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).
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
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