Staring Directly At The Sun: That's Where The Science Is

October 05, 1998

Indeed, Mom probably did tell you never to stare directly at the Sun. However, scientists led by a team at NASA/Marshall have proposed a new scientific instrument called JASMIN that would do just that - and reveal scientific details never before seen on our nearest stellar neighbor.

JASMIN - the Japanese American Solar Magnetograph INstrument - is a collaboration between scientists across the nation, from Hawaii to the East Coast. If selected for flight, JASMIN will be built by a consortium of midwest industries, and will ride to orbit on board Japan's Solar-B satellite scheduled to launch in the year 2004.

"The Sun still holds many mysteries," said Dr. John Davis, JASMIN's Principal Investigator at NASA/Marshall's Space Sciences Laboratory, "especially how its atmosphere and magnetic field work together as a single system. JASMIN would be one of three instruments on Solar-B, which together can give us a better understanding of the detailed physics, in order to learn how, why, where, and when things happen on the Sun."

The Solar-B Partnership

ISAS - Institute of Space and Astronautical Science (JAPAN)
NASA - National Aeronautics and Space Administration (USA)
PPARC - Particle Physics and Astrophysics Research Council (UK)
JASMIN Science Collaborators

NASA/Marshall Space Flight Center
Montana State University, Bozeman, MT
National Optical Astronomy Observatories, Tucson, AZ
Naval Research Laboratory, Washington, DC
New Jersey Institute of Technology, Newark, NJ
NorthWest Research Associates, Inc., Bellevue, WA
SAIC, San Diego, CA
Tennessee State University, Nashville, TN
University of California, Berkeley, CA
University of Hawaii at Manoa, Manoa, HI
University of Michigan, Ann Arbor, MI
JASMIN Industry Partners

ITT Industries, Fort Wayne, IN
Michigan Aerospace Corporation, Ann Arbor, MI
Wyle Laboratories, Dayton, OH

The Sun-Earth Connection

Nearly all of the phenomena we observe on the Sun are controlled by magnetic fields, from the heating of the outer solar atmosphere (the corona), to sunspots, solar flares, and huge eruptions of material called coronal mass ejections. These events are more than just scientific curiosities because they can - and do - disrupt communications and navigation in space and on the Earth. Even small variations in heating in the outer layers of the Sun can change the amount of light and heat the Earth receives by enough to change our climate.

"Solar flares are explosive events," Davis explained. "They release huge amounts of energy in a very short time. Exactly how they do this is not clear." Since the flares are always associated with strong magnetic fields, it is thought that the flares' energy is stored in the form of magnetic energy.

JASMIN will help unravel these mysteries with the first measurements of the direction and strength of these magnetic fields taken from space."

"Solar flares always happen at night or on cloudy days," is a common lament for solar scientists. Ground-based telescopes can see the Sun only during clear days. And even then, Earth's turbulent atmosphere distorts the view most of the time.

Solar-B and JASMIN

The solution is to put the telescope above the sky in an orbit that shifts a little each day so the Sun never sets on the satellite. This is the plan for the Japanese Solar-B mission. Its 50 cm (20 in.) telescope, the largest solar telescope ever orbited, would feed images to JASMIN, which can operate in three distinct scientific modes.

Inside JASMIN - about the size of two large suitcases - will be a complex arrangement of filters and other optics that will resolve the strength and direction of the magnetic field more than ten times better than anything done before.

"We'll be able to map the magnetic field over large areas of the Sun with a resolution corresponding to a box with 100 km (60 miles) sides. This might sound like a rather large area, but it's nearly 20 times better than anything we can do from the ground," Davis said. "Currently our ability to resolve details of the Sun's magnetic field is like trying to watch a football game from a mile up. You can see the field, but all the interesting action is taking place on scales smaller than you can see."

"Solar-B will be the first mission to 'break the resolution barrier' and observe the Sun on the spatial scales where the physics is happening."

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The Science of Solar-B/JASMIN

JASMIN will study several fundamental questions of Solar Science critical to developing an accurate and quantitative understanding of the Sun's dynamics. For example,

Heat waves

When we go upward from the surface into the Earth's atmosphere, the air generally gets cooler. The Sun works in reverse. Its outermost layer, the corona, is hotter than 1,000,000 ºC (1.8 million degrees F) while the visible surface, or photosphere - where the atmosphere originates - has a temperature of only about 6,000 ºC (11,000 ºF) . How the corona is heated is one of the great mysteries of solar physics.

"It should be possible to heat the corona with waves," Davis said. "All kinds of waves are generated in the photosphere - such as acoustic waves, from mechanical motions, or Alfven waves, from shaking magnetic fields. These waves spread upward into the corona which absorbs energy from the waves.

"The trouble with this idea is that none of the waves likes to be absorbed by the corona. They either go right through, or are reflected back to their starting point. This is a long-standing problem and nobody has a good solution for it.

Alternatively, Marshall scientists have suggested that energy is pumped into the corona through a series of little explosive effects - microflares - that occur all over the place."

Microflares, if they are real, are small-scale versions of well-known solar flares. The question is: Are there enough of them to do the job? They are too small to understand with current telescopes, but will be within Solar-B's grasp.

Living with stress

Virtually all of the dynamic phenomena we observe on the Sun can be traced back to magnetic fields which form deep in the Sun's interior. These fields can emerge in large active regions that would swallow the entire Earth many times over, or on scales smaller than we can currently see - about 100 km across. As these fields rise, they are twisted and stretched, and store energy much like a rubber band stores energy when it is twisted or stretched.

"Very accurate measurements are needed to tell how much energy is stored in the field." Davis said. "JASMIN will make these measurements and will allow us to determine what happens to the field when it releases its energy. Most scientists think that the field reconnects. When a twisted rubber band is cut, the band untwists, leaving two open ends. The magnetic field behaves the same way, but with one important difference. Magnetic field lines can't have open ends, and so if a field line is cut, it has to rapidly reconnect either to itself or to another field line that has undergone a similar process."

The Japanese Solar-B mission will provide the first real opportunity to study this 'reconnection' process, which scientists think releases the tremendous amounts of energy that fuels solar flares. Solar-B will follow these events from the build-up of energy in the field, to its release during reconnection, into the X-rays and charged particles that can affect the Earth.

Measuring the Magnetic Field

Determining the strength and direction of the magnetic field on Earth is not difficult. A compass can tell us the direction, and other simple devices can measure its strength. We can't put a compass on the Sun, but since the structure of the magnetic field is encoded in the light we collect from the Sun; we can try to decipher it.

"This is an area that Marshall solar scientists pioneered," Davis said. "It is not an easy measurement, and the theoretical interpretation is difficult."

Like much of what happens in physics, it comes down to understanding how individual atoms behave.

When atoms are heated, they collide and knock electrons into higher energy orbits. The electrons can then return to a lower energy level by releasing energy in the form of light. The color of the light emitted depends on the amount of energy released. Each element has its own unique color set, because of its unique set of energy levels. If light from heated atoms is studied with a spectrometer - an instrument that analyzes light in its different colors - these very narrow lines of color, called spectral lines, are observed.

When heated atoms are also placed in a magnetic field, the energy levels of the electron orbits split, causing two lines with slightly different colors or spectral positions. Furthermore, the emission is polarized, which means that the oscillations of the electric and magnetic fields components of the light have specific orientations.

Certain crystals have properties that allow them to transmit light only when its oscillations are lined up in a certain way. These crystals are known as polarizers. When polarized light is passed through a polarizer the light can be transmitted or blocked by rotating the polarizer, so the degree of polarization can be measured.

Solar Vector Magnetographs do just this. They contain optical systems that spread the light enough to show spectral lines, and then measure how much the lines are polarized. This allows scientists to then calculate the strength and direction of the magnetic fields at the point on the Sun from which the light originated.

In 1973 Marshall scientists, under Dr. Mona Hagyard's direction, put their Solar Vector Magnetograph (left) into operation, just in time to support studies with the solar telescopes aboard the Skylab space station. It has supported all the solar space missions since that time including the Japanese Yohkoh (Solar-A) mission.

"One of the unexpected and most important results from this instrument was that flares almost always occurred in the regions where the magnetic field was stressed," Davis said. "When scientists saw that, it made perfect sense. Those observations were the ground truth of what we were doing, and it reinvigorated solar physics in this area."

"JASMIN, on board Solar-B, will show how the three dimensional field patterns change with time on very fine spatial scales. With this, we will be in a good position to solve many of the puzzles of solar physics," Davis said.
-end-


NASA/Marshall Space Flight Center--Space Sciences Laboratory

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