W. Curdt

SUMER - Solar Ultraviolet Measurements of Emitted Radiation

The instrument SUMER — Solar Ultraviolet Measurements of Emitted Radiation is designed to investigate structures and associated dynamical processes occurring in the solar atmosphere, from the chromosphere through the transition region to the inner corona, over a temperature range from 104 to 2 x 106 K and above. These observations will permit detailed spectroscopic diagnostics of plasma densities and temperatures in many solar features, and will support penetrating studies of underlying physical processes, including plasma flows, turbulence and wave motions, diffusion transport processes, events associated with solar magnetic activity, atmospheric heating, and solar wind acceleration in the inner corona. Specifically, SUMER will measure profiles and intensities of EUV lines; determine Doppler shifts and line broadenings with high accuracy; provide stigmatic images of the Sun in the EUV with high spatial, spectral, and temporal resolution; and obtain monochromatic maps of the full Sun and the inner corona or selected areas thereof. SUMER will be flown on the Solar and Heliospheric Observatory (SOHO), scheduled for launch in November, 1995. This paper has been written to familiarize solar physicists with SUMER and to demonstrate some command procedures for achieving certain scientific observations.

The SUMER spectral atlas of solar-disk features

A far-ultraviolet and extreme-ultraviolet (FUV, EUV) spectral atlas of the Sun between 670 Aa nd 1609 A in the rst order of diraction has been derived from observations obtained with the SUMER (Solar Ultraviolet Measurements of Emitted Radiation) spectrograph on the spacecraft SOHO (Solar and Heliospheric Observatory). The atlas contains spectra of the average quiet Sun, a coronal hole and a sunspot on the disk. Dierent physical parameters prevalent in the bright network (BN) and in the cell interior (CI) { contributing in a distinct manner to the average quiet-Sun emission { have their imprint on the BN/CI ratio, which is also shown for almost the entire spectral range. With a few exceptions, all major lines are given with their identications and wavelengths. Lines that appear in second order are superimposed on the rst order spectra. These lines are clearly marked in the atlas. The spectra include emissions from atoms and ions in the temperature range 6 10 3 Kt o 2 10 6 K, i.e., continua and emission lines emitted from the lower chromosphere to the corona. This spectral atlas, with its broad wavelength coverage, provides a rich source of new diagnostic tools to study the physical parameters in the chromosphere, the transition region and the corona. In particular, the wavelength range below 1100 A as observed by SUMER represents a signicant improvement over the spectra produced in the past. In view of the manifold appearance and temporal variation of the solar atmosphere, it is obvious that our atlas can only be a { hopefully typical { snapshot. Brief descriptions of the data reduction and calibration procedures are given. The spectral radiances are determined with a relative uncertainty of 0.15 to 0.30 (1) and the wavelength scale is accurate to typically 10 m A. The atlas is also available in a machine readable form.

FIRST RESULTS OF THE SUMER TELESCOPE AND SPECTROMETER ON SOHO – I. Spectra and Spectroradiometry

SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 Å (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 Å. The second-order spectra of detectors A and B cover 330 to 805 Å and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 106 K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mÅ is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.SUMER – the Solar Ultraviolet Measurements of the Emitted Radiation instrument on the Solar and Heliospheric Observatory (SOHO) – observed its first light on January 24, 1996, and subsequently obtained a detailed spectrum with detector B in the wavelength range from 660 to 1490 A (in first order) inside and above the limb in the north polar coronal hole. Using detector A of the instrument, this range was later extended to 1610 A. The second-order spectra of detectors A and B cover 330 to 805 A and are superimposed on the first-order spectra. Many more features and areas of the Sun and their spectra have been observed since, including coronal holes, polar plumes and active regions. The atoms and ions emitting this radiation exist at temperatures below 2 × 106 K and are thus ideally suited to investigate the solar transition region where the temperature increases from chromospheric to coronal values. SUMER can also be operated in a manner such that it makes images or spectroheliograms of different sizes in selected spectral lines. A detailed line profile with spectral resolution elements between 22 and 45 mA is produced for each line at each spatial location along the slit. From the line width, intensity and wavelength position we are able to deduce temperature, density, and velocity of the emitting atoms and ions for each emission line and spatial element in the spectroheliogram. Because of the high spectral resolution and low noise of SUMER, we have been able to detect faint lines not previously observed and, in addition, to determine their spectral profiles. SUMER has already recorded over 2000 extreme ultraviolet emission lines and many identifications have been made on the disk and in the corona.

Doppler Shift Oscillations of Hot Solar Coronal Plasma Seen by SUMER: A Signature of Loop Oscillations?

We report observations of strongly damped Doppler shift oscillations detected in a flare line, Fe XIX, with the Solar Ultraviolet Measurement of Emitted Radiation spectrometer. Spectra were recorded above an active region at the western limb of the Sun, from lines with formation temperatures ranging from 0.01 to 10 MK. However, the oscillations were seen only in the hot plasma (>6 MK) lines. The Doppler oscillations have periods of 14-18 minutes, with an exponential decay time of 12-19 minutes, and show an initial large blueshift pulse with peak velocities up to 77 km s-1. Several indications suggest that the Doppler oscillations are incompressible coronal loop oscillations that are excited impulsively by a flarelike event that also produced a strong increase in Fe XIX emission.

The Sunrise Mission

The first science flight of the balloon-borne Sunrise telescope took place in June 2009 from ESRANGE (near Kiruna/Sweden) to Somerset Island in northern Canada. We describe the scientific aims and mission concept of the project and give an overview and a description of the various hardware components: the 1-m main telescope with its postfocus science instruments (the UV filter imager SuFI and the imaging vector magnetograph IMaX) and support instruments (image stabilizing and light distribution system ISLiD and correlating wavefront sensor CWS), the optomechanical support structure and the instrument mounting concept, the gondola structure and the power, pointing, and telemetry systems, and the general electronics architecture. We also explain the optimization of the structural and thermal design of the complete payload. The preparations for the science flight are described, including AIV and ground calibration of the instruments. The course of events during the science flight is outlined, up to the recovery activities. Finally, the in-flight performance of the instrumentation is discussed.

FIRST RESULTS OF THE SUMER TELESCOPE AND SPECTROMETER ON SOHO – II. Imagery and Data Management

SUMER – Solar Ultraviolet Measurements of Emitted Radiation – is not only an extreme ultraviolet (EUV) spectrometer capable of obtaining detailed spectra in the range from 500 to 1610 Å, but, using the telescope mechanisms, it also provides monochromatic images over the full solar disk and beyond, into the corona, with high spatial resolution. We report on some aspects of the observation programmes that have already led us to a new view of many aspects of the Sun, including quiet Sun, chromospheric and transition region network, coronal hole, polar plume, prominence and active region studies. After an introduction, where we compare the SUMER imaging capabilities to previous experiments in our wavelength range, we describe the results of tests performed in order to characterize and optimize the telescope under operational conditions. We find the spatial resolution to be 1.2 arc sec across the slit and 2 arc sec (2 detector pixels) along the slit. Resolution and sensitivity are adequate to provide details on the structure, physical properties, and evolution of several solar features which we then present. Finally some information is given on the data availability and the data management system.

Comet P/Halley’s nucleus and its activity

The Halley Multicolour Camera on board ESA’s Giotto spacecraft observed the nucleus of comet P/Halley and its environment and returned more than 2000 images. The observations are summarized, their calibration is described, the status of the analysis and the results are discussed. Topographic features on the nuclear surface and areas of activity are identified. The optical thickness of the dust produced in jet-like features is estimated. The impact and constraints of the observations on cometary nucleus models are discussed.

SEMIEMPIRICAL MODELS OF THE SOLAR ATMOSPHERE. III. SET OF NON-LTE MODELS FOR FAR-ULTRAVIOLET/EXTREME-ULTRAVIOLET IRRADIANCE COMPUTATION

Semiempirical atmospheric models of solar surface features as observed at moderate resolution are useful tools for understanding the observed solar spectral irradiance variations. Paper I described a set of models constructed to reproduce the observed radiance spectrum for solar surface features at ~2 arcsec resolution that constitute an average over small-scale features such as granulation. Paper II showed that a revision of previous models of low-chromospheric inter-network regions explains the observed infrared CO lines in addition to the UV and radio continuum from submillimeter to centimetric wavelengths. The present paper (1) shows that the Ca II H and K line wing observations are also explained by the new quiet-Sun-composite model, (2) introduces new low-chromospheric models of magnetic features that follow the ideas in Paper II, (3) introduces new upper chromospheric structures for all quiet-Sun and active-region models, and (4) shows how the new set of models explains EUV/FUV observations of spectral radiance and irradiance. This paper also discusses the chromospheric radiative-loss estimates in each of the magnetic features. The new set of models provides a basis for the spectral irradiance synthesis at EUV/FUV wavelengths based on the features observed on the solar surface.

Hot coronal loop oscillations observed with SUMER: Examples and statistics

We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50-100 arcsec off the limb of the Sun in ultraviolet lines, mainly Fe XIX and Fe XXI, with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops ( or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7-31 min, with decay times 5.7-36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s(-1). The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2-3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: ( 1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; ( 2) the intensity fluctuation lags the Doppler shifts by 1/4 period; ( 3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro- or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8-102 km s(-1), together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure.

Slow-mode standing waves observed by SUMER in hot coronal loops

We report the first detection of postflare loop oscillations seen in both Doppler shift and intensity. The observations were recorded in an Fe xix line by the SUMER spectrometer on SOHO in the corona about 70 min after anM-class flare on the solar limb. The oscillation has a period of about 17 min in both the Doppler velocity and the intensity, but their decay times are different (i.e., 37 min for the velocity and 21 min for the intensity). The fact that the velocity and the intensity oscillations have exactly a 1/4-period phase difference points to the existence of slow-mode standing waves in the oscillating loop. This interpretation is also supported by two other pieces of evidence: (1) the wave period and (2) the amplitude relationship between the intensity and velocity are as expected for a slow-mode standing wave.