Chapter 11: Our Sun: The Nearest Star

Life on Earth would be impossible without the Sun. As we explore the inhospitable environments of the rest of the Solar System, we learn to appreciate the Earth's beneficial relationship with the Sun. All human resources derive from the Sun's energy. This is true for all of our food since the Sun is at the bottom of the food chain. It's also true for our energy sources since most of them come from fossil fuels, which are derived from the photosynthesis of the Sun's radiation. Solar power is an obvious example, but even other alternative fuels such as wind power take advantage of processes powered by the Sun. The Sun also affects the Earth in more subtle ways.

Aurora borealis: the northern lights.

Those who live at far northern and southern latitudes on the Earth's surface are occasionally treated to a spectacular light show. This is called an aurora (plural aurorae). It is caused by energetic charged particles emitted from the Sun crashing into the Earth's atmosphere near the north and south magnetic poles. This produces intermittent auroras in the Arctic and in Antarctica. These auroras are also called the northern and southern lights (or aurora borealis and aurora australis, respectively).

 

The Sun as seen by the SOHO Satellite

What makes the aurorae such colorful displays? Solar flares don't just emit radiation, such as X-rays, but also streams of atomic particles, such as protons and electrons. These join and enhance the solar wind. If a flare directs material toward the Earth, the enhanced solar wind hits our atmosphere after a few days' travel. During solar flares, the surge in the solar wind can be strong enough to seriously distort the Earth's magnetic field, affecting the motions of charged particles throughout the Earth's vicinity. As the solar wind sweeps around the outer limits of the upper atmosphere, or ionosphere, it builds up voltages of 100,000 volts or more between the outer regions of the magnetic field and the atmosphere. This voltage drives some charged particles along the magnetic field lines toward the poles. These particles crash into the upper atmosphere, excite the gas atoms there and cause them to glow. This glow is powerful enough to be seen from the ground and from space. We can even observe this phenomenon on other planets — Jupiter and Saturn both have active aurorae.

 

The Sun featuring several large sunspots

In addition to bringing us the haunting lights of the aurora, the Sun may also profoundly affect Earth's climate. Researchers suspect that long-term changes in solar radiation cause substantial variations in global climate. For example, during the period from 1645 to 1715, sunspot numbers were unusually low. Tree-ring patterns and other evidence suggest that the Earth's climate altered during this period. In particular, records show that northern Europe was gripped by a "Little Ice Age" during this period. Similarly, scientists have found a correlation between low sunspot activity and severe Ethiopian droughts that occurred over a period of more than four centuries. It's not as simple as more sunspots making the Sun dimmer and fewer sunspots making the Sun brighter. The Sun's visual brightness only varies by 0.1% across the solar cycle, which is not enough to explain large changes in weather or climate. Partially motivated by the importance of these climatic effects, astronomers are observing the Sun more closely, and not just its sunspots. A spacecraft called the Solar and Heliospheric Observatory (SOHO) was launched in 1995 and it is still studying the solar wind, the Sun's interior, and its atmosphere from a vantage point between the Earth and the Sun (as well as discovering 4000 comets!). SOHO was the main source of real-time data for predicting space weather for decades, but its capabilities have been eclipsed by the Solar Dynamics Observatory (SDO), launched by NASA in 2010.

 

Every second, 1370 Joules of the Sun's energy reach every Sun-facing square meter of Earth. Although this overall rate shows variations of only a tenth of a percent per year, the variations in certain parts of the spectrum, such as X-rays or ultraviolet radiation, can be much larger. Since ultraviolet radiation is absorbed by the ozone layer, a change in the ultraviolet output of the Sun can affect atmospheric heating and structure. Small modifications of the atmosphere's heat budget can have serious effects. If the mean annual temperature shifts by only one degree the result can be dramatic changes in climate and food production. This is why signs of global warming cause such concern. The issue of solar influence on climate and agriculture is a hot research area. Solar studies are increasingly relevant to astronomy, meteorology, agriculture, and world economics. The relationship between variations in the Sun and the climate on Earth is a scientific mystery that is still being unraveled.

Scientists have found a correlation between low sunspot activity and severe Ethiopian droughts that occurred over a period of more than four centuries. Partially motivated by the importance of these climatic effects, astronomers are observing the Sun more closely - and not just its sunspots. A spacecraft called the Solar and Heliospheric Observatory (SOHO) is currently studying the solar wind, the Sun's interior, and its atmosphere from a vantage point between the Earth and the Sun.

Every second, 1370 Joules of the Sun's energy reach every Sun-facing square meter of Earth. Although this overall rate shows variations of only a tenth of a percent per year, the variations in certain parts of the spectrum, such as X-rays or ultraviolet radiation, can be much larger. Since ultraviolet radiation is absorbed by the ozone layer, a change in the ultraviolet output of the Sun can affect atmospheric heating and structure. Small modifications of the atmosphere's heat budget can have serious effects. If the mean annual temperature shifts by only one degree the result can be dramatic changes in climate and food production. This is why signs of global warming cause such concern. The issue of solar influence on climate and agriculture is attracting new research. Solar studies are increasingly relevant to astronomy, meteorology, agriculture, and world economics. The relationship between variations in the Sun and the climate on Earth is a scientific mystery that is still being unraveled.