Fe XXI Line Research Articles

Spectral diagnostics with the SDO EVE flare lines

The diagnostic use of the soft X-ray and EUV lines observed with the Solar Dynamics Observatory (SDO) Extreme ultraviolet Variability Experiment (EVE) is discussed. We focus on all the flare lines o bserved in the 80‐640 A range (mainly due to Fexviii‐ Fexxiv), showing their use to measure temperatures, emission measures, densities, and chemical abundances. We discuss their identification at the EVE resolution, by using the latest atomic data, and by assess ing possible sources of blending, taking into account higher-resolution solar spectra. We present observations of four fla res, and study in more detail the gradual phase peak of the 7 March 2012 X5.6 flare. Good agreement between observations and theory is found in most cases, and the best lines for diagnostics are recommended. We found reasonable densities (10 11.2 cm −3 from Fexxi lines), but isothermal temperatures (12 MK) lower than those estimated with GOES. We show that EVE can be used to measure relative elemental abundance, and find photospheric argon /iron and calcium/iron abundances. We also show that lines normally formed in the quiet Sun in the low transition region such as O III are the best to study the impulsive phase.

Spectral Indications of Density Variability in the Corona of AD Leonis

Direct comparison of high-resolution X-ray spectra of the active dMe star AD Leo, observed three times with Chandra, shows variability of key density-sensitive lines, possibly due to flaring activity. In particular, a significant long-duration enhancement of the coronal density is indicated by the Ne IX and Si XIII He-like triplets, and possibly also by density-sensitive Fe XXI line ratios.

On the sizes of stellar X-ray coronae

Spatial information from stellar X-ray coronae can be inferred by use of scaling laws from the solar corona. This requires measurement of the physical parameters temperature and density. We concentrate on the newly available density measurements from line fluxes of X-ray lines measured for a large sample of 64 grating spectra of 42 stellar coronae obtained with Chandra and XMM-Newton. Line counts of strong H-like and He-like ions and FeXXI lines were measured with the CORA line fitting tool by Ness and Wichmann 2002. Densities are estimated from He-like f/i flux ratios of OVII and NeIX representing only cooler (1-6MK) plasma components. The densities scatter between log n_e = 9.5-11 from the OVII triplet and between log n_e = 10.5-12 from the NeIX triplet, but we caution that the latter triplet may be biased by contamination from FeXIX and FeXXI lines. We find that low-activity stars tend to show densities of no more than a few times 10^10cm^-3, whereas no definitive trend is found for the more active stars. Investigating the densities of the hotter plasma with various FeXXI line ratios, we found no consistent proof for the presence of very high densities. This result is in line with constant pressure in the emitting active regions. We focus on the commonly used RTV78 scaling law to derive loop lengths from temperatures and densities assuming loop-like structures as identical building blocks. We derive emitting volumes from direct measurements of ion-specific emission measures and densities. Available volumes are calculated from the loop-lengths and stellar radii. The ratios of these volumes depict volume filling factors, which we find similar up to 0.1 for all stages of activity for OVII and increasing for NeIX.

X‐Ray Spectroscopy of the Unsteady Quiescent Corona of AD Leonis withChandra

We present the analysis and interpretation of an observation of the flare star AD Leo (dM3e) with the Low Energy Transmission Grating of Chandra. The high-resolution X-ray spectrum—dominated by emission lines from O VII-VIII, Ne IX-X, and Fe XVII-Fe XIX—allowed us to infer the plasma emission measure distribution (EMD) versus temperature, as well as the abundances of individual elements in the corona of this magnetically active star, during a typical state characterized by significant variability but no evident flaring event. We have also measured plasma densities at various temperatures using spectroscopic diagnostics provided by He-like triplets and Fe XXI lines. We show that the present EMD is similar, in terms of overall shape and temperature of the peak, to those previously obtained from Extreme Ultraviolet Explorer (EUVE) spectra during quiescent and flaring states confirming the long-term stability of the corona of AD Leo. At variance with the case of other active stars, the EMD of AD Leo is characterized by a significantly shallower slope, compatible with that predicted by static models of isobaric loops with constant cross section and uniform heating. We discuss such coronal modeling to infer the average properties of the corona in terms of loop populations, including estimates of the surface filling factor derived by comparison between the model and the observed EMD. We also show that the EMD is compatible with the model of a corona continuously heated by flares, which predicts an EMD slope slightly steeper than observed, but that can be accommodated by observational uncertainties. The coronal composition is such that the element abundances, relative to solar values, tend to increase with the first ionization potential, with few exceptions. The line-to-continuum ratios suggest a nearly solar metallicity, a result difficult to reconcile with previous determinations based on global fitting of X-ray spectra.

Observations of 1000 km s-1Doppler shifts in 107 K solar flare supra-arcade

An X1.5 flare on the west limb of the Sun on 21 April 2002 developed a large supra-arcade about 30 min after flare onset. The growth of the supra-arcade can be followed in both TRACE 195 A images and SUMER spectra. Its growth seems to be associated with dark (in TRACE images), sunward moving channels that descend onto the arcade from above. SUMER recorded Doppler shifts of 800–1000 km s−1 in Fe xxi 1354 A from positions where this sunward flow interacts with the arcade tops. We describe the observations, focusing on the relationship of the high Fe xxi line shifts to the sunward moving dark flows.

Space Telescope Imaging Spectrograph Survey of Far‐Ultraviolet Coronal Forbidden Lines in Late‐Type Stars

We describe an extensive search with the Space Telescope Imaging Spectrograph (STIS) for ultraviolet coronal (T > 10 6 K) forbidden lines in a sample of 29 F-M dwarfs, giants, and supergiants. Measuring coronal lines in the 1150-1700 Aband with STIS has important advantages of superior velocity resolution and an absolute wavelength calibration compared with using the Chandra or XMM-Newton grating spectrometers to observe permitted transitions of the same ion stages in the kilovolt X-ray region. Fe xii �� 1242, 1349 (T � 2 � 10 6 K) and Fe xxi � 1354 (10 7 K) are well known from solar studies and have been reported in previous stellar work. A search for other coronal forbidden lines in the 1200-1600 Aregion was largely negative. The few candidate identifications (e.g., Ar xiii � 1330 and Ca xv � 1375) are too faint to be diagnostically useful. We add new dwarfs to the list of Fe xii detections, including the nearby solar twin � Cen A (G2 V). Clear detections of Fe xxi were obtained in dMe stars, active giants, a short-period RS CVn binary, and possibly in active solar-type dwarfs. We developed a semiempirical method for removing the C i blend that partially affects the Fe xxi � 1354 profile. As discussed recently by Johnson et al., Capella (� Aur; G8 III+G1 III) displays clear Fe xxi variability between Goddard High-Resolution Spectograph (GHRS) and STIS observations 4 yr apart, which is apparently due to a substantial decline in the contribution from the G8 primary. We present an alternative model of the GHRS and STIS era profiles using information in the two sets of line shapes jointly, as well as knowledge of the behavior of Fexxi profiles of other late-G '' clump '' giants similar to Capella G8. The full survey sample also provides a context for the apparent variability: the Fe xxi flux of the G8 star in the GHRS spectrum is nearly identical (in LFe XXI=Lbol) to other clump giants of similar LX=Lbol, but it had dropped at least a factor of 6 in the STIS measurement. The He ii � 1640 B� feature—which is thought to be responsive to coronal irradiation—also showed significant changes between the GHRS and STIS epochs, but the decrease in the G8 star was much smaller than Fe xxi. The Fe xii flux displays a correlation with the ROSAT 0.2-2 keV X-ray flux that can be described by an � ¼ 0:5 power law. Fe xxi exhibits a steeper, perhaps linear (� ¼ 1), correlation with the ROSAT flux down to an activity level of LX=Lbol � 10� 5, below which detections of the coronal forbidden line are rare. There is no evidence of large, systematic Doppler shifts in either Fe xii � 1242 or Fe xxi � 1354. This suggests that the emissions arise dominantly in confined structures, analogous to magnetic loops on the Sun, rather than, say, in a hot wind. The Fe xii and Fe xxi line widths generally are close to thermal (FWHM � 40 90 km s � 1 at T � 10 6:2 10 7:0 K), except for the Hertzsprung-gap giants 31 Comae (G0 III) and Capella G1 and the K1 subgiant primary of HR 1099, all of which show evidence for excess broadening in Fe xxi (Fe xii is obscured in these objects by broad N v � 1242 features). If the excess broadening is rotational, it implies that the hot coronae of '' X-ray- deficient '' 31 Com and Capella G1 are highly extended, contrary to the compact structures suggested by recent density estimates in a number of active coronal sources. Subject headings: stars: coronae — ultraviolet: stars — X-rays: stars

Chandra LETGS observation of the active binary Algol

A high-resolution spectrum obtained with the low-energy transmission grating onboard the Chandra observatory is presented and analyzed. Our analysis indicates very hot plasma with temperatures up to T 15 20 MK from the continuum and from ratios of hydrogen-like and helium-like ions of Si, Mg, and Ne. In addition lower temperature material is present since O vii and N vi are detected. Two methods for density diagnostics are applied. The He-like triplets from N vii to Si xiii are used and densities around 10 11 cm 3 are found for the low temperature ions. Taking the UV radiation eld from the B star companion into account, we nd that the low-Z ions can be aected by the radiation eld quite strongly, such that densities of 3 10 10 cm 3 are also possible, but only assuming that the emitting plasma is immersed in the radiation eld. For the high temperature He-like ions only low density limits are found. Using ratios of Fe xxi lines produced at similar temperatures are sensitive to lower densities but again yield only low density limits. We thus conclude that the hot plasma has densities below 10 12 cm 3 . Assuming a constant pressure corona we show that the characteristic loop sizes must be small compared to the stellar radius and that lling factors below 0.1 are unlikely.

Dissecting Capella's Corona: GHRS Spectra of the Fexxiλ1354 and Heiiλ1640 Lines from Each of the Capella Stars

We report on moderate (λ/Δλ = 20,000) and high (λ/Δλ = 90,000) resolution spectra of the 104-day period Capella binary system (HD 34029) obtained with Hubble Space Telescope's Goddard High-Resolution Spectrometer (GHRS) on 1995 September 9 and 1996 April 9. The observations include a long-duration, moderate-resolution spectrum of the coronal Fe XXI λ1354 line and both moderate- and high-resolution spectra of the He II λ1640 multiplet. Our objective in observing the Fe XXI line formed at T = 1 × 107 K is to determine for the first time the line shape parameters and the contribution of each star's corona. This is feasible because the GHRS can resolve the 53 km s-1 radial velocity separation of the stars. Our analysis led to four surprising results: (1) The contribution of the slowly rotating G8 III star to the total Fe XXI λ1354 flux is similar to that of the more active rapidly rotating G1 III star, in contrast to other UV lines formed at lower temperatures. (2) The centroid velocities of the Fe XXI lines from both stars are near their respective photospheric radial velocities. Thus, there is no evidence for downflows or winds, and the hot coronal plasma must be confined, presumably by strong, closed magnetic fields. This is the first direct kinematic evidence for magnetic confinement in the corona of a giant star. (3) The line widths are thermal, indicating very low turbulence (ξ < 23 km s-1) compared with the 54 km s-1 thermal speed. (4) Our analysis of Extreme-Ultraviolet Explorer (EUVE) spectra that include four Fe XXI lines shows that the volume emission measures of these lines are about a factor of 3 smaller than for the λ1354 line, which was observed several months after the closest EUVE observation. We consider possible explanations for this discrepancy and conclude that variability of the high-temperature coronal emission is the most likely explanation. We observed the He II λ1640 line twice with a time separation of 7 months, corresponding to about 2 orbital periods. The profiles are nearly identical, except for a large difference in flux near the expected radial velocity of the G8 III star. We believe that the broad He II emission produced by the G1 star is formed mostly by collisional excitation in its transition region, while the emission from the G8 star is produced predominantly by a photoionization/recombination process driven by EUV radiation from the G8 star's corona. The He II λ1640 flux that we measure from the G8 star is consistent with predictions based on EUVE flux measurements. The decrease in the λ1640 emission from the G8 star between the phase 0.73 and 0.78 measurements indicates that its EUV radiation is variable, as is observed by EUVE.

A Time-resolved Extreme-Ultraviolet Spectroscopic Study of the Quiescent and Flaring Corona of the Flare Star AU Microscopii

view Abstract Citations (46) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Time-resolved Extreme-Ultraviolet Spectroscopic Study of the Quiescent and Flaring Corona of the Flare Star AU Microscopii Monsignori Fossi, B. C. ; Landini, M. ; Del Zanna, G. ; Bowyer, S. Abstract The extreme-ultraviolet emission measured during a 1992 July 14-17 observation of AU Mic has been studied with lines detected with the short-, medium-, and long-wavelength spectrometers on the Extreme Ultraviolet Explorer. Flares were observed during this period. The time evolution of the differential emission measure is presented. The synthetic spectrum derived from the differential emission measure is compared with the observed data. An estimate of the density and volume of the emitting plasma is obtained by use of Fe XXI lines. Publication: The Astrophysical Journal Pub Date: July 1996 DOI: 10.1086/177522 Bibcode: 1996ApJ...466..427M Keywords: STARS: CORONAE; STARS: FLARE; STARS: INDIVIDUAL CONSTELLATION NAME: AU MICROSCOPII; ULTRAVIOLET: STARS full text sources ADS | data products MAST (2) SIMBAD (1) HEASARC (1)

Observing stellar coronae with the Goddard High Resolution Spectrograph. 1: The dMe star AU microscopoii

We report on an observation of AU Mic taken with the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope. The data consist of a rapid sequence of spectra covering the wavelength range 1345-1375 A with a spectral resolution of 10,000. The observations were originally intended to search for spectral variations during flares. No flares were detected during the 3.5 hr of monitoring. A method of reducing the noise while combining the individual spectra in the time series is described which resulted in the elimination of half of the noise while rejecting only a small fraction of the stellar signal. The resultant spectrum was of sufficient quality to allow the detection of emission lines with an integrated flux of 10(exp -15) ergs/sq cm(sec) or greater. Lines of C I, O I, O V, Cl I, and Fe XXI were detected. This is the first indisputable detection of the 1354 A Fe XXI line, formed at T approximately = 10(exp 7) K, on a star other than the Sun. The line was well resolved and displayed no significant bulk motions or profile asymmetry. From the upper limit on the observed line width, we derive an upper limit of 38 km/s for the turbulent velocity in the 10(exp 7) K plasma. An upper limit is derived for the flux of the 1349 A Fe XII line, formed at T approximately = 1.3 x 10(exp 6) K. These data are combined with contemporaneous GHRS and International Ultraviolet Explorer (IUE) data to derive the volume emission measure distribution of AU Mic over the temperature range 10(exp 4)-10(exp 7) K. Models of coronal loops in hydrostatic equilibrium are consistent with the observed volume emission measures of the coronal lines. The fraction of the stellar surface covered by the footprints of the loops depends upon the loop length and is less than 14% for lengths smaller than the stellar radius. From the upper limit to the estimated width of the Fe XXI line prfile we find that the we cannot rule out Alfven wave dissipation as a possible contributor to the required quiescent loop heating rate.

Enhanced Turbulence and Heating Observed Preceding the Impulsive Phase in a Solar Flare

AbstractWe studied the unique EUV spectra of the 21 Jan. 1974 flare observed by the NRL Normal-Incidence Slit Spectrometer (SO 82B) onboard the Skylab. The results show that the pre-impulsive transition region plasmas exhibited enhanced turbulence and heating before the acceleration of energetic particles. The absence of blue-shifted components in the Fe XXI line profiles shows that the chromospheric evaporation is not important in this flare.

SKYLAB XUV observations of densities, thermal structure, and mass motions in a compact flare

The extensive series of slit spectra of the energetic limb flare of December 17, 1973 obtained by Skylab is studied, and the December 17, 1973 flare is reconsidered in the light of these new data. Examples of the flare morphology observed in He II, NeIV, and Mg VII are contrasted with the morphology observed in Fe XVII. Slit profiles of Fe XXI 1354 A, O V 1371 A, and Si IV 1403 A are presented, and Fe XXI line widths are compared with the Doppler-widened kernel widths observed with the spectroheliograph. The electron density at selected phases is derived from diagnostic line ratios of O IV, O V, S IV, and S V observed in the slit spectra, and the density distribution in the flare between 10 to the 5th-6th K is obtained. The flare morphology is reexamined, and the arch structure within the 5 x 5 arcsec kernel is resolved. An improved picture of the density spatial structure of the flare kernel is derived. 26 references.

Energy release topology in a multiple-loop solar flare

The temporal and spatial structures of the UV and X-ray emissions and the magnetic field configuration in the November 12, 1980 flare observed from SMM have been studied. The UV observations were done in the O V and Fe XXI lines with a spatial resolution of 10 arcsec. The observations show that the impulsive UV bursts, and also the hard X-ray bursts by their temporal correlation with the impulsive O V emission, occurred in small localized kernels. By comparing the O V, Fe XXI, and X-ray raster images of the flare with the magnetogram, these emission kernels were identified as footpoints of interacting magnetic flux loops. The temporal evolution of the O V/Fe XXI emission shows that there was considerable preheating in the flare plasma some 8-9 minutes prior to the onset of the main hard X-ray bursts. The results are interpreted as indicating that the primary flare energy release occurred in a highly sheared multiloop structure, which lies along a magnetic neutral line. By either beam particle propagation or convective motion, flare energy is transported via a common footpoint to another loop which brightened later. The preheating of the flare plasma is shown to create a more favorable environment for energetic particle acceleration which resulted in the main impulsive hard X-ray bursts.

Diagnostic application of highly ionized iron lines in the XUV spectrum of a solar flare

Recent atomic data have been used to analyze a solar flare spectrum obtained with the Goddard Space Flight Center's grating spectrometer on the OSO-5 satellite. There exist in the wavelength region 90–200 A strong lines from each of the ions Fe xviii-Fe xxiv. The Fe xxi lines can be used as an electron density diagnostic for the 107 K plasma. From our analysis of a particular flare, we find a steep positive slope in the emission measure between 106.5 and 107.2 K and an electron density of ∼4 × 1011 cm−3 at 107 K. We emphasise the need for high spectral and spatial resolution observations of solar flares in this wavelength region, which has to date been largely neglected.

Transitions 2s'2pk -2s2p"I of the N i and C i Isoelectronic Sequences

view Abstract Citations (29) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Transitions 2s'2pk -2s2p"I of the N i and C i Isoelectronic Sequences Feldman, U. ; Doschek, G. A. ; Cowan, R. D. ; Cohen, Leonard Abstract Transitions of the type +1 have been identified for the elements from titanium through iron for ions of the nitrogen isoelectronic sequence and for the elements titanium through nickel for ions of the carbon isoelectronic sequence. Wavelengths, intensity estimates and energies are given. The lines were identified from EUV spectra obtained from laser-produced plasmas. The energy differences of levels of the ground configuration for the C I isoelectronic sequence are compared with extrapolations based on semiempirical equations derived by Edlen. Wavelengths of forbidden Fe XXI lines that can be seen in low-density plasmas such as solar flare plasmas are predicted. The laser plasma spectrum is briefly compared with available solar flare spectra. Subject headings: flares, solar - line identifications - spectra, laboratory - spectra, ultraviolet Publication: The Astrophysical Journal Pub Date: March 1975 DOI: 10.1086/153445 Bibcode: 1975ApJ...196..613F full text sources ADS |