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Studies of Fine Scale Structures in Coronal Holes -- Polar Plumes

 

Polar plumes, which appear prominently in white light coronagraph observations of coronal holes as distinct strongly collimated flow tubes, might carry the bulk of the mass and energy of the solar wind emanating from high-latitude regions of the sun's corona (polar coronal holes). These structures have been studied in detail in the past (see, e.g., Dziubenko, 1957; Saito, 1965; Newkirk and Harvey, 1969; Ahmad and Withbroe, 1977). More recently, Thieme et al (1990) have proposed that remnants of solar wind flows in plumes can be detected below 1 AU. It seems, therefore, timely to explore the physical properties of polar plumes by coordinating observations between different experiments on SOHO. Polar plumes are regions of relatively simple magnetic topology; thus a detailed understanding of the plasma dynamics within, and between, polar plumes is considered a crucial preliminary step in developing models for the solar wind outflow in the more complex geometries known to exist at lower heliographic latitudes (streamers, active regions, etc.).

Observations of plumes in the past have been of limited spatial resolution. The superior spatial resolution of the SOHO instrumentation will allow us to amass a much-improved database on plume morphology (shapes, widths, radial extents) and on plasma parameters such as proton temperature, flow speed and electron density. Furthermore, it is important to establish where the divergence of the field lines actually occurs. To achieve this goal, UVCS should observe very close to the coronal base (below 1.2 R) and the observations should extend out to at least a few solar radii, with an azimuthal angular resolution of 2 degrees, with steps of 0.05 R, as it is expected that the divergence of the field lines will occur very rapidly, although we do not know exactly at which radial distance. These observations should be coordinated with the inner coronagraph (C1) on LASCO, which has a field of view of 1.1 to 3 R. These observations will also be interpreted within the framework of a dynamical model of polar plumes, which is based on the lateral balance of plasma and magnetic pressures and takes into account the outward flow of plasma along nonradial flux tubes.

Studies of Fine Scale Structures in Coronal Holes -- Polar Plumes



next up previous contents
Next: Modeling the Solar Up: Examples of UVCS Previous: The Temperature Gradient



Peter Smith
Fri Jan 17 12:11:15 EST 1997