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Armagh Observatory

Occurrence and lifetime of spicules seen at the solar limb

As seen during the Venus transit, the solar limb (the visual edge of the solar disc) appears neat and clear in white (visible) light. But higher in the solar atmosphere, in the ultra-violet light, the solar limb proves to be fuzzy and tremendously dynamic.

What we see in the higher solar atmosphere, mainly at the solar poles, are rapidly evolving jet-like structures, called spicules. Compared to other solar features, they are quite small: their height usually ranges between 5,000 and 15,000 km, occasionally stretching to over 30,000 km (that is, more than twice the diameter of the Earth).

spicules

This figure illustrates some recent research completed at the Armagh Observatory by Prof. J.G. Doyle (Armagh Observatory), Dr. L.D. Xia (Univ. of Science & Technology of China, Hefei), M.D. Popescu (Astronomical Institute of the Romanian Academy, Bucharest and PhD student at Armagh Observatory) and J. Giannikakis (Univ. of Athens, Greece).

The two top plots are artificially coloured images of the solar atmosphere taken in the ultraviolet region of the electromagnetic spectrum, by two instruments from the SoHO (Solar and Heliospheric Observatory) spacecraft: EIT (Extreme Ultraviolet Imaging Telescope) and SUMER (Solar Ultraviolet Measurements of Emitted Radiation). They show the solar spicules continuously appearing and then fading away.

The first figure shows the Sun in the light emitted by the Fe XII line at the wavelength of 195 Å. This ionization level of iron only exists at a temperature of 1,600,000 K, and comes from the hottest and the highest layer of the solar atmosphere, called the corona. The dark region near the bottom of the image is a coronal hole.

The middle figure represents the time evolution of a small region at the solar limb, inside the coronal hole (the narrow red rectangle in the upper image). The image is a time sequence taken repeatedly every minute for 7 hours (time is shown on the x-axis of the lower plot). Brighter colours represent higher intensity of the ultraviolet N IV line, originating at a temperature of approximatively 140,000 K in the solar transition region.

Distances on the Sun are measured in arcseconds, one arcsecond on the Sun being approximatively 715 km. This figure has the highest spatial resolution currently achievable for the Sun at this wavelength: one arcsecond. The observed region is one arcsecond wide and 60 arcseconds long (the red rectangle had been exaggerated in width for clarity).

The lower figure represents the intensity at one point within the narrow rectangle, namely at 10 arcseconds above the limb. This corresponds to the dotted white line at Y=-10 in the middle plot; the green line represents the limb.

Solar ultraviolet spicules have never previously been seen with such a clarity or for such a long single observing sequence. The data reveal the short lifetime of spicules (5 to 15 minutes), as well as many other important dynamic characteristics of those structures, some of which have never been observed before.

What is even more interesting about spicules is that it is still not known how they appear on the solar disc, compared with how they look at the solar limb. Preliminary work suggests that spicules are actually seen on the disc as brightenings, sudden increases of intensity, which themselves are made up from bursts of jet-like features caused by small-scale magnetic flux cancellations.

Miruna D. Popescu
Armagh Observatory
16 June 2004

Last Revised: 2009 November 2nd