3-D Study of Solar Activity Vital "Next Step" in Space Weather Prediction

10/17/2006 - 33-06r
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NRL SECCHI Experiment to fly aboard
NASA STEREO observatory in October

STEREO is scheduled to launch at 8:38 PM October 25.
(The launch window is 8:38-8:53PM.)

Coronal Mass Ejections (CMEs) are large clouds of gas ejected into space by the Sun. Violent eruptions with temperatures upwards of two-million degrees and travelling anywhere from 50 to 3,000 kilometers per second, CMEs are known to cause geomagnetic storms that can present hazards for satellites, radio communications and power systems.

Researchers have already learned much about CMEs from instruments such as the Naval Research Laboratory's Large Angle Spectrometric Coronagraph (LASCO) experiment on the NASA/European Space Agency's Solar and Heliospheric Observatory (SOHO), but now scientists will use NASA's new Solar Terrestrial Relations Observatory (STEREO) to obtain new and unprecedented views of the solar corona. STEREO is a two-year mission scheduled to launch in October 2006 that will use two nearly identical observatories to provide 3-D measurements of the Sun to study the nature of CMEs.

What sets a CME in motion? How do CMEs accelerate? How do they cause space weather disturbances? These are some of the fundamental questions that an international team of investigators led by NRL hope to answer with a new suite of remote sensing instruments, which make up the Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI). SECCHI will observe CMEs as they form at the Sun and then follow them as they travel through the corona and interplanetary space.

"The elemental questions we hope to answer using SECCHI data cannot be addressed conclusively with observations from a single viewpoint," says NRL principal investigator, Dr. Russell Howard. In order to really understand and forecast CMEs, researchers need 3-D images of the CMEs and of the surrounding solar corona and heliosphere. "Combining observations from two distinct views allows us to finally see the other dimension that has been missing in all of the previous observations. We will be able to explore all facets of CMEs, as they begin at the source and as they travel toward earth," notes Dr. Howard.

SECCHI consists of two separate packages of optical instruments - one that is pointed toward the center of the Sun and one that is pointed at the line between Sun and Earth. The Sun-Centered Instrument Package contains an extreme ultraviolet imager that will look at the upper chromosphere and innermost corona, and two white-light coronagraphs that explore the outer regions of the solar corona, all with higher spatial resolution and higher time resolution than the LASCO instruments. The Heliospheric Imager package contains two telescopes, which will obtain the first direct imaging observations of CMEs along the Sun-Earth line.

Simultaneous image pairs will be obtained by STEREO as the two spacecraft gradually separate during the course of the mission. Substantial new physical insight will be gained simply from visual examination of the stereo images. The importance of coordinating the observations, as well as their analyses, can hardly be overemphasized, says Dr. Howard. "We have never before had an opportunity to observe from two viewing points. All of our previous observations of CMEs have been two-dimensional images from platforms close to the earth and we have had to make educated guesses about the third dimension. The stereoscopic techniques and their results will be entirely new. Furthermore, the opportunity will not be a static one as we had with the SOHO mission. Here the viewing angles will change continuously as the two spacecraft drift away from Earth. We will be able to learn different things as the two spacecraft separate, and the combination of the LASCO and EIT observations on the SOHO spacecraft will give us a third viewpoint. Unfortunately we can't reproduce the many viewpoints that are used by the medical profession when taking a CAT scan, but thankfully the solar corona is simpler than the human body."

Numerical modelling is integral to the SECCHI experiment. Given the complex spatial and temporal structure of the Sun and heliosphere, the investigators will use models to extract from the data an accurate three-dimensional picture of the corona, CMEs, and the heliosphere. Dr. Howard says that significant progress has been made recently in developing modelling techniques that will enable maximum use to be made in understanding the SECCHI observations. We expect to be able to deliver a steady state, three-dimensional description of the magnetic field, electron density, and velocity of the solar wind, as well as descriptions of the CME itself and how it interacts with the solar wind.

In addition to deciphering the imaging observations, the numerical models will enable direct comparisons between SECCHI observations and observations of the solar wind at each spacecraft. For example, the description of the solar wind generated by SECCHI's numerical model can be used by other instrument teams to predict the arrival of energetic particles.

The SECCHI team assembled for this effort has designed, built and flown primary instruments on the Solar and Heliospheric Observatory (SOHO), Yohkoh, TRACE, Spartan 201, SMEI, and members are currently fabricating instruments for SXI and Solar B. The team is made up of scientists from NRL, NASA's Goddard Space Flight Center and the Jet Propulsion Laboratory, Lockheed Martin Solar and Astrophysics Laboratory, the Smithsonian Astrophysical Observatory, and Space Applications International Corporation. Domestic university support includes Stanford University, the University of Michigan, and Boston College. International contributing partners include the Max Planck Institute for Solar System Research, the University of Gottingen and the University of Kiel (Germany); the Institute d'Astrophysique Spatiale and the Laboratoire d'Astronomie Spatiale (France); the University of Birmingham, University College-London and Rutherford Appleton Laboratory (UK); the Royal Observatory of Belgium and the Centre de Spatiale de Liege (Belgium). SECCHI was partly sponsored by the DoD Space Test Program in support of the U.S. Navy Coronal Mass Ejection Warning System (CMEWS) experiment.

SECCHI is one of four experiments on STEREO. Learn more about SECCHI and the STEREO mission at http://www.nasa.gov/stereo and http://secchi.nrl.navy.mil/.

Artist's rendering of the STEREO spacecraft imaging a coronal mass ejection.
Credit: NASA.
A view of the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument being built at the Naval Research Laboratory in Washington, DC.
Credit: NRL/NASA/Chris Gunn.
The Sun-Centered Instrument Package

The Extreme Ultraviolet Imager (EUVI) provides full Sun coverage with twice the spatial resolution and dramatically improved cadence over its predecessor EIT.

EUVI observes the photospheric magnetic field, chromosphere, and innermost corona underlying the same portions of the corona and the heliosphere observed by two coronagraphs (COR 1 and COR 2) and a heliospheric imager (HI).

The Guide Telescope acts as a fine sun sensor for the EUVI and provides the error signal for the EUVI fine pointing system.

The Heliospheric Instrument

The most novel instrument, the Heliospheric Imager will obtain the first direct imaging observations of coronal mass ejections in interplanetary space.

Credit: NASA/NRL
In an Applied Physics Lab clean room at Johns Hopkins University, STEREO team members from the Naval Research Labratory prepare SECCHI's Heliospheric Imager for installation aboard the "A" observatory (the one that will be placed "ahead" of Earth in its orbit around the sun). Credit: NASA/Johns Hopkins University Applied Physics Lab.
Using a crane, engineers lower the Sun Centered Imaging Package (SCIP) into one of the twin STEREO observatories.
Credit: NASA/Johns Hopkins Applied Physics Laboratory
Engineers unpack a portion of the Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) instrument, led by the Naval Research Laboratory. The Sun Centered Imaging Package (SCIP) consists of two white light coronagraphs and an extreme ultraviolet imager.
Photo credit: NASA/Johns Hopkins Applied Physics Laboratory
Location of the NRL Secchi instruments on the STEREO spacecraft, built by the Johns Hopkins University Applied Physics Lab.
Credit: NASA

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