Sunspots, Solar Prominences and Solar Flares

Through telescope images we can see several interesting features on the sun that can have effects here on Earth. Let's look at sunspots, solar prominences and solar flares.

Sunspots
Dark, cool areas called sunspots appear on the photosphere. Sunspots always appear in pairs and are intense magnetic fields (about 5,000 times greater than the Earth's magnetic field) that break through the surface; field lines leave through one sunspot and re-enter through the other one. The magnetic field is caused by movements of gases in the sun's interior. Sunspot activity occurs as part of an 11-year cycle called the solar cycle where there are periods of maximum and minimum activity; we are currently in a solar maximum time (Figure 3).

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 3. The sun's 11-year solar cycle as reflected by the number of sunspots recorded to date and projected (dotted line). Selected EIT 195 angstrom (green) and MDI magnetogram (gray) images are shown. In this cycle, the sun undergoes a period of activity (solar maximum) followed by a period of quiet (solar minimum). The rising level can be clearly seen in the comparison of EIT and MDI images.

It is not known what causes this 11-year cycle, but two hypotheses have been proposed:

• Uneven rotation of the sun distorts and twists magnetic field lines in the interior. The twisted field lines break through the surface forming sunspot pairs. Eventually, the field lines break apart and sunspot activity decreases. The cycle starts again.
• Huge tubes of gas circle the sun's interior at high latitudes and begin to move toward the equator. When they roll against each other, they form spots. When they reach the equator, they break up and sunspots decline.

Solar Prominences
Occasionally, clouds of gases from the chromosphere will rise and orient themselves along the magnetic lines from sunspot pairs. These arches of gas are called prominences (Figure 4). Prominences can last two to three months and can extend 30,000 miles (50,000 km) or more above the sun's surface. Upon reaching this height above the surface, they can erupt for a few minutes to hours and send large amounts of material racing through the corona and outward into space at 600 miles per second or 1,000 km/s; these eruptions are called coronal mass ejections.

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 4. Large eruptive solar prominence in helium-2 image at 304 angstroms with an image of Earth added for size comparison. This prominence on July 24, 1999 was particularly large and looping, extending over 35 Earths out from the sun. Erupting prominences, when directed toward Earth, can affect communications, navigation systems,and even power grids, while producing auroras visible in the night sky.

Solar Flares
Sometimes in complex sunspot groups, abrupt, violent explosions from the sun occur. These are called solar flares. Solar flares are thought to be caused by sudden magnetic field changes in areas where the sun's magnetic field is concentrated. Solar flares are accompanied by the release of gas, electrons, visible light, ultraviolet light and X-rays. When this radiation and particles reach the Earth's magnetic field, they interact with it at the poles to produce the auroras (borealis, australis) as shown below (Figure 5). Solar flares can also disrupt communications, satellites, navigation systems and even power grids. The radiation and particles ionize the atmosphere and prevent the movement of radio waves between satellites and the ground or between the ground and the ground. The ionized particles in the atmosphere can induce electric currents in power lines and cause power surges. These power surges can overload a power grid and cause blackouts.

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 5. The sun's magnetic field and releases of plasma directly affect the Earth and the rest of the solar system. Solar wind shapes the Earth's magnetosphere, and magnetic storms are illustrated here as approaching Earth. These storms, which occur frequently, can disrupt communications and navigational equipment, damage satellites and even cause blackouts. The white lines represent the solar wind; the purple line is the bow shock line; and the blue lines surrounding Earth represent its protective magnetosphere. The magnetic cloud of plasma can extend to 30 million miles or 50 million km wide by the time it reaches Earth.