NP-119 Science in Orbit: The Shuttle & Spacelab Experience, 1981-1986

 

Chapter 4

Observing the Sun: Solar Physics

 

[44] The Shuttle and Spacelab have been used very successfully as an observatory for studying the sun, the nearest and best known star and the source of energy for Earth's environment.

A manned observatory in space has several advantages for viewing the sun. From space, all the sun's radiance, including that normally absorbed by the Earth's atmosphere, can be observed and measured; ultraviolet and X-ray images reveal important features and processes that cannot be viewed through telescopes on the ground. In comparison to a rocket flight that lasts only a few minutes, many more solar images and much larger data sets can be obtained during a week-long Shuttle mission and subsequent reflights. By comparison to an unmanned orbiting observatory, scientists aboard the

Shuttle can monitor the sun, select targets for viewing, point and focus and fine tune the instruments, and explore interesting phenomena, exercising the same kind of real-time control that is common in a ground observatory.

The Space Shuttle is an ideal location for a solar observatory for two more reasons. Because it is above the turbulence of the Earth's atmosphere, which seriously degrades the quality of images obtained at ground-based observatories, photos from the Shuttle have far better spatial resolution, enabling us to see much smaller details in the sun's surface. Furthermore, since nighttime on the Shuttle lasts only about 40 minutes, it is much easier to follow the evolution of solar phenomena without long interruptions.

The solar telescopes and detectors flown to date have benefited from the adaptability that is possible on a Shuttle/Spacelab mission. The onboard scientists, the ease of instrument commanding, the availability of realtime data and images to scientists on the ground, and the ability to communicate with the crew and replan observations in response to unexpected events have resulted in very successful use of the Shuttle as a solar observatory.

Solar experiments on Spacelab 2 for the first time used a sophisticated mount for telescopes and detectors; the Instrument Pointing System (IPS), built by the European Space Agency, provided precision pointing and stability independent of spacecraft motion and attitude, making it possible to obtain very high-resolution solar...

 


a view of the sun over the earth's horizon

 
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Spacelab 2 experiments monitored the sun from the Instrument Pointing System, a sophisticated mechanism for aiming telescopes and detectors.

Spacelab 2 experiments monitored the sun from the Instrument Pointing System, a sophisticated mechanism for aiming telescopes and detectors.

 


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Crewmembers used controls in a Shuttle workstation to point telescopes at specific areas of the sun.

Crewmembers used controls in a Shuttle workstation to point telescopes at specific areas of the sun.

 

....images and spectral data from this fastmoving observatory.

As the following summaries indicate, Shuttle-based solar investigations are making significant contributions to our understanding of the sun as a star and the effects of solar events on the Earth's environment.

 

Images of the Sun: Both still photography and video techniques have been used to gain some of the best solar images ever obtained. The telescopes and cameras themselves are designed for high-resolution imaging, and the IPS provides necessary pointing control and stability to achieve clear, detailed images of solar features.

The complement of solar instruments flown on the Spacelab 2 mission functioned collectively as an observatory for detailed examination of the sun. Scientists watched areas as small as 350 kilometers (200 miles) for an entire orbit (as long as an hour) without distortion. From the ground, the limit for unblurred observation is only a few seconds or minutes at a time and then only rarely under ideal observing conditions. Seeing the small, rapidly changing features in sharp focus without distortion on a routine basis from the Shuttle was an exciting, new experience for solar observers.

The extended solar atmosphere (corona), the visible surface (photosphere), and the chromosphere and transition region between the hot corona and the much cooler photosphere came under careful scrutiny. The resultant images reveal very small, very faint structures (solar gases shaped by magnetic fields), slight changes in brightness, small-scale motions, and other details that are improving our knowledge of the sun's behavior. These details provide critical clues to the origin of larger, more turbulent solar changes and thus a better understanding of precursor events, which will result in better predictions of the explosive solar events that affect Earth's atmosphere and the nearby space environment.

Movies of tiny, bubble-like convection cells (granules) also contained surprises. Turbulence in Earth's atmosphere blurs ground observatory images of the sun so much that fine details or subtle changes from one image to the next cannot be seen. From Spacelab, however, scientists could see for the first time that granules in magnetic regions (sunspots, pores, and network boundaries) are quite different than in the quiet, undisturbed sun. The shapes of the very small magnetic pores are irregular, scalloped, and rapidly changing as they attempt to maintain their structure against the encroachment of turbulent surrounding granules.

The movies also provided the first undistorted histories of granule evolution, which will help scientists determine normal and abnormal patterns of development. Cinematography has....

 


During the mission, a solar operations center was set up on the ground, and scientists used the latest data from satellites and ground observatories to pinpoint interesting solar features for closer scrutiny by Spacelab 2 telescopes.

During the mission, a solar operations center was set up on the ground, and scientists used the latest data from satellites and ground observatories to pinpoint interesting solar features for closer scrutiny by Spacelab 2 telescopes.


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This hydrogen-alpha image from the High Resolution Telescope and Spectrograph (HRTS) shows a sunspot and a wispy filament on the sun. These features are precursors to explosive solar events that affect the Earth's atmosphere and the nearby space environment.

 

This image from a single frame of the Solar Optical Universal Polarimeter (SOUP) movie shows a sunspot surrounded by granules.

This image from a single frame of the Solar Optical Universal Polarimeter (SOUP) movie shows a sunspot surrounded by granules. For the first time, scientists could see that the activity of granules in magnetic regions such as sunspots differs from those in quiet, undisturbed regions.

 

The suite of Spacelab 2 solar instruments observed the layers of the sun's surface and atmosphere.

The suite of Spacelab 2 solar instruments observed the layers of the sun's surface and atmosphere.

 

[48] ...shown that more than half of all granules die a violent death instead of quietly fading away. They either expand until they reach a critical size and explode into many tiny fragments, or they are destroyed by a nearby explosion. The Spacelab 2 movies also disclosed that granules stream radially outward from the center of a sunspot into the surrounding quiet photosphere, a phenomenon never before seen and still unexplained.

Scientists are thrilled with the new images of the sun. The movies are far more consistent in quality from frame to frame than any yet obtained. The Solar Optical Universal Polarimeter (SOUP) instrument, for example, recorded several hours of sunspot and active region observations; the 6,400 frames collected are unique for their extreme image stability. Eight hours of video and 500 still photographs of the sun made by the High Resolution Telescope and Spectrograph (HRTS) instrument in hydrogen-alpha ultraviolet light, plus another 1,300 ultraviolet spectroheliograph exposures, reveal interesting new features of spicules, spiky structures seen along the edge of the sun. While spicules are well recognized from ground based visible light observations, from the Shuttle scientists observed ultraviolet superspicules that rise twice as high as ordinary spicules. They recorded, for the first time, dramatic changes in the size and shape of the superspicules that may provide the key to understanding these mysterious features.

In addition to these discoveries, postflight film processing and image enhancement techniques are being used to bring to light many features and motions that are completely invisible to ground observers. For example, granules were previously thought to remain roughly in place or have only small random motions during their lifetimes. After sophisticated analysis of the SOUP movies, it has been learned....

 


Solar granules that appear to expand radially at the end of their lifetimes are called exploding granules. An example is shown in this sequence.

Solar granules that appear to expand radially at the end of their lifetimes are called exploding granules. An example is shown in this sequence. The bright granule in the center of the first image expands to a puffy state and then explodes, leaving a black circle seen in later images. From ground-based observations, exploding granules were considered rare, but in the SOUP data, it is hard to find areas of quiet sun without them.

 

The quiet solar limb was recorded by the HRTS ultraviolet spectroheliograph. The image shows the first observation of superspicules, spikes that stand out above the solar limb.

The quiet solar limb was recorded by the HRTS ultraviolet spectroheliograph. The image shows the first observation of superspicules, spikes that stand out above the solar limb.


 

[49] ...not only that granules are in almost continual motion (with speeds of 3,000 to 4,000 kilometers per hour (1,900 to 2,500 miles per hour) but also that they float like corks on top of a much larger flow pattern (called supergranulation and mesogranulation), which consists of giant convective cells 10,000 to 40,000 kilometers (6,000 to 25,000 miles) in diameter. Solar physicists have known about supergranules for over 25 years, but the SOUP observations have provided the first detailed measurements of their flows and their relationships to the large magnetic structures in the sun's atmosphere.

 

Spectral Data: Spectral analysis - separation of radiation into discrete wavelengths - is another technique used to understand the chemistry and physics of the sun and other stars. Since different chemicals absorb or emit radiation at certain characteristic wavelengths (spectral lines), these "signatures" can reveal much about the composition and motion of solar gases. Spectrometers flown on the Spacelab 2 mission recorded a variety of spectra from features on the solar disk and in the corona. The harvest from the HRTS instrument, which can differentiate 2,000 spectral lines in the....

 


Some of the elements found on the sun (silicon, carbon, and helium) are shown in this HRTS spectrum (1,500 to 1,700 Angstroms).

Some of the elements found on the sun (silicon, carbon, and helium) are shown in this HRTS spectrum (1,500 to 1,700 Angstroms). This spectral region is particularly useful for understanding the outer layer of the sun where high temperatures are maintained despite large radiative energy losses.

 

The lower image shows flow patterns (arrows), outflowing material (red), and inflowing material (blue), superimposed on a SOUP sunspot image. Granules may float like corks on these surface flows, as represented by crosses in the upper image. These flow patterns are impossible to calculate from ground-based images.

The lower image shows flow patterns (arrows), outflowing material (red), and inflowing material (blue), superimposed on a SOUP sunspot image. Granules may float like corks on these surface flows, as represented by crosses in the upper image. These flow patterns are impossible to calculate from ground-based images.


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By measuring the signatures and intensities of various elements, solar physicists can tell more about the sun's physical makeup.

By measuring the signatures and intensities of various elements, solar physicists can tell more about the sun's physical makeup. These images are part of a catalog made by the Coronal Helium Abundance Spacelab Experiment (CHASE) instrument. Each solar quadrant reveals the spectral intensity of a specific element; the bright areas represent more intense solar activity characteristic of hot, active regions.

 

Shown in false color are small portions of an ultraviolet emission spectrum obtained by HRTS.

Shown in false color are small portions of an ultraviolet emission spectrum obtained by HRTS. The long horizontal streaks marked "EE" are caused by high-speed motions of the solar plasma. Material that moves toward the observer shifts its emission toward higher frequencies (area labeled blue). The emission of material moving away from the observer shifts toward the area labeled red. The velocities of solar material imaged in this spectra are several hundred kilometers per second.

 

....ultraviolet range, was about 19,000 exposures of sunspots, spicules, explosive events, and jets, representing a large new data base for studying the structure and evolution of these features. The HRTS spectral survey of the disk also increased the statistical data base for studying solar features globally.

The Coronal Helium Abundance Spacelab Experiment (CHASE) obtained one of the most accurate measurements of the abundance of solar helium relative to solar hydrogen. By recording ultraviolet emissions from hydrogen and ionized helium, both on the solar disk and in the corona above the limb, an abundance ratio of helium to hydrogen of 10% ±2% was measured. Understanding several important astrophysical processes depends on an accurate accounting of helium in the universe. Since all the helium in the surface layers of the sun is thought to be primitive in origin, data collected on the Spacelab 2 mission are of great importance to cosmologists as well as solar physicists.

From the new spectral information about rapidly changing solar features and the composition of solar gas, scientists are learning more about the physics of energy transfer through the solar atmosphere. Because of the ability to see the ultraviolet sun, high-resolution spectral observations from the Shuttle are especially effective for investigating high-velocity events in the upper solar atmosphere, chromosphere, transition zone, and corona.

The CHASE instrument was able to study the structure and development of active regions in the solar atmosphere. Images in a variety of spectral lines were compiled. These images clearly show that hot active region material forms a bridge between the hot outer layer of the sun (the corona) and the somewhat cooler layer of the sun (the chromosphere) sandwiched between the solar disk and the corona.

Another Spacelab 2 instrument, the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) measured the sun's energy output across the ultraviolet spectrum. The measurements produced a highly accurate spectrum which will be used as a baseline in studying how solar output varies as the sun goes through cycles of minimum and maximum activity. These measure meets also are being used by atmospheric physicists.

 

Solar Models: Both images and spectral data contribute to theoretical modeling of the sun's structure and dynamics. Scientists are attempting to understand how magnetic fields on the sun form and change, how they interact with solar gases, how the various layers of the solar atmosphere differ and interact, and how to predict the occurrence of explosive solar flares.

Data from all the solar investigations mentioned above are affecting solar....

 


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careful planning and close coordination between scientists in orbit and on the ground

the Spacelab 2 crew took advantage of solar observing opportunities.

Through careful planning and close coordination between scientists in orbit and on the ground, the Spacelab 2 crew took advantage of solar observing opportunities.

 

....physics theories and models. In addition, the X ray flare investigation flown on the OSS 1/STS-3 mission was specifically designed to discriminate between competing theories. Despite a contamination problem that complicated data analysis, the experiment gained the most sensitive flare polarization data set ever obtained, placing important experimental constraints on theories of the acceleration and propagation of energetic atomic particles on the sun.

 

Extending Observations: The dazzling Spacelab 2 images prove that from low Earth orbit solar instruments do have a clearer view of the sun. These early experiments also> show the value of using the eyes and brains of the onboard crew to analyze results and focus instruments on interesting solar events. Without the close interaction between the Spacelab 2 solar physicist crewmembers and scientists on the ground, many observing opportunities would have been lost..

The Spacelab 2 workstation is serving as a model for the controls and monitors that are being designed for the Space Station solar observatory. Like Spacelab, the Space Station will have a solar physicist on board to operate solar instruments and coordinate detailed observing plans with scientists on the ground. Space Station, however, will expand current capabilities by providing additional work areas for repairing and calibrating instruments.

Space Station will provide the continuous, long duration observations that are obtainable only from a permanent space facility. Several different modes of operation will be possible: around-the-clock, automated observations for a solar cycle or more; scheduled campaigns that are planned months in advance and last from days to months; and unscheduled campaigns that are initiated on short notice in response to solar activity.

It will be possible to control Space Station instruments from the ground. Instruments will generate up to several hundred million bits of data each second, transmitting some to the Station and some to the ground for real time analysis. Data will be archived and distributed worldwide. International cooperation will be important since solar activity affects Earth across the globe.

[52] Several types of spacecraft and observatories are planned to study diverse solar phenomena. High-resolution telescopes will observe detailed solar features, and low-resolution instruments will study solar variability. A solar observatory may be formed on or near the Space Station. Smaller instruments for studying the acceleration and propagation of high-energy particles, low-frequency radio antennas for studying high-energy electrons accelerated by flares in the solar atmosphere, and other high-resolution telescopes may be included in this observatory to make observations in all wavelengths with full spectral and temporal coverage. This will extend the Spacelab 2 data across the entire electromagnetic spectrum, resulting in the first detailed observations of processes that control many astrophysical phenomena.

The next step will be to deploy a geosynchronous platform several thousand kilometers above the Space Station. At these altitudes, there are no day/night cycles, and solar viewing is uninterrupted. Scientists will be able to track the detailed evolution of solar phenomena across the entire solar disk. Instruments on the platform may be remotely controlled from the Space Station or the ground.

As solar physicists' understanding of the sun progresses, it will be very important to share information with scientists studying the atmosphere and the plasma environment enveloping Earth. Space Station will provide the first chance to make a coordinated set of measurements of the sun, the space plasma, and the atmosphere from low-Earth orbit. As solar physicists monitor events on the sun, plasma physicists and atmospheric physicists will measure the impacts closer to home. This will result in a valuable model of the workings of a star system, a model which can be applied to astrophysical systems throughout the universe.

 


Instruments on a proposed Solar-Terrestrial Observatory platform will be able to monitor events such as solar flares and subsequent effects on Earth's environment.

Instruments on a proposed Solar-Terrestrial Observatory platform will be able to monitor events such as solar flares and subsequent effects on Earth's environment.

 

[53] Solar Physics Investigations

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OSS-1/STS-3

Solar Flare X-Ray Polarimeter (SFXP)

R. Novick, Columbia University, Columbia, Missouri

Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)

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Spacelab 2/51-F

Coronal Helium Abundance Spacelab Experiment (CHASE)

A.H. Gabriel, Rutherford and Appleton Laboratory, Chilton, United Kingdom

J.L.Culhane, University College, London, United Kingdom

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High Resolution Telescope & Spectrograph (HRTS)

G.E. Brueckner, Naval Research Laboratory, Washington, D.C.

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Solar Optical Universal Polarimeter (SOUP)

A.M. Title, Lockheed Solar Observatory, Palo Alto, California

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Solar Ultraviolet Spectral Irradiance Monitor (SUSIM)*

G.E. Brueckner, Naval Research Laboratory, Washington, D.C.

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* reflight


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