15.4 Space Weather

What is Space Weather and How it Affects Earth

  • Learning Objective: Explain what space weather is and how it affects Earth.

  • Space weather refers to the conditions in space caused by particles from the Sun, such as the solar wind and coronal mass ejections (CMEs), reaching Earth's magnetosphere.

  • Scientists face the challenge of predicting the effects of solar storms on Earth.

  • When space weather turns stormy, it puts our technology at risk, including thousands of satellites, the International Space Station, cell phones, GPS, wireless communication, and dependable electrical power.

  • Governments are investing significantly in predicting solar storms and their impacts on Earth.

Some History: The Carrington Event (1859)

  • Space weather effects were first recognized in 1859, known as the Carrington Event.

  • Amateur astronomers, including Richard Carrington, independently observed a solar flare in early September 1859.

  • A day or two later, a major solar storm reached Earth's magnetic field, overloading it with charged particles.

  • This resulted in intense aurora activity, with northern lights visible as far south as Hawaii and the Caribbean.

  • The storm severely affected the newly established telegraph system, causing sparks from wires and machines.

  • This event led to the scientific understanding of a connection between solar activity and impacts on Earth, marking the beginning of space weather research.

Sources of Space Weather

  • Three main solar phenomena account for most space weather:

  1. Coronal Holes:

    • Areas where the solar wind flows freely away from the Sun, unhindered by magnetic fields.

    • When this solar wind reaches Earth, it causes the magnetosphere to contract and expand.

    • Results in usually mild electromagnetic disturbances on Earth.

  2. Solar Flares:

    • Shower Earth's upper atmosphere with X-rays, energetic particles, and intense ultraviolet radiation.

    • X-rays and UV radiation ionize atoms in the upper atmosphere, freeing electrons.

    • These freed electrons can build up a static charge on spacecraft surfaces.

    • Discharge of this static charge can damage spacecraft electronics.

  3. Coronal Mass Ejections (CMEs):

    • Erupting bubbles of tens of millions of tons of gas blown from the Sun into space.

    • Reach Earth a few days after leaving the Sun.

    • Heat the ionosphere, causing it to expand further into space.

    • Increases friction between the atmosphere and spacecraft, dragging satellites to lower altitudes.

    • During a strong flare and CME in March 1989, 11,000 out of 19,000 orbiting objects were temporarily lost due to orbital changes.

    • During solar maximum, some satellites are destroyed by atmospheric friction; the Hubble Space Telescope and International Space Station require reboosts.

Solar Storm Damage on Earth

  • When a CME reaches Earth, it distorts Earth's magnetic field, inducing electrical currents that accelerate electrons to high speeds.

    • These “killer electrons” can penetrate and destroy satellite electronics, permanently disabling communication satellites.

  • Disturbances in Earth’s magnetic field can disrupt communications, especially cell phone and wireless systems. These disruptions can occur several times a year during solar maximum.

  • Changes in Earth’s magnetic field due to CMEs can cause surges in power lines, burning out transformers and leading to major power outages.

    • For example, in 1989, parts of Montreal and Quebec Province experienced a 9-hour power outage due to a major solar storm.

  • CMEs can distort signals from satellites, reducing the accuracy of GPS-derived positions.

    • This has forced the Federal Aviation Administration to restrict flights due to GPS inaccuracies.

  • Solar storms also expose astronauts, passengers in high-flying airplanes, and people on Earth’s surface to increased radiation.

    • A single ill-timed solar outburst can end an astronaut’s career; protecting astronauts from high-energy radiation is a major challenge for Mars exploration.

Predicting Space Weather

  • Advance warning of solar storms would minimize disruptive effects:

    • Power networks could run at reduced capacity to absorb surges.

    • Communications networks could prepare for malfunctions and have backup plans.

    • Spacewalks could be timed to avoid major solar outbursts.

  • Scientists are working to predict when and where flares and CMEs will occur, and their severity.

    • Current predictive capability is still poor; the main warning comes from direct observation of CMEs and flares.

  • A CME travels outward at about 500 ext{ kilometers per second}, providing several days' warning at Earth's distance.

    • Example 15.1 The Timing of Solar Events

      • Equation: ext{distance} = ext{velocity} \times \text{time} or D = v \times t

      • Rearranging for time: T = D/v

      • If the average solar wind speed is 400 ext{ km/s} and the distance to the Sun is 1.496 \times 10^8 ext{ km}, the time for particles to reach Earth is:
        T = \frac{1.496 \times 10^8 ext{ km}}{400 ext{ km/s}} = 3.74 \times 10^5 ext{ s}
        3.74 \times 10^5 ext{ s} \div (60 ext{ s/min} \times 60 ext{ min/h} \times 24 ext{ h/d}) \approx 4.3 ext{ days}

  • The severity of impact depends on the CME's magnetic field orientation, which can only be measured by a satellite about an hour upstream from Earth.

  • Space weather predictions are available online, with week-ahead outlooks, bulletins for public interest, and warnings/alerts for imminent or ongoing events.

  • Future efforts include launching more satellites and developing models to predict CME impacts, aiming for predictive capabilities similar to terrestrial weather forecasting, though the largest storms remain challenging to predict.

Earth's Climate and the Sunspot Cycle

  • The Sun's energy output varies slightly (less than 1%) over centuries, and the number of sunspots follows an approximate 11-year cycle.

  • Maunder Minimum (1645-1715):

    • An interval of significantly low sunspot numbers, first noted by Gustav Spӧrer and E. W. Maunder.

    • Coincided with the “Little Ice Age” in Europe, characterized by exceptionally low temperatures (e.g., River Thames froze 11 times, low summer temperatures, poor harvests).

    • Auroral activity was abnormally low during this period.

  • Little Maunder Minimum: A period of somewhat lower sunspot numbers during the early 19th century.

  • Historical accounts of Norse exploration and colonization of Iceland and Greenland, and visits to North America, show a correlation with periods of high and low solar activity, suggesting potential climate impacts.

  • Debate on Causation:

    • Scientists are cautious about directly attributing the Little Ice Age solely to low sunspot numbers or variations in the Sun's energy output.

    • No satisfactory model explains how such small solar variations might cause significant global temperature changes (Sun's total energy output varies by only ~0.1% during a solar cycle).

    • Alternative possibility: Volcanic activity, which ejects aerosols (like SO_2 from the 1991 Pinatubo eruption) that reflect sunlight and can reduce global temperatures (e.g., by 0.4 \text{ °C}).

  • Other potential solar effects:

    • Extreme ultraviolet radiation is 10 times higher at solar maximum, affecting the chemistry and temperature structure of the upper atmosphere.

    • This could lead to a reduction in the ozone layer, cooling of the stratosphere near the poles, and changes in wind circulation patterns and storm tracks.

    • Some recent evidence suggests regional rainfall may correlate with solar activity better than global temperature.

  • Important Idea: Solar variability is not the cause of the global warming observed during the past 50 years, as consistently shown by climate change data and models.