15.2 Solar Cycle

The Ever-Changing Sun

• Before the telescope, the Sun was thought to be a perfect, unchanging sphere.

• We now know the Sun is in a constant state of change:

  • Its surface is a "seething, bubbling cauldron of hot gas."

  • Darker, cooler areas (sunspots) appear and disappear.

  • Vast plumes of gas erupt into the chromosphere and corona.

  • Giant explosions occasionally send charged particles and energy toward Earth, potentially causing power outages.

Sunspots

• Definition: Large, dark features on the Sun's surface caused by increased magnetic activity.

• Appearance: Look darker because they are cooler than surrounding regions.

  • Sunspot temperature: approximately $3800 K$

  • Surrounding bright regions (photosphere) temperature: approximately $5800 K$

• Visibility: Occasionally large enough to be seen with the unaided eye (with proper safety filters or natural haze/mist).

• Structure: If a spot lasts, it typically consists of two parts:

  • Umbra: Inner, darker core.

  • Penumbra: Surrounding, less dark region.

• Size and Grouping:

  • Many are larger than Earth; some reach diameters over $140,000$ kilometers.

  • Frequently occur in groups of $2$ to $20$ or more, with complex groups having over $100$ spots.

• Movement and Rotation (Galileo's Discovery):

  • Sunspots are not fixed; they drift slowly.

  • Galileo, in $1612$, used sunspot motions to demonstrate that the Sun rotates on its axis.

  • Rotation Period: Approximately $1$ month.

  • Direction: West-to-east, similar to planetary orbital motions.

• Differential Rotation: The Sun is a gas, so it doesn't rotate rigidly.

  • Rotation speed varies by latitude:

  • Equator: about $25$ days

  • Latitude $40^ ext{o}$: about $28$ days

  • Latitude $80^ ext{o}$: about $36$ days

The Sunspot Cycle

• Learning Objective: Describe the sunspot cycle and, more generally, the solar cycle.

• Discovery: Heinrich Schwabe (German pharmacist and amateur astronomer) discovered the sunspot cycle between $1826$ and $1850$ by observing sunspots daily.

• Cycle Periodicity: The total number of sunspots visible varies systematically in cycles.

  • Average interval between maxima: $11$ years.

  • Range of intervals: From $9$ to $14$ years.

• Phases of the Cycle:

  • Sunspot Maxima: Periods when more than $100$ spots can often be seen at once (covering less than $0.5\%$ of the Sun's surface).

  • Sunspot Minima: Periods when sometimes no spots are visible.

• Current Cycle: The Sun's activity is expected to reach its next maximum in the $2024$–$2025$ timeframe.

Magnetism and the Solar Cycle

• Learning Objective: Explain how magnetism is the source of solar activity.

• Core Mechanism: The Sun's changing magnetic field is the fundamental driver of all solar activity.

• Measuring Solar Magnetic Fields (Zeeman Effect):

  • Zeeman Effect Definition: In the presence of a strong magnetic field, atomic energy levels split into several closely spaced levels. Spectral lines formed in a magnetic field appear as a series of closely spaced lines, not a single one.

  • Application: Measurements of the Zeeman effect in sunspot regions reveal strong magnetic fields.

• Magnetic Polarity of Sunspots:

  • Sunspots typically appear in pairs or groups with opposite magnetic polarities (north-seeking and south-seeking poles).

  • During a given $11$-year cycle, the leading spots in the Northern Hemisphere tend to have one polarity, while those in the Southern Hemisphere have the opposite polarity.

  • Polarity Reversal: In the next $11$-year cycle, the polarity of the leading spots in each hemisphere reverses.

  • Full Magnetic Cycle: Strictly speaking, the sunspot cycle doesn't repeat its magnetic polarity until two $11$-year cycles (a $22$-year cycle) have passed.

• Magnetogram: A visual representation used to show the magnetic polarity of sunspots and the Sun's magnetic field (e.g., black areas for magnetism pointing toward the core, white for pointing away).

• The Solar Dynamo:

  • Mechanism: A "machine" within the Sun that converts the kinetic energy of turbulent, ionized gas into electricity (moving electrons), which in turn generates magnetic fields.

  • Location: Most researchers believe it's in the convection zone or the interface layer between the convection zone and the radiative zone.

• Magnetic Field Dynamics and Sunspot Formation:

  • Generation: Differential rotation and convection just below the surface twist and distort the magnetic fields.

  • Evolution: Fields grow stronger, then decay, and regenerate with opposite polarity approximately every $11$ years.

  • Sunspot Creation: As fields strengthen near solar maximum, they flow from the interior to the surface in the form of loops. Where a large loop emerges, it creates regions of sunspot activity.

  • Polarity Explanation: This magnetic loop idea naturally explains why leading and trailing sunspots in an active region have opposite polarities, corresponding to the two ends of a loop.

• Why Sunspots are Cooler and Darker:

  • Inhibition of Convection: The strong forces produced by the magnetic fields resist the upward motions of hot gas columns that transfer heat via convection.

  • Cooling Effect: By inhibiting convection, magnetic fields prevent heat from reaching the surface in these regions, allowing them to cool.

  • Appearance: Cooler areas appear darker in contrast to the hotter, brighter surrounding photosphere.

• Unresolved Questions in Solar Physics:

  • Why are the magnetic fields so large?

  • Why does the field's polarity in each hemisphere flip every cycle?

  • Why does the length of the solar cycle vary?

  • What causes phenomena like the Maunder Minimum (a period of greatly reduced sunspot activity)?