EUV Spectra Research Articles

The element abundance FIP effect in the quiet Sun

The Mg/Ne abundance ratio in the quiet Sun is measured in both network and supergranule cell centre regions through EUV spectra from the Coronal Diagnostic Spectrometer on SOHO. Twenty four sets of data over the period 1996 March to 1998 June (corresponding to solar minimum and the onset of the next cycle) are studied. Emission lines of the sequences Ne IV–VII and Mg V–VIII are simultaneously analysed by comparing with theoretical emissivities from the CHIANTI database to yield the Mg/Ne abundance and emission measure over the temperature region . The average enhancements over the photospheric Mg/Ne abundance are found to be (network) and (cell centres), implying that the structures making up the transition region have close-to photospheric abundances. In particular, this implies that only a small fraction of the quiet Sun can connect to the solar wind where the Mg/Ne abundance ratio is found to be factors 4–5 greater than the photospheric value. The quiet Sun spectra are also utilised to determine the coronal density and temperature, leading to average values of cm-3 and . No significant trend with the rise in solar activity during 1996–98 is found for any of the derived quantities, implying that quiet Sun regions show little dependence on the solar cycle.

Observational Evidence of the Kink Instability in Solar Filament Eruptions and Sigmoids

Two lines of observational evidence are used to infer that the MHD helical kink instability is associated with solar eruptions. The senses of twist and writhe are determined in images of seven erupting filaments obtained at 10830, 1600, 195, and 171 ?. In every case the sense of twist is the same as the sense of writhe, as required for a kink. From images in the soft X-ray and EUV spectrum, measurements of the height/width ratio of 623 sigmoids show a mean value of 5.47, which is the ratio expected for kinked flux ropes.

EUV Line Identifications and Lifetime Measurements in Highly Charged Iron Ions

When beam-foil experiments on ions of the iron group were done at the Bochum Dynamitron tandem accelerator, line identifications of intercombination lines in Mg- and Al-like Fe matched previously unclassified lines that appear in the EUV spectrum of the solar corona. Beam-foil work continued on Si- to Cl-like Fe ions, identifying in particular decays of high-J levels with nanosecond lifetimes. Millisecond lifetimes in the same ions were then measured at a heavy-ion storage ring and at an electron beam ion trap. Remaining problems and prospects for solutions are discussed.

APEX, the Astrophysical Plasmadynamic EXplorer: An EUV High Resolution Spectroscopic Observatory

AbstractAPEX is a proposed Small Explorer satellite that will obtain high-resolution EUV spectra of white dwarfs, CVs, stellar coronae, and the local ISM. The APEX effective area (30-50 cm2) and resolution (~10,000) are an order of magnitude improvement over Chandra.

Cassini UVIS observations of the Io plasma torus: I. Initial results

During the Cassini spacecraft's flyby of Jupiter (October, 2000–March, 2001), the Ultraviolet Imaging Spectrograph (UVIS) produced an extensive dataset consisting of 3349 spectrally dispersed images of the Io plasma torus. Here we present an example of the raw data and representative EUV spectra (561–1181 Å) of the torus, obtained on October 1, 2000 and November 14, 2000. For most of the flyby period, the entire Io torus fit within the UVIS field-of-view, enabling the measurement of the total power radiated from the torus in the extreme ultraviolet. A typical value for the total power radiated in the wavelength range of 580–1181 Å is 1.7×10 12 W, with observed variations of up to 25%. Several brightening events were observed. These events lasted for roughly 20 hours, during which time the emitted power increased rapidly by ∼20% before slowly returning to the pre-event level. Observed variations in the relative intensities of torus spectral features provide strong evidence for compositional changes in the torus plasma with time. Spatial profiles of the EUV emission show no evidence for a sharply peaked “ribbon” feature. The ratio of the brightness of the dusk ansa to the brightness of the dawn ansa is observed to be highly variable, with an average value of 1.30. Weak longitudinal variations in the brightness of the torus ansae were observed at the 2% level.

Diagnostics of Coronal Heating in Solar Active Regions

We study the relationship between EUV spectral line intensities and the photospheric magnetic field in solar active regions, using magnetograms from SOHO-MDI and EUV spectra of the Fe XVI 360.8 Â line (2 × 106 K) and the O V 629.7 A line (220,000 K) from the Coronal Diagnostic Spectrometer on SOHO, recorded for several active regions. We overlay and compare spatial patterns of the O V emission and the magnetic flux concentrations, with a 4″ x 4″ spatial resolution, and search for a relationship between the local O V line intensity and the photospheric magnetic flux density in each active region. While this dependence exhibits a certain amount of scatter, it can be represented by a power law fit. The average power index from all regions is 0.7 ± 0.2. Applying static loop models, we derive the dependence of the heating rate on the magnetic flux density, Eh ∝ B0.8, and compare it to the dependence predicted by the coronal heating models.

FIP Effect and FIP-Dependent Bias in the Solar Corona

Using EUV spectra of an active region observed off the solar disk by the SOHO/SUMER spectrograph on the SOHO spacecraft, we investigate the dependence of the FIP effect on the height above the photosphere, and its relation to plasma magnetic structures present in the field of view. We also investigate the possibility of the FIP bias in the low-FIP elements to be FIP-dependent, so that different abundance anomalies must be found even within the low-FIP class of elements, which can provide important constraints on the FIP effect models.

The Coronae of AR Lacertae

We observed the coronally active eclipsing binary AR Lac with the High Energy Transmission Grating on Chandra for a total of 97 ks, spaced over five orbits, at quadratures and conjunctions. Contemporaneous and simultaneous EUV spectra and photometry were also obtained with the Extreme Ultraviolet Explorer. Significant variability in both X-ray and EUV fluxes were observed, dominated by at least one X-ray flare and one EUV flare. We saw no evidence of primary or secondary eclipses, but exposures at these phases were short and intrinsic variability compromised detection of any geometric modulation. X-ray flux modulation was largest at high temperature, indicative of flare heating of coronal plasma rather than changes in emitting volume or global emission measure. Analysis of spectral line widths interpreted in terms of Doppler broadening suggests that both binary stellar components are active. On the basis of line fluxes obtained from total integrated spectra, we have modeled the emission measure and abundance distributions. The EUV spectral line fluxes were particularly useful for constraining the parameters of the cool (≤2 × 106 K) plasma. A strong maximum was found in the differential emission measure, characterized by two apparent peaks at log T = 6.9 and 7.4, together with a weak but significant cooler maximum near log T = 6.2 and a moderately strong hot tail from log T = 7.6-8.2. Coronal abundances have a broad distribution and show no simple correlation with first ionization potential. While the resulting model spectrum generally agrees very well with the observed spectrum, there are some significant discrepancies, especially among the many Fe L lines. Both the emission measure and abundance distributions are qualitatively similar to prior determinations from other X-ray and ultraviolet spectra, indicating some long-term stability in the overall coronal structure.

Chemical fractionation and abundances in coronal plasma

Much of modern. astrophysics is grounded on the observed chemical compositions of stars and the diffuse plasma that pervades the space between stars, galaxies and clusters of galaxies. X-ray and EUV spectra of the hot plasma in the outer atmospheres of stars have demonstrated that these environments are subject to chemical fractionation in which the abundances of elements can be enhanced and depleted by an order of magnitude or more. These coronal abundance anomalies are discussed and some of the physical mechanisms that might be responsible for producing them are examined. It is argued that coronal abundances can provide important new diagnostics on physical processes at work in solar and stellar coronae. It seems likely that other hot astrophysical plasmas will be subject to similar effects.

The Extreme-Ultraviolet Continuum of a Strong Stellar Flare

We present the serendipitous detection of an extreme-ultraviolet flare on EUVE J0613-23.9B. The flare showed over a 200-fold increase above the quiescent emission in the DS/Lexan 60-200 Å wavelength band. Optical spectroscopy revealed that the event was associated with an active dM3.5e star. The EUVE spectra are dominated by emission lines formed at temperatures in excess of 107 K. The observation is unique as we have detected, for the first time, a strong Lyman continuum in the EUVE long-wavelength range (320-650 Å). The flare in the continuum (T ≈ 20,000-30,000 K) was extremely short, lasting for less than 500 s, while in the DS (T ≈ 107 K) its duration was ≈28 ks. The total energy of the flare in the DS is ~3 × 1034 ergs. We have made a fit to the continuum using semiempirical model atmospheres and derived the time-averaged temperature and density structures.

Explosive events and transition region blinkers: Time variability of non-Gaussian quiet Sun EUV spectra

The transition region (TR) from the chromosphere to the corona of the Sun and solar type stars is a very dynamic regime. On the Sun at least two major observational classes of transients can be seen: explosive events and blinkers. Besides these transients there is also a more steady spectral component in transition region lines that accounts for the deviation of the line profile from a (single) Gaussian shape, i.e. for enhanced wings that are well described by a second broad Gaussian, a tail component. The present paper discusses the spectral properties of these features in order to learn more about a possible connection between blinkers, explosive events and tail components. This paper will show that explosive events are most probably not related to the tail components or to transition region blinkers. During a blinker not only the intensity rises, as described by many CDS studies, but also the line width and shift are changing, as we show with SUMER spectra. The variation in line shift and width is closely related to the intensity variation. The line width drops to very small values at the midst of a blinker, almost reaching the thermal line width, while at the same time the Doppler shifts reach a maximum value. From this we may conclude that blinkers are driven by events in the chromosphere heating a loop asymmetrically and powering a strong laminar flow through the loop.

Spectroscopic diagnostics of an Hα and EUV filament observed with THEMIS and SOHO

A long filament has been observed with THEMIS/MSDP and SOHO/CDS - SUMER, during a coordinated campaign (JOPs 131/95) on May 5, 2000. The data were (a) 2-D Hα spectra, observed using THEMIS, (b) Lyman series spectra and Lyman continuum, observed using SOHO/SUMER, and (c) EUV spectra (in O  629 A, Mg  624 A, Si  520 A, Ca  557 A and He  584 A) observed using SOHO/CDS. A large depression of the line emissions in CDS images represents the EUV filament. A computed model shows that the EUV filament consists of an extended in height cloud of low gas pressure at an altitude lower than the top of the Hα filament, volume-blocking and absorbing coronal emission and absorbing transition region line emission. The optical thickness of the Lyman continuum is estimated by using the ratio of O  intensity inside and outside the EUV filament, while the optical thickness of Hα is computed from the Hα line profile by using an inversion technique. Using simultaneous Hα, Lyman lines and Lyman continuum spectroscopic data, we performed detailed, non-LTE radiative transfer diagnostics of the filament plasma conditions. The optical thickness of the Lyman continuum is larger than that of the Hα line by one to two orders of magnitude. This could be of a great importance for filament formation modeling, if we consider that more cool material exists in filament channels but is optically too thin to be visible in Hα images.

The Structure of Stellar Coronae in Active Binary Systems

A survey of 28 stars using EUV spectra has been conducted to establish the structure of stellar coronae in active binary systems from the EMD, electron densities, and scale sizes. Observations obtained by the EUVE during 9 years of operation are included for the stars in the sample. EUVE data allow a continuous EMD to be constructed in the range log T~5.6-7.4, using iron emission lines. These data are complemented with IUE observations to model the lower temperature range. Inspection of the EMD shows an outstanding narrow enhancement, or ``bump'' peaking around log T~6.9 in 25 of the stars, defining a fundamental coronal structure. The emission measure per unit stellar area decreases with increasing orbital (or photometric) periods of the target stars; stars in binaries generally have more material at coronal temperatures than slowly rotating single stars. High electron densities (Ne>10^12 cm^-3) are derived at ~10 MK for some targets, implying small emitting volumes. The observations suggest the magnetic stellar coronae of these stars are consistent with two basic classes of magnetic loops: solar-like loops with maximum temperature around log T~6.3 and lower electron densities (Ne>10^9-10.5), and hotter loops peaking around log T~6.9 with higher electron densities (Ne>10^12). For the most active stars, material exists at much higher temperatures (log T>6.9) as well. However, current ab initio stellar loop models cannot reproduce such a configuration. Analysis of the light curves of these systems reveals signatures of rotation of coronal material, as well as apparent seasonal changes in the activity levels.

Inversion of the intensity-magnetic field relationship in solar active regions

We discuss the relationship between the EUV spectral line intensities and the photospheric magnetic flux in solar active regions. Since the histograms of the magnetic flux density in active region plages can be approximated by an exponential function, the equation describing how the observed total line intensity integrated over an active region area arises from the magnetic field, can be approximated by a Laplace integral. We use this property to solve an inverse problem and derive a function relating the line intensity from individual loops to the photospheric magnetic flux density at their footpoints. We propose a simple model in which the intensity of a coronal line Fe XVI 360.8 A in an individual coronal loop is proportional to the footpoint magnetic flux density to the power of δ and explore how well the value of δ is constrained by the observations. Using EUV spectra from the Coronal Diagnostic Spectrometer (CDS) on SOHO and magnetograms from SOHO Michelson Doppler Imager for 26 active regions without sunspots, we find that the value of δ depends on the magnetic flux density threshold used to define active region magnetic area. When even the weakest fields are included, we obtain δ = 1.3, where 1.0 <δ <1.6 with 90% confidence. This result can be used to provide constraints on coronal heating models.

Theoretical EUV Spectrum of Near Pd-like Xe

The EUV spectrum of multiple charged Xe ion is theoretically investigated. The strong emission in the 11 nm band is attributed to 4d-4f transitions of Xe7+ to Xe18+. The 4d-5p transition of Xe10+ contributes to the emission in the 13.5 nm band from low density plasma.

Extreme Ultraviolet ExplorerPhase‐resolved Spectroscopy of V834 Centauri

The Extreme Ultraviolet Explorer (EUVE) satellite was employed for 5.46 days, beginning on 1999 February 9.03 UT, to acquire phase-resolved EUV photometric and spectroscopic observations of the AM Her type cataclysmic variable V834 Centauri. The resulting data are superior to those obtained by EUVE beginning on 1993 May 28.14 UT, because the source was approximately 3 times brighter, the observation was 4 times longer and dithered, and ASCA observed the source simultaneously. Although we do not understand the EUV light curves in detail, they are explained qualitatively by a simple model of accretion from a ballistic stream along the field lines of a tilted ([β,ψ] ≈ [10°,40°]) magnetic dipole centered on the white dwarf. In 1993, when the EUV flux was lower, accretion was primarily along the ≈ ψ ≈ 40° field line, whereas in 1999, when the EUV flux was higher, accretion took place over a broad range of azimuths extending from ≈ ψ ≈ 40° to ≈ 76°. These changes in the accretion geometry could be caused by an increase in the mass accretion rate and/or the clumpiness of the flow. The 75-140 A EUVE spectra are well described by either a blackbody or a pure H stellar atmosphere absorbed by a neutral hydrogen column density, but constraints on the size of the EUV emission region and its UV brightness favor the blackbody interpretation. The mean 1999 EUV spectrum is best fitted by an absorbed blackbody with temperature kT ≈ 17.6 eV, hydrogen column density NH ≈ 7.4 × 1019 cm-2, fractional emitting area f ≈ 10-3, 70-140 A flux ≈ 3.0 × 10-11 ergs cm-2 s-1, and luminosity Lsoft ≈ 7.2 × 1032(d/100 pc)2 ergs s-1. The ratio of the EUV to X-ray luminosities is Lsoft/Lhard ≈ 40, signaling that some mechanism other than irradiation (e.g., blob heating) dominates energy input into the accretion spot. The 1999 short-wavelength (SW) hardness ratio variation can be explained by minor variations in kT and/or NH, but instead of tracking the SW count rate variation, the hardness ratio variation was sinusoidal, with a minimum (maximum) when the accretion spot was on the near (far) side of the white dwarf, consistent with the trend expected for an atmosphere with an inverted temperature distribution.

Comparison of the spectrum of a sunspot-like star with a sunspot

A typical sunspot umbra has an effective temperature, gravity, and strong magnetic field strength similar to the dM1e star AU Microscopii. We compare STIS, Chandra, EUVE, and FUSE spectra of AU Mic with typical spectra of sunspots to characterize the major differences and then compare models of AU Mic with sunspot models. We find that the emission, heating rates, and differential emission measure distributions are very different for AU Mic and sunspots, and conclude that the root cause of the difference lies in the very different geometries of the two magnetic fields.

Relativistic fine structure oscillator strengths for Li-like ions: C IV - Si XII, S XIV, Ar XVI, Ca XVIII, Ti XX, Cr XXII, and Ni XXVI

Ab initio calculations including relativistic effects employing the Breit-Pauli R-matrix (BPRM) method are reported for fine structure energy levels and oscillator strengths upto n = 10 and 0.leq. l .leq.9 for 15 Li-like ions: C IV, N V, O VI, F VII, Ne VIII, Na IX, Mg X, Al XI, Si XII, S XIV, Ar XVI, Ca XIII, Ti XX, Cr XXII, and Ni XXVI. About one hundred bound fine structure energy levels of total angular momenta, 1/2 .leq. J .leq. 17/2 of even and odd parities, total orbital angular momentum, 0 .leq L .leq. 9 and spin multiplicity (2S+1) = 2, 4 are considered for each ion. The levels provide almost 900 dipole allowed and intercombination bound-bound transitions. The BPRM method enables consideration of large set of transitions with uniform accuracy compared to the best available theoretical methods. The CC eigenfunction expansion for each ion includes the lowest 17 fine structure energy levels of the core configurations 1s^2, 1s2s, 1s2p, 1s3s, 1s3p, and 1s3d. The calculated energies of the ions agree with the measured values to within 1% for most levels. The transition probabilities show good agreement with the best available calculated values. The results provide the largest sets of energy levels and transition rates for the ions and are expected to be useful in the analysis of X-ray and EUV spectra from astrophysical sources.

Debris from tape-target irradiated with pulsed YAG laser

For high brightness and debris-free laser-produced plasma EUV source, EUV spectra and debris were measured. The experimental debris scattering angle distribution and the amount of piled debris were estimated in the thickness of debris on silicon wafers from tape-target irradiated with pulsed YAG laser (SHG, E=0.8 J, t=7 ns) in more than 200,000 laser shots. The shape and size of debris were observed using scanning electron microscope. EUV spectra were obtained by using the grazing-incidence XUV spectrometer with the Au-coated toroidal mirror, diffraction grating and X-ray CCD camera. In 200,000 laser shots, for copper tape-target, 50 nm thick debris at 50° from target normal and 30 nm thick debris at more than 70° from target normal were obtained. More than 20 μm in diameter debris of aluminum and less than 1 μm in diameter debris of copper were observed. Laser-produced plasma EUV source system with low debris tape-target of thin metal and polymer films, debris-shield and debris-free EUV filter has been developed. We demonstrated successive 8 h generation of EUV with the developed laser-produced plasma EUV source system for EUV lithography.

ADAS analysis of the differential emission measure structure of the inner solar corona

The differential emission measure (DEM) of a solar active region is derived from SERTS-89 rocket data between 170 and 450 i.. (Thomas and Neupert 1994). The integral inversion to infer the DEM distribution from spectral line intensities is performed by the data adaptive smoothing approach (Thompson 1990, 1991). Our analysis takes into account the density dependence of both ionisation fractions and excitation coefficients according to the collisional-radiative theory as implemented in ADAS, the Atomic Data and Analysis Structure (McWhirter and Summers 1984; Summers 1994; Summers 2001). Our strategy aims at checking, using observational data, the validity and limitations of the DEM method used for analysing solar EUV spectra. We investigate what information it is possible to extract, within defined limitations, and how the method can assist in a number of cases, e.g. abundance determination, spectral line identification, intensity predictions, and validation of atomic cross-sections. Using the above data and theory, it is shown that a spurious multiple peak in the DEM distribution between and 6.7, where is the electron temperature, may derive from an inaccurate treatment of the population densities of the excited levels and ionisation fractions or from using an integral inversion technique with arbitrary smoothing. Therefore, complex DEM structures, like those proposed for solar and stellar coronae by several authors, must be considered with caution. We address also the issue of systematic differences between iso-electronic sequences and show that these cannot be unambiguously detected in the coronal lines observed by SERTS. Our results indicate that a substantial improvement is required in the atomic modelling of the complex element Fe. The elemental abundance ratio Si/Ne is found to be close to its photospheric value. The same result may be true for the Fe/Ne abundance, but this latter result is uncertain because of the problems found with Fe.