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The Sun Part 3

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Dynamics of the Sun

• The Sun is a hot ball of Plasma with imbedded

magnetic fields frozen into it.

• The magnetic fields carry charged particles and

the charged particles move magnetic fields.

• The Convection layer carries large currents of

plasma, magnetic fields, and energy from lower

to higher regions of the Sun.

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• These are the underlying dynamics of the Sun.

• But we have an additional complication:

• The Sun rotates differentially with the central

portion rotating in ~ 26 days and the pole regions

rotating in ~ 35 days.

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During the time of minimum solar activity, the Sun

has relatively weak internal magnetic field lines that

are lined up with the geographic poles (a poloidal

field).

And the Sun is rotating differentially…..

The result is that the poloidal magnetic field starts

to wind up into a toroidal field in the direction of

rotation.

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The Sun rotates differentially, winding up

the magnetic field of the Sun.

poloidalfield

toroidalfield

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As the magnetic field lines are pulled along the

equator, the line density increases and

magnetic flux tubes are formed.

Kinks develop in the flux tubes and the kinks

may protrude through the photosphere as the

flux tubes rise.

The flux lines become tangled and instabilities

develop in the outer regions of the Sun.

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During the time of maximum disruption of the

magnetic field, the Sun is most active.

Sunspots develop where flux tubes rise up through

the photosphere.

Flares, prominences, and coronal mass ejections

occur frequently during the Sun’s most active time.

The Sun continues its differential rotation.

The magnetic field begins to smooth out and the

intense solar activity subsides.

After 11 years, the magnetic field weakens, the

field becomes more uniform and poloidal,

But after 11 years the field is reversed!

This period of 11 years is called the Sunspot cycle.

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Sunspots

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Sunspots are cooler regions in the

photosphere where the rising flux tubes

interfere with the heat flow in the convection region.

They are still very hot

and bright, but look dark against the much brighter photosphere

surface.

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Typical

sunspots

Penumbra

Umbra(dark center)

magnetic

North pole

magnetic

South pole

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A recent

Solar flare

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Solar Flare Characteristics• A solar flare is a sudden, intense increase in brightness on

the Sun’s surface, usually near a sunspot.

• It occurs when magnetic energy in the Sun’s lower

atmosphere is suddenly released.

• Charged particles are accelerated to high speeds, and radiation at all wavelengths is emitted.

• The energy released in a typical flare is ~ 1020 J/s, but some

flares have energies as high as 1026 J/s.

This is equivalent to 10 million volcanic explosions, about a tenth of the total energy the Sun generates every second.

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1. Precursor stage: Built up magnetic energy is

triggered (flux lines break and reconnect).

2. Impulsive stage: charged particles are

accelerated to high speeds, with typical

temperatures in the flare of 20 million degrees.

3. Decay: build up and decay of soft x-rays

These stages last from seconds up to an hour.

During this stage, radio waves, x-rays and gamma

rays are emitted.

Stages of a Solar Flare

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Solar prominence, observed in uv

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Solar prominence, observed in uv

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• A prominence is a bright loop (a magnetic flux tube)

filled with hot plasma that grows up through the

chromosphere and extends into the corona.

• A prominence appears dark against the bright Sun

because it does not glow in visible light.

• The loop typically contain around 1014 kg of material,

develops in a day and can last for months.

• The loop extends over 10,000 km, and sometimes

as much as 400,000 km.

Characteristics of Prominences

20Massive coronal ejection, observed in x-rays

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Coronal Mass Ejection (CME)

• A typical CME consists of three parts:

- a cavity of low (plasma) particle density

- a dense core of plasma (a prominence)

- a bright leading edge

• CME’s are initiated in the Corona which is

dominated by magnetic fields.

• They are thought to result from large scale

restructuring of the Sun’s magnetic field.22

• A CME will rise up with speeds that can be as high as

2700 km/sec, but typically around 500 km/sec as they

accelerate upward.

• A CME usually starts to rise slowly, then it rapidly

accelerates to a near-constant speed.

• The average mass ejected in a CME is about 1012 kg

• CME’s occur about three times per week during the

solar minimum and up to 5-6 times per day during the

solar maximum.

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The Sun’s conona during a Sun cycle peak.24

Sunspot cycles last eleven years

Butterfly diagram

During that time the magnetic field of the Sun reverses

A few sunspots occur far from the equator at the beginning of the

sunspot cycle. Sunspots occur

more frequently and closer to the equator later in the sunspot cycle.

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The full cycle of two magnetic field reversals is called the Solar Cycle

Two sunspot cycles = 1 solar cycle

(22 years).

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