SPICE is an imaging spectrometer operating at vacuum ultraviolet (VUV) wavelengths, 70.4 – 79.0 nm and 97.3 - 104.9 nm. It is a facility instrument on the Solar Orbiter mission, which carries 10 science instruments in all, to make observations of the Sun’s atmosphere and heliosphere, at close proximity to the Sun, i.e to 0.28 A.U. at perihelion. SPICE’s role is to make VUV measurements of plasma in the solar atmosphere. SPICE is designed to achieve spectral imaging at spectral resolution >1500, spatial resolution of several arcsec, and two-dimensional FOV of 11 x16arcmins. The many strong constraints on the instrument design imposed by the mission requirements prevent the imaging performance from exceeding those of previous instruments, but by being closer to the sun there is a gain in spatial resolution. The price which is paid is the harsher environment, particularly thermal. This leads to some novel features in the design, which needed to be proven by ground test programs. These include a dichroic solar-transmitting primary mirror to dump the solar heat, a high in-flight temperature (60deg.C) and gradients in the optics box, and a bespoke variable-line-spacing grating to minimise the number of reflective components used. The tests culminate in the systemlevel test of VUV imaging performance and pointing stability. We will describe how our dedicated facility with heritage from previous solar instruments, is used to make these tests, and show the results, firstly on the Engineering Model of the optics unit, and more recently on the Flight Model. For the keywords, select up to 8 key terms for a search on your manuscript's subject.
SPICE is a high resolution imaging spectrometer operating at extreme ultraviolet wavelengths, 70.4 – 79.0 nm and 97.3 -
104.9 nm. It is a facility instrument on the Solar Orbiter mission. SPICE will address the key science goals of Solar
Orbiter by providing the quantitative knowledge of the physical state and composition of the plasmas in the solar
atmosphere, in particular investigating the source regions of outflows and ejection processes which link the solar surface
and corona to the heliosphere. By observing the intensities of selected spectral lines and line profiles, SPICE will derive
temperature, density, flow and composition information for the plasmas in the temperature range from 10,000 K to
10MK. The instrument optics consists of a single-mirror telescope (off-axis paraboloid operating at near-normal
incidence), feeding an imaging spectrometer. The spectrometer is also using just one optical element, a Toroidal Variable
Line Space grating, which images the entrance slit from the telescope focal plane onto a pair of detector arrays, with a
magnification of approximately x5. Each detector consists of a photocathode coated microchannel plate image
intensifier, coupled to active-pixel-sensor (APS). Particular features of the instrument needed due to proximity to the Sun
include: use of dichroic coating on the mirror to transmit and reject the majority of the solar spectrum, particle-deflector
to protect the optics from the solar wind, and use of data compression due to telemetry limitations.
Coronal mass ejections (CMEs) and corotating interaction regions (CIRs) as well as their source regions are important
because of their space weather consequences. The current understanding of CMEs primarily comes from the Solar and
Heliospheric Observatory (SOHO) and the Solar Terrestrial Relations Observatory (STEREO) missions, but these
missions lacked some key measurements: STEREO did not have a magnetograph; SOHO did not have in-situ
magnetometer. SOHO and other imagers such as the Solar Mass Ejection Imager (SMEI) located on the Sun-Earth line
are also not well-suited to measure Earth-directed CMEs. The Earth-Affecting Solar Causes Observatory (EASCO) is a
proposed mission to be located at the Sun-Earth L5 that overcomes these deficiencies. The mission concept was recently
studied at the Mission Design Laboratory (MDL), NASA Goddard Space Flight Center, to see how the mission can be
implemented. The study found that the scientific payload (seven remote-sensing and three in-situ instruments) can be
readily accommodated and can be launched using an intermediate size vehicle; a hybrid propulsion system consisting of
a Xenon ion thruster and hydrazine has been found to be adequate to place the payload at L5. Following a 2-year transfer
time, a 4-year operation is considered around the next solar maximum in 2025.
Dissipation in the solar corona is expected to occur in extremely thin current sheets of order 1-100 km. Emission from
these current sheets should be visible in coronal EUV emission lines. However, this spatial scale is far below the
resolution of existing imaging instruments. Conventional optics cannot be easily manufactured with sufficient surface
figure accuracy to obtain the required < 0.1 arcsec resolution. A photon sieve, a diffractive imaging element similar to a
Fresnel zone plate, can be manufactured to provide a few 0.001 arcsec resolution, with much more relaxed tolerances
than conventional imaging technology. A simple design for a sounding rocket payload is presented that obtains 80 mas
(0.080 arcsec) imaging with a 100 mm diameter photon sieve to image Fe XIV 334 and Fe XVI 335. These images will
not only show the structure of the corona at a resolution never before obtained, they will also allow a study of the
temperature structure in the dissipation region.
The measurements of velocity and temperature of coronal electrons are of immense importance to the study of coronal dynamics, especially in the low solar corona. In this lies interesting physics yet to fully explain the theoretical reasoning for the million degree hot coronal plasma and the cause for the initial acceleration of this coronal plasma. In this regard it would be equally important if both of these coronal electron parameters, namely the velocity and the temperature of these coronal electrons, could be determined simultaneously and globally all around the low solar corona. The purpose of this paper is twin fold. First, to lay out an instrumental procedure that allows for the measurement of a coronal signature that could measure all around the low solar corona simultaneously. Second, to describe a theoretical procedure that allows for deriving both the coronal electron temperature and its bulk flow velocity from the measured coronal signature.
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.
The Solar Terrestrial Relations Observatory (STEREO) is a pair of identical satellites that will orbit the Sun so as to drift ahead of and behind Earth respectively, to give a stereo view of the Sun. STEREO is currently scheduled for launch in November 2005. One of the instrument packages that will be flown on each of the STEREO spacecrafts is the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI), which consists of an extreme ultraviolet imager, two coronagraphs, and two side-viewing heliospheric imagers to observe solar coronal mass ejections all the way from the Sun to Earth. We report here on the inner coronagraph, labeled COR1. COR1 is a classic Lyot internally occulting refractive coronagraph, adapted for the first time to be used in space. The field of view is from 1.3 to 4 solar radii. A linear polarizer is used to suppress scattered light, and to extract the polarized brightness signal from the solar corona. The optical scattering performance of the coronagraph was first modeled using both the ASAP and APART numerical modeling codes, and then tested at the Vacuum Tunnel Facility at the National Center for Atmospheric Research in Boulder, Colorado. In this report, we will focus on the COR1 optical design, the predicted optical performance, and the observed performance in the lab. We will also discuss the mechanical and thermal design, and the cleanliness requirements needed to achieve the optical performance.
GSFC is in the process of assembling a solar EUV Normal-Incidence Spectrometer called EUNIS, to be flown as a sounding rocket payload. This instrument builds on the many technical innovations pioneered by our highly successful SERTS experiment over its past ten flights. The new design has improved spatial and spectral resolutions, as well as 100 times greater sensitivity, permitting EUV spectroscopy with a temporal resolution near 1 second for the first time ever. To achieve such high time cadence, a novel Active-Pixel-Sensor detector is being developed as a key component of our design. The high sensitivity of EUNIS allows entirely new studies of transient coronal phenomena, such as the rapid loop dynamics seen by TRACE, and searches for non-thermal motions indicative of magnetic reconnection or wave heating. The increased sensitivity also permits useful EUV spectra at heights of 2-3 solar radii above the limb, where the transition between the static corona and the solar wind might occur. In addition, the new design features two independent optical systems, more than doubling the spectral bandwidth covered on each flight. Its 300-370A bandpass includes He II 304A and strong lines from Fe XI-XVI, extending the current SERTS range of 300-355A to further improve our ongoing series of calibration under-flights for SOHO/CDS and EIT. The second bandpass of 170-205A has a sequence of very strong Fe IX-XIII lines, and allows under-flight support for two more channels on SOHO/EIT, two channels on TRACE, one on Solar-B/EIS, and all four channels on the STEREO/EUVI instrument. First flight of the new EUNIS payload is presently scheduled for 2002 October.
Traditional magnetographs measure the solar magnetic field at the visible 'surface' of the Sun, the photosphere. The Solar Ultraviolet Magnetograph Investigation (SUMI) is a hardware development study for an instrument to measure the solar magnetic field higher in the atmosphere, in the upper chromosphere and in the transition region at the base of the corona. The magnetic pressure at these levels is much stronger than the gas pressure (in contrast to the situation at the photosphere), so the field controls the structure and dynamics of the atmosphere. Rapid changes in the magnetic structure of the atmosphere become possible at this height, with the release of energy. Measurements of the vector magnetic field in this region will significantly improve our understanding of the physical processes heating the Sun's upper atmosphere and driving transient phenomena such as flares and coronal mass ejections. The instrument will incorporate new technologies to achieve the polarization efficiencies required to measure the magnetic splitting of lines in the VUV an UV (CIV at 1550 angstrom and MgII at 2800 angstrom). We describe the scientific goals, the optical components that are being developed for a sounding rocket program, and the SUMI baseline design.
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.
In the solar corona, the density scale height is large, a considerably fraction of a solar radius. Because of this, observations of the Sun from a single vantage point produce images which show an unavoidable overlapping of many structures along the line of sight. This makes it difficult, and sometimes impossible, to determine the true nature of the feature being observed. This difficulty can be overcome by obtaining simultaneous observations from multiple vantage points. Using these observations, and a reconstructions process similar to that used in medical imaging applications, the true 3D nature of the solar corona can be deduced. The same process can be used to follow the formation of coronal mass ejections (CME's) in the low corona and the propagation of CME's through interplanetary space.
A STEREO mission concept requiring only a single new spacecraft has been proposed. The mission would place the new spacecraft in a heliocentric orbit and well off the Sun- Earth line, where it can simultaneously view both the solar source of heliospheric disturbances and their propagation through the heliosphere all the way to the earth. Joint observations, utilizing the new spacecraft and existing solar spacecraft in earth orbit or L1 orbit would provide a stereographic data set. The new and unique aspect of this mission lies in the vantage point of the new spacecraft, which is far enough from Sun-Earth line to allow an entirely new way of studying the structure of the solar corona, the heliosphere and solar-terrestrial interactions. The mission science objectives have been selected to take maximum advantage of this new vantage point. They fall into two classes: those possible with the new spacecraft alone and those possible with joint measurements using the new and existing spacecraft. The instrument complement on the new spacecraft supporting the mission science objectives includes a soft x-ray imager, a coronagraph and a sun-earth imager. Telemetry rate appears to be the main performance determinant. The spacecraft could be launched with the new Med-Lite system.
Spartan Lite is a proposed series of very low-cost spacecraft missions which offer potential flight opportunities for pointed solar experiments. Early versions will be launched as Space Shuttle attached payloads with the capability of being released for free flight. They would not be recovered, allowing useful lifetimes of six months to one year. An expendable launch vehicle option will be added later. The spacecraft is 3-axis stabilized with a cylindrical instrument cavity 100 cm long and 36 cm in diameter. If approved, the program would provide multiple launch opportunities during the upcoming solar maximum. A conceptual instrument design for a solar pointed mission on Spartan Lite is shown and discussed. The Extreme-Ultraviolet Normal Incidence Spectrograph will observe the solar spectrum between 290 and 466 A with high spatial and spectral resolutions. The large bandpass is due to the compact design, fitting two optical systems into the instrument cavity, each observing a different, but overlapping, wavelength range.
The solar output changes on a variety of timescales, from minutes, to years, to tens of years and even to hundreds of years. The dominant timescale of variation is, of course, the 11-year solar cycle. Observational evidence shows that the physics of solar output variation is strongly tied to changes in the magnetic field, and perhaps the most dramatic manifestation of a constantly changing magnetic field is the Coronal Mass Ejection (CME). On August 5 - 6, 1996 the Second Workshop to discuss missions to observe these phenomena from new vantage points, organized by the authors, was held in Boulder, Colorado at the NOAA Space Environmental Center. The workshop was attended by approximately 20 scientists representing 13 institutions from the United States and Europe. The purpose of the Workshop was to discuss the different concepts for multi- spacecraft observation of the Sun which have been proposed, to develop a list of scientific objectives, and to arrive at a consensus description of a mission to observe the Sun from new vantage points. The fundamental goal of STEREO is to discover how coronal mass ejections start at the Sun and propagate in interplanetary space. The workshop started with the propositions that coronal mass ejections are fundamental manifestations of rapid large-scale change in the global magnetic structure of the Sun, that CME's are a major driver of coronal evolution, and that they may play a major role in the solar dynamo. Workshop participants developed a mission concept that will lead to a comprehensive characterization of CME disturbances through build-up, initiation, launch, and propagation to Earth. It will also build a clear picture of long-term evolution of the corona. Participants in the workshop recommended that STEREO be a joint mission with the European scientific community and that it consist of four spacecraft: `East' at 1 AU near L4, 60 deg from EArth to detect active regions 5 days before they can be seen by terrestrial telescopes. `West' at L5 views the sources of energetic particle events reaching Earth. `Earth Orbiter' to view the Sun, solar plasma and Earth's magnetosphere, and `North-South' in a 1 AU orbit tilted 30 deg from the ecliptic plane to provide measurements of polar fields and high-latitude activity. All spacecraft will carry solar activity imagers (e.g., EUV telescope and white-light coronagraph) and radio burst detectors to support a tomography program. All will carry sensitive polarimeters that will image CME's from 40 solar radii to 1 AU, and all will carry instruments for situ plasma and energetic particle sampling. East and North-South have solar vector magnetographs.
A multilayer coated high density toroidal grating was flown on a sounding rocket experiment in the Solar EUV Rocket Telescope and Spectrograph (SERTS) instrument. To our knowledge this is the first space flight of a multilayer coated grating. Pre-flight performance evaluation showed that the application of a 10-layer Ir/Si multilayer coating to the 3600 l/mm blazed toroidal replica grating produced a factor of 9 enhancement in peak efficiency near the design wavelength around 30 nm in first order over the standard gold coating, with a measured EUV efficiency that peaked at 3.3 percent. In addition, the grating''s spectral resolution of better than 5000 was maintained. The region of enhanced grating efficiency due to the multilayer coating is clearly evident in the flight data. Within the bandpass of the multilayer coating, the recorded film densities were roughly equivalent to those obtained with a factor of six longer exposure on the previous flight of the SERTS instrument.