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A Closer Look

When we examine the Sun in more detail, we find a such a variety of phenomena that we are able to view the Sun not only as a source of energy for the Earth but as a laboratory for physics unlike any that could be created on Earth. The surface of the Sun shows us a pattern of boiling gas arranged in a distinctive cellular pattern known as granulation.

  Figure 3:   The solar granulation pattern, taken in white light at the vacuum tower telescope at Sacramento Peak Observatory. The characteristic size is about 2,000 kilometers (1,250 miles). The bright component is hot, upward-moving gas and the dark component is cool, downward-moving gas. Courtesy of George Simon NSO/SP.

The so-called `effective' temperature of the Sun is 5770 Kelvin (10,420F) which corresponds to the visible band of the electromagnetic spectrum. Above the ``surface'' actually defined by this effective temperature, there extends the solar atmosphere. Curiously, even though the amount of material in the solar atmosphere decreases rapidly with distance above the surface, its temperature actually increases, up to about 2,000,000 Kelvin in the part of the atmosphere known as the corona. A primary difference between the solar surface and the atmosphere is that strong magnetic fields occupy only a small fraction of the surface, compared to filling a very large fraction of the solar atmosphere. Even though they are weak in strength, their influence on their surroundings is strong, i.e., they expand dramatically after emerging through the solar surface. The magnetic field of the Sun shapes the interplanetary magnetic field and interacts with the Earth's magnetic field to influence many parts of the terrestrial system.

The Sun emits radiation across almost the entire electromagnetic spectrum, from X-rays to radio wavelengths. This radiation originates from many different levels in the solar atmosphere and thus is influenced by the local conditions.

  Figure 4: The spectrum of the Sun, ranging from the EUV (extreme ultraviolet) to the IR (infrared). The integration of this curve over all wavelengths is defined as the total solar irradiance. Both quantities need to be understood. The dashed curve shows the expected spectrum if the Sun radiated as a black body of temperature 5770 Kelvin. The departures from this curve, particularly in the UV, are very significant.

Before proceeding with a discussion of variations known to occur on the Sun it is important to distinguish between two terms: solar weather and solar climate. These terms have a familiar meaning: weather is viewed as a short-term prediction, most reliable over the course of hours and sometimes days or weeks, whereas climate emphasizes variations across years, decades, and centuries.

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Approved by Peter Fox
Last revised: Wed May 17 11:28:21 MDT 2000