Dynamics of coronal hole regions. I - Steady polytropic flows with multiple critical points

Details Dynamics of coronal hole regions. I - Steady polytropic flows with multiple critical points Dynamics of coronal hole regions. I - Steady polytropic flows with multiple critical points

Jul 1976

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1976soph...49...43k&link_type=abstract

Solar Physics, vol. 49, July 1976, p. 43-56.

Physics

198

Critical Point, Polytropic Processes, Solar Corona, Steady Flow, Critical Flow, Dynamic Characteristics, Solar Atmosphere, Solar Magnetic Field, Solar Physics, Subsonic Flow, Supersonic Flow

Scientific paper

Dynamical properties are examined for a coronal-expansion model in which the flow tubes are oriented radially to the sun but may have a radial dependence of cross-sectional area appreciably different from a square-of-distance form. Results based on the polytrope law show that more than one critical point may exist in the flow, that these critical points may be either X-type or O-type, and that only X-type points afford a smooth transition from subsonic to supersonic flow and vice versa. It is found that the physically realistic solution for the case of the solar corona begins with a subsonic flow near its base and traverses that X-type critical point for which the critical-point function is an absolute minimum, thereby yielding supersonic expansion at large distances. Application of this theory to the type of geometry expected for coronal-hole regions strongly suggests that coronal holes may be unique among open-field coronal regions since the solution which becomes supersonic at a critical point located in the region of strong divergence is the only physically acceptable solution allowed by the rapid spreading of the field lines threading hole regions at low coronal heights. It is proposed that the flow in coronal-hole regions is supersonic everywhere outside the first critical point encountered (i.e., everywhere except in a narrow region just above the coronal base).

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