Photosphere

1. Definition and Location

• The photosphere is the Sun’s lowest visible layer and is considered its “surface” because it is the layer from which sunlight escapes into space.

• It lies just above the Sun’s convective zone and beneath the chromosphere.

2. Physical Characteristics

• Thickness: Roughly 400–500 kilometers (about 250–310 miles) thick — extremely thin compared to the Sun’s total radius (~700,000 km).

• Temperature: Around 5,500–6,000 °C (about 9,932–10,832 °F). This temperature is sufficient to emit visible light.

• Density: The gas density is about 10^-4 times that of Earth’s atmosphere at sea level — very thin plasma.

• Composition: Mainly hydrogen (~74%) and helium (~24%) with trace amounts of heavier elements.

3. Visual Appearance

• The photosphere looks granular due to convection cells called granules.

• Granules are about 1,000 km across and last for about 8 to 20 minutes. They are caused by hot plasma rising and cooler plasma sinking.

• Sunspots appear on the photosphere as dark patches — these are cooler regions caused by intense magnetic fields that inhibit convection.

4. Radiation and Energy

• The photosphere emits the majority of the Sun’s visible light and infrared radiation, which provides the energy that sustains life on Earth.

• The light from the photosphere takes about 8 minutes and 20 seconds to reach Earth.

5. Phenomena

• Sunspots: Cooler, darker spots, sometimes large enough to be visible from Earth without a telescope.

• Faculae: Bright areas surrounding sunspots, hotter and more luminous.

• The photosphere's magnetic activity influences solar flares and coronal mass ejections originating above.

Chromosphere

1. Definition and Location

• The chromosphere lies directly above the photosphere and beneath the corona.

• Its name means “color sphere” because it appears reddish due to strong hydrogen alpha emissions during solar eclipses.

2. Physical Characteristics

• Thickness: Roughly 2,000 to 3,000 kilometers thick.

• Temperature: Starts at about 4,500 °C near the photosphere and rises sharply to around 20,000 °C near the top (transitioning to the corona).

• Composition: Mainly hydrogen gas, ionized at higher levels, producing emission lines especially in hydrogen-alpha (red) light.

3. Visual Appearance and Emission

• The chromosphere glows with a reddish light, which is mostly emission from hydrogen atoms.

• It’s usually not visible except during total solar eclipses or with special instruments like spectrometers and filters (hydrogen-alpha filters).

4. Structure and Dynamics

• The chromosphere is highly dynamic and contains features like:

  • Spicules: Thin jets of rising gas, lasting a few minutes, extending up to 10,000 km.

  • Solar Prominences: Large loops or arcs of glowing gas, held by magnetic fields, extending thousands of kilometers into the corona.

  • Filaments: Prominences viewed against the solar disk, appearing dark.

5. Temperature Gradient and Heating

• Unlike the photosphere, the temperature in the chromosphere increases with altitude, a phenomenon not fully understood but believed to be related to magnetic wave heating and magnetic reconnection.

• The sharp temperature rise leads to ionization of hydrogen and the emission of light.

6. Role in Solar Activity

• The chromosphere acts as an interface layer where much of the Sun’s magnetic activity manifests.

• Solar flares, prominences, and other solar phenomena are rooted in this layer.

• It plays a key role in transporting energy from the photosphere to the corona.

Summary Table