On the subadiabatic magnetohydrodynamic overstability in background polytropes

Journal Article · Tue Jan 01 04:00:00 EST 1991 · Astrophysical Journal; (USA)

DOI:https://doi.org/10.1086/169635· OSTI ID:6002322

Lou, Y ^[1]

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1. Alaska Univ., Fairbanks (USA)

The instability properties are studied of nonadiabatic MHD oscillations in background polytropes permeated by a uniform vertical magnetic field with the effects of thermal conduction, viscosity, and resistivity included. In particular, the very existence of subadiabatic MHD overstabilities for the quasi-fast modes over a wide range of field strengths are shown, and it is demonstrated that their presence originates from the subadiabatic acoustic overstability. The magnetic Lamb mode based on the previous results is identified. It is also shown that, in the strong-field limit, isothermal oscillations are neutral. Since the convective stability is enhanced by the presence of vertical magnetic field, MHD overstabilities can thus occur in a wide range of temperature gradients (subadiabatic as well as superadiabatic) while the system retains convective stability. The growth rates of oscillations are larger for steeper adverse temperature gradients. These results are important for understanding umbral oscillations and the heat transport process in sunspots, and for determining possible mechanisms to excite rapid oscillations of magnetic Ap stars. 54 refs.

• OSTI ID: 6002322
• Journal Information: Astrophysical Journal; (USA), Journal Name: Astrophysical Journal; (USA) Vol. 367; ISSN ASJOA; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

640102 -- Astrophysics & Cosmology-- Stars & Quasi-Stellar
Radio & X-Ray Sources
640104* -- Astrophysics & Cosmology-- Solar Phenomena
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
ENERGY TRANSFER
FLUID MECHANICS
HEAT TRANSFER
HYDRODYNAMICS
MAGNETIC FIELD CONFIGURATIONS
MAGNETIC FIELDS
MAGNETIC STARS
MAGNETOHYDRODYNAMICS
MAIN SEQUENCE STARS
MECHANICS
OSCILLATIONS
SOLAR ACTIVITY
STABILITY
STARS
STARSPOTS
STELLAR ACTIVITY
SUN
SUNSPOTS
TEMPERATURE GRADIENTS
THERMAL CONDUCTION
VISCOSITY