HR: 0800h
AN: SH11A-0245    [Abstracts]
TI: Influence of Thermal Conduction on Acoustic Waves in Coronal Loops
AU: * Bogdan, T J
EM: tom@ucar.edu
AF: High Altitude Observatory National Center for Atmospheric Research , P.O. Box 3000, Boulder, CO 80307-3000 United States
AB: Observations of intensity oscillations along coronal loops have sparked considerable interest for their potential contributions to the nascent field of coronal seismology. The prevailing interpretation is that magnetic field-guided longitudinal acoustic waves are responsible for the loop intensity oscillations. This contribution assesses the influence of classical (Spitzer) thermal conduction on longitudinal acoustic waves in the solar corona through an idealized but exactly solvable model. The model consists of an isothermal, stratified (g=constant) atmosphere in which a vertically propagating acoustic wave of prescribed frequency and amplitude, traveling in the direction of decreasing density, is imposed throughout the lower half of the atmosphere. Based on the linearized equations of motion the complete steady-state solution is obtained. In addition to the imposed acoustic wave, this solution contains reflected acoustic and thermal conduction waves in the lower half of the atmosphere, and transmitted acoustic and conduction waves in the upper half of the atmosphere. The acoustic waves in the lower half of the atmosphere have almost no entropy fluctuations, while the transmitted acoustic wave in the upper half of the atmosphere has almost no temperature fluctuations. The boundary between the two halves of the atmosphere is located where the gas pressure passes through a critical value determined by the thermal conductivity. This critical pressure is proportional to the wave period and the three-halves power of the temperature. In c.g.s. units, the critical pressure is 4.1 10^-4 for a 5-minute oscillation in a million degree plasma. Except in the immediate vicinity of the coronal acoustic cutoff frequency (0.43-0.47 mHz) the energy flux carried by the reflected wave is negligible. The fraction of the energy flux carried by the transmitted acoustic wave (relative to that carried by the imposed acoustic wave) has a maximum value of 44% for a wave period of approximately 29 min, and it decreases to zero as the wave frequency approaches infinity and the cutoff frequency. The remainder of the incident wave energy flux is dissipated by the two conduction waves. The transmitted conduction wave causes the entire upper half of the atmosphere to oscillate uniformly about the equilibrium temperature with the prescribed incident wave frequency, while the transmitted acoustic wave is nearly isothermal. This raises the curious possibility that intensity oscillations in coronal loop tops---where the gas pressure is much less than the critical value---might be the transmitted conduction waves, while any transmitted acoustic waves would only be detectable through their Doppler shifts.
DE: 3384 Acoustic-gravity waves
DE: 7509 Corona
DE: 7522 Helioseismology
SC: SPA-Solar and Heliospheric Physics [SH]
MN: Fall Meeting 2005