They look like little whiskers on the Sun's face when seen through a telescope, but the jets of solar gas known as spicules are thousands of kilometers long and shoot material up from the Sun's surface at 20 kilometers per second. They are transient — each one lives just a few minutes before disappearing — but more than 100,000 of them cover the Sun's surface at any moment. Solar physicists have struggled to explain spicules since they were discovered in 1877. Now a new theory based on some of the sharpest-ever images of the Sun may finally provide the answer.
According to the new theory developed by Bart De Pontieu (Lockheed Martin Solar and Astrophysics Lab) and his collaborators, the spicules are intimately related to the slight pulsations of the Sun's visible surface (photosphere) caused by giant sound waves called p-modes ricocheting in its interior. The p-modes are normally trapped near the photosphere, but they can leak into the atmosphere above with the help of strong magnetic-field structures called magnetic flux tubes.
The magnetic flux tubes act as channels, focusing the waves as they move outward from the photosphere through the atmosphere. The waves accelerate as they move higher, pushing gas before them and eventually squirting it out thousands of kilometers above the photosphere. These columns of squirted gas are visible as spicules from Earth.
A video captured by the researchers using the 1-meter Swedish Solar Telescope (SST) shows spicules flickering above the photosphere. The telescope captures features as small as 70 km across on the Sun, using a shape-changing mirror that corrects blurring caused by Earth's atmosphere.
Using their new theory, the researchers have built a computer model that can predict where and when spicules will form based on the location of the magnetic flux tubes and timing of the sound waves. The model's minute-by-minute predictions for specific locations on the Sun's visible surface match up well with recent data obtained with the orbiting Solar and Heliospheric Observatory and the SST.