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Дата изменения: Wed Jun 2 13:46:47 1999 Дата индексирования: Tue Oct 2 02:26:50 2012 Кодировка: Поисковые слова: lightning |
The measurement of line widths can provide information concerning ion
temperatures, sub-resolution turbulent motions and velocity fluctuations
associated with magnetohydrodynamic (MHD) waves in the corona.
Line width variations combined with simultaneous electron density estimates,
provides a very powerful diagnostic for the solar corona.
In two previous papers we have reported such observations
off the Western limb (Doyle et al. 1998; hereafter DBP) and above a Coronal Hole
(Banerjee et al. 1998; hereafter BTDW) based on Si VIII lines observed
with SUMER (Wilhelm et al. 1995) onboard SOHO. Doschek et al.
(1997) have derived electron densities as a function of height in the north and
south polar coronal holes. They find that for distances of a few arc seconds
outside the solar limb, the average line of sight electron densities in the
coronal holes are about a factor of 2 lower than in quiet sun regions.
Electron densities similar to those derived from the Si VIII lines are
reported by Fludra et al. (1999a,b) and Doyle et al. (1999) based on Si IX
data obtained with the CDS instrument onboard SOHO (Harrison et al. 1995).
Wilhelm et al. (1998) have deduced electron temperatures, densities and ion
velocities in plumes and inter-plume regions of coronal holes. Recently Warren
& Hassler (1999) have used Si III, Mg VIII, Si VIII and
Mg IX line ratios for electron density measurements. In an earlier study,
Hassler et al. (1990) presented line profiles of coronal lines from
Mg X 609/625 Å up to
140,000 km above the limb.
In the coronal hole (see BTDW), the line width data show that the non-thermal
line-of-sight velocity increases from
at 27 arc sec
above the limb to
some 250 arc sec (i.e.
180,000 km) above the limb. The electron density shows a decrease from
to
over the same
distance. It was shown by DBP and BTDW that the Si VIII
non-thermal velocity was inversely
proportional to the quadratic root of the electron density, in excellent
agreement with that predicted for undamped radially propagating Alfvén waves.
The Western limb data showed a similar trend. Several questions arise from this
work, e.g. how representative are the Si VIII lines of coronal hole
conditions near the limb? How do these observations of line width and electron
density out to
compare with measurements further off-limb?
We address these issues by looking at additional SUMER data out to 1.38
coupled with measurements
obtained from LASCO and UVCS. This allows us to investigate physical conditions
of the solar corona and the coronal-heliospheric interface. Earlier observations
of Skylab and more recently SOHO/UVCS
observations (Cranmer et al. 1999) have established that the fast solar wind
originates from coronal holes, while the slow wind is associated with bright
equatorial streamers and a number of dramatic transient events such as coronal
mass ejections (CMEs). We concentrate here on the coronal hole region in the
solar minimum phase (during Nov-Dec '96). All solar wind modelling requires
knowledge of both the electron density and non-thermal velocity at the base of the
coronal hole as boundary conditions. We hope that our results will provide
input parametric values for such solar wind modelling.