1.24: Sunspots, Coronal Mass Ejections, and the Solar Wind

Sunspots and Coronal Mass Ejections

Sunspots are relatively dark patches that appear temporarily on the Sun's photosphere (Figures 1-59 and 1-60). Sunspots are cause by a flux in magnetic fields that appear to inhibit convection. Sunspots usually occur in pairs, like the two ends of a U-shaped magnet. Sunspots last a few days to a few months before they dissipate. The concentration of sunspots on the solar surface tend to follow an 11 year cycle that also flows a small variation is the total amount of solar energy output.

Coronal mass ejections are unusually large eruptions of streaming plasma and radiation (composed of charged particles) under the influence of solar magnetism. Eruptions result in the formation of solar flares and prominences (arching flares) that erupt from the Sun's surface (Figures 1-61 and 1-62).

Figure 1.59. Sunspots appear as dark splotches on the Sun's surface.

Figure 1.60. Comparison of the relative size of sunspots to the size of the Earth.

Figure 1.61. A solar flare erupting on the Sun's surface.

Figure 1.62. A large solar prominence erupting on the Sun's surface.

The Solar Wind

The solar wind is a stream of energized, charged particles (mostly electrons and protons) flowing outward from the Sun's upper atmosphere. The ionized particles are released into space from the Sun's corona and by coronal mass ejections (prominences and flares). The solar wind moves through solar system at speeds roughly 500 miles per second (800 km/sec); about 10 days from Sun to Earth) and can reach temperatures of about 1 million degrees (Celsius). The solar wind is what blows a tail away from the bodies of comets as they go through the solar system. Estimates suggest the Sun loses the equivalent of one Earth mass about every 150 million years (which isn't much considering the size of the Sun). Large corona mass ejections from the Sun’s surface result in solar storms that frequently impact Earth and other planets.

The Earth’s magnetic field shields the planet from the erosive effects of the solar wind (Figure 1.63). Particles trapped by Earth’s magnetic field flow into the upper atmosphere producing the aurora borealis (Northern Lights) and aurora australis (Southern Lights) (Figure 1.64). Over geologic time, the solar wind also erodes the atmosphere of planets with weak magnetic fields (this includes Mercury, Mars, and the Moon). Strong auroras have been observed on the gas planets (Jupiter, Saturn, Uranus, and Neptune)—all of which have a dense atmosphere and a strong magnetic field.

Solar storms associated with coronal mass ejections can interfere with radio communications, cause damage to satellites, and impact electrical transmission lines and facilities (resulting in power outages). During strong solar storms long lines of metal (like electrical power lines, pipelines, and railroad lines in northern regions can overload with electrical charges which and spark to nearby objects and have been reported to have started brush fires. Because massive solar ejections can be observed, the possible impacts of solar storms can be predicted.

Figure 1.63. Coronal mass ejections result in the solar wind which is deflected and captured by the Earth's magnetic field.

Figure 1.64. The aurora borealis are streaming light displays in the northern hemisphere.