Topics : Sun

A Prodigious Celestial Furnace

If not for the Sun, we wouldn’t have any weather. Sunlight provides the energy that drives atmospheric circulation and creates such weather events as precipitation and wind. The Sun also has profound impacts on our upper atmosphere, shaping the magnetic field that surrounds our planet. Brief ejections of mass and energy from the Sun sometimes set off electrified disturbances in this magnetic field that affect sensitive communications and energy systems worldwide. Solar researchers at NCAR and elsewhere use an array of instruments to determine the complex processes in the Sun's interior and atmosphere, as well as its impacts on Earth’s atmosphere.

The Sun is often described as an average star, but it is actually a prodigious and very stable producer of energy. Earth, at a distance of 93 million miles (150 million kilometers), receives less than a billionth of the Sun’s energy output. But that’s enough to warm the planet, enable photosynthesis in plants, and sustain our web of life.

Is the Sun’s output always the same? Scientists talk about the solar constant, which amounts to 1,368 watts of energy per square meter (9 square feet) at the outer edge of the Earth’s atmosphere. (An average-sized person positioned there, facing the Sun, would intercept the energy equivalent of more than 13 100-watt light bulbs.)

In fact, however, precise satellite measurements show the Sun’s total energy output rises and falls by as much as 0.1 percent over the 11-year solar cycle. This total output is dominated by visible, near-ultraviolet, and near-infrared radiation, but radiation in the extreme ultraviolet can vary a hundred times more over the solar cycle. It is this extreme ultraviolet radiation that controls Earth’s upper atmosphere, and NCAR scientists use observations of its variability, along with theoretical atomic physics and computer modeling, to study its effects on the upper atmosphere.

As powerful and stable as the Sun is, it won’t be around forever in its current form. In another several billion years, it will transform into a red giant star. Its surface will likely expand and encompass the inner planets, including Earth. Eventually, it may contract into a relatively small, cool star known as a white dwarf. But, thankfully, that’s all far in the future.

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