Space Physics Final

Defintion of plasma

gas of charged particles behaving quasineutral. Hot, and highly ionized gas.

3 Plasma Criterion

1) Lambda_d << L

2) N_de >> 1

3) w_pe*Tau_n >> 1

Solar Wind Basics

The Sun emits a supersonic plasma (~500 km/s) into space: this is the solar wind.

- Composed mainly of electrons and protons, with ~5% helium ions.

- Originates from the expanding solar corona.

- Plasma is highly conductive, so the solar magnetic field is "frozen-in" and carried outward.

Near Earth:

Electron density ≈ 5 cm⁻³

Temperature ≈ 10⁵ K

Magnetic field ≈ 5 nT

The solar wind is a stream of charged particles released from the solar corona, carrying the solar magnetic field into space.

Solarwind Earth Basics (bow shock, magnetosheath)

Earth's dipolar magnetic field resists solar wind penetration.

The solar wind is deflected and slowed, forming a bow shock due to its supersonic speed.

Kinetic energy → thermal energy at the bow shock.

Behind the shock lies the magnetosheath:

Hotter and denser than solar wind

Stronger magnetic field

Contains subsonic thermalized plasma

Magnetosphere & Magnetopause

• Magnetosphere: cavity formed by Earth's magnetic field in the solar wind.

• Magnetopause: boundary between magnetosphere and solar wind.

• Solar wind can't penetrate easily due to frozen-in magnetic field lines.

• Frontside: compressed by solar wind pressure.

• Nightside: stretched into a long magnetotail (extends beyond Moon).

Magnetospheric Plasma Sources

• Plasma mainly electrons and protons.

• Sources: solar wind + terrestrial ionosphere.

• Minor ions:

  • He+, O+ from ionosphere

  • He++ from solar wind

• Plasma is not uniformly distributed—forms distinct regions with varying densities and temperatures.

Radiation Belts

• Located along dipolar field lines between ~2–6 Earth radii (RE).

• Filled with energetic electrons and ions oscillating between hemispheres.

• Typical values:

  • Electron density: ~1 cm⁻³

  • Temperature: ~5 × 10⁷ K

  • Magnetic field strength: 100–1000 nT

Magnetotail and Plasma Sheet

• Magnetotail: extended nightside region of magnetosphere.

• Plasma sheet:

  • ~10 RE thick

  • Concentrated near tail’s midplane

  • Connects to high-latitude auroral ionosphere

  • Average:

    • Electron density ≈ 0.5 cm⁻³

    • Temperature: moderate (unspecified in your text)

Magnetotail Lobes

• Outer part of the magnetotail.

• Contains highly rarified plasma:

  • Electron density: ~10⁻² cm⁻³

  • Temperature: ~5 × 10⁵ K

  • Magnetic field: ~30 nT

Ionosphere Basics

• Formed by solar UV ionization of Earth's upper atmosphere.

• Exists above ~80 km, where collisions are too rare for rapid recombination.

• Creates a permanent ionized region.

• Mid-latitude ionosphere:

  • Electron density ≈ 10⁵ cm⁻³

  • Temperature ≈ 10³ K

  • Magnetic field strength ≈ 10⁴ nT

Plasmasphere

• A torus-shaped region inside the radiation belt.

• Contains cool, dense plasma of ionospheric origin.

• Corotates with the Earth.

• In the equatorial plane:

  • Extends to ~4 Earth radii (RE)

  • Beyond that, density sharply drops at the plasmapause.

  • Plasmasphere values:

    • nₑ ≈ 5 × 10² cm⁻³

    • T ≈ 5 × 10³ K

Aurora Formation

• Plasma sheet electrons follow magnetic field lines down to ionosphere.

• They collide with neutral particles, causing ionization and light emission.

• This creates the aurora (polar light).

• Seen in the auroral oval:

  • Region where field lines connect to the plasma sheet.

  • Polar cap lies inside the oval and connects to the tail lobe field lines.

Plasma Motion & Currents

• Plasmas often move under external forces.

• When ions and electrons move differently, electric currents are generated.

• These currents:

  • Transport charge, mass, momentum, and energy

  • Create magnetic fields that can distort existing ones

Magnetospheric Currents

The magnetosphere's shape is maintained by large-scale currents:

• Magnetopause current: flows across the dayside boundary, compressing the field

• Tail current: flows along the magnetotail surface

• Neutral sheet current: central plasma sheet, contributes to a “θ-like” current system viewed along Earth–Sun line

Ring Current

• Encircles Earth westward, at several Earth radii.

• Formed by radiation belt particles drifting around Earth:

  • Protons drift west

  • Electrons drift east → net charge transport

• Influences inner magnetospheric field configuration

Ionospheric Currents

• Occur at 100–150 km altitudes in the conducting ionosphere.

• Key systems:

  • Auroral electrojets: inside auroral oval

  • Sq (solar quiet) currents: dayside mid-latitudes

  • Equatorial electrojet: near magnetic equator

Field-Aligned Currents

• Flow along magnetic field lines

• Link magnetospheric currents to those in the polar ionosphere

• Carried mainly by electrons

• Crucial for energy & momentum exchange between regions

ExB Drift

A drift effect caused by the electric field and magnetic field, resulting in charged particles moving perpendicular to both fields. It plays a significant role in the dynamics of plasma in space environments.

No net current, e- and ions in same direction