Opacity effects in soft X-ray spectral lines of the solar corona

Current literature suggests that several lines in the soft X-ray portion of the coronal spectrum may not be optically thin. Here, we confirm the results of Schmelz et al. (1996) who find no significant opacity effects for three of the brightest non-iron resonance lines in this part of the spectrum — O VIII at 18.97Å, Ne IX at 13.45Å, and Mg XI at 9.17Å. A comparison is made between each of these lines and an optically thin “reference” line produced by the same element in the same ionization state — O VIII at 15.18Å, Ne IX at 13.55Å, and Mg XI at 9.23Å. In the latter two cases, the comparison line is the intersystem line of the He-like triplet. 33 spectra from the Solar Maximum Mission Flat Crystal Spectrometer are analyzed, all of which were obtained from non-flaring, quasi-stable active regions.     

Multiband echo tomography of Sco X-1

We present preliminary results of a simultaneous X-ray/optical campaign of the prototypical LMXB Sco X-1 at 1–10 Hz time resolution. Lightcurves of the high excitation Bowen/HeII emission lines and a red continuum at λ_c  6000 Å were obtained through narrow interference filters with ULTRACAM, and these were cross-correlated with simultaneous RXTE X-ray lightcurves. We find evidence for correlated variability, in particular when Sco X-1 enters the Flaring branch. The Bowen/HeII lightcurves lag the X-ray lightcurves with a light travel time which is consistent with reprocessing in the companion star while the continuum lightcurves have shorter delays consistent with reprocessing in the accretion disc.     

Fe XVII X-ray lines in solar coronal plasmas

Calculated intensities of the Fe X-ray lines due to transitions 2p⁶ − 2p⁵3d lines (near 15 Å) and 2p⁶ − 2p⁵3s lines (near 17 Å) are compared with measured line intensities in solar and tokamak spectra. For the solar spectra, temperature T_e is obtained from the ratio of the Fe 16.776 Å line to a nearby Fe line. We find excellent agreement for all the major Fe line features in the 15–17 Å region except the Fe 15.015 Å line, the observed flux of which is less than the theoretical by a factor f. We find that f strongly depends on the heliocentric angle θ of the emitting region, being smallest (0.2) when the region is nearest Sun centre, but nearly 1 near the limb. Attributing this to resonance scattering, we are able to deduce the path length and electron density from the observations. Possible application to stellar active regions is given.     

Observations of stars with the Hopkins Ultraviolet Telescope

The Hopkins Ultraviolet Telescope (HUT) was flown aboard the space shuttle Columbia as part of the Astro-1 mission during December 1990. During the nine-day flight, HUT carried out 3 Å resolution spectrophotometry of a wide variety of astronomical objects, including a number of stellar targets, in the 912–1860 Å range of the far ultraviolet. A few nearby stars were observed in the 415–912 Å range of the extreme ultraviolet as well. For nearly all of these targets, the spectra obtained by HUT are the first ever obtained in the spectroscopically rich region between Lyman (1216 Å) and the Lyman limit (912 Å). Here, we present highlights of the results obtained by HUT in a variety of areas of stellar astronomy.     

Energy deposition in the upper atmosphere in the EXCEDE III experiment

The EXCEDE III sounding rocket flight of April 27, 1990 used a 18 Ampere 2.5 keV electron beam to produce an artificial aurora in the region 90 to 115 km. A “daughter” sensor payload remotely monitored the low-energy X-ray spectrum while scanning photometers measured the spatial profile of prompt emissions of N₂⁺ (1N) and N₂ (2P) transitions (3914Å and 3805Å, respectively). Two Ebert-Fastie spectrometers measured the spectral region from 1800 to 8000Å. On the “mother” accelerator payload, the return current electron differential energy spectra were monitored by an electrostatic analyzer (up to 10 keV) and by a retarding potential analyzer (0 eV to 100 eV). We present an overview of the results from this experiment.     

Results of XUV full sun imaging spectroscopy for eruptive and transient events by the spirit spectroheliograph on the CORONAS-F mission

SPIRIT (SPectroheliograph Ic soft X-Ray Imaging Telescope) is the current experiment on board theCORONAS-F satellite launched on July 31, 2001 (Oraevskii & Sobelman, 2002). The main goal of this experiment is to study a structure and dynamics of the solar atmosphere in the wide scale of heights (from the chromosphere to a far corona) and of temperatures (from ten thousands through thirty millions Kelvins) by means of the XUV imaging spectroscopy. Since the launch of the CORONAS-F satellite more than three hundred thousands of images and spectroheliograms have been recorded. For the first time continuous series of monochromatic full Sun images in MgXII lines at 8.42 Å (doublet: 8.418 and 8.423 Å) were obtained. These series include long-term continuous observations of duration up to 10 days with the cadence of 100 sec as well as temporal sequences with duration of a few minutes and high resolution of 7 sec, synchronized with flares. The spectroheliograms for the whole disk and off-limb regions are also recorded in the spectral bands 177 – 207 and 285 – 335 Å providing spectra with high resolution of various coronal structures including eruptive and transient events. This paper presents preliminary results of quick look analysis of some observational data obtained by means of the SPIRIT spectroheliographs.     

First results from UVCS/SOHO

We present here the first results obtained by the Ultraviolet Coronagraph Spectrometer (UVCS) operating on board the SOHO satellite. The UVCS started to observe the extended corona at the end of January 1996; it routinely obtains coronal spectra in the 1145 Å – 1287 Å, 984 Å – 1080 Å ranges, and intensity data in the visible continuum. Through the composition of slit images it also produces monocromatic images of the extended corona. The performance of the instrument is excellent and the data obtained up to now are of great interest. We briefly describe preliminary results concerning polar coronal holes, streamers and a coronal mass ejection, in particular: the very large r.m.s. velocities of ions in polar holes (hundreds km/sec for OVI and MgX); the puzzling difference between the HI Ly- image and that in the OVI resonance doublet, for most streamers; the different signatures of the core and external layers of the streamers in the width of the ion lines and in the OVI doublet ratio, indicating larger line-of-sight (l.o.s.) and outflow velocities in the latter.     

An investigation of small GOES flares with intense hard X-ray bursts

Most solar flare observations show that intense hard X-ray bursts come from large flares that have a large GOES classification (large peak 1 – 8 Å flux). This correlation, known as the “Big Flare Syndrome”, suggests that more intense flares tend to have harder spectra. We have observed 7 flares that are exceptions to this. These flares have small GOES classifications ranging from B1.4 to C5.5 and peak hard X-ray count rates similar to those often observed from M class flares. This paper examines the cause of this anomoly using the Yohkoh Soft X-Ray Telescope, Hard X-Ray Telescope, and Bragg Crystal Spectrometer. Two hypotheses are proposed for the exceptions: (1) flares with multiple magnetic loops and common footpoints, producing multiple hard X-ray emission regions and low density thermal plasma distributed over a large volume, and (2) high densities in the magnetic loops restricting the propagation of the non-thermal electrons in the loop after magnetic reconnection has occurred and suppressing chromospheric evaporation. Two of the flares support the first hypothesis. The other flares either have data missing or are too small to be properly analysed by the Yohkoh instruments.     

High spectral resolution Fabry-Perot interferometer measurements of comet Halley at H-alpha and 6300 Å

A 40.6 cm Newtonian telescope has been interfaced to the Fabry-Perot interferometer at the Arecibo Observatory to make high spectral resolution measurements of Comet Halley emissions at 6562.72 Å (H-alpha) and 6300.3 Å (OI). In March 1986 the H-alpha surface brightness for a 5′.9 field of view centered on the comet nucleus decreased from 39±7.8 rayleighs on 12 March to 16±3.8 rayleighs on 23 March. The atomic hydrogen production rate on 12 March 1986 was 1.62±0.5 × 10³⁰ s⁻¹, and on 23 March 1986 it was 6.76±2.3 × 10²⁹ s⁻¹. Using spectral resolution of 0.196 Å, we found the atomic hydrogen outflow velocity to be approximately 7.9±1.0 km s⁻¹. In general, the H-alpha spectra are highly structured, and indicative of a multiple component atomic hydrogen velocity distribution. An isotropic outflow of atomic hydrogen at various velocities is not adequate to explain the spectra measured at H-alpha. The 6300.3 Å emission of O(1D) had a surface brightness of 81±16 rayleighs on 15 March 1986, and 95±11 rayleighs on 17 March 1986. After adjustment for atmospheric extinction, the implied O(1D) production rate on 15 March is 6.44±3.0 × 10²⁸ s⁻¹, and the production rate on 17 March is 5.66±2.7 × 10²⁸ s⁻¹. These spectra included a feature at 6300.8 Å that we attribute to NH₂. The brightness of this emission feature was 37±11 rayleighs on 15 March.     

Initial results from the extreme ultraviolet explorer

Data obtained during the first five months of calibration and science operation of the Extreme Ultraviolet Explorer (EUVE) are presented. Spectra of an extragalactic object were obtained; the object is detectable to wavelengths longer than 100 Å, demonstrating that extragalactic EUV astronomy is possible. Spectra of a hot white dwarf, and a late-type star in quiescence and flaring are shown as examples of the type of spectrographic data obtainable with EUVE. Other objects for which broad band photometric mode data have been obtained and analyzed include an RS CVn star and several late-type stars. The backgrounds in the EUVE detectors are quite low and the character of the diffuse astronomical EUV background has been investigated using these very low rates. Evidence is presented showing that, contrary to previously published reports, EUVE is about three times more sensitive than the English Wide Field Camera in the short wavelength bandpass covered by both instruments. Only limited information has been extracted from the longer bandpasses covered only by EUVE. Nonetheless, the brightest EUV source in the sky, a B star, has been discovered and is detected only in these longer bandpasses.     

ROSAT-WFC detection of the Lyman continuum flux of β CMa (B1 II–III)

We report the first detection of Lyman continuum (≈ 600Å) emission from a non-degenerate star. The WFC on-board ROSAT detected emission from β CMa (B1 II–III) which has a very low interstellar column. Both Kurucz LTE line-blanketed and Mihalas NLTE un-blanketed model atmospheres can match the observed count rate within the very considerable absolute calibration errors. We also find very marginal evidence for variability, although not at the dominant period of the known optical pulsations.     

UVSTAR, a spectrographic telescope for the shuttle Hitchhiker-M bridge

UVSTAR is an EUV spectral imager intended as a facility instrument devoted to solar system astronomy and to astronomy. It covers the wavelength range of 500 to 1250 Å, with sufficient spectral resolution to separate atomic emission lines and to form spectrally resolved images of extended plasma sources. Targets include the Io plasma torus at Jupiter, hot stars, planetary nebulae and extragalactic sources. UVSTAR will make useful measurements of emissions from the Earth's atmosphere as well. UVSTAR consists of a pair of telescopes and concave-grating spectrographs that cover the overlapping spectral ranges of 500–900 Å and 850–1250 Å. The telescopes use two 30 cm diameter off-axis paraboloids having a focal length of 1.4 m. An image of the target is formed at the entrance slits of two concave grating spectrographs. The gratings provide dispersion and re-image the slits at the detectors, intensified CCDs. The readout format of the detectors can be chosen by computer, and three slit widths are selectable to adapt the instrument to specific tasks. The spectrograph package has internal gimbals which allow rotation of ±3° about each of two axes. Dedicated finding and tracking telescopes will acquire and track the target after rough pointing is achieved by orienting the Orbiter. Responsibilities for the implementation and utilization of UVSTAR are shared by groups the U.S. and Italy. UVSTAR is scheduled for flight in early 1994.     

Imaging the solar corona in the EUV

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 Å images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 10⁴ K; the Fe IX–X filter at 171 Å images the corona at a temperature of 1.3 × 10⁶ K; the Fe XII filter at 195 Å images the quiet corona outside coronal holes at a temperature of 1.6 × 10⁶ K; and the Fe XV filter at 284 Å images active regions with a temperature of 2.0 × 10⁶ K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection.     

High-frequency slowly drifting structures in the November 25, 2000 and April 15, 2001 solar flares

Three slowly drifting structures observed during the November 25, 2000 and April 15, 2001 solar flares arepresented. Their relationship to the hard X-ray emission is shown. While the April 15, 2001 X14.4 flare started with the high-frequency drifting structure associated with a plasmoid ejection observed by TRACE in the 171 Å line, the November 25, 2000 event commenced with two drifting structures in two different frequency ranges; the high-frequency one consists of fast positively drifting features with rapid frequency variations of their high-frequency boundary. Due to fast drifting features the slowly drifting structures are interpreted in the flare model with a sequence of fast electron beams accelerated in the current sheet below the ejected plasmoid. Drifting structures in the November 25, 2000 flare are explained by beams injected into the plasmoid and, at the same time, by beams propagating downwards against the evaporation or termination shocks.     

EUVITA — An extreme UV imaging telescope array with spectral capability

EUVITA is a set of 8 extreme UV normal incidence imaging telescopes, each of them sensitive in a narrow band (λ/Δλ = 15 to 80), centered at wavelengths between 50 and 175 Å. Each telescope has an effective area of a few cm²; a field of view of 1.2° and a spatial resolution of 10 arcsec.

EUVITA will be flown on the Russian mission SPECTRUM X-G. This satellite will be launched in a highly eccentric orbit with a period of 4 days, allowing long, uninterrupted observations (e.g. 10⁵ seconds). EUVITA's narrow spectral bands allow the measurement of source parameters such as temperature or power law index as well as interstellar absorption, and will resolve groups of strong lines emitted by optically thin hot plasmas.     

Wide field ultraviolet observations of Comet Halley with the FAUST Spacelab I instrument

The wide field (7.5°), arc minute imaging, and spectroscopic capabilities of the Far Ultraviolet FAUST telescope which will be flown on Spacelab I can provide valuable information on Comet Halley. The use of the FAUST instrument in obtaining images of the hydrogen coma at 1216 Å, and in obtaining objective grating spectroscopy from 1300–3300 Å of the comet and tail, are described. The FAUST images would provide large field of view data that are required for model calculations of gas production rates and the determination of scale lengths and lifetimes of ion species.     

ROSAT all-sky-survey detections of classical novae and related objects

In the ROSAT all-sky-survey 7 classical novae were detected, but only 1 of the 26 that had an outburst in the 10 years before the survey. 3 are new X-ray detections; 1 source is a known intermediate polar and 4 are suspect “magnetic novae”. 3 objects show a very soft spectrum. Among the objects related to novae we found 2 dwarf-novae with unusually long periodicity, the peculiar source KR Aur and 4 X-ray novae. One of these, V404 Cyg, was detected at quiescence for the first time.     

Spectroscopic observations of comet Giacobini-Zinner during the ice encounter

A set of 5 sequential spectra of Comet Giacobini-Zinner was obtained during the encounter of the ICE spacecraft with that comet on September 11, 1985. The observations were initiated 20 min before the encounter and were conducted in 10 min-long intervals until approximately 40 min after the event. The spectra were taken along a direction nearly coincident to the axis of the cometary tail and cover a distance of approximately 74 000 km at the comet. The preliminary analysis of the data indicates that the measured brightness intensity of CN at 3884 Å and that of the continuum at 5215 Å was nearly constant throughout the period of observation.     

What is the role of magnetic null points in large flares?

We have performed the analysis of the magnetic topology of active region NOAA 10486 before two large flares occurring on October 26 and 28, 2003. The 3D extrapolation of the photospheric magnetic field shows the existence of magnetic null points when using two different methods. We use TRACE 1600 Å and 195 Å brightenings as tracers of the energy release due to magnetic reconnections. We conclude on the three following points:

1. The small events observed before the flares are related to low lying null points. They are long lasting and associated with low energy release. They are not triggering the large flares.

2. On October 26, a high altitude null point is found. We look for bright patches that could correspond to the signatures of coronal reconnection at the null point in TRACE 1600 Å images. However, such bright patches are not observed before the main flare, they are only observed after it.

3. On October 28, four ribbons are observed in TRACE images before the X17 flare. We interpret them as due to a magnetic breakout reconnection in a quadrupolar configuration. There is no magnetic null point related to these four ribbons, and this reconnection rather occurs at quasi-separatrix layers (QSLs).

We conclude that the existence of a null point in the corona is neither a sufficient nor a necessary condition to give rise to large flares.     

Extreme ultraviolet observations of the solar corona: First results from the coronal diagnostic spectrometer on SOHO

We present first results from the Coronal Diagnostic Spectrometer (CDS) aboard the ESA/NASA Solar and Heliospheric Observatory (SOHO). CDS is a double spectrometer operating in the 151–785 Å range. This region of the solar spectrum is rich in emission lines from trace elements in the solar atmosphere, which can be used to derive diagnostic information on coronal and transition region plasmas. Early spectra are presented and well identified lines are listed. In addition, examples of images in selected wavelength ranges are shown, for a prominence, a loop system and a bright point, demonstrating well the power of such extreme ultraviolet observations.