15.3 Solar Activity above the Photosphere

Manifestations of the Solar Activity Cycle

• The solar activity cycle drives dramatic changes beyond just sunspots, affecting the chromosphere and corona.

• Observations:

  • Chromosphere: Studied using emission lines from elements like hydrogen and calcium.

  • Corona: Investigated through X-rays, extreme ultraviolet, and other high-energy wavelengths.

Plages

• Description: Bright "clouds" found in the chromosphere, typically surrounding sunspots.

• Observation: Routinely photographed using filters that transmit light at specific wavelengths corresponding to hydrogen and calcium emission lines.

• Characteristics:

  • Regions within the chromosphere exhibiting higher temperature and density compared to their surroundings.

  • While containing all solar elements, the spectral lines of hydrogen and calcium from these regions are distinct and easily observable.

  • (Figure 15.18 illustrates these bright, cloud-like regions observed in singly ionized calcium light.)

Prominences

• Description: Spectacular phenomena that originate near sunspots and extend high into the corona.

• Observation: Often appear as red features rising above the eclipsed Sun during an eclipse.

• Types and Characteristics:

  • Quiescent Prominences: Graceful, stable loops of plasma (ionized gas) that can persist for many hours or even days.

  • Eruptive Prominences: Send matter rapidly upward into the corona.

  • Surge Prominences: The most active type, capable of moving at speeds up to $1300\text{ kilometers per second}$ (almost $3\text{ million miles per hour}$).

• Scale: Some eruptive prominences have been observed reaching heights exceeding $1\text{ million kilometers}$ above the photosphere, an immense scale where Earth would be dwarfed.

• Composition: Consist of relatively cool (e.g., $60,000\text{ K}$), dense gas suspended within the much hotter corona.

  • (Figure 15.19 provides examples of eruptive prominences and illustrates their structure and temperature profiles in ultraviolet light.)

Solar Flares

• Description: The most violent and rapid eruptive events occurring on the surface of the Sun.

• Duration and Energy Release:

  • Typical flares last $5\text{ to }10\text{ minutes}$.

  • Release energy equivalent to approximately a million hydrogen bombs.

  • The largest flares can last several hours, emitting enough energy to power the entire United States at its current electrical consumption rate for $100,000\text{ years}$.

• Frequency: More frequent near sunspot maximum, with small flares several times daily and major ones every few weeks.

• Observation: While often observed in the red light of hydrogen, this visible emission represents only a minute fraction of the total energy released.

• Temperature and Radiation: At the moment of explosion, flare material heats to temperatures as high as $10\text{ million K}$, resulting in a flood of X-ray and ultraviolet radiation.

• Mechanism: Occur when opposing magnetic fields within the solar corona interact, destroy each other, and release immense amounts of electromagnetic radiation.

  • (Figure 15.20 depicts a solar flare observed by the Solar Dynamics Observatory.)

Coronal Mass Ejections (CMEs)

• Description: Massive expulsions of coronal material, primarily protons and electrons, into interplanetary space.

• Association: Often occur in conjunction with solar flares.

• Speed: Material is ejected at high speeds, typically ranging from $500\text{ to }1000\text{ kilometers per second}$.

• Impact: CMEs can have significant effects on Earth, influencing space weather.

  • (Figure 15.21 illustrates the sequence of a flare leading to a coronal mass ejection expanding into the solar system.)

Solar Active Regions

• Definition: Specific areas on the Sun where sunspots, flares, and bright regions in the chromosphere and corona are observed to occur together in time and space.

• Spatial Relationship: These phenomena are located at similar longitudes and latitudes but manifest at different heights within the Sun's atmosphere.

• Solar Cycle Correlation: The occurrence and intensity of active regions vary in phase with the sunspot cycle.

  • Flares, for example, are more probable during sunspot maximum, a period when the corona is also more prominent.

• Underlying Cause: Active regions are fundamentally associated with strong magnetic fields.

  • (Figures 15.22 and 15.23 demonstrate how active regions, observed across different wavelengths and through magnetograms, intensify and diminish during the solar cycle and are linked to strong magnetic fields below.)