Section 3 - The Sun

Question 1

What type of star is the Sun?

Answer 1

Typical main-sequence star, mid-sized and middle-aged. It takes up 99.8% of the mass of the solar system.

Question 2

Describe the spectrum of the Sun.

Answer 2

Similar to a black body.
Spectrum of the Sun at the surface of the Earth shows absorption by molecules dominating the infrared, with Rayleigh scattering in the optical.

Question 3

Define photosphere

Answer 3

Where the Solar atmosphere transitions from being optically thick to transparent (optically-thin).

Question 4

What is granulation of the photosphere caused by?

Answer 4

Due to convection that extends about 30% of the radius of the Sun into its interior.

Question 5

Describe the process of convection in the Sun.

Answer 5

Convection arises when the temperature gradient is steep. As relatively hot blobs of material rise, they expand adiabatically. If the temperature gradient is shallow they lose buoyancy and stall, but if the temperature gradient is steep, they remain buoyant and continue to rise.

Question 6

What sort of energy transport occurs in the deeper interior of the Sun?

Answer 6

Deeper interior stable to convection, so energy transport is radiative.

Question 7

Helioseismology - What can we find out from analysing the mode amplitudes? And what is the high frequency noise detected?

Answer 7

• Can probe interior density structure, composition, rotation, and depth of convection zone.
• Oscillation of Sun due to standing soundwaves.

Question 8

Describe a p-mode.

Answer 8

Restoring force is pressure (sound waves). They are refracted back to the surface due to temperature gradient which increases sound speed.

Question 9

Describe a g-mode.

Answer 9

Restoring force is buoyancy (related to gravity). Only propagate in radiative zone, so not directly visible. Potentially detectable through perturbations of p-modes.

Question 10

What do numerical models of the Sun’s evolution indicate?

Answer 10

The Sun was smaller and cooler in the past, and significantly less luminous. Poses the ‘faint young Sun paradox’. (Describes the apparent contradiction between observations of liquid water early in Earth’s history and the astrophysical expectation that the Sun’s output would be only 70 percent as intense then as it is now.)

Question 11

What is a Sunspot? Name two ways they differ from other regions in the Sun.

Answer 11

Relatively dark regions where the temperature of the photosphere is much lower. This indicates convection is suppressed in these regions. The granulation (convection) pattern around the spot is distorted.

Question 12

What effect is observed in optical spectra of Sunspots, and describe how it occurs.

Answer 12

Zeeman splitting of atomic absorption lines due to magnetic field.
- Magnetic field lifts the degeneracy of atomic energy levels with different angular momentum states, splitting individual lines into multiple components.

Question 13

Why are sunspots in hydrostatic equilibrium with their surroundings despite being cooler?

Answer 13

Thermal pressure is partially replaced with magnetic pressure.

Question 14

How can the magnetic field of the Sun be mapped and monitored in real time? What does this show?

Answer 14

By spatially resolving the Zeeman splitting.

Shows sunspot groups are location of magnetic flux loops that emerge through the solar surface.

Question 15

Describe the Sun’s magnetic field. What is required to have a magnetic dynamo?

Answer 15

The Sun has a large-scale dipole field, together with higher order flux loops associated with Sunspot groups. Believed to be actively generated by a magnetic dynamo.

A conducting fluid with differential rotation and convection.

Question 16

How does the surface of the Sun move?
What more have we learnt about this through helioseismology?
What is believed to drive the Solar dynamo?

Answer 16

It rotates differentially, with the equator rotating more rapidly than the poles.

Helioseismology shows this differential rotation extends through the convective zone, but the radiative zone rotates as a solid body.

The strong shearing flows at the tacholine between the convective and radiative zones.

Question 17

What is the solar corona, and in what way, and why is it visible?

Answer 17

The extended atmosphere of the Sun, visible with the naked eye during a solar eclipse due to photons from the photosphere are scattering off free electrons in the extended atmosphere (the corona).

Question 18

What are the three main components of the Solar atmosphere in order of increasing distance from the Sun’s interior, and increasing temperature?

Answer 18

Photosphere (6000K), Chromosphere (50,000K), (Transition region), Corona/flare plasma (1,000,000K).

Question 19

What is the X-ray spectrum of the Solar corona like and what does this show?

Answer 19

Dominated by collisionallly-excited emission lines, shows optically-thin thermal emission from hot plasma.

Question 20

Over what cycle does the number of sunspots vary?

Answer 20

11 years (technically 22 years due to polarity changes)

Question 21

What happens to the Solar dipole field at the end of each cycle of sunspot numbers? (thought to be fundamental property of the Solar magnetic dynamo)

Answer 21

The polarity reverses, so does the polarity of active regions.

Question 22

Describe the 5 stages of the solar dynamo model of the solar cycle.

Answer 22

1) Beginning of each cycle - few sunspots and magnetic field close to a dipole.
2) Differential rotation drags field lines, converting poloidal field into torodial components.
3) Twisted magnetic flux loops emerge through Solar photosphere, causes sunpots activity regions, and coronal heating.
4) Meridional flows drag active regions towards equator - opposite polarity toroidal components cancel.
5) Solar magnetic field reverts to dipole configuration with opposite polarity.

Question 23

How is the large-scale Solar magnetic field, although similar to a dipole, different to a dipole?
Why is this?
What can this cause?

Answer 23

Has many more open field lines than a dipole.

Competition between magnetic field confining the heated corona, and its thermal pressure.

Coronal plasma can escape along open field lines in a solar wind of high velocity charged particles.

Question 24

What is the main energy source of coronal heating to drive the solar wind, and what is believed to be the source of required additional energy?

Answer 24

Adiabatic expansion.

Alfven waves propagating along magnetic field lines, excited by convection at photosphere. (Probably also magnetic reconnection events.

Question 25

What can the solar wind do to planets not protected by their own magnetic field (e.g. Mars)?

Answer 25

The Solar wind can erode their atmospheres.

Question 26

What is magnetic breaking?

Answer 26

The rotation of the Sun exerts a torque on the Solar wind that transfers angular momentum and slows the Solar rotation.

Question 27

What is magnetic reconnection?

What does this cause?

Answer 27

When the complex magnetic field loops in the Solar corona can reconfigure suddenly to a simpler morphology.

Magnetic energy is converted into kinetic energy via particle acceleration and then to thermal energy through particle collisions. Results in fast intense emission of X-rays and gamma-rays. (solar flare)

Question 28

What causes a coronal mass ejection (CME)?

What can these cause?

Answer 28

At most energetic reconnection events, coronal loops can burst open, ejecting hot plasma from the corona.

Extreme space weather on Earth, damaging spacecraft and national power grids.