Other
Scientific paper
May 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...427..459p&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 427, no. 1, p. 459-474
Other
118
Astronomical Models, Gamma Rays, Magnetic Field Reconnection, Magnetohydrodynamic Waves, Solar Corona, Solar Wind, X Rays, Magnetic Fields, Magnetic Signatures, Skylab Program, Solar Granulation, Solar Maximum Mission, Solar Observatories
Scientific paper
X-ray bright points are an important part of the solar corona and therefore of the coronal heating problem. When it was first realized that bright points are always situated above opposite polarity magnetic fragments in the photosphere, it was natural to suggest that such fragments represent emerging flux and that an X-ray bright point is caused by reconnection of the emerging flux with an overlying coronal magnetic field. However, a recent important discovery at the Big Bear Solar Observatory is that the magnetic fragments of opposite polarity are usually not emerging but are instead coming together and disappearing and so are referred to as canceling magnetic features. Sometimes a tiny filament is observed to form and erupt at the same time. A unified model is here proposed which explains these observational features and has several phases: (1) a preinteraction phase, in which two photospheric fragments are unconnected magnetically and begin to approach one another, until eventually oppositely directed fields from the fragments come into contact at a second-order null point; (2) an interaction phase, in which the null point becomes an X-point and rises into the corona; an X-ray bright point is created for typically 8 hr by coronal reconnection, driven by the continued approach of the photospheric sources; long hot loops and Yohkoh X-ray jets may be created by the reconnection, and rapid variability in bright point emission may be produced by an impulsive burst regime of reconnection; the explosive events seen with High Resolution Telescope and Spectrograph (HRTS) may be produced as the X-point passes through the upper chromosphere; (3) a cancellation phase, in which a canceling magnetic feature is produced by photospheric reconnection as the fragments come into contact and decrease in strength; above the canceling fragments a small filament may form and erupt over typically an hour. An important role is played by the interaction distance (d), which is proportional to the magnetic flux of the fragments and inversely proportional to the overlying magnetic field strength. It determines the fragment separation at which the interaction phase begins and the resulting maximum height of the reconnection point. It is suggested that coronal reconnection driven by footpoint motion represents an elementary heating event that may be heating normal coronal loops and may be at the root of the nanoflare/microflare process. Bright points may well be at the large-scale end of a broad spectrum of events of the type modeled in this paper, which are heating the solar corona. At very small scales, such events in 'furnaces' in the coronal hole network may even produce high-frequency waves that propagate out and drive the solar wind (Axford 1993).
Martin Sara F.
Parnell Clare E.
Priest Eric R.
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