Issue |
A&A
Volume 455, Number 2, August IV 2006
|
|
---|---|---|
Page(s) | 697 - 708 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20054693 | |
Published online | 04 August 2006 |
Solar coronal-hole plasma densities and temperatures
Max-Planck-Institut für Sonnensystemforschung (MPS), 37191 Katlenburg-Lindau, Germany e-mail: wilhelm@mps.mpg.de
Received:
15
December
2005
Accepted:
13
April
2006
Polar plumes extending from the Sun into the solar corona have long been seen
during eclipses, and can now be studied without this restriction with telescopes
and spectrometers on board of spacecraft. Despite the large amount of
observational data available on this prominent phenomenon, it is not clear
whether plumes contribute substantially to the fast solar-wind streams emanating
from coronal holes. An understanding of the processes leading to the formation
of bright plumes and the surrounding darker inter-plume regions in coronal holes
requires a good knowledge of the physical conditions in plumes and their
environment. This investigation aims at measuring the electron densities and
temperatures in these regions with the help of radiance ratios of ultraviolet
emission lines obtained by SUMER on SOHO.
It finds densities of about cm-3 in
bright plumes and
cm-3 in inter-plume lanes
at ≈45 Mm above the limb. At this height, the total plume
cross-section relative to the size of the coronal hole was found to be
less than 8%. The densities drop by a factor of roughly two over the next 80 Mm in height, in lanes a little less than seen in plumes.
In this height range, the electron temperatures in plumes are ≈
K and ≈
K in inter-plume
regions. The effective ion temperatures, deduced from the line widths,
are higher and nearly independent of the altitude in plumes,
whereas they increase in inter-plume regions, starting from an even higher
level. No systematic dependence of the line-of-sight bulk velocities
on the brightness could be found in the coronal-hole plasma.
Key words: Sun: UV radiation / Sun: corona / Sun: solar wind
© ESO, 2006
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