A NASA rocket carrying a next-generation ultraviolet spectrograph for solar physics research will help answer some of the mysteries of the Sun
By Donald M. Hassler, Ph.D.
The past decade has been a Golden Age of space-based solar physics, with significant improvements in our understanding of the Sun from missions such as Yohkoh, SOHO, TRACE and RHESSI. However, many unanswered questions remain in our understanding of the physical mechanisms, dynamics and flow of energy through the solar atmosphere. Scientists at Southwest Research Institute (SwRI) are developing, building and preparing to fly a next-generation ultraviolet imaging solar spectrograph to help answer some of these questions as part of a NASA Low Cost Access to Space (LCAS) Program.
The Rapid Acquisition Imaging Spectrograph Experiment (RAISE) will serve as a prototype for the next generation of space-based solar spectrographs, specifically addressing several major limitations of existing and planned solar missions.
The NASA sounding rocket program, which supports the RAISE mission, plays a vital role in the nation’s space program, historically providing two important functions. First, it develops new technologies and instrumentation and qualifies them for space at relatively low cost and with a rapid turnaround. It also trains graduate students and the next generation of experimentalists and instrument scientists. RAISE, scheduled to fly in the fall of 2006, will incorporate both of these functions, developing instrumentation and validating observational techniques as a precursor of future orbital missions.
The primary scientific objectives of the experiment focus on three areas that are accessible only with the instrument’s unique capabilities and can be advanced with a single, six-minute rocket flight: Small-scale dynamics of coronal loops, the nature of high-frequency waves in the solar atmosphere and the nature of transient brightenings in the solar network. Each of these objectives is intended to answer specific questions that are important to our understanding of the mechanisms of energy conversion in the solar atmosphere. For example, how are temperature and velocity related in hot loop structures? And are there high-frequency waves in the solar atmosphere with sufficient energy to heat the corona or accelerate the solar wind?
The solar atmosphere can be described as layered, but it is neither evenly stratified nor stationary. Both ground- and space-based observations have demonstrated the dynamic nature of the “quiet” Sun. Observations using the TRACE spacecraft show that the solar chromosphere, the corona above it and the transition zone in-between are not well separated, but rather interleaved in turbulent boundary zones that are highly structured and organized along the local magnetic field.
These boundary zones are central to many of the outstanding mysteries of solar physics, including the nature of small-scale coronal heating, the origin of the solar wind, the origin of coronal holes and the location dependence and governing mechanisms of small-scale reconnection. RAISE is designed to quantify the physics of these boundary layers and the dynamics of the interactions that take place there.
It has been believed that heating occurs within flux tubes that reach from the photosphere upward into the corona, where coronal energy is conducted downward again to form a thin, conductively heated transition region, or coronal loop. Unfortunately, this picture is flawed at a fundamental level. TRACE observations have revealed bright emissions in the transition region, which fluctuate and move about, as well as dark intrusions of chromospheric material that reach to heights formally associated with the corona, revealing the intrinsically dynamic character of this interface. However, a comparison of TRACE and ground-based observations suggests that coronal and chromospheric heating are not spatially correlated. Either the magnetic topology is different from what is expected, or the two domains are heated along different field lines. The transition region may be composed of a collection of magnetically and thermally isolated structures rather than a unified layer. With spatial resolution comparable to the smallest observed photospheric magnetic features, and the ability to detect rapid fluctuations and mass flows, RAISE will be able to identify the physics and structure of the interleaved transition region and identify the energy that flows into the corona.
Not only do the gross magnetic morphology and force balance change throughout the solar atmosphere, but the chromospheric dynamics also can influence overlying magnetic structures, typically up to 1,000 kilometers above the photosphere. For example, the heating of the magnetized component of the chromosphere is poorly understood, yet it must play a prominent role in the generation of magnetic wave energy that propagates up into the coronal extensions of the canopy fields. Identifying the coronal response to photospheric forcing requires one to follow the disturbances as they traverse the chromosphere. The times for wave-crossing from the upper photosphere to the base of the corona are on the order of 100 seconds, determined mostly by the speed of sound through the lower chromosphere. Because this time is comparable to the time scale of both photospheric oscillations and the smallest observed magnetic features, dynamic effects must be measured and understood. Because of the extremely broad temperature range of this dynamic activity, plus the difficulty that narrow temperature-band imagers have in distinguishing between propagating wave effects and genuine mass flows, high-speed spectroscopy — with instruments such as RAISE — will be needed to ultimately capture the plasma behavior of this region.
Current Technology Limitations
Existing spectrometers on SOHO have yielded intriguing measurements of motion and heating in the solar atmosphere, but they have been unable to observe simultaneously all of the layers of the atmosphere sufficiently to couple and fully understand the mechanisms responsible for either the motions or the heating. To explain these physical processes that underlie coronal activity, future researchers will need to use high-speed imaging spectroscopy not only to observe intensity fluctuations and morphological changes, but also to identify bulk flows, waves, heating, cooling and non-LTE (local thermodynamic equilibrium) velocity distributions both when and where they occur. Future spectrometers will need to be able to observe and capture this spectral information simultaneously from all layers of the solar atmosphere, on time scales comparable to the dynamics that are occurring.
The next generation RAISE spectrograph is specifically designed to do this by providing simultaneous density (intensity), velocity (Doppler shift) and kinetic temperature (line width) information of solar structures at all heights in the solar atmosphere — that is, in the chromosphere, transition region and cool corona — with the high time-resolution required for rapid wave motion studies.
Instrument and Payload
The optical design of RAISE is based on a new class of ultraviolet and extreme ultraviolet imaging spectrographs that use only two reflections to provide quasi-stigmatic imaging (meaning good-focused imaging over the full field of view of the spectrograph), simultaneously over multiple wavelengths and spatial fields. The design uses an off-axis telescope to form an image of the Sun on the spectrograph entrance aperture. A slit in the aperture then selects a portion of the solar image, passing its light onto a diffraction grating, which re-images the spectrally dispersed radiation onto an array detector. Two full regions of the spectrum from the same one-dimensional spatial field are recorded instantaneously onto two intensified Active Pixel Sensor (APS) detectors whose focal planes are individually adjusted for optimal performance. The instrument also includes a “slit-jaw” camera to image the telescope focal plane and co-align the spectra obtained by the spectrograph on these images. RAISE will read out the full field of all three detectors at 10 Hz, allowing the team to record more than 3,600 complete spectral observations in a single six-minute rocket flight. This will open up a new domain of high time-resolution spectral imaging and spectroscopy.
In addition to the innovative design and operational strategy, two new technologies are being developed to enable this enhanced performance. Both will be tested and flown for the first time on RAISE. The first new technology is a diffraction grating that uses a new toroidal, variable line-spaced (TVLS) design to provide the quasi-stigmatic imaging simultaneously over multiple wavelengths and spatial fields. This new technology enables RAISE to observe simultaneously the emission lines that are formed at many different layers in the solar atmosphere. The second new technology is the miniature digital camera system, which uses large-format, complementary metal-oxide semiconductor APS arrays that are fed by a microchannel plate intensifier. The team chose APS sensor technology for the RAISE cameras because of their extremely fast readout, direct digital output and variable gain, making them ideally suited for spectroscopic applications. Other advantages of the APS camera technology are low mass, low power demand and high radiation tolerance.
The scientific payload of the RAISE rocket consists of two sections: a 22-inch vacuum section with a black-anodized shutter door (provided by NASA Wallops Flight Facility) containing the RAISE spectrograph, and an electronics section containing the flight processor, telemetry interface and system batteries. The payload is designed to have the air removed from it prior to launch so that dust, water vapor and other contaminants will not affect the performance or radiometric sensitivity of the instruments.
RAISE is optimized for time-resolved imaging spectroscopy to diagnose coronal plasma dynamics. The two wavelength passbands contain strong emission lines that provide broad, continuous temperature coverage from the chromosphere (formed at 10,000 degrees K) to the low corona (formed at more than 1 million degrees K). An exposure time of only 100 milliseconds is sufficient to record line profiles in all of the bright lines throughout this range. Because the APS detectors can read out and expose simultaneously, downtime between exposures is negligible.
The scientific instruments will obtain roughly five minutes of data above an altitude of 200 kilometers, reaching a maximum altitude of 350 km. RAISE will be launched on a Terrier-Black Brant sounding rocket from the White Sands Missile Range in the fall of 2006. The exact launch date and time will be coordinated with other NASA spacecraft (TRACE, SOHO, Solar-B) and ground-based observatories to observe the same target on the Sun (active region, flare) and to maximize the likelihood of observing significant dynamic activity.
RAISE will provide an important opportunity to explore a new observational domain in solar physics with the capability to observe solar activity with an order of magnitude higher time resolution than for any previous or current spectroscopic instrument. The observational insight that RAISE will provide into the ionization states and temperatures of the solar atmospheric plasma, combined with imaging information about the locations of the emitting regions, will be a major contribution to NASA’s Heliophysics Division and the Living with a Star program in particular.
Questions about this article: Contact Hassler at (303) 546-0683 or firstname.lastname@example.org.
Published in the Spring 2006 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.