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Proposal ID Moses_6

Date of Proposal

D Moses


Code 7660.1
Washington,DC 20375




EIT Observing Program:

Joint LASCO/EIT Observations of Coronal Holes


D. Moses and J.W. Cook (;

Brief Scientific Summary:

Coronal holes are one of the few manifestations of the Sun's
large scale magnetic field which present a clear signature in the
structure of the lower (x-ray) corona. While the majority of
structures seen in the inner corona differentially rotate, with
rates comparable with the surface rate (XBPs) or the upper
convection zone rate (ARs), the coronal holes rotate rigidly. The
boundary of the coronal hole is a region where outside small
scale flux elements, because of their differential rotation, move
into or away from the coronal hole boundary. Elements entering
the hole interact with the exisiting large scale unipolar field.
Although the general evolution of magnetic flux on a wide range
of scales is beginning to be understood (Nash, Sheeley, and Wang
1988, Solar Phys., 177, 359), the specific details of the
transition from small scale to large scale structures are not
understood well enough to determine if the reconnection occurs in
a way that can heat significant amounts of plasma. Skylab soft
x-ray observations of coronal holes indicate that XBPs are
instrumental in the evolution of coronal hole boundaries (Kahler
and Moses 1990, Ap. J., 362, 728) and the disintegration of
coronal holes (Davis, 1985, Solar Phys., 95, 73). It often
appears that XBPs occur along the boundaries of coronal holes,
and the evolution of such XBPs is directly connected with changes
in the local coronal hole boundary.

Joint LASCO and EIT coronal hole observations will allow the
determination of the interaction of the small scale and large
scale structures associated with the evolution of coronal holes.
EIT observations can track the evolution of XBPs on the disk. We
plan to study a limb coronal hole, either one of the always
present polar coronal holes, or a disk coronal hole rotating over
the west limb (easier to pre-plan in the SOHO observations than a
hole coming over the east limb). The LASCO C1 coronagraph field
begins 0.1 Rsolar (approximately 100 arc sec) above the limb.
LASCO C1 observations will show the structure above the coronal
hole at approximately 10 arc sec spatial resolution, and in
particular, will show the relationship of disk XBPs and polar
plumes or "fountains" (YOHKOH nomenclature), and the general
large scale magnetic field as shown by the bright coronal loop

This is a different approach than JOP 14 since it is a data use
proposal for all data of LASCO and EIT for which there is an
appropriately configured coronal hole. The JOP 14 only considers
those times specifically in the JOP campaign.

Suggested EIT Observations:

All 4 wavelengths are useful, but 195 A (Fe XII) shows best both
the coronal hole and boundaries, and the XBPs and EUV bright
points. We can use a reduced field of 1/4 the full field,
covering the appropriate solar limb. We would like simultaneous
magnetograph observations, either ground based (NSO/Kitt Peak),
or SOHO MDI observations, if appropriate. From ground based
magnetograph observations, 15 minute temporal resolution follows
the magnetic field evolution adequately, but XBPs can flare and
evolve on minute timescales, and presumably the EUV bright points
even more quickly. The synoptic observations can follow the 8
hour time scale evolution, but we would also like a period of
much higher temporal coverage.



EIT Observing Program:

Joint LASCO/EIT Observations of Coronal Holes

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