6. The Extended Solar Atmosphere

    • The visible disk of the Sun is enveloped by a million-degree corona.

    • The corona is seen at the edge of the Sun during a total solar eclipse.

    • The high temperature of the Sun’s corona is inferred from forbidden emission lines and from its x-ray radiation.

    • Dark sunspots seen on the visible disk of the Sun are regions of intense and varying magnetic fields.

    • The number and position of sunspots vary over an 11-year period known as the sunspot cycle. It is also called the solar activity cycle.

    • Intense magnetic fields loop out into the corona and trap million-degree, x-ray-emitting gas. The looping magnetic features are known as coronal loops.

    • The corona is heated when coronal loops merge and magnetically reconnect, releasing magnetic energy.

    • Coronal holes are low-density regions in the corona that do not emit detectable x-ray radiation.

    • Comet ion tails, which always point away from the Sun, provided the first evidence for a charged solar wind.

    • The solar winds consist of electrons, protons, other ions, and magnetic fields always streaming away from the Sun.

    • There are two solar winds, a fast wind moving at about 750 kilometers per second and a slow wind moving at about half that speed.

    • The fast solar wind originates in coronal holes, and the slow solar wind originates from regions of closed magnetic field. Both the open and closed magnetic regions vary in location during the 11-year solar activity cycle.

    • A solar flare lasts about 10 minutes. It emits intense radiation and energetic charged particles that move away from the Sun at nearly the speed of light. Solar flares are Earth-sized regions whose temperature can become as hot as 20 million K, or slightly hotter than the center of the Sun.

    • Coronal mass ejections are magnetic bubbles that carry about 10 billion tons of the corona out into surrounding space. A coronal mass ejection becomes larger than the Sun.

    • When directed at the Earth, it takes a coronal mass ejection about 4 days to arrive at our planet.

    • The Earth’s magnetosphere deflects the solar wind away from the planet.

    • Strong magnetic storms and intense auroras occur when a coronal mass ejection strikes the Earth’s magnetosphere with the right magnetic alignment that permits magnetic reconnection.

    • Protons ejected into space by solar flares threaten astronauts. Intense x-rays from a solar flare alter the Earth’s ionosphere and radio communication on the planet.

Copyright 2010, Professor Kenneth R. Lang, Tufts University