What will the Solar Eclipse of 2017 look like?
A Coronal Prediction
Our Sun, and other stars, are
surrounded by an atmosphere, a brightly glowing plasma that is usually
overwhelmed by the Sun's radiance.
Like a pearly aura, this atmosphere, or corona, will become visible
during the total solar eclipse of 21 August 2017. Our Sun is a dynamic,
constantly-changing, magnetic star.
So the corona of each total solar eclipse has a unique and beautiful
shape that is determined by the Sun's magnetic fields. About a month before the eclipse,
solar scientists are able to predict that shape!
It's All About Magnetism
| The Earth's magnetic field resembles that of a bar magnet with its poles near the geographic poles of the Earth The field is generated deep in the Earth's core, where the iron is as hot as the Sun's surface, but the crushing pressure caused by gravity prevents it from becoming liquid Surrounding this core is a thick layer of primarily iron and nickel.Under less pressure than the inner core, these metals can liquefy. This flow of liquid iron generates electric currents, which in turn produce magnetic fields. The Earth's magnetic field remains fairly steady over thousands of years. |
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The Sun's magnetic field is quite different. There is a polar field somewhat similar to the Earth's, with lines of force that fan out into space from the north and south poles. But there are also equatorial fields generated by the bubbling and roiling gases. The Sun's magnetic fields are constantly changing on timescales ranging from a fraction of a second to billions of years> |
How does this work?
The Sun is made of plasma, a gas-like state of matter in which electrons are stripped from their nuclei. Both the electrons and the stripped atoms now carry an electrical charge. The result is a super-hot mix of charged particles, and when charged particles move, they create magnetic fields. When these magnetic fields rise from the solar interior, they emerge through the Sun's surface and reach high into the corona.
Magnetic fields, of course, are invisible. But plasma can be trapped in them, and
hence show us the shape, just as we use iron filings to trace the field of a
bar magnet. The Sun's magnetic
fields are responsible for solar activity.
Sunspots result from magnetic fields emerging through the solar
surface. And, it is the breaking
and reconnecting of these magnetic fields that cause solar flares and the
resulting coronal mass ejections (plasma shot into space)!
It is the shape of the Sun's magnetic fields at the time of
the eclipse that determines how the corona will appear.
The Solar Cycle Plays a Role
The shape of the corona is also determined by the solar
cycle. Every 11 years or so, the
Sun moves from a period of high solar activity (Solar Maximum), with lots of
magnetic fields, sunspots, and flares, to a period of low solar activity (Solar
Minimum), with few sunspots and less activity. During solar maximum, the ubiquitous
magnetic fields hold the plasma in tightly, so the corona appears more
circular. During solar minimum,
with fewer magnetic fields, the plasma is freer to move out, especially in the
equatorial regions, and the corona appears more elliptical. We are currently approaching solar
minimum, so we expect to see more of an elliptical coronal pattern in our total
eclipse.
The Predicted Shape
By observing the Sun's magnetic fields[1]
solar scientists at Predictive Science, Inc. were able to predict the shape of
the corona on July 25, one complete solar rotation (about 27 Earth days)
before August 21 –when the same
side of the Sun will be facing us as on July 25. The fields may change a little, but the
basic structure should remain fairly constant.
Streams of light protruding from the Sun's north and south
pole will be visible, as well as concentrated bubbles of light close to the
surface.
Additional Resources:
- Newsweek article on corona prediction http://www.newsweek.com/total-solar-eclipse-2017-sun-corona-646528
- Understanding the Magnetic Sun https://www.nasa.gov/feature/goddard/2016/understanding-the-magnetic-sun
- National Solar Observatory predicts
shape of solar corona for August eclipse https://phys.org/news/2017-07-national-solar-observatory-corona-august.html
- >Coronal Prediction (technical description) http://www.predsci.com/corona/aug2017eclipse/aug2017eclipse.html
- Magnetograms from the HMI instrument
(Stanford University) on NASAÕs Solar Dynamics Observatory spacecraft https://www.nasa.gov/content/goddard/sdo-hmi-magnetogram/
- Solar activities and resources for
educators and students https://solar-center.stanford.edu
- Solar Storms and Space Weather: https://www.nasa.gov/mission_pages/sunearth/spaceweather/index.html
Image credits:
- Fhgure 1: Peter Reid, The University of Edinburgh via NASA (https://www.nasa.gov/mission_pages/sunearth/news/gallery/Earths-magneticfieldlines-dipole.html)
- Figure 2: NASA
- Figure 3: Nature of the Universe (http://lifeng.lamost.org/courses/Hongkong/Hongkong_En/lecture/ch11/ch11.html)
- Figure 4: TRACE mission/NASA
- Figure 5: National Solar Observatory/NSF
- Figure 6: Predictive Science, Inc.