Solar Rotation

Lab 3

Differential Rotation

• The sun lacks a fixed rotation rate

• Since it is composed of a gaseous plasma, the rate of rotation is fastest at the equator (24 days), and decreases as latitude increases (30 days at polar regions)

• ~27 days

Sidereal Rotation

• At the equator the solar rotation period is 25.38 days and is called the sidereal rotation period

• Sidereal rotation period – time it takes to rotate 360 degrees on its axis

• Not to be confused with synodic rotation

Synodic Rotation

• Sun has a synodic rotation period of 27.2753 days, which is the time for a fixed feature on the sun to rotate to the same apparent position as viewed from earth

• The synodic period is longer because the sun must rotate for a sidereal period plus an extra amount due to the orbital motion of the earth around the sun

Use of Tracers to Measure Rotation

• Sunspots are used as tracers to measure rotation rates of Sun

• A tracer is a fixed feature on the Sun

• Sunspots had been observed since ancient times, but after Lipperhey/Galileo invented the telescope it was seen that they turn with the Sun, and thus could define the period of the solar rotation

Sunspots

• magnetic elements• Every ~11 years the number of sunspots

seen on the Sun increases from ~0 to ~100, then decreases to ~0 again as the next cycle starts

• The polarity is reversed also• So a complete sunspot cycle is ~22 years • Seen in a band, 5-40˚ latitude above and

below the solar equator

What are Sunspots?

• Sunspots appear as dark spots on the surface of the Sun• Sunspots are magnetic regions on the Sun with magnetic

field strengths thousands of times stronger than the Earth's magnetic field

• Temperatures in the dark centers of sunspots drop to about 3700 K (compared to 5700 K for the surrounding photosphere)

• They typically last for several days, although very large ones may live for several weeks

• Sunspots usually come in groups with two sets of spots. One set will have positive or north magnetic field while the other set will have negative or south magnetic field

• The field is strongest in the darker parts of the sunspots (umbra), and weaker and more horizontal in the lighter part (penumbra)

Sunspots

Solar Magnetic Field• The Sun’s magnetic field is produced by the flow of

electrically charged ions and electrons• Sunspots are places where very intense magnetic

lines of force break through the Sun's surface• The sunspot cycle results from the recycling of

magnetic fields by the flow of material in the interior• The prominences seen floating above the surface

of the Sun are supported, and threaded through, with magnetic fields

• The streamers and loops seen in the corona are shaped by magnetic fields

• Magnetic fields are at the root of virtually all of the features seen on and above the Sun

X-ray image of the sun (corona)

Corona

• The Sun's outer atmosphere (corona) is hotter than 1,000,000ºC

• Usually temperatures fall as distance increases from a heat source

• This is true in the Sun's interior right up to the visible surface

• BUT THEN, over a relatively small distance, the temperature suddenly rises to extremely high values

• Areas on the Sun near sunspots often flare up, heating material to millions of degrees in just seconds (solar flares) and blasting billions of tons of material into space (coronal mass ejections)

Other features of the Sun - Faculae

• Faculae are bright areas that are usually most easily seen near the limb, or edge, of the solar disk

• These are also magnetic areas but the magnetic field is concentrated in much smaller bundles than in sunspots

• While the sunspots tend to make the Sun look darker, the faculae make it look brighter

Faculae

Other features of the Sun - Granules

• Granules are small (~1000 km across) cellular features that cover the entire Sun (except for those areas covered by sunspots)

• They are the tops of convection cells, where hot fluid rises up from the interior in the bright areas, spreads out across the surface, cools, and then sinks inward along the dark lanes

• Individual granules last for only ~20 minutes • The granulation pattern continually evolves, old

granules get pushed aside by new emerging ones• The flow within the granules can reach supersonic

speeds of more than 15,000 mph and produce sonic "booms" on the Sun's surface

Granules

• Movie - http://science.nasa.gov/ssl/pad/solar/feature1.htm

Other features of the Sun - Supergranules

• Supergranules are much larger versions of granules (~35,000 km across)

• These features also cover the entire Sun and continually evolve

• Individual supergranules last for ~1-2 days and have flow speeds of ~1000 mph

• The fluid flows observed in supergranules carry magnetic field bundles to the edges of the cells where they produce the chromospheric network

Supergranules

Other Features of the Sun - Chromospheric Network

Other Features of the Sun - Filaments and Plages

Other Features of the Sun - Prominences

Prominence eruption

• http://science.nasa.gov/ssl/pad/solar/images/granddaddy.mpg

Other Features of the Sun - Spicules

Magnetosphere

Solar Wind

• The Sun is the source of the solar wind; a flow of gases from the Sun that streams past the Earth at speeds of more than a million miles per hour

• Disturbances in the solar wind shake the Earth's magnetic field and pump energy into the radiation belts

• Solar flares and give off ultraviolet light and x-rays that heat up the Earth's upper atmosphere