Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986apj...311.1031e&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 311, Dec. 15, 1986, p. 1031-1042. Research supported by the University of C
Astronomy and Astrophysics
Astrophysics
16
Cometary Atmospheres, Ionopause, Magnetohydrodynamic Stability, Solar Wind, Sunlight, Giacobini-Zinner Comet, Halley'S Comet, Kelvin-Helmholtz Instability, Solar Activity Effects
Scientific paper
The stability of the sheared MHD flow at the sunlit cometary ionopause at several heliocentric distances is examined, taking into account the effects of ion-neutral drag, sources, curvature, and compressibility. If the ionosphere is regarded as a magnetic field-free cavity, whether it is stable or unstable to the Kelvin-Helmholtz (K-H) mode depends on the value of the velocity shear assumed. Also, for any assumed value of the shear the instability is greater for smaller wavelengths (about 100 km) than for larger (at least 1000 km). It is also shown that even if the ionopause is initially unstable, a modest penetration of the interplanetary magnetic field into the ionosphere could lead to its restabilization. Which of the three instabilities, K-H, flute, or drag, dominates depends on the relative magnitude of the shear, the rather uncertain ion-neutral drag coefficient, and the density ratio across the ionopause. The apparent marginal instability of the ionopause of comet Giacobini-Zinner and the stability of the ionopause of comet Halley are explained essentially in terms of the different solar wind conditions encountered by the two comets.
Ershkovich A. I.
Flammer K. R.
Mendis Asoka D.
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