A problem of fundamental importance for future space travel to the Moon and Mars is the determination and prediction of the radiation environment generated by the Sun. The sources of solar energetic particles (SEP) and the physical processes associated with their acceleration and propagation are not well understood. Ultraviolet coronagraphic spectroscopy uniquely has the capabilities for determining the detailed plasma properties of the likely source regions of such particles. This information can be used to develop empirical models of the source regions for specific events, and it can provide the key information needed to identify and understand the physical processes that produce SEP hazards. UVCS/SOHO observations have provided the first detailed diagnostics of the plasma parameters of coronal mass ejections (CMEs) in the extended corona. These observations have provided new insights into the roles of shock waves, reconnection and magnetic helicity in CME eruptions. Next generation ultraviolet coronagraph spectrometers could provide additional diagnostic capabilities. This paper summarizes past observations, and discusses the diagnostic potential of advanced ultraviolet coronagraphic spectroscopy for characterizing two possible sites of SEP production: CME shocks and reconnection current sheets.
The Advanced Spectroscopic and Coronagraphic Explorer (ASCE) was proposed in 2001 to NASA's Medium-Class Explorer (MIDEX) program by the Smithsonian Astrophysical Observatory in collaboration with the Naval Research Laboratory, Goddard Space Flight Center
and the Italian Space Agency. It is one of four missions selected for Phase A study in 2002. ASCE is composed of three instrument units: an Advanced Ultraviolet Coronagraph Spectrometer (AUVCS), an Advanced Large Aperture visible light Spectroscopic Coronagraph (ALASCO),
and an Advanced Solar Disk Spectrometer (ASDS). ASCE makes use of a 13 m long boom that is extended on orbit and positions the external occulters of AUVCS and ALASCO nearly 15 m in front of their respective telescope mirrors. The optical design concepts for the instruments
will be discussed.
We have carried out measurements of efficiency as functions of position across the surfaces of replica grating made from the same masters as the UVCS/SOHO flight units. Variations in first order efficiency which significantly affect the interpretation of UVCS data are found along the direction perpendicular to the grooves. Variations are also found along the direction parallel to the grooves, but these do not seriously affect UVCS data interpretation. The measurements and their application to the radiometric calibration of UVCS/SOHO are discussed.
The Advanced Solar Coronal Explorer (ASCE) is one of five missions selected for a Phase A Concept Study in the current round of proposed MIDEX missions. ASCE's instrument complement is supported by a SPARTAN 400 reusable carrier. The spacecraft is carried into orbit and deployed by the Space Shuttle; at mission's end, nominally 2 years later, it is retrieved and returned to earth for post-flight calibration. ASCE comprises two instrument modules, the Spectroscopic and Polarimetric Coronagraph (SPC) and the Extreme Ultraviolet Imager (EUVI). The external occulter for the coronagraph is supported on a boom, which is extended 10 meters beyond the instrument apertures once the spacecraft is on station. Large aperture optics can therefore be used, and this, in combination with improvements in optical and photon detection efficiencies, will provide spectroscopy of the extended solar corona with unprecedented sensitivity and spatial resolution, routine measurements of the electron temperature, and polarimetry of the H I Lyman lines. SPC also extends the short wavelength limit to 28 nm. As a consequence, SPC will be able to perform the first He II 30.4 nm and He I 58.4 nm spectroscopy of the extended corona. In the visible part of the spectrum (450 - 600 nm), SPC's Large Aperture Spectroscopic Coronagraph (LASCO) channel will provide polarimetric images with 1.8 arc second resolution elements, which will allow the determination of polarized brightness of the coronal plasma. In a separate parallel channel LASCO will also provide images at single minor ion line wavelengths from which can be determined the shapes and Doppler shifts of those lines. The distant external occulter provides for major improvement in stray light suppression. The EUVI instrument will take high cadence images of the full disk and low corona at four selectable wavelengths with 0.9 arc second resolution elements. A description of the instrument design and performance capabilities is presented.
The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) comprises two telescopes and two spectrometer channels for spatially resolved ultraviolet spectral diagnostics of the solar corona. The principal lines for which the two channels are optimized are the H I 'Lyman-(alpha) ' line at 121.5 nm and the O VI (O<SUP>5+</SUP>) doublet at 103.2 and 103.7 nm. An 'in-flight' method, using observations of stars and scattered solar disk light, has been devised to determine the flat field function, i.e., the relative detection efficiency of the detector pixels. We present the details and results of this process. Local pixel-to-pixel efficiency variation is found to be, typically, about plus or minus 9% to plus or minus 17% (1 (sigma) ) for the H I Lyman-(alpha) channel and plus or minus 9% for the O VI channel.
The Advanced Solar Coronal Explorer (ASCE) is a mid-explorer (MidEx) mission selected, together with other five, for the a Phase A Concept Study in the 1999 round of MidEX proposal. ASCE's spacecraft bus is a SPARTAN 400 reusable carrier deployed in low Earth orbit by the Space Shuttle. ASCE's payload comprises two instrument modules, the Spectroscopic and Polarimetric Coronagraph (SPC) and the Extreme Ultraviolet Imager (EUVI). The scientific objective of the mission is the investigation, through spectroscopic and polarimetric techniques, of the physics of the coronal heating and of the solar wind acceleration. A critical physical parameter of the corona is the magnetic field. Polarimetric measurements of UV coronal radiation and their interpretation through the Hanle effect can be used for coronal magnetic field diagnostics. One of the SPC spectrometers, the Spectroscopy/Polarimetry channel (SPCH), includes a reflecting Brewster-angle polarimeter for measurements of the linear polarization of the HI Lyman series lines (i.e., Ly-(alpha) , -(beta) , and (gamma) ) and of the O VI 1032 Angstrom line. In this paper, the optical design of the SPCH polarimeter is described. A relevant element of this design is the external occulter (EXO) that is supported on a boom, which is extended 10 m beyond the instrument aperture, once the instrument is in station. The analysis of the stray- light reduction provided by this occulting system is described in this paper. The principal source of stray light is solar disk light that is diffracted from the edge of the EXO and scattered from the telescope mirror. The analysis shows that the stray-light is less than 10<SUP>-2</SUP> the coronal signal. This level of stray-light rejection minimizes the polarized stray light that may be introduced by the EXO's straight edge. The most appropriate material for the polarizer has been found to be CaF<SUB>2</SUB>. The material selection criteria are described. Finally, the paper illustrates with an example that if the linear polarization can be measured better than 1%, then the instrumental sensitivity to magnetic fields may reach a few gauss (greater than 2 gauss), in coronal active regions.
The optical configurations studied for the EUV channels on board the Advanced Solar Coronal Explorer (ASCE) mission are presented. ASCE has been proposed to study fundamental physical processes as heating of coronal holes and streamers, driving of solar wind and coronal mass ejections. It comprises three EUV channels: the HEII channel for high-resolution spectroscopic measurements in the 57.3 - 64.3 nm region, the Te channel for the determination of the electron temperature by measuring the shapes of the electron-scattered wings of the coronal HI Ly(alpha) (121.6 nm) and the S/P channel for spectroscopic measurements in the regions near HI Ly(alpha) and O VI (103.2 nm) lines and for polarimetric measurements of the HI Lyman series lines, that will explore the possibility of measuring coronal magnetic fields. The possibility of using toroidal gratings or spherical varied line-space ones has been analyzed. The results of the ray-tracing simulations on the three channels will be presented in details.
During the late stages of integration at MATRA-Marconi in Toulouse, France of the Ultraviolet Coronagraph Spectrometer (UVCS) for the joint NASA/ESA (European Space Agency) Solar and Heliospheric Observatory (SOHO), project management for the International Solar and Terrestrial Physics Project (ISTP) at Goddard Space Flight Center (GSFC) became concerned that the instrument's elaborate stray light rejection system had not been tested and might possibly be misaligned such that the instrument could not deliver promised scientific returns. A white light stray light test, which would place an upper bound on UVCS's stray light rejection capability, was commissioned, conceived, and carried out. This upper bound value would be indicative of the weakest coronal features the spectrometer would be capable of discerning. The test was rapidly developed at GSFC, in parallel with spacecraft integration, in coordination with science team members from Harvard- Smithsonian Center for Astrophysics (CFA) and was carried out at MATRA in late February 1995. The outcome of this test helped justify later impact to integration schedule to conduct similar much needed testing with visible and far ultraviolet light at CFA in a facility specifically designed to perform such tests.
Chemically-vapor-deposited (CVD) silicon carbide (SiC) has become a popular mirror material for spaceborne solar instrumentation for the vacuum ultraviolet wavelength range due to its appreciable broadband reflectance and favorable thermal and opto-mechanical properties. Scatter from surfaces of mirrors operating in this wavelength range can destroy otherwise good image contrast especially for extended targets such as the sun. While valid far ultraviolet (FUV) scatter measurements are entirely non-trivial to conduct and so are rarely performed, visible light scatter measurements are comparatively easy. Unfortunately, it is not straightforward to predict FUV scatter performance based on visible light scatter measurements for mirrors made of CVD SiC. It is hoped that by carrying out scatter measurements in both wavelength regimes for the same CVD SiC mirror, that the ability to make such predictions may be enhanced. Visible light (633 nm) scatter measurements were performed at Goddard Space Flight Center (GSFC) by two different means on CVD SiC telescope mirrors (from the same process and same vendor) for two instruments on the Solar and Heliospheric Observatory (SOHO) - - the Ultraviolet Coronagraph Spectrometer (UVCS) and Solar Ultraviolet Measurement of Emitted Radiation (SUMER). Additionally, extensive FUV scatter measurements were made for SUMER telescope mirrors. In this paper, we correlate the results for those FUV and visible light scatter measurements for this important material.
The Ultraviolet Coronagraph Spectrometer (UVCS) of the Solar and Heliospheric (SOHO) mission has been developed for spectroscopic and polarimetric determinations of temperatures, densities and flow velocities in the extended solar corona. The instrument consists of a visible light (VL) polarimeter, and two ultraviolet (UV) spectrometers, optimized for the H I Lyman (alpha) (Ly-(alpha) ) line ((lambda) 1216 angstrom) and for the O VI doublet ((lambda) (lambda) 1032/1037 angstrom). The stray-light profile of the Ly-(alpha) holographic grating has been measured, in a laboratory set-up, in both the spatial and spectral directions. The observed profile has been found to be a combination of two components: the scattering from the grating surface, and the Fraunhofer diffraction due to the vignetting of the grating. An analytical expression for the scattering component of the grating point spread function (PSF) has been derived from a simple model of the grating surface roughness. The stray-light profile generated by the analytical expression of the grating PSF gives a good fit of stray-light profile measured in the laboratory. This instrument function has been used in the analysis of in-flight UVCS observations of the profile of electron scattered Ly- (alpha) from the solar corona. These observations have resulted, for the first time, in the most direct measurement of the coronal electron temperature.
The Ultraviolet Coronagraph Spectrometer is one of the instruments on board the Solar and Heliospheric Observatory spacecraft, which was launched in December, 1995. The instrument is designed to make ultraviolet spectrometric measurements and visible polarimetric measurements of the extended solar corona. Prior to launch laboratory measurements were carried out to determine system level values for many of the key performance parameters. Further measurements on instrument performance have been carried out since launch. Presented are descriptions of measurement techniques and representative results.
The SOHO ultraviolet coronagraph spectrometer (UVCS/SOHO) is composed of three reflecting telescopes with external and internal occultation and a spectrometer assembly consisting of two toric grating spectrometers and a visible light polarimeter. The UVCS will perform ultraviolet spectroscopy and visible polarimetry to be combined with plasma diagnostic analysis techniques to provide detailed empirical descriptions of the extended solar corona from the coronal base to a heliographic height of 12 R. In this paper, the salient features of the design of the UVCS instrument are described. An overview of the UVCS test and calibration activities is presented. The results from the calibration activity have demonstrated that the UVCS can achieve all its primary scientific observational goals.
The diffraction efficiency of holographic gratings depends on the degree and orientation of the linear polarization of the incident light. A test has been run to measure the reflectance efficiency at Kr 1236 angstrom of two holographic gratings in the orders +1, 0, $min1, as a function of the angle of the incident beam polarization plane. To this purpose, a triple-reflection polarizer, optimized for Ly(alpha) (1216 angstrom) has been fabricated and characterized. The test has been run to demonstrate that the coronal resonantly scattered Ly(alpha) can be measured using a holographic grating in the Ly(alpha) channel of the ultraviolet coronagraph spectrograph (UVCS) of the Solar and Heliospheric Observatory (SOHO) without introducing relevant polarization biases inside the instrument.
The Ultraviolet Coronagraph Spectrometer (UVCS) of the Solar and Heliospheric (SOHO) mission has been developed for spectroscopic and polarimetric determinations of temperatures, densities and flow velocities in the extended solar corona. The instrument consists of two ultraviolet (UV) spectrometers and a visible light (VL) polarimeter. The grating of the UV channel optimized for the HI Lyman (alpha) (Ly-(alpha) ) line ((lambda) 1236 angstrom) and that for the OVI doublet ((lambda) (lambda) 1032/1037 angstrom) have been characterized.