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| SPARTAN Solar Studies (WLC, UVCS) |
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| Overview |
| SPARTAN 201: Studying the Sun's Corona The SPARTAN 201 series is investigating the solar corona--the thin upper layers of the sun's atmosphere, which reach temperatures of about 2 million degrees Fahrenheit. Scientists hope to determine the mechanisms that cause the heating of the corona and the acceleration of the solar wind, which originates in the corona, and explain why the corona is so much hotter than the rest of the sun. The first three SPARTAN 201 flights have already advanced our understanding of the solar wind and its origin. SPARTAN 201-05 observations will be coordinated with observations made from the Solar and Heliospheric Observatory (SOHO) satellite, a cooperative mission of the European Space Agency (ESA) and NASA. The second and third missions were coordinated with the passage of the Ulysses spacecraft over the sun's south and north poles. The sun's corona is difficult to study because its light is relatively dim compared to the sun's total luminance. The white light corona can be viewed from Earth only during a solar eclipse, which strongly reduces the brightness of the scattered sunlight. Ground-based astronomers are never able to see the ultraviolet radiation because of interference from the Earth's atmosphere. SPARTAN Solar Telescopes SPARTAN 201 is equipped with two complementary telescopes that can measure these emissions from the sun's corona. The white light coronagraph (WLC), provided by the High-Altitude Observatory in Boulder, Colo., will measure the density of the electrons in the coronal white light. The ultraviolet coronal spectrometer (UVCS) from the Smithsonian Astrophysical Observatory at Harvard will measure the velocities, temperatures, and densities of the coronal plasmas. The ultraviolet instrument and the WLC are housed in an evacuated cylinder with an aperture door that is opened after SPARTAN is released from the shuttle. By comparing the data collected by the two telescopes and combining the observations of the SPARTAN 201 missions and Ulysses and observations made by ground-based instruments, scientists expect to gain a much more complete picture of the origin of the solar wind. White Light Coronograph The SPARTAN WLC is an externally occulted coronagraph that will image the solar corona, providing measurements of the intensity and polarization of the electron-scattered white light. The WLC will also be used to cross-calibrate the SOHO satellite's UVCS white light channel and large-angle and spectrometric coronagraph. The LISS located in the front of the WLC points the telescope at the sun's center. An external occulting disk at the front of the telescope blocks sunlight from the solar disk entering the WLC aperture with coronal light. The primary objective lens, located behind the external occulting disk, forms images of the corona and external occulting disk. The image of the occulting disk is blocked by an internal occulting disk behind the primary objective lens. The coronal image is then reimaged through a half-wave plate, which measures the intensity and polarization of coronal radiation, and a charge-coupled device (CCD) detector. Baffles between the telescope's aperture and the primary objective lens block direct sunlight from interfering with the coronal light, which is about 10-9 times fainter than the light emitted from the solar disk. The WLC also contains an optical path that allows in-flight calibrations of the intensity of the coronal light. This calibration path is aimed directly at the solar disk. Direct sunlight passes through mirrors, opal glass and a wheel with three different neutral-density filters and onto the CCD detector. Ultraviolet Coronal Spectrometer The UVCS measures the characteristics of ultraviolet light from atomic hydrogen and the brightness of light emitted by ionized oxygen in the sun's corona to determine the velocities of the coronal plasma blown away from the sun and the temperatures and densities of some of the major constituents of the corona and solar wind. The telescope's measurements are also used to identify regions where the solar wind originates and map the sources of solar wind streams detected by Ulysses. The UVCS will focus its observations on the north coronal hole, a region of exceptionally low density and temperature, and the areas between coronal holes and streamers, which are bright regions formed by particles trapped in the sun's magnetic field. One of the major goals of the SPARTAN 201-05 mission is to continue tracking the changes in the morphology and physical conditions of the corona as it changes during the solar cycle. Missions 201-01, 02, and 03 were flown during the declining phase of solar activity. H I Lyman alpha profiles in coronal holes observed during the first three missions seemed to become less complex as the corona evolves toward solar minimum, when there are fewer high-latitude streamers intersecting the view of coronal holes. The dominant light emission in the ultraviolet spectrum of the extended solar corona is called H I Lyman alpha. It is formed as Lyman alpha radiation from the chromospheric layer of the sun passes through the solar corona, where it is scattered by neutral hydrogen. The SPARTAN UVCS measures the variation of intensity with wavelength of H I Lyman alpha and the intensities of the light emitted and scattered by the oxygen ions at wavelengths of 103.2 and 103.7 nm. In addition to verifying the profile shapes, the SPARTAN UVCS will provide a fresh radiometric calibration of the SOHO instrument. This is needed in order to distinguish changes in the observed intensities of the corona from changes in the efficiency of the SOHO UVCS. For this calibration, the SPARTAN instrument has been retrofitted with newly coated optics, and the entire optical paths of the Lyman alpha and O VI spectrometer channels have been accurately characterized. The planned joint observations of the corona by the two instruments will allow past and future SOHO UVCS observations to be compared with earlier SPARTAN observations. The data from both instruments will provide valuable information on how the corona changes throughout the solar cycle. Measurements by the SOHO UVCS show that Lyman alpha profiles in polar coronal holes and coronal streamers are wider than those predicted by theoretical models. The broad Lyman alpha profiles correspond to hydrogen kinetic temperatures that are many times the temperature expected if the hydrogen atoms were in thermal equilibrium with the electrons in the corona. One explanation is that transverse waves in the corona are responsible for the hydrogen transverse velocities. Other evidence from the broad O VI profiles observed by the SOHO telescope suggest that there is strong heating of highly charged oxygen perpendicular to the coronal magnetic field. If this is also true for protons, the neutral hydrogen atoms would also be affected since they are coupled to the protons by charge transfer and collisions. Charged particles are believed to be accelerated by high-frequency magnetohydrodynamic waves that propagate through the corona much like a cyclotron accelerates charged particles. The SOHO UVCS can look at spectral lines from ions of different masses and charge states to determine the basic properties of the waves that are accelerating these particles. Simultaneous observations by the SPARTAN and SOHO spectrometers will be used to confirm the profile shapes. Observing Strategy for UVCS The UVCS has three slits for taking measurements. The smallest slit measures the profile of the H Lyman alpha spectral line. The largest measures the total intensity of the alpha line and a few nearby bright spectral lines at 124.2 nm. The O VI slit measures the total intensities of the O VI lines at 103.2 and 103.7 nm. Different regions of the corona can be observed by scanning the UVCS telescope mirrors to look at different heights and by rolling the SPARTAN 201 spacecraft to look at different position angles about the sun. Observations are planned by looking at the latest images from ground-based and space-based telescopes just before the shuttle launch. Four primary coronal targets have been set in the SPARTAN UVCS flight software. The actual pointing of the spacecraft is not decided until a few hours before the SPARTAN 201 satellite is released from the shuttle. The first target is a coronal streamer on the east (left) limb of the sun. Hydrogen Lyman alpha profiles and intensities will be measured at 1.7 and 2.1 solar radii on the streamer axis. Then the SPARTAN 201 spacecraft will roll about its sun-pointing axis to put the O VI slit on the axis of the streamer at the same heights. The second target will also be a coronal streamer, but observations will be made at heliocentric distances out to 3.5 solar radii. Measurements of the H I Lyman alpha profile will be made along the streamer axis at 1.7, 2.1, 2.5, 3.0, and 3.5 solar radii. The Lyman alpha field of view at each height is positioned along the streamer axis by adjusting the spacecraft roll about its sun-pointing axis. For the measurements at 1.8 solar radii, the O VI field of view will be placed on the streamer axis. These measurements will provide information on the kinetic velocities and bulk outflow velocities in closed magnetic field regions. The magnetic field configuration is expected to open up at the streamer edges and at the larger heights in the streamer. The plasma in these regions of open magnetic field may have different characteristics from the plasma in the closed regions. The third target is above the sun's North Pole in a polar coronal hole. Observations will be made at seven heights near the axis of a coronal hole: 1.7, 1.8, 1.9, 2.1, 2.5, 3, and 3.5 solar radii. The last target is at a single height (1.7 solar radii) in the south polar coronal hole with the Lyman alpha detector only. (The O VI detector door is closed just before the last orbit in order to preserve the efficiency of the O VI photocathode.) Solar Disk Studies In addition to observing the solar corona, SPARTAN will take a look at the solar disk on two orbits and record background/geocoronal measurements during nighttime portions of each orbit. The solar disk observations will be used to probe the Earth's exosphere in order to correct for its absorption and scattering effects on the spectral data. During a typical SPARTAN 201 orbit, the light from the sun goes through exospheric layers of different thicknesses. By following the variation of the disk light through an entire orbit, researchers can probe the effects of the exosphere. Since the SOHO satellite is outside the exosphere, comparing its observations with those of the SPARTAN UVCS will help researchers make necessary corrections in the SPARTAN Lyman alpha intensities and profiles and O VI intensities. Data Collection & Recording All of the coronal images and UV spectral data (line intensities and profiles) will be recorded on board the spacecraft for postflight analysis. The SPARTAN 201 spectrometer is a more sensitive version of the instrument that was flown on rockets in 1979, 1980, and 1982. It consists of an occulted telescope, dual spectrometer, detectors for 1.216-angstrom radiation (atomic hydrogen) and 1,032- and 1,037-anstrom radiation (ionized oxygen), and electronics. Because the SPARTAN UVCS instrument is retrieved by the shuttle, its in-flight performance can be inferred from preflight and postflight calibrations. Laboratory tests and experiments performed for the first three missions have provided detailed knowledge of the instrument's performance characteristics. |
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| Benefits |
| Discoveries about the unknown source of the energy that heats the solar corona and accelerates the solar wind may help scientists understand the winds that carry mass and momentum away from other stars and why the sun's rotation has slowed. The results may also help them to explain how the Earth's magnetism and, ultimately, its climate and weather are affected by variations in the radiation and particles emitted by the sun. |
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