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Slide 1

Basic facts about the Sun is 93 million miles from Earth is made of 92% Hydrogen, 7% Helium, and 1% other gases is 5 billion years old, another 5 billion to go is a medium-size star, one of about 2 billion in our galaxy has an 11-year cycle of solar activity

Solar Observing in the Space Age Certain spacecraft orbits have unique advantages for solar observing. The orbit of the ESA/NASA SoHO spacecraft, shown here, allows solar scientists to observe the Sun continuously, without interuption by periods of night, when the spacecraft is in the shadow of Earth.

Names of things on the Sun

We probe the Sun’s interior using sound waves Turbulence near the surface of the Sun generates sound. Sound waves of certain frequencies are trapped inside the Sun.  These waves are refracted and reflected.  Certain wave lengths resonate,  like tones and overtones inside a musical instrument.

The patterns of resonating waves reveal the Sun’s interior structure and motions The motions on the surface of the Sun are organized into patterns in both space and time, which reflect those waves which resonate inside the Sun.  Analysis of these patterns (blue means upward, red downward) allows us to infer the structure and motions hidden from direct view. The pattern shown – like a certain musical note --corresponds to a certain integral number of wavelengths in latitude, longitude and radial depth.

The Doppler effect reveals ripples on the surface Spectroscopic observations show up and down motions on the surface with no obvious pattern.   However, careful harmonic analysis shows that these motions are organized in millions of individual “notes”, or modes. Four of these modes are shown here.

The Sun rotates faster at its Equator than its Poles In the early 17th century, Galileo used sunspots to determine that the Sun rotates in about 27 days.  We now know that the  Sun is not a solid body.  Its equatorial rotation period rate is about 25 days, while the polar period is about 35 days. Color code: Red is faster, blue is slower. Helioseismology tell us that this “differential rotation” extends far below the surface.

Earlier helioseismology results: plasma rivers Rivers of gas flow under the surface.  Their motions are systematically faster and slower than average.  However, early observations did not span enough time to be able to see variations on the scale of the 11-year solar cycle.

Recent results: rotation is variable at all depths Rotation rate fluctuations are seen at depths from the surface to below the bottom of the convection zone. Fluctuations with 1.6 year period have been observed over the last five years.

Implications for magnetic fields: a dynamo A dynamo is a mechanism for amplifying magnetic fields. In this model of the dynamo, differential rotation inside the Sun amplifies the primordial dipolar solar magnetic field by wrapping it up. When the field reaches a strength about 100,000 times greater than the Earth’s magnetic field, it buoys up to form active regions in the photosphere and corona.

The deep origins of the Sun Earth connection Motions in the Sun’s interior are ultimately responsible for the dynamic eruptions of magnetic fields that are so important to the the Sun Earth connection.

Magnetic fields observed on the solar surface This movie shows just over one solar rotation period.  In the black and white areas magnetic fields respectively emerge from, and descend back into, the solar interior. Watch for: New active regions emerging Hemispheric differences Faster rotation at the equator

Magnetic fields control the Sun’s atmosphere The Sun’s atmosphere is strongly controlled by magnetic forces.  Where strong magnetic fields erupt from below (orange), they hold the gas down and the corona is bright.  In the polar regions, where no significant amounts of magnetic field emerge, the corona is dark because it can expand freely (red arrows) to form the solar wind.

We see different parts of the Sun’s atmosphere by looking in different colors Different colors of light come from gases at different temperatures.  The white light you see with your eye comes from the cool photosphere and sunspots; X-rays come from the hot corona.

The Sun’s corona is filled with magnetic loops This dramatic Yohkoh X-ray image shows many magnetic field lines arching into the Sun’s corona from below, forming active regions.  Readers of Sky and Telescope magazine (January 2000) chose it to be one of the 10 most inspiring astronomical images of the century!

Sunspot activity follows an 11-year cycle Traditionally the number of sunspots defines the rise and fall of solar activity. This chart shows that sunspot activity reached its peak level or “solar maximum” around mid-2000.

Comparing the Sun: 1996 to 1999 By comparing images of the Sun over two years apart, you can see a dramatic difference in its activity levels. This increase is caused by the emergence of more and stronger magnetic fields. (Sun observed in extreme ultraviolet light.)

A Coronal Mass Ejection (CME) A CME is a cloud of hot magnetized  gas, weighing billions of tons, which is launched from the Sun at a speed up to millions of miles per hour. The CME shown in the sequence at the right was launched over an 8-hour period in 1996. CMEs are caused by the breaking apart of magnetic forces. These occur almost every day, but most go off in other directions, not toward Earth.

CMEs busting out all over This 16-day sequence during a very active period shows that CMEs occur often and in many directions..

How to recognize an Earth-bound CME

Sigmoids in the X-ray corona CME’s tend to be launched from regions shaped like an S or reverse-S, called “Sigmoids”.  The sigmoid shown here erupted several times during the two days of the movie clip.

S marks the spot animation The twisted magnetic fields of sigmoids store energy.

Space weather: a CME approaching Earth It takes 2 to 4 days  for a CME blast to reach Earth, as  it expands up to 30 million miles wide. The white lines show the space filled by Earth’s magnetic field – the magnetosphere.

Earth’s Magnetosphere and a CME Normal shape

Earth’s magnetosphere reacting to a CME This supercomputer model was created by taking data from a number of instruments and developing a visualization of how the magnetosphere responded to a CME on Jan. 10, 1997. The clip shows a four hour period.

Auroras often occur when a CME hits Earth     Often called the northern or southern lights, these often colorful and flashing light displays in the night sky are mostly seen at high latitudes near the poles.  They are caused by the impact of fast moving electrons from outer space (our magnetosphere) on the Earth's upper atmosphere.

Powerful magnetic storm: 1989 In 1989, near the last solar maximum, a very strong solar storm impacted the Earth’s magnetosphere.  Quebec, Canada lost power  for 9 hours and power, navigational and communications systems around the world were upset.  This computer model shows how the storm moved in just a few minutes. (Red means highest level).

When CMEs impact our magnetosphere .

What are the major questions of solar physics? How is the sun’s magnetic field generated? Shear flows at the base of the convection zone? Shear flows within the convection zone? What makes the solar corona so hot? Dissipation of shock waves? Many tiny flares? What causes solar flares and CMEs? Reconnection of magnetic fields? Just the right magnetic topology?

Research Opportunities for physics majors

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