J.-P. Delaboudiniere

EIT: Extreme-UltraViolet Imaging Telescope for the SOHO Mission

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R⊙ above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 Å), Fe XII (195 Å), Fe XV (284 Å), and He II (304 Å) to provide sensitive temperature diagnostics in the range from 6 × 104 K to 3 × 106 K. The telescope has a 45 x 45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.

EUVI: the STEREO-SECCHI extreme ultraviolet imager

The Extreme Ultraviolet Imager (EUVI) is part of the SECCHI instrument suite currently being developed for the NASA STEREO mission. Identical EUVI telescopes on the two STEREO spacecraft will study the structure and evolution of the solar corona in three dimensions, and specifically focus on the initiation and early evolution of coronal mass ejections (CMEs). The EUVI telescope is being developed at the Lockheed Martin Solar and Astrophysics Lab. The SECCHI investigation is led by the Naval Research Lab. The EUVI’s 2048 x 2048 pixel detectors have a field of view out to 1.7 solar radii, and observe in four spectral channels that span the 0.1 to 20 MK temperature range. In addition to its view from two vantage points, the EUVI will provide a substantial improvement in image resolution and image cadence over its predecessor SOHO-EIT, while complying with the more restricted mass, power, and volume allocations on the STEREO mission.

EIT OBSERVATIONS OF THE EXTREME ULTRAVIOLET SUN

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops.Variability of solar EUV structures, as observed in the EIT time sequences, is pervasive and leads to a re-evaluation of the meaning of the term ‘quiet Sun’. Intensity fluctuations in a high cadence sequence of coronal and chromospheric images correspond to a Kolmogorov turbulence spectrum. This can be interpreted in terms of a mixed stochastic or periodic driving of the transition region and the base of the corona. No signature of the photospheric and chromospheric waves is found in spatially averaged power spectra, indicating that these waves do not propagate to the upper atmosphere or are channeled through narrow local magnetic structures covering a small fraction of the solar surface. Polar coronal hole observing campaigns have identified an outflow process with the discovery of transient Fexii jets. Coronal mass ejection observing campaigns have identified the beginning of a CME in an Fexii sequence with a near simultaneous filament eruption (seen in absorption), formation of a coronal void and the initiation of a bright outward-moving shell as well as the coronal manifestation of a ‘Moreton wave’.

Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5) × 106 K

The three-dimensional structure of solar active region NOAA 7986 observed on 1996 August 30 with the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) is analyzed. We develop a new method of dynamic stereoscopy to reconstruct the three-dimensional geometry of dynamically changing loops, which allows us to determine the orientation of the mean loop plane with respect to the line of sight, a prerequisite to correct properly for projection effects in three-dimensional loop models. With this method and the filter-ratio technique applied to EIT 171 and 195 A images we determine the three-dimensional coordinates [x(s), y(s), z(s)], the loop width w(s), the electron density ne(s), and the electron temperature Te(s) as a function of the loop length s for 30 loop segments. Fitting the loop densities with an exponential density model ne(h) we find that the mean of inferred scale height temperatures, Tλe=1.22 ± 0.23 MK, matches closely that of EIT filter-ratio temperatures, TEITe=1.21 ± 0.06 MK. We conclude that these cool and rather large-scale loops (with heights of h≈30-225 Mm) are in hydrostatic equilibrium. Most of the loops show no significant thickness variation w(s), but we measure for most of them a positive temperature gradient (dT/ds>0) across the first scale height above the footpoint. Based on these temperature gradients we find that the conductive loss rate is about 2 orders of magnitude smaller than the radiative loss rate, which is in strong contrast to hot active region loops seen in soft X-rays. We infer a mean radiative loss time of τrad≈40 minutes at the loop base. Because thermal conduction is negligible in these cool EUV loops, they are not in steady state, and radiative loss has entirely to be balanced by the heating function. A statistical heating model with recurrent heating events distributed along the entire loop can explain the observed temperature gradients if the mean recurrence time is 10 minutes. We computed also a potential field model (from SOHO/MDI magnetograms) and found a reasonable match with the traced EIT loops. With the magnetic field model we determined also the height dependence of the magnetic field B(h), the plasma parameter β(h), and the Alfven velocity vA(h). No correlation was found between the heating rate requirement EH0 and the magnetic field Bfoot at the loop footpoints.

Observations of Correlated White-Light and Extreme-Ultraviolet Jets from Polar Coronal Holes

Time-lapse sequences of white-light images recorded with the Large Angle Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) frequently show long, narrow structures moving outward over the Suns polar regions at high apparent speeds. By comparing the LASCO observations with Fe XII λ195 spectroheliograms made with the Extreme-ultraviolet Imaging Telescope (EIT) on SOHO between 1997 April and 1998 February, we have identified 27 correlated white-light and extreme-ultraviolet (EUV) jet events. In each case, the EUV jet was observed near the limb of the polar coronal hole 20-60 minutes before the corresponding white-light jet was registered in the coronagraphs 2-6 R☉ field of view. The jets originate near flaring EUV bright points and are presumably triggered by field line reconnection between magnetic bipoles and neighboring unipolar flux. The leading edges of the white-light jets propagate outward at speeds of 400-1100 km s-1, whereas the bulk of their material travels at much lower velocities averaging around 250 km s-1 at heliocentric distances of 2.9-3.7 R☉. These lower velocities may reflect the actual outflow speeds of the background polar wind.

Evolution of Chromospheric Structures: How Chromospheric Structures Contribute to the Solar He II 30.4 Nanometer Irradiance and Variability

The bright He II 30.4 nm solar emission is an important energy source for ionization and heating of the Earths upper atmosphere. The analysis of the Solar and Heliospheric Observatory (SOHO) Extreme-Ultraviolet Imaging Telescope (EIT) He II 30.4 nm images provides an improved understanding of how the solar surface structures, i.e., plage, enhanced network (plage remnants), active network, and the quiet chromosphere, contribute to the solar He II 30.4 nm irradiance and its variability. We first normalize the intensities of each image to the background quiet-chromosphere intensity with a global fit that preferentially weights network cell intensities. The resulting quiet-chromosphere intensity scale is stable to within 0.7% (1 σ) over the 2 yr data set. The plage, enhanced-network, active-network, and quiet-chromosphere structures are then identified on each EIT He II image with an algorithm that uses criteria of intensity, size, filling factor, and continuity. This decomposition leads to time series of structure area and integrated intensity, their spatial distribution on the solar disk, and their intensity contrast relative to the quiet-chromosphere intensity; thus, these time series show how the solar surface structures contribute to the He II 30.4 nm irradiance. For example, we find that the active network contributes as much as the plage and enhanced network to the solar He II 30.4 nm irradiance variability during solar minimum. Conversely, the quiet-chromosphere irradiance does not vary during this time period; thus we conclude that long-term He II 30.4 nm irradiance variations can be traced purely to magnetic activity during this time period. We also find that the plage, enhanced-network, active-network, and quiet-network intensity contrasts, relative to the quiet chromosphere and averaged over the full area of each structure, are 4.8, 3.3, 2.1, and 1.6, respectively, and these contrasts remain essentially constant with time.

A Spectroscopic Observation of a Magnetic Reconnection Site in a Small Flaring Event

We have observed two types of coronal bidirectional flows in a flare with a small energy release through a spectroscopic observation of the Fe X emission line at 6374 A with a ground-based coronagraph at the Norikura Solar Observatory. We find a bidirectional flow of ±3 km s-1 above the top of a flare loop. Remarkable increases of the line intensity and line width are not observed in the flow. From the loop geometry and sign of the Fe X Doppler velocity we conclude that the bidirectional flow is reconnection inflow above the flare loop. We estimate the reconnection rate to be ~0.003 for this event. The other bidirectional flow is observed along postflare loops with significant increases of the line intensity and Doppler velocity. This flow is interpreted as a cooling upflow having a velocity of ~10 km s-1 along a postflare loop from its lower part. We also find that the increase of the nonthermal line width in the loop-top region starts when the line intensity reaches its peak. This supports the presence of a mechanism to enhance turbulent plasma motions in the loop-top region.

Imaging the solar corona in the EUV

Abstract 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 Suns corona in 4 EUV wavelengths. The He II filter at 304 A 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 A images the corona at a temperature of ∼ 1.3 × 106 K; the Fe XII filter at 195 A images the quiet corona outside coronal holes at a temperature of ∼ 1.6 × 106 K; and the Fe XV filter at 284 A 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 Suns corona, and in particular, magnetic field reconnection.

Calibration of the EIT instrument for the SOHO mission

Optical characteristics in the wavelength range 15 - 75 nm of the EUV imaging telescope to be launched soon on the SOHO mission are discussed. Bandpasses and photometric sensitivity of the multilayered optics telescope have been measured by a dedicated synchrotron light source at Orsay, France.

Recent Helioseismoiogicai Results from Space: Solar p-mode Oscillations from IPHIR on the PHOBOS Mission

IPHIR (Interplanetary Helioseismology by IRradiance measurements) is a solar irradiance experiment on the USSR planetary mission PHOBOS to Mars and its satellite Phobos. The experiment was built by an international consortium including PMOD/WRC, LPSP, SSD/ESA, KrAO and CRIP. The sensor is a three channel sunphotometer (SPM) which measures the solar spectral irradiance at 335, 500 and 865 nm with a precision of better than 1 part-per-million (ppm). It is the first experiment dedicated to the investigation of solar oscillations from space. The results presented here are from a first evaluation of data gathered during 160 days of the cruise phase of PHOBOS II, launched on July, 12th 1988. The long uninterrupted observation produces a spectrum of the solar p-mode oscillations in the 5-minute range with a very high signal-to-noise ratio, which allows an accurate determination of frequencies and line shapes of these modes.