Outside The

Above the photosphere is an almost translucent region known as the chromosphere due to its scarlet coloration. This color results from its hydrogen content, which emits visible radiation largely at a specific red wavelength. The chromosphere is quite thin: a few thousand miles wide, which is large on the scale of a planet, but less than 1 percent of the solar diameter.

Penetrating the chromosphere are spikes of gas that rapidly jet upwards and then fall back again; these are termed spicules. Larger ejections of mass are called prominences, as seen in Figures 1-3 to 1-5. Such prominences provide one of the highlights of a total eclipse.

Another vivid feature seen in an eclipse is the corona (or aureola). This is a rarified region of extremely hot gas, stretching millions of miles out into space, consisting of ionized atoms speeding away from the Sun. Like the chromosphere, the corona can only be seen by eye during a total eclipse, although there are other technical ways to observe it between times. One of the great puzzles of solar physics is how the corona is heated to such a high temperature—over a million degrees Celsius (2 million degrees Fahrenheit)—given that the underlying regions are much cooler.

Flowing outwards from the Sun is a continuous stream of particles known as the solar wind. These particles zip by the Earth at a speed of about 300 miles per second. The Sun has an intense but dynamically changing magnetic field that is carried outwards by the solar wind. By dint of their own magnetic fields, the planets interact with this solar wind, producing effects both beautiful, like the auroras, and disruptive, such as interference with radio communications, navigation systems, TV and cell phone services, and manned space walks. The density and other characteristics of the solar wind are quite variable. For example, there are gradual ebbs and flows with the 11-year solar cycle, but also spasmodic solar flares may be seen, associated with ejections of large amounts of matter into the solar wind, intersecting the Earth a day or two later.

The photographs in Figure 5-2, which were obtained using an instrument known as a coronagraph (see below), show the corona and solar wind, both of which are highly uneven. Coronal streamers are the most obvious features in those images, their shapes varying over the several hours of the data collection.

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