PHYC 2452: Intro to Stellar & Galactic Astrophysics - Detailed Lecture Notes

The Chromosphere & Corona

The Chromosphere

  • Lies just above the photosphere, with temperatures ranging from 10,000 K to 20,000 K.

  • Named for the visible jets of red gas (prominences) seen during solar eclipses, due to its radiation in the Hα line of hydrogen at 6563 Å.

Corona

  • Above the chromosphere, temperatures rise sharply, reaching millions of degrees K.

  • The corona feeds into a hot, tenuous solar wind.

  • Despite the high temperature, energy is low due to low gas densities in these outer layers.

Temperature Profile of Solar Atmosphere

  • Photosphere: 5,700 °C

  • Chromosphere: ~10,000 °C - 20,000 °C

  • Transition Region: 30,000 K - 50,000 K

  • Corona: Millions of degrees K.

Heating Mechanisms of the Chromosphere & Corona

Magnetic Fields

  • Chromospheric and coronal gas heating is attributed to magnetic field interactions with current loops in the ionized gas (plasma).

  • The structure of the chromosphere is dynamic and three-dimensional, contrary to simplistic diagrams.

Plasma Dynamics

  • Partial ionization of gas above the photosphere leads to dominance of magnetic field structures.

  • Convective motions in the Sun's interior wind up the magnetic fields, creating strong, braided structures.

Sunspots

  • Formed where magnetic fields suppress convection, leading to cooler areas (around 4,500 K).

  • Sunspots consist of regions with opposite magnetic polarity, linked by magnetic loops extending to the corona.

  • Current flow along these loops causes resistive heating, raising the temperature of the surrounding gas.

CME and Solar Flares

Coronal Mass Ejections (CMEs)

  • Dynamic processes associated with the solar corona include CMEs, explosive ejections of hot, ionized gas.

  • CMEs originate from magnetic flux loops that connect sunspots, twisting and forming intricate magnetic structures.

Mechanisms of Energy Release

  • During reconnection events, magnetic loops short-circuit, causing explosive heating of gases and ejecting material.

  • This process contributes to both solar flares and CMEs.

Interaction with Earth

Effects of Solar Wind

  • The solar wind is a stream of charged particles that have implications for Earth and its atmosphere.

  • Interaction with Earth's magnetic field can lead to auroras.

  • A strong CME can disrupt electrical systems and communications on Earth.

Auroras

  • Result from energetic particles from the solar wind hitting Earth's atmosphere, causing photonic emissions from oxygen and nitrogen molecules.

  • Typically observed in polar regions where they form an auroral oval.

Formation and Evolution of Earth

Accretion and Early Earth

  • Earth formed ~4.6 billion years ago from an accretion disk surrounding a young protostar.

  • Details of early Earth include a molten surface and a carbon dioxide-rich atmosphere.

  • The Hadean Era marks Earth's initial formation stage, characterized by extreme thermal conditions.

Moon Formation

  • The large impact theory suggests a Mars-sized body, Theia, collided with early Earth, leading to the formation of the Moon.

  • This impact explains several characteristics of the Earth-Moon system: high angular momentum, Moon's lower density, and similar isotopic compositions between Earth and Moon materials.

Summary of Key Concepts

  • Chromosphere: a thin layer above the photosphere, characterized by high temperatures and dynamic magnetic interactions.

  • Corona: the Sun’s outer atmosphere, very hot but low in density, driving solar wind and CMEs.

  • Solar Wind: influences space weather, protecting Earth while also posing risks during solar storms.

  • Early Earth: distinguished by high temperatures and volcanic activity, leading to diverse geological formations and the eventual formation of the Moon through impact.