Solar Structure and Activity: Atmosphere Layers and Magnetic Cycles

The Sun's Structure and Activity

The Solar Atmosphere: Layers and Characteristics

The Sun's atmosphere consists of three primary layers: the Photosphere, the Chromosphere, and the Corona.

Photosphere

• Part of the Sun we see: Covers the distance over which the Sun changes from transparent to opaque.
• Thin layer of gas, 300–400 km deep.
• Below this layer, the gas is denser and hotter, but light does not escape.
• Produces an absorption line spectrum.

Chromosphere

• Lies immediately above the photosphere.
• Discovered during eclipses in the 17th century.
• Approximately 2,000–3,000 km thick.
• Density is about 1/10,000 that of the photosphere.
• Produces an emission line spectrum.
• Its red color results from hydrogen (H) emission (n=3 to n=2 transition line).

Corona

• Observed during total solar eclipses (normally overpowered by light from the photosphere).
• Extends millions of kilometers above the photosphere, thinning out to form the solar wind.
• Extremely hot (millions of degrees), but low density. This means particles are fast-moving but carry low heat content.
• High temperature leads to X-ray emission.

Atmospheric Activity, Sunspots, and Magnetic Cycles

The Sun's magnetic field drives virtually all the structure and activity observed in its atmosphere.

Sunspots and the Maunder Minimum

The best-known features on the surface of the Sun are sunspots—relatively dark blemishes in the solar photosphere that appear and disappear over time. These spots are regions where material is trapped at the surface by intense magnetic field lines.

The Maunder Minimum (1645–1715) was a period corresponding to a significant drop-off in sunspots, which coincided with unusually cold temperatures observed in Europe.

The Sunspot and Magnetic Cycles

The number and distribution of sunspots change over time in a pronounced 11-year pattern called the sunspot cycle.

In the early 20th century, solar astronomer George Ellery Hale (1868–1938) demonstrated that the 11-year sunspot cycle is actually half of a larger 22-year magnetic cycle, during which the direction of the Sun’s magnetic field reverses.

Key characteristics of the magnetic cycle:

• In one 11-year cycle, the leading sunspot in each pair tends to be a North magnetic pole, while the trailing sunspot tends to be a South magnetic pole.
• In the subsequent 11-year cycle, this polarity is reversed: the leading spot in each pair becomes a South magnetic pole.
• The transition between these two magnetic polarities occurs near the peak of each sunspot cycle.

The predominant theory explaining this magnetic cycle involves a dynamo mechanism operating in the interior of the Sun, similar to the dynamos that generate the magnetic fields of planets.

Solar Radiation and Global Temperature Trends

Regarding the potential solar contribution to global warming, scientific data shows that in recent years, there have been opposite trends in solar radiation levels and global temperature increases, suggesting solar activity is not the primary driver of recent warming.