U. Schühle

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.

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.

SUMER MEASUREMENTS OF NONTHERMAL MOTIONS: CONSTRAINTS ON CORONAL HEATING MECHANISMS

We have determined nonthermal velocities in the quiet Sun at temperatures between 104 K and 2 × 106 K by measuring the widths of a number of EUV and far-ultraviolet (FUV) lines taken with SUMER on board the SOHO spacecraft. The broadenings owing to the SUMER instrument and the finite opacity in each line have been carefully examined. The nonthermal velocity at temperatures below 2 × 104 K is smaller than 10 km s-1. The velocity increases with temperature, reaches a peak value of 30 km s-1 around 3 × 105 K, and then decreases with the temperature. The coronal nonthermal velocity is about 20 km s-1. There exists a strong correlation between intensity and nonthermal velocity at temperatures 2 × 104-1 × 105 K. The correlation at higher temperatures weakens as temperature increases. Furthermore, there is a spatial correlation between the nonthermal velocities inferred from a set of any two lines with temperatures below 2 × 105 K. Neither significant center-to-limb variation nor meaningful dependence on the integration time was found from the measured nonthermal velocities. We have discovered the existence of high-velocity components in the observed S VI λ933.4 line profiles. The average nonthermal velocity and intensity fraction of this S VI line high-velocity component are found to be 55 km s-1 and 0.25, respectively. Observational characteristics of nonthermal motions carry some problems that should be solved when interpreting observed nonthermal motions in terms of either unresolved loop flows or Alfven waves. The isotropic and very small scale nature of the observed nonthermal motions appears to be suited to the MHD turbulence interpretation of nonthermal motions. The turbulent heating rates inferred from the measured nonthermal motions can account for the radiative loss throughout the transition region and corona if the nonthermal motions are truly turbulent motions whose mechanical energy is injected at a scale of 1000 km (Kolmogorov-type turbulence) or 15 km (Kraichnan-type turbulence). The existence of high-velocity components at temperatures 6 × 104-2 × 105 K appears as observational evidence supporting nanoflare heating at these temperatures.

The coronal composition above the solar equator and the north pole as determined from spectra acquired by the SUMER instrument on SOHO

Using spectra obtained by the SUMER instrument on the Solar and Heliospheric Observatory (SOHO) we have determined the composition of the bulk of the coronal plasma in the vicinity of the solar surface over a polar coronal hole and an equatorial region. Our measurements show that although low first ionization potential (FIP) elements are enriched by about a factor of 4 in the corona above the quiet equatorial region, little or no enrichment exists above the north polar coronal hole. These observations are in good agreement with the Ulysses in situ observations in both fast speed and slow speed winds.

Electron Densities in the Solar Polar Coronal Holes from Density-Sensitive Line Ratios of Si VIII and S X

We derive electron densities as a function of height in the north and south polar coronal holes from a forbidden spectral line ratio of Si VIII. Si VIII is produced at about 8 × 105 K in ionization equilibrium. We also derive densities from a similar line ratio of S X (1.3 × 106 K). The spectra were obtained with the Solar Ultraviolet Measurements of Emitted Radiation spectrometer flown on the Solar and Heliospheric Observatory spacecraft. In addition to the primary mechanism of electron impact excitation, the derivation of theoretical level populations for Si VIII and S X includes both proton and resonance capture excitation. We compare the coronal hole results to quiet-Sun coronal measurements obtained outside the east and west limbs. We find for distances of a few arcseconds outside the solar limb that the average line-of-sight electron densities in the coronal holes are about a factor of 2 lower than in quiet-Sun regions. The decrease of density with height is exponential in the polar holes. We also confirm the result known from a variety of earlier observations that the temperature of most of the plasma in coronal holes does not exceed about 106 K.

On-Orbit Degradation of Solar Instruments

We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about On-Orbit Degradation of Solar and Space Weather Instruments that took place at the Solar Terrestrial Centre of Excellence (Royal Observatory of Belgium) in Brussels on 3 May 2012. The aim of this workshop was to open discussions related to the degradation observed in Sun-observing instruments exposed to the effects of the space environment. This article summarizes the various lessons learned and offers recommendations to reduce or correct expected degradation with the goal of increasing the useful lifespan of future and ongoing space missions.

Electron density diagnostics for the solar upper atmosphere from spectra obtained by SUMER/SOHO

We evaluate the electron density in various solar regions above the limb observed by the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument on SOHO. We find in general good agreement among line ratios from Be-, B-, N-, and Mg-like ions, giving densities of order 108 cm-3.

Radiometric calibration of SUMER: refinement of the laboratory results under operational conditions on SOHO

The radiometric calibration of the solar telescope and spectrometer SUMER was carried out in the laboratory before delivery of the instrument for integration into the SOHO (Solar and Heliospheric Observatory) spacecraft. Although this effort led to a reasonable coverage of the wavelength range from 53.70 to 146.96 nm, uncalibrated portions of the sensitivity curves remained before SUMER became operational in early 1996. Thereafter it was possible to perform extrapolations and interpolations of the calibration curves of detector A to shorter, longer, and intermediate wavelengths by using emission line pairs with known intensity ratios. The spectra of the stars alpha and rho Leonis were also observed on the KBr (potassium bromide) photocathode and the bare microchannel plate (MCP) in the range from 120 to 158 nm. In addition, the sensitivity ratios of the KBr photocathode to the bare MCP were determined for many solar lines as well as the H i Lyman and the thermal continua. The results have been found to be consistent with published laboratory data. The uncertainty is +/-15% (1 varsigma) in the wavelength range from 54 to 125 nm.