Solar Prominence

Solar prominences are large, glowing clouds of extremely hot ionized gases (or plasma) suspended in magnetic field loops above the sun’s photosphere, primarily in the corona. Although impossible to see in white light (because the brilliance of the photosphere blots them out), they are easily visible in hydrogen alpha images (pictures taken in light emitted by hydrogen atoms, the principal constituent of the sun). Prominences have been observed during eclipses for hundreds of years, but it was not until the twentieth century that they were observed in detail.

Prominences arise as products of the solar activity cycle. The hot gas that comprises the sun is magnetized, and as the sun rotates and the heat of its interior churns its subsurface layer in great convective bubbles, the magnetic field becomes increasingly tangled. Large magnetic loops burst through the sun’s photosphere and into its atmosphere. At the focal points of these loops one often finds sunspots, while trapped in the upper part of the loop is hot (about 10,000 K), glowing hydrogen gas. These glowing loops are prominences, and not surprisingly, they are most common at the height of the solar activity cycle, and decrease in number as the complex magnetic field rearranges itself into simpler configurations and the activity cycle declines.

Because the magnetic loops are not static, prominences evolve on time scales of days. As a magnetic loop expands, the pressure of the material inside it may become sufficient to break through the field, and the prominence will then dissipate. The gas inside a prominence flows from one part of the loop to the other as well, making prominences dynamic objects for study from Earth-based and satellite telescopes. When prominences break apart they are called coronal mass projections.

Prominences are typically huge; several Earth-sized planets could fit inside a typical prominence loop. Graceful quiescent prominences last for up to several days, while their more violent cousins, the eruptive prominences, only last for a matter of hours. Prominences do not appear to be confined to the sun; evidence exists for gigantic, prominence-like structures on other stars. Some stellar prominences have been suggested to extend as far as an entire stellar radius from the surface of their parent star. Such a structure would dwarf even the largest solar prominences.

In December 1995, the Solar and Heliospheric Observatory (SOHO) was jointly launched by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). SOHO’s mission is to study the sun, including solar prominences. As of 2006, the largest solar prominence observed by SOHO was a 216,000 mi (350,000 km) long prominence that was seen in 1997. The diameter of this prominence was almost thirty times Earth’s diameter. Its two-year mission is still on-going as of October 2006.

Jeffrey Hall