Lecture 10: Solar Activity Study Notes

Lecture 10: Solar Activity

Overview

  • Lecture by: Shelley Wright

  • Course: ASTR 01: Stars and Black Holes

  • Date: October 30, 2025

Course Announcements

  • Homework #4: Due Friday at midnight.

  • Discussion Section: This week will review topics and problems from Homework #4.

  • Research Choice Selection: Due Sunday, November 9th via Canvas. Review Research Guidelines document posted on Canvas.

Lecture Outline

  • Sunspots (Chapter 14)

  • Coronal Mass Ejections (Chapter 14)

  • Space Weather (Chapter 14)

1. Nuclear Fusion in the Sun’s Core

  • The Sun generates energy through nuclear fusion, where four hydrogen nuclei (protons) combine to form one helium nucleus.

    • Inputs and Outputs:

    • IN: 4 protons

    • OUT: 1 helium nucleus (24extHe^{4}_{2} ext{He}), 2 gamma rays, 2 positrons, 2 neutrinos.

    • Total mass is reduced by approximately 0.7extextperthousand0.7 ext{ extperthousand}, resulting in energy production.

1.1 Neutrinos

  • Neutrinos produced during fusion escape directly from the solar core.

  • Key Observation: Observing solar neutrinos provides information about nuclear fusion processes.

  • Neutrinos travel near the speed of light, arriving on Earth 8 minutes post-fusion event.

  • Speed of Neutrinos: Approximately 0.999999999imesc0.999999999 imes c, where cc is the speed of light.

2. Solar Activity Defined

  • Analogous to Weather: Solar activity is likened to weather on Earth and includes:

    • Sunspots

    • Solar Flares

    • Solar Prominences

  • All these phenomena are connected to the Sun's magnetic fields.

3. Solar Magnetic Fields

  • The Sun’s magnetic field consists of a complex structure resembling a dipolar field.

  • Generation of Magnetic Fields:

    • Magnetic fields are produced by moving electric currents.

  • Regions of the Sun:

    • Core

    • Radiative Zone

    • Tachocline

    • Convective Zone

    • Photosphere

    • Chromosphere

    • Corona

3.1 Magnetic Activity

  • Magnetic field lines indicate the direction of compass needles.

  • Closer lines denote stronger magnetic fields.

  • Charged particles spiral along these magnetic field lines.

4. Sunspots

  • Characteristics of Sunspots:

    • Cooler than the surrounding solar surface with temperatures around 4000extK4000 ext{ K} (compared to 5800extK5800 ext{ K} elsewhere).

    • Associated with strong magnetic fields.

  • Structure:

    • Umbra: The darker core of the sunspot.

    • Penumbra: The lighter surrounding area.

5. Solar Flares

  • Solar flares are explosive releases of energy that emit bursts of X-rays and charged particles.

  • Classification: Solar flares are categorized by severity on a scale of A, B, C, M, and X.

6. Coronal Mass Ejections (CMEs)

  • CMEs release significant amounts of energetic charged particles into the solar system, typically containing 1013−1014extkg10^{13}-10^{14} ext{ kg} of material.

  • Impact: CMEs can disrupt electrical power grids, satellites, and pose threats to astronauts in space.

7. Historical Context of Solar Activity

  • Carrington Event (1859): A large CME event observed, recognized for its significant impact on Earth’s magnetic environment.

  • Long-term Solar Cycle Observations: Sunspots have been recorded for nearly 400 years; patterns include regular cycles with deviations.

  • Maunder Minimum: Coincided with the “Little Ice Age.”

7.1 Solar Cycle

  • The number of sunspots varies in an approximately 11-year cycle.

  • Current Cycle (2025): Solar Cycle 25 is at its peak, marked by increased sunspots and solar activity.

7.2 Sunspot Migration

  • Sunspots migrate from the poles towards the equator throughout the cycle due to the Sun’s differential rotation (the equator spins faster).

  • This migration leads to a twisting of magnetic lines and an eventual flip in magnetic polarity.

7.3 Climatic Implications

  • Despite the cyclical nature of sunspots, total sunlight has remained approximately constant while Earth's temperature is warming, indicating that other factors are at play in climate change.

8. Current Solar Research Missions

  • Solar and Heliospheric Observatory (SOHO)

  • Solar Terrestrial Relations Observatory (STEREO)

  • Solar Dynamics Observatory (SDO)

  • Interface Region Imaging Spectrograph (IRIS)

  • Parker Solar Probe: Currently traveling rapidly towards the Sun, has a trajectory that brings it closer than Mercury.

9. Other Observations and Measurements

  • Luminosity: The total power output of the Sun, measured in watts.

  • Apparent Brightness: The amount of starlight reaching an observer on Earth, based on distance.

  • Inverse Square Law: The relationship of brightness and distance, reflecting how luminosity is perceived based on the observer's distance from the star.

10. Conclusion

  • Space weather, driven by solar activity, can have far-reaching implications for modern technology and Earth's climatic conditions, highlighting the need for continued observation and research into solar phenomena.

Questions for Discussion

  • Topics for further exploration and clarification.