In solar physics, a spicule is a dynamic jet of about 500km diameter on the Sun. It moves upwards at about 20 km/s from the photosphere. They were discovered in 1877 by Father Angelo Secchi of the Vatican Observatory in Rome. The chromosphere is entirely composed of spicules.

Description

Spicules live for about 5-10 minutes; at the solar limb they appear elongated (if seen on the disk, they are known as "mottles" or "fibrils"). They are usually associated with regions of high magnetic flux; their mass flux is about 100 times that of the solar wind.

Prevalence

At any one time there are around 100,000 active spicules on the Sun; an individual spicule typically reaches 3,000-10,000 km altitude above the photosphere.

Causes

Bart De Pontieu (Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, California ), Robert Erdélyi and Stewart James (both from the University of Sheffield, Sheffield, UK) hypothesised in 2004 that spicules formed as a result of p-mode oscillations in the Sun's surface, sound waves with a period of about five minutes that causes the Sun's surface to rise and fall at several hundred meters per second (see helioseismology). Magnetic flux tubes that tilted away from the vertical can focus and guide the rising material up into the solar atmosphere to form a spicule. There is still however some controversy about the issue in the solar physics community.

Literature

• De Pontieu, B., Erdélyi, R. and James, S: Solar chromospheric spicules from the leakage of photospheric oscillations and flows In: Nature. 430/2004, p. 536–539,