calculations of radiative rates and electron impact excitation cross sections for Fe XI are used to derive emission-line intensity ratios involving 3s23p4-3s23p33d transitions in the 180-223 Å wavelength range. These ratios are subsequently compared with observations of a solar active region obtained during the 1995 flight of the Solar Extreme Ultraviolet Research Telescope and Spectrograph (SERTS). The version of SERTS flown in 1995 incorporated a multilayer grating that enhanced the instrumental sensitivity for features in the ~170-225 Å wavelength range, observed in second order between 340 and 450 Å. This enhancement led to the detection of many emission lines not seen on previous SERTS flights, which were measured with the highest spectral resolution (0.03 Å) ever achieved for spatially resolved active region spectra in this wavelength range. However, even at this high spectral resolution, several of the Fe XI lines are found to be blended, although the sources of the blends are identified in the majority of cases. The most useful Fe XI electron density diagnostic line intensity ratio is I(184.80 Å)/I(188.21 Å). This ratio involves lines close in wavelength and free from blends, and it varies by a factor of 11.7 between Ne = 109 and 1011 cm-3 yet shows little temperature sensitivity. An unknown line in the SERTS spectrum at 189.00 Å is found to be due to Fe XI, the first time (to our knowledge) this feature has been identified in the solar spectrum. Similarly, there are new identifications of the Fe XI 192.88, 198.56, and 202.42 Å features, although the latter two are blended with S VIII/Fe XII and Fe XIII, respectively.

Emission lines of Fe xv in spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph

Recent R-matrix calculations of electron impact excitation rates in Mg-like Fe XV are used to derive theoretical emission-line ratios involving transitions in the 243‐418 A wavelength range. A comparison of these with a data set of solar active region, subflare and off-limb spectra, obtained during rocket flights by the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS), reveals generally very good agreement between theory and observation, indicating that most of the Fe XV emission lines may be employed with confidence as electron density diagnostics. In particular, the 312.55-A line of Fe XV is not significantly blended with aC oXVII transition in active region spectra, as suggested previously, although the latter does make a major contribution in the subflare observations. Most of the Fe XV transitions which are blended have had the species responsible clearly identified, although there remain a few instances where this has not been possible. We briefly address the long-standing discrepancy between theory and experiment for the intensity ratio of the 3s 21 S‐3s3p 3 P1 intercombination line at 417.25 A to the 3s 21 S‐3s3p 1 P resonance transition at 284.16 A. Ke yw ords: atomic data ‐ Sun: activity ‐ Sun: flares ‐ ultraviolet: general.