Week 8 (2): Planetary Magnetosphere

Question 1

Draw a labelled diagram of the magnetosphere and list out its components

Answer 1

• Bow shock
• Magnetotail: The extension of the magnetosphere on the planet’s nightside
• Magnetopause: Outer boundary of the magnetosphere
• Magnetosheath: A layer between the bow shock and the magnetopause

Question 2

Explain why:
- Jupiter’s magnetic field lines elongate outwards
- Neptune’s magnetosphere strongly vary as it rotates

Answer 2

Jupiter’s magnetic field lines elongate outwards because Jupiter rotates rapidly, causing the magnetic field lines to be loaded with additional mass of ionized materials erupted from Io’s volcanoes.

Neptune’s magnetosphere strongly vary as it rotates due to the large angle between Neptune’s spin axis and magnetic field axis.

Question 3

Explain how helical motion arise in charged particles, and what happens when a charged particle move in the direction of increasing B

Answer 3

The trajectory of charged particle consists of a circular component acting perpendicularly to magnetic field lines, and a motional component acting parallel to magnetic field lines. Thus, the circular motion from circular component combined with the linear motion from motional component results in a helical path.

As the charge move in the direction of increasing B, the magnetic moment must be conserved. This means that v_perp must increase. However, as energy must also be conserved, increase in v_perp must lead to decrease in v_par. So, the particle will eventually stop moving along B and reverse direction/ bounce along the field.

Question 4

Define the term “radiation belts” and give example of these in the solar system.

Answer 4

Radiation Belts: When charged particles trapped inside a magnetosphere exhibits a bounce motion between the poles and a drift motion around the planet. This results in toroidal belts.

Jupiter’s Radiation belt:
- Detectable in radio wavelengths due to synchroton radiation, which occurs when high energy e gyromotion leads to emission

Saturn’s Radiation belt:
- Saturn’s magnetic field almost perfectly aligns with its spin axis, so the radiation belt don’t wobble with planet’s rotation
- This leads to gaps in its radiation belt, as the radiation belt particles are absorbed by moons and ring particles

Question 5

Explain how Aurora forms

Answer 5

• e and ions are accelerated into the upper atmosphere in the aurora oval at each pole, ionizing atmospheric particles
• Excitations of these particles release photons. The colors of the aurora is due to the excitation of atmospheric constituents
• The ions are then accelerated up field lines into the magnetosphere, where they escape as plasma along open field lines

Question 6

Explain how gradual loss of Mars’ atmosphere occurs

Answer 6

• Mars have no global magnetic field, only crustal fields in some regions
• So, solar wind strikes the upper atmosphere directly
• This generates an electric field that accelerates ions out of the upper atmosphere.

Question 7

Give and explain the evidences for subsurface ocean on Europa

Answer 7

• The evidences comes from the magnetic field signature near Europa
• The rotation of Jupiter + dipole tilt causes Europa to experience a continually-changing magnetic field
• This induces eddy current that flow along its surface, which only possible if Europa possess a conducting layer, implying that Europa has a global salt water ocean underneath its surface. It can not be a pure water ocean as pure water’s conductivity is too low.