Insolar physics, aspicule, also known as afibril ormottle,[a] is a dynamic jet of plasma in theSun'schromosphere about 300 km in diameter.[1] They move upwards with speeds between 15 and 110 km/s from thephotosphere and last a few minutes each[1] before falling back to the solar atmosphere.[2] They were discovered in 1877 byAngelo Secchi, but the physical mechanism that generates them is still hotly debated.
Spicules last for about 15 minutes;[3] at thesolar limb they appear elongated (if seen on the disk, they are known as "mottles" or "fibrils"). They are usually associated with regions of highmagnetic flux; theirmass flux is about 100 times that of thesolar wind. They rise at a rate of 20 km/s (or 72,000 km/h) and can reach several thousand kilometers in height before collapsing and fading away.
There are about 3,000,000 active spicules at any one time on theSun'schromosphere.[3] An individual spicule typically reaches 3,000–10,000 km altitude above the photosphere.[4]
Spicules are generally found to move in a parabolic fashion, eg <Pereira et al, 2012>. This means that after their ejection from the solar surface at high speed they decelerate at a constant rate until they reach their maximum height (or length, as most are inclined to the vertical). They then descend back down towards the surface, accelerating at the same rate, until reaching their starting point with the same speed downwards as they originally had in the upward direction.
 | This sectionis missing information about numerous other proposed mechanisms. Please expand the section to include this information. Further details may exist on thetalk page.(October 2019) |
Bart De Pontieu (Lockheed Martin Solar and Astrophysics Laboratory,Palo Alto, California, United States),Robert Erdélyi and Stewart James (both from theUniversity of Sheffield, United Kingdom) hypothesised in 2004 that spicules form as a result ofP-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 (seehelioseismology). Magnetic flux tubes that are tilted away from the vertical can focus and guide the rising material up into the solar atmosphere to form a spicule.[5] However, there is still some controversy about the issue in the solar physics community.
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