Proceedings Volume 2804

Missions to the Sun

David M. Rust
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Proceedings Volume 2804

Missions to the Sun

David M. Rust
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 25 November 1996
Contents: 5 Sessions, 29 Papers, 0 Presentations
Conference: SPIE's 1996 International Symposium on Optical Science, Engineering, and Instrumentation 1996
Volume Number: 2804

Table of Contents

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Table of Contents

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  • A Close Look at the Sun
  • Solar Observations in Stereo
  • New Missions and Instrument Designs
  • Solar Image Processing
  • Recent Instruments and Results
  • New Missions and Instrument Designs
  • A Close Look at the Sun
  • Recent Instruments and Results
  • Solar Image Processing
  • A Close Look at the Sun
  • New Missions and Instrument Designs
  • Recent Instruments and Results
  • New Missions and Instrument Designs
  • Recent Instruments and Results
  • New Missions and Instrument Designs
  • Solar Observations in Stereo
A Close Look at the Sun
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Advanced Solar Probe Experiment Module (AD SOLEM)
Ralph L. McNutt Jr., Robert E. Gold, Edwin P. Keath, et al.
A small, low-power suite of fields and particles and imaging experiments is required for fulfilling the critical science objectives for a near-sun flyby mission. We discuss how an integrated instrument suite using novel sensors and advanced detector/microelectronics/packaging techniques can be implemented for such a payload. Critical tradeoffs between science requirements, measurement strategies and these resource limits are discussed, and critical enabling components are identified. The instrument site consists of 6 major investigations, some with multiple sensors, power conditioners for both high and low voltages and a common DPU. The concept design is essentially a dress-rehearsal of how a payload could realistically make the measurements needed to answer the critical science questions while operating within a real-world physics, engineering and technology context.
Concept for a miniature solar magnetograph
We present a novel concept for a solar magnetograph that uses a photo-refractive crystal to reflect and focus the light from the wings of many spectral lines onto a camera. The crystal acts simultaneously as multiple, narrow-band filters and as an off-axis telescope. Polarization measurements are performed close to the final focus. Since this approach uses the light from many spectral lines simultaneously, the required telescope aperture is substantially reduced and exposure times can be so short that accurate tracking is not necessary. Such a concept is particularly attractive for NASA's Minimum Solar Mission where very compact, light-weight instruments are required.
Solar Observations in Stereo
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Heliospheric Links Explorer (HELIX)
David M. Rust, N. U. Crooker, Leon Golub, et al.
The proposed HELIX mission consists of two spacecraft that will enable stereoscopic imaging of solar mass ejections, starting with their origins on the Sun and continuing to 1 AU and beyond. With a complement of telescopes and plasma detectors, the HELIX spacecraft will test magnetic helicity conservation and other approaches to understanding the physics of solar mass ejections. The mission will help explain how and why solar ejections occur and how they evolve in interplanetary space. 3D images and velocity maps and in-situ space plasma and magnetic field measurements will allow identification and tracking of ejected plasma. Detection of eruptions aimed at Earth will be an immediate practical benefit of the mission. The HELIX mission should lead to the development of a reliable storm prediction capability that will be of significant value to communications systems operators, electric power networks, NASA operators and others.
STEREO: a solar terrestrial event observer mission concept
Dennis George Socker, S. K. Antiochos, Guenter E. Brueckner, et al.
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.
Tomography of the solar corona and the inner heliosphere: the solar imaging of coronal extended structures (SLICES)
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.
High-spectral-resolution x-ray measurements of solar flares and gama ray bursts on two new small satellites
Upendra D. Desai, Mario H. Acuna, Thomas L. Cline, et al.
Two new small satellites will be launched in late 1996, each carrying x-ray and gamma-ray detectors capable of high spectral resolution while operating at or near room temperature. The Argentinean Satellite de Aplicaciones Cientificas B (SAC-B) and the Small Spacecraft Technology Institute Clark mission will each carry several arrays of x- ray detectors primarily to study solar flares and gamma ray bursts. Arrays of small (8 mm X 8 mm X 2 mm thick) cadmium zinc telluride detectors will provide x-ray measurements in the 5 - 60 keV range with an energy resolution of 2 - 4 keV. Arrays of both silicon avalanche photodiodes (APDs) and P-intrinsic-N photodiodes will provide energy coverage from 2 to 20 keV with approximately 1 keV resolution. High energy data from 30 to 300 keV will be obtained on SAC-B using CsI(TI) scintillators coupled to silicon APDs, resulting in significant savings in weight, power, and volume relative to conventional CsI- photomultiplier systems.
Solar Mass Ejection Imager (SMEI)
Stephen L. Keil, Richard C. Altrock, Stephen Kahler, et al.
The Solar Mass Ejection Imager (SMEI) experiment is designed to detect and measure transient plasma features in the heliosphere, including coronal mass ejections, shock waves, and structures such as streamers which corotate with the Sun. SMEI will provide measurements of the propagation of solar plasma clouds and high-speed streams which can be used to forecast their arrival at Earth from one to three days in advance. The white light photometers on the HELIOS spacecraft demonstrated that visible sunlight scattered from the free electrons of solar ejecta can be sensed in interplanetary space with an electronic camera baffled to remove stray background light. SMEI promises a hundred-fold improvement over the HELIOS data, making possible quantitative studies of mass ejections. SMEI measurements will help predict the rate of energy transfer into the Earth's magnetospheric system. By combining SMEI data with solar, interplanetary and terrestrial data from other space and ground-based instruments, it will be possible to establish quantitative relationships between solar drivers and terrestrial effects. SMEI consists of three cameras, each imaging a 60 degree(s) X 3 degree(s) field of view for a total image size of 180 degree(s) X 3 degree(s). As the satellite orbits the earth, repeated images are used to build up a view of the entire heliosphere.
Problem of designing a solar stereoscopic observatory
Victor M. Grigoryev, P. Papushev, V. Konovalov, et al.
This paper presents the results derived by exploring the possibilities of creating an interplanetary stereoscopic observatory to investigate the 3D structure solar features from granules and spicules to coronal structure. A preliminary stage was made of the passive motion of two spacecrafts in the vicinity of Lagrangian libration points L4 and L5. The version of ballistic scheme of setting-up of the system with a minimal deployment time is considered. For preliminary development of stereoscopic SC the main parameters of scientific payload have been taken: mass - 600 kg, power's - 1 kw, summary data - 5 Gbits per day. The chief results of this work are: (1) the stereoscopic observatories can be realized with a complete set of achievable objectives, (2) launching of SC with the mass 2000 kg into both libration points is possible by using Soviet rocket-vehicle `Proton' in a time 1.17 year, (3) transmission of information from stereoscopic observatory amount 5 Gbit per day is possible by the ground-based antenna 70 m in diameter and using, aboard the SC's, a transmitting-receiving phase-array antenna of size 5 X 5 m.
New Missions and Instrument Designs
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Conceptual design for a solar instrument on the proposed Spartan Lite spacecraft
Michael James Amato, Roger J. Thomas, Joseph M. Davila, et al.
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.
Solar Image Processing
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Measuring storage and release of magnetic free-energy
Juan M. Fontenla
The discovery of the large scale patterns of the solar magnetic field has roots in the Babcock model of the sunspot cycle. This explanation of developing toroidal field from the differential rotation, is equivalent to the development of a global poloidal electric current distribution such as shown in Figure 1. We sketched the currents' path as mostly closing through the photosphere but the actual path could well go through the corona, and can also change during the solar cycle. The picture of Fig. 1 is consistent with the induction of poloidal EMF as result of the poloidal field and toroidal plasma velocity. There is still pending the question of how the 22 years magnetic cycle works. Several ideas have been proposed, but so far none is very well developed and tested. Schematically, the surface field reversal is believed to result from the disappearance of the previous cycle flux, and replacement by emerging opposite directed flux. The closed (by Maxwell's laws) magnetic field and associated toroidal electric current must either be expelled from the Sun or decay and dissipate in place. Field reconnection may play a role for eliminating the old field, but does not destroy the field. Eliminating the field implies destroying the associated electric currents. The scheme of Figure 2 shows the implications of these ideas. Both Figs. I and 2 deal with cylindrical symmetric schemes with base latitudinal mode. After observing many solar cycles, it is clear now that there is no such simple symmetry in the solar fields, thus, although these schemes may be partially valid there is a far richer spectrum of possibilities. These possibilities arise from added degrees of freedom due to field structures of sizes smaller than the whole Sun. A few of such possible temporal evolution schemes are shown in the Figure 3 for some low latitudinal and longitudinal modes. Since higher angular mode cases are also distorted by global differential rotation, a number of features of the symmetric models also show up. However, the closed field loop bundles can not only become distorted, but also move and rotate in many ways.
Recent Instruments and Results
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Software configurable CCD camera for SOHO mission
J. Patrick Haas, Leslie J. Payne
Goddard's Laboratory for Astronomy and Solar Physics has developed a CCD camera design for space flight use with multi-mission capability as part of its design philosophy. The first application of this camera is being demonstrated as part of the Coronal Diagnostic Spectrometer onboard the Solar and Heliospheric Observatory launched on December 2, 1995. The multi-mission design philosophy allows for the architecture of the camera electronics to be largely independent of the actual CCD selected for use as the detector. It is the CCD device itself that is typically the mission dependent part of the instrument. This is of great importance to the overall camera design since CCD topologies can vary widely from one manufacturer to another. Such factors as aspect ratio, number of pixels, readout mechanisms, clock phasing and clocking levels are all variables that must be incorporated into the hardware design of the camera. Among the advantages of this approach are the reduction of dedicated hardware and CCD development time. Also, since the camera can be reprogrammed from the ground, it is possible to continually optimize CCD performance in orbit. This may prove useful in offsetting degradation of the device due to factors such as radiation damage.
Online analysis and compression of spectra-spectroheliograms
Harrison P. Jones
The on-line algorithm used by the NASA/National Solar Observatory Spectromagnetograph for analysis of long-slit spectrum-line profiles is presented with a view towards future application to processing large volumes of data from spacecraft and other remote instruments. At every spatial position, the procedure finds continuum intensity, equivalent width, and wavelength position and intensity of the central extremum of absorption or emission. The wavelength position of line center is found from the zero- crossing of the convolution of the line profile with a fixed anti-symmetric kernel. As currently used with the spectromagnetograph, images based on the line parameters computed at every spatial position are saved, but the original data are discarded. This paper explores the possibility of preserving the data in compressed form by saving, in addition to the derived line parameters, the differences between the data and model profiles synthesized from the on-line analysis.
New Missions and Instrument Designs
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Compact particle detector for low-energy particle measurements
Ralph L. McNutt Jr., Donald G. Mitchell, Edwin P. Keath, et al.
We are developing an Energetic Particle detector capable of making particle measurements in a variety of heliospheric environments, and, in particular in the outer corona of the sun. The current concept is a miniature ion composition/electron telescope that will measure the energy spectra, mass composition and detailed pitch angle distribution of energetic ions and electrons from approximately 10 keV per nucleon to approximately 3 MeV total energy. The telescope is made up of two main components: a time of flight section and a solid state detector array. A collimator defines the acceptance angles for the incoming particles while the time of flight and solid state detector sections measure the velocity and energy of the ions. Electrons are recorded as essentially zero time of flight particles. The miniaturized detector is approximately 100 mm in diameter and measures particles in 6 angular sectors across 140 degree(s); each sector has an opening angle of 12 degree(s) in the orthogonal direction. Extensive use of VLSI techniques and chip on board design allows all electronics to be mounted on circular boards that mate directly to the detector. The entire instrument is less than 0.5 kg in mass and requires less than 0.5 W of regulated power.
A Close Look at the Sun
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New high-thermal-conductivity composite material for high-precision space optics
Tsuyoshi Ozaki, Chihiro Ikeda, Minoru Isoda, et al.
This paper reports on the newly developed graphite-cyanate composite pipes for high-precision space optics such as the Solar-B optical telescope. Fundamental mechanical, thermal, and hygroscopic properties of unidirectional graphite- cyanate laminates were evaluated, first. The orientation of fibers in the pipe was designed to minimize longitudinal thermal deformation. Model pipes were fabricated based on the design, and have conducted a series of measurements to evaluate the thermal expansion behavior, the hygroscopic performance, the thermal conductivity, and the long-term stability. Excellent performance of the pipe was successfully verified and the material was found to be the most promising candidate for space optics structures.
Recent Instruments and Results
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General purpose solid state camera for SERTS
Leslie J. Payne, J. Patrick Haas
The Laboratory for Astronomy and Solar Physics at Goddard Space Flight Center uses a variety of CCD's and other solid state imaging sensors for its instrumentation programs. Traditionally, custom camera systems are built around the imaging device to optimize the circuitry for the particular sensor. This usually produces a camera that is small, uses little power and is elegant. Although these are desirable characteristics, this approach is also expensive and time consuming. An alternative approach is to design a `universal' camera that is easily customized to meet specific mission requirements. This is the approach our team used for SERTS. The camera design used to support the SERTS mission is a general purpose camera design that is derived from an existing camera on the SOHO spacecraft. This camera is designed to be rugged, modest in power requirements and flexible. The base design of the camera supports quadrant CCD devices with up to 4 phases. Imaging devices with simpler architectures are in general supportable. The basic camera is comprised of a main electronics box which performs all timing generation, voltage level control, data processing and compression. A second unit, placed close to the detector head, is responsible for driving the image device control electrodes and amplifying the multichannel detector video. Programmable high voltage units are used for the single stage MCP type intensifier. The detector head is customized for each sensor type supported. Auxiliary equipment includes a frame buffer that works either as a multi-frame storage unit or as a photon counting accumulation unit. This unit also performs interface buffering so that the camera may appear as a piece of GPIB instrumentation.
LASCO spectrometric Lyot coronagraph tunable passband filter
Dennis George Socker, Guenter E. Brueckner, Clarence M. Korendyke, et al.
Spectrometric and spectropolarimetric aspects of the Lyot coronagraph flown aboard the ESA/NASA SOlar Heliospheric Observatory (SOHO) are presented. The coronagraph is one of the three channels comprising the LASCO coronagraph and the only channel with spectroradiometric capabilities. Among the primary science objectives assigned to the Lyot coronagraph are the determination of the mechanisms responsible for the acceleration of the solar wind and the heating of the corona. Spectrometric and spectropolarimetric coronal observations made with the Lyot coronagraph are used in support of these and other objectives. We describe the Lyot instrument design from the imaging coronal spectrometer perspective. The rationale for use of a tunable Fabry-Perot interferometer as the spectral resolving element is outlined. The relationships between spectral resolving power, interferometer diameter, telescope entrance stop diameter and coronal field of view as it applies to LASCO is reviewed. Performance requirements imposed on the interferometer by the coronal source and the science objectives are described. The optical, mechanical, electronic and semi-automated control designs as well as the interferometer modes of operation are summarized. The actual flight model Fabry-Perot interferometer performance allows the instrument to operate with high luminosity and with finesse values high enough to provide approximately optimal passband widths and reasonable tunable ranges about useful spectral features. We conclude with some early results indicative of the flight performance of the instrument.
Solar Image Processing
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Time series restoration from ground-based solar observations
Many processes of interest in the solar atmosphere have spatial scales of much less than one second of arc. If the processes are related to magnetic fields, the relevant scales are even smaller. Noticeable evolutions of solar features occur on time-scales of less than a minute if a spatial resolution of better than one second of arc is reached. It is, therefore, of great interest to recover time-series imagery with near diffraction-limited spatial resolution and good temporal resolution on a consistent basis and over extended periods of time using ground-based techniques. Phase diversity is a post-collection technique for restoring fine-resolution detail when imaging in the presence of phase aberrations such as atmospheric turbulence. Incident energy is split into two channels: one is collected at the conventional focal plane, the other is intentionally defocussed a known amount and collected by a second detector array. Phase-diverse speckle is an extension of phase diversity whereby a time sequence of short-exposure image pairs is collected. The maximum-likelihood estimate of a common object and a set of phase aberrations is performed jointly using all images. A phase-diverse speckle set of images of a plage region was collected over a span of 13.5 minutes using the 76-cm Vacuum Tower Telescope at the National Solar Observatory on Sacramento Peak. A phase- diverse pair of broad-band images at 6563 angstroms was collected along a third, narrow-band image in the wing of H- (alpha) . A set of restorations was made into a movie depicting the highly dynamic photosphere at scales below 0.3 arcsec. We conclude that the combination of fine spatial and temporal resolution achieved with phase-diverse speckle opens a new window to the study of the dynamics of the solar atmosphere from ground-based observatories.
Enhancing the spatial resolution of solar coronagraph images using dynamic imaging
Margarita Karovska, T. S. Zaccheo, John W. Cook, et al.
The LASCO C1 mirror coronagraph onboard the SOHO satellite (launched on 2 December 1995) was designed to observe the fine structure of the solar corona from 1.1 to 3.0 R. Even though the optical resolution is approximately 3 arc sec, the nominal achieved resolution is set by the CCD pixel size of 5.6 arc sec. A pixel size of 1.5 arc sec or less is needed to obtain diffraction limited observations according to the Nyquist criteria, and therefore the actual coronagraph images are under sampled by a factor of 4. We have explored improving the spatial resolution of the LASCO C1 images using the technique of dynamic imaging. Successive images are obtained with sub-pixel displacements of the steerable primary mirror. Typically a set of 4 images is obtained with 1/2 pixel displacements in the x and y axes. Using simulated data we have studied the improvement resulting both from simple co-addition of the multiple observations, and from a deconvolution algorithm we call Fractional Pixel Restorations (FPR). We studied the effects in numerical simulations of noise, contrast variations, modest differences in the scene observed in the multiple images, etc. WE have also applied co-addition and the FPR algorithm to laboratory pre-flight images of a wire mesh target, which significantly improved the resolution. Using dynamic imaging with 16 images and 1/4 pixel steps, it would in principle be possible to reach the diffraction limit of the telescope in some circumstances (low noise, sufficient image contrast, no temporal changes in the observed scene, a well characterized instrumental point response function). By the time of this meeting we hope to have high resolution solar images from the LASCO C1 telescope to show. LASCO is a cooperative project of an international group of scientists at the Naval Research Laboratory, Washington, DC, the Max- Planck Institute fur Aeronomie, Germany, the Laboratoire d'Astronomie Spatiale, France, and the Space Research Group at the University of Birmingham, Great Britain.
Evaluation of the total dose and single event upset effects on JPEG/DPCM LSI device and 16Mb-DRAM for space application
Masatoshi Tohno, Hideo Masuzawa, Yukio Sohma
Recently many satellites have missions to transmit image data to the ground through realtime downlink. Since the satellites are linked to the ground stations through limited bandwidth, the image data must be compressed to meet mission requirements. Meanwhile, the Institute of Space and Astronautical Science and other public establishments have begun to appropriate commercially available devices to their projects to reduce development costs and to apply the latest technologies. Under these circumstances, we evaluated the radiation tolerance characteristics of a large scale integrated (LSI) device having both the JPEG (lossy) and DPCM (lossless) functions and a large scale integrated dynamic random access memory (16 Mb-DRAM) necessary to carry out the compression. To gather the radiation tolerance characteristics, we carried out the single event upset tolerance test and total dose tolerance test for JPEG/DPCM LSI and 16 Mb-DRAM. In this paper, using the data gathered from the radiation tests, we describe the feasibility of applying JPEG/DPCM LSI and 16 Mb-DRAM to satellites.
A Close Look at the Sun
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Notes on the filter magnetograph project
Nikolai I. Kobanov, Victor M. Grigoryev
A simple design is proposed for a filter magnetograph, based on the bichromatic image methodusing magnetooptic filter. The scheme ensures an optimal harnessing of the light flux and the computermemory. A push-pull modulation, utilized in the instrument, is readily adjusted to CCD operating in the mode of a two-step image detector. There is good reason to hope that the signal/noise ratio can be improved considerably. In whole disk observations the magnetograph provides measurements of the general magnetic field simultaneously with a usual magnetogram. The possibility that the proposed magnetograph can be operated in the mode of measuring the steepness of the spectral line wing, is considered. This would contribute, in these authors' opinion, to more reliable calibration of conventional filter magnetograms as well as aiding a more correct evaluation of theintegral magnetic field flux in areas of the image with a transverse field and mixed polarity. Keywords : magneto-optical filter , magnetograph
New Missions and Instrument Designs
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High-Energy Solar Spectroscopic Imager (HESSI)
Brian R. Dennis, Robert P. Lin, Richard C. Canfield, et al.
The primary scientific objective of the High Energy Solar Spectroscopic Imager (HESSI) is to understand particle acceleration and explosive energy release in the magnetized plasmas at the Sun. HESSI will provide the first hard X-ray imaging spectroscopy, the first high-resolution spectroscopy of solar gamma-ray lines from a spacecraft, the first imaging above 100 keV, and the first imaging of solar gamma- ray lines. The gamma-ray imaging spectroscopy will provide the first information on the spatial distribution of energetic (>1 MeV) protons, heavy ions, and relativistic electrons, and the first information on the angular distribution of the energetic ions. It will also provide detailed information on elemental abundances for both the accelerated ions and the ambient ions in the interaction region. HESSI uses Fourier-transform imaging spectroscopy to cover the broad energy range from soft X-rays (2 keV) to gamma-rays (20 MeV) with spatial resolutions down to 2 arcseconds and spectral resolutions down to 1 keV. This capability is achieved with 12 bi-grid rotating modulation collimators located in front of a corresponding set of 12 pairs of cooled germanium and silicon (Si(Li)) detectors to provide the wide spectral coverage. HESSI has been selected by NASA as an alternate Medium-class Explorer (MIDEX) mission, for launch in the year 2000. If it does not get funded as a flight mission, it will be descoped and proposed at a Small Explorer mission for launch in 2000 at half the MIDEX cost.
Measurement of the Sun's changing sizes
Jean-Pierre Rozelot
Since about a decade, it has been suggested that the solar diameter may vary in time. The amplitude of the observed variations does not exceed 0.7' over 15 years and has been probably less (0.3') during the last five years. Recently, new measurements of the solar oblateness made at the Pic du Midi Observatory (France) seem to confirm a possible oscillation with the solar cycle as previously conjectured by a Princeton group. Over a one year period (July 1993 - July 1994), the reported difference between the equatorial diameter and the polar one is 11.5 +/- 3.4 arc ms, consistent with previous observations made in 1983 and 1984, giving support for a solar activity dependence. However, the question is not yet settled as very few solar observatories are able to undertake unambiguous and accurate measurements. In order to free the data from atmospheric disturbances, it is here suggested to conduct a space experiment with the following objectives: (1) to confirm (or invalidate) the temporal (equatorial) diameter variations; in the mean time trying to interpret past--or simultaneously--obtained data to understand effects of the atmosphere; (2) to measure the solar oblateness and its time variations if any; (3) to determine the true shape of the helioid and its possible changing sizes; (4) to correlate the data with the solar irradiance variations and produce a forecasting model; (5) to study the response on the Earth climatic system. As innovative instrument of about 50 Kg could be built, based on the experience of our scanning heliometer. A 0.002 arc s accuracy must be achieved, which seems to be within our grasp with up-to-date technologies. All cooperation is welcome.
High-resolution spectral imaging of the Sun in the far ultraviolet
Marilyn E. Bruner, Theodore D. Tarbell, Alan M. Title, et al.
The Transition Region and Coronal Explorer instrument (TRACE) will use narrow-band interference filters together with other appropriate band limiting elements to make high resolution images of the Sun in the C IV lines at 154.8 and 155.0 nm. Filter observations of solar C IV emission are complicated by the presence of UV Continuum and nearby chromospheric lines because of the relatively wide bandpasses of the narrowest currently available interference filters. TRACE will use a series of filters to estimate the effects of the UV continuum and the long-wavelength `leaks' in the blocking filters which we show are the most important contaminants in the C IV images. Further improvements in filtergraph performance may be realized through the use of tunable Fabry-Perot etalons, which have been under development at Lockheed-Martin. We present test data from a cultured quartz etalon designed for 155 nm, and will discuss the prospects for etalons operation at substantially shorter wavelengths.
Recent Instruments and Results
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Flare Genesis Experiment
Graham A. Murphy, David M. Rust, Kim Strohbehn, et al.
In January 1996, the Flare Genesis Experiment was carried for 19 days by a 29.4 M cu. ft helium-filled balloon in the stratosphere above Antarctica, during which over 14000 images of the Sun were recorded. Long-duration ballooning provides a relatively inexpensive means to observe the Sun under near-space conditions and to develop instrumentation and techniques that will be used on future solar space missions. The purpose of the flight was to improve understanding of the mechanisms involved in many different types of solar activity, particularly flares and solar filament eruptions. Achieving this goal demanded the development of a platform for an 80-cm F/1.5 optical telescope that would be stable to 10 arcseconds. In addition, we developed an image motion compensation system capable of holding the Sun's image to better than the system's 0.2 arcsecond diffraction limit. Other key elements on board included a lithium-niobate Fabry-Perot etalon filter to provide a tunable 0.016-nm bandpass over a wide wavelength range, a fast 1534 X 1024-pixel Kodak CCD camera, and 180 GBytes of on-board storage. There was also a system for sending commands and receiving telemetry and a high-speed downlink for sending images during periods when the payload was in line of sight of the ground station. On- board computers provided a command and control system capable of near-autonomous operation. During most of the flight, contact with the payload was sporadic, so operation was primarily under autonomous control.
New Missions and Instrument Designs
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Next-generation EUV imaging spectrometer for solar flare observations
J. Daniel Moses, Guenter E. Brueckner, Kenneth P. Dere, et al.
The Naval Research Laboratory Skylab SO82A slitless spectrograph provided solar flare observations that have never been equaled in diagnostic capabilities for interpreting thermal flare physics. Improvements in detector technology, optics and optical coating technology, and almost two decades of analysis of SO82A data can be combined with the basic concept of an EUV objective grating spectrograph to build an instrument to address many of the remaining mysteries of solar flares. This next generation instrument incorporates two sets of two identical, orthogonally mounted slitless spectrographic Cassegrain telescopes. Each telescope consists of a multilayer coated, Wadsworth mount objective grating and multilayer coated spherical secondary mirror; a backside illuminated CCD detector is installed at the focal plane. The orthogonal mounting changes the dispersion direction by 90 degrees on the disk image; processing on the two resulting images allows recovery of the undispersed disk image and spectral line profiles. The resulting instrument will obtain high time cadence, spectrally-dispersed images with improved spatial resolution, dynamic range, signal-to-noise ratio, and velocity discrimination.
Recent Instruments and Results
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Japanese sounding rocket experiment with the solar XUV Doppler telescope
Taro Sakao, Saku Tsuneta, Hirohisa Hara, et al.
We present an overview of an ongoing Japanese sounding rocket project with the Solar XUV Doppler telescope. The telescope employs a pair of normal incidence multilayer mirrors and a back-thinned CCD, and is designed to observe coronal velocity field of the whole sun by measuring line- of-sight Doppler shifts of the Fe XIV 211 angstroms line. The velocity detection limit is estimated to be better than 100 km/s. The telescope will be launched by the Institute of Space and Astronautical Science in 1998, when the solar activity is going to be increasing towards the cycle 23 activity maximum. Together with the overview of the telescope, the current status of the development of each telescope components including multilayer mirrors, telescope structure, image stabilization mechanism, and focal plane assembly, are reviewed. The observation sequence during the flight is also briefly described.
New Missions and Instrument Designs
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Solar x-ray imaging telescope
Carl C. Gaither III, Upendra D. Desai
The study of high energy, transient astrophysical phenomena (solar flares, pulsars, X-ray bursters, gamma ray bursts) require new instrumentation capable of simultaneously performing high spatial, temporal and spectral observations. Currently, there are no elements such as lenses or mirrors capable of reflecting or refracting X- and gamma rays. Shadow-casting techniques must be employed to image such sources. These techniques rely on the total absorption of X- and gamma rays to indirectly give images of the sources. We propose a design for a shadow-caster based on dual Fresnel Zone Plate (FZP) coders suitable for solar observations from a satellite or balloon-based platform. Most shadow-casters require an image plane detector with a spatial resolution comparable to the smallest features ct into the coder for the best angular resolution. The image plane detector for a telescope based on dual FZPs does not have such a requirement since the coders measure almost the exact Fourier transform of the source distribution. We present here the results of simulations that demonstrate the telescope's capability to produce images with an angular resolution of a few arcseconds, a temporal resolution dependent only on the source intensity, and high spectral resolution obtained using an array of solid state X-ray detectors.
Solar Chromospheric and Coronal Explorer
Arthur B. C. Walker II, Philip H. Scherrer, Richard B. Hoover, et al.
The presence of the solar magnetic field has a profound effect on the structure of the lower chromosphere, and is responsible for the formation of the upper chromosphere and the corona, and the acceleration of the solar wind. The variation of the field induces variations in the chromosphere and the corona on time scales from 0.001 seconds to centries. SOHO, and subsequent approved solar missions such as TRACE will bring powerful observational capabilities to bear on critical questions relating to solar variability. However, the most fundamental question--how energy is transferred from the magnetic field into the solar plasma--will require observations of diagnostic quality on a spatial scale of 50 - 100 kilometers; this is an order of magnitude beyond the capability of any planned mission. Our mission concept, the Solar Chromospheric and Coronal Explorer (SCCE) is designed to investigate the mechanisms underlying the variability of the solar atmosphere, by attaining spectroscopic observations of the solar atmosphere over a wide range of temperatures (4,500 K to 100,000,000 K), with very high angular (0.1 arcseconds) and temporal (0.001 seconds) resolution, that will permit models of the physical processes that underlie the phenomena of solar activity to be formulated and tested at the scale, 50 - 75 kilometers that appears to be fundamental. The architecture of the SCCE is based on advances in multilayer optics, which permit broad spectral response, and high angular and spectral resolution to be achieved in a volume, and at a cost that is compatible with deployment within the fiscal and physical constraints of the MIDEX program.
Solar Observations in Stereo
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Solar Terrestrial Relations Observatory (STEREO)
Joseph M. Davila, David M. Rust, Victor J. Pizzo, et al.
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.